Embed Size (px)
Citation preview
University of WollongongResearch Online
University of Wollongong Thesis Collection University of Wollongong Thesis Collections
1992
Flow assessment of powders in pneumaticconveying : a bench top assessmentMukeshchandra Kantilal DesaiUniversity of Wollongong
Research Online is the open access institutional repository for theUniversity of Wollongong. For further information contact ManagerRepository Services: [email protected].
Recommended CitationDesai, Mukeshchandra Kantilal, Flow assessment of powders in pneumatic conveying : a bench top assessment, Doctor of Philosophythesis, Department of Mechanical Engineering, University of Wollongong, 1992. http://ro.uow.edu.au/theses/1579
FLOW ASSESSMENT OF POWDERS
IN PNEUMATIC CONVEYING
- A BENCH TOP ASSESSMENT
A thesis submitted in fulfilment of the requirements for the award of the degree of
DOCTOR OF PHILOSOPHY
from
THE UNIVERSITY OF WOLLONGONG
IVVOUONGO**
by \ UBfc^l
MUKESHCHANDRA KANTILAL DESAI
B.E.(MECH.), M.E.(HONS.), M.I.E. (AUST.)
DEPARTMENT OF MECHANICAL ENGINEERING 1992
CERTIFICATE
This is to certify that this work has not been submitted
for a degree to any other university or institution.
(Mukeshchandra K.Desai)
To my parents
iv
ACKNOWLEDGEMENTS
The author extends his most sincere thanks and appreciation to his thesis
supervisor, Dr. A.G. McLean, for the skillful guidance, the deep interest and
enthusiasm, numerous invaluable comments and encouragement throughout the course
of this investigation as well as for his personal generosity and assistance.
Thanks are also due to Prof. P. C. Arnold, Head of Department of Mechanical
Engineering for his assistance and support for this study.
The author wishes to thank the laboratory and workshop staff for helping to
construct and revise the experimental equipment.
The author wishes to convey his appreciation for moral support, patience and
understanding from his wife Dipika, daughter Ashka and son Vishal w h o have missed
week-end fun for years.
The scholarship provided by the Wollongong University which enabled me to
pursue this study is gratefully acknowledged.
v
ABSTRACT
This thesis outlines the use of powder properties, determined from bench tests to
predict powder flow behaviour in pneumatic conveying particularly in dense phase and
super dense phase systems. The bench test powder properties examined included,
particle size and distribution, bulk density and particle density, surface characteristics,
fluidization and deaeration characteristics, powder cohesiveness, tensile strength and
wall friction characteristics.
The surface characteristics of various powders pneumatically conveyed were
observed by an Electron Scanning Microscope. This information provides a good insight
into flow characteristics. Differences in shape and surface explain vastly different
conveying characteristics of supposedly similar powders.
Particle size and distribution of various powders were measured by a Malvern
Particle Sizer. It was found that particle size determines, to a large extent, powder
cohesiveness, fluidization and deaeration characteristics. Furthermore, the effect of
particle density, bulk density, permeability and compressibility on flow behaviour were
examined. These properties were determined using a Beckman Pycnometer, a Jenike
Compressibility Tester and a Jenike Permeability Tester. The observed fluidization
characteristics not only revealed the powder classification with respect to Geldart's
diagram, but also revealed the extent of powder cohesiveness and ease of segregation.
An estimate of a powder's cohesion was conveniently evaluated by use of an
Arch Tester. This tester consists of a perspex silo with a variable width slotted outlet
opened by a chain drive mechanism. The powder remains undisturbed as the outlet is
opened. Each powder bed examined was subject to a set deaeration time before opening
the outlet. These results were compared with the cohesion measured in a direct shear
tester. It was found that arch length and deaeration characteristics assists prediction of
powder behaviour and cohesion.
Tensile strengths were measured using an Ajax W.S.L. Tensile Tester under
different extents of consolidation. This property was found to indicate cohesiveness and
indirectly the air retention capacity of powders.
The deaeration characteristics of powders were observed in a perspex cylinder
suitably instrumented. In particular, pressure transducers were connected at the middle
vi
and bottom of the cylinder fitted with permeable and impermeable bases, respectively.
For effective measurement of deaeration characteristics, it was found that the fill rate
should be as fast as possible. These characteristics are important for assessing air
retention characteristics of powders.
Knowledge of the powder velocity is very important parameter in pneumatic
conveying. For instance, if powders travel too slow, they drop out from the suspension
and settle at the bottom of the pipe. This may lead to a blockage. Hence, it is necessary to
convey powders above the critical settling velocity and they should not be conveyed
with excessive velocity, which leads to powder degradation, pipeline wear and increased
energy consumption. %
To effect velocity measurement, an optical fibre probe was developed on the
cross-correlation principle using two sensing probes, a fixed distance apart. The probe
consisted of eight fibres in total. Six projector fibres are connected to a light source,
which emits light to the powder passing through a sight glass fitted in the conveying
line. In this case, the conveying line formed part of an actual pilot scale pneumatic
conveying rig. Reflected light from the travelling powder was transmitted by two
receiver fibres and cross-correlated, using a HP3721A correlator, to determine the
particle transit time between the two fibres and thus predict the powder velocity.
Wall friction is another important factor contributing to the pressure drop in dense
phase pneumatic conveying. The frictional properties of powders have an adverse effect
in pneumatic conveying. These properties were evaluated under aerated conditions in a
perspex tube by pushing powders upwards for different column lengths.
Due to the importance of cohesion in governing a powder's dense phase flow
characteristics or whether it can be transported by pneumatic conveying, the cohesion
strengths predicted by the Arch Tester, Jenike Shear Tester and the Tensile Tester.
Hence, cohesiveness ranking of the various powders tested was possible.
A new phase diagram incorporating powder properties is proposed to predict a
powder's an optimal pneumatic conveying mode.
This thesis concludes by correlating the described bench measured powder
properties with reported actual powder pneumatic conveying characteristics. The
identified correlations provide useful information for future pneumatic conveying system
designs.
vii
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENT iv
ABSTRACT v
TABLE OF CONTENTS vii
LIST OF FIGURES x
LIST OF PLATES xvi
LIST OF TABLES xvii
CHAPTER 1 INTRODUCTION
1.1 Introduc tion 1
1.2 Pneumatic Conveying Systems 1
1.3 Flow Patterns 2
1.4 Types of Dense Phase Pneumatic Conveying Systems 6
1.5 Advantages and Disadvantages of Pneumatic Conveying
Systems 8
1.6 Powder Properties 9
1.7 The Objectives of the Research 10
CHAPTER 2 BLOW TANKS, DENSE PHASE FLOW AND
WALL FRICTION
2.1 Blow Tank 13
2.2 System Design 20
2.3 Dense Phase and Super Dense Phase Flow 20
2.4 Powder Properties 24
2.5 Wall Friction 27
CHAPTER 3 PNEUMATIC CONVEYING SYSTEM
3.1 Gas Solid Suspension 37
viii
3.2 Instability 39
3.3 Pneumatic Conveying Models 39
3.4 Particle Velocity 42
3.5 Particle Concentration 47
CHAPTER 4 COEFFICIENT OF RESTITUTION, BENDS
AND WEAR
4.1 Coefficient of Restitution 49
4.2 Bends 56
4.3 Wear and Abrasion 61
4.4 Attrition 62
4.5 Piping 65
CHAPTER 5 POWDER PROPERTIES
5.1 Introduction 68
5.2 Salient Powder Properties 68
CHAPTER 6 TEST EQUIPMENT AND PROCEDURES
6.1 Pneumatic Conveying Rigs 96
6.2 Velocity Measurement 103
6.3 Powder Concentration Ill
6.4 Hewlett Packard 3497A Data Acquistion System 112
6.5 Test Procedures for Pneumatic Conveying Rigs 117
6.6 Wall Friction Rig 120
6.7 Coefficient of Restitution Rig 122
6.8 Solid Density, Loose Poured Bulk Density and...
Compressibility Tests 122
6.9 Particle Size Measurement 128
6.10 Jenike Direct Shear Tester 130
ix
6.11 Tensile Tester 131
6.12 Cohesion Arch Tester 133
6.13 Deaeration Tester 135
6.14 Fluidization Rig 135
CHAPTER 7 RESULTS
7.1 Scanning Electron Microscope Photographs 139
7.2 Coefficient of Restitution 157
7.3 Particle Size Analysis 161
7.4 Bulk Density I 6 6
7.5 Solids Density 168
7.6 Arch Length and Drained Angle of Repose 168
7.7 Flow Function 180
7.8 Tensile Strength 182
7.9 Wall Friction 191
7.10 Deaeration 204
7.11 Fluidization and Deaeration 211
7.12 Pneumatic Conveying Flow Characteristics 217
CHAPTER 8 DISCUSSION 235
CHAPTER 9 CONCLUSIONS 304
BIBLIOGRAPHY 315
PUBLICATIONS 329
APPENDICES 330
X
LIST OF FIGURES FIGURE DESCRIPTION PAGE NO.
NO.
1.1 Basic Components of Pneumatic Conveying Systems 2
1.2 Pneumatic Conveying System Layout 3
1.3 Phase Diagram for Pneumatic Conveying of Solids 5
1.4 Flow Patterns in a Horizontal Pipe 5
1.5 Classification of Dense Phase Pneumatic Conveying Systems 7
2.1 A Single Blow Tank System 14
2.2 Parallel Arrangement 17
2.3 Series Arrangement 17
2.4 Aerated Blow Tank 19
2.5 Column of Material 29
2.6 Pressure Distribution 29
2.7 Force Analysis of a Column of Bulk Material 29
2.8 Forced Flow Apparatus 34
2.9 Conveying Force Results for Millet 35
2.10 Rademacher Wall Friction Tester 35
2.11 Front View of the Coefficient of Friction Test Rig 36
2.12 Variation of Frictional Force and the Normal Load for Brown Coal 36
4.1 Variation of the Coefficient of Restitution of Perspex with Temperature 54
4.2 Particle Trajectories for Quartz and Lime Impacting Various Pipe 54
Materials
4.3 Variation of Coefficient of Restitution versus Impact Height 55
4.4 Test Rig for Particle/Wall Collision 55
4.5 Variation of Coefficient of Restitution versus Impact Angle 56
4.6 Examples of Bend Geometries 59
5.1 Critical Arching Diameters 91
5.2 Critical Arching Diameters 91
5.3 Tensile Tester 92
5.4 Geldart's Classification of Powders 92
5.5 Deaeration Test Rig 93
5.6 Deaeration Experiment 93
5.7 Filling-Deaeration Plot for Zyolite Powder 94
5.8 Pressure Variation in a Hopper; Permeable and Impermeable Base 94
5.9 Deaerated Bed Settling for Group A Powder 95
5.10 Deaerated Bed Settling for Group C Powder 95
6.1 Configuration of Sturtevant Blow Tank 97
XI
6.2 Schematic Layout of the Sturtevant Pneumatic Conveying Test Rig 99
6.3 Types of Bends 100
6.4 Low Velocity Rig Blow Tank 102
6.5 L o w Velocity Test Rig Layout 105
6.6 T.200 Series Transducer Block Schematic 106
6.7 Pin Diode Amplifier Circuit 111
6.8 The Block Schematic of the T.300 Transducer 113
6.9 The Layout of the Front Panel T.300 Concentration Meter 114
6.10 T.300 Transducer Board 115
6.11 Jenike Compressibility Tester 127
6.12 Jenike Permeability Tester « 127
7.1 Coefficient of Restitution Rig 157
7.2 Coefficient of Restitution of Wheat, Millet and Bean (Co-rotation) 159
7.3 Coefficient of Restitution of Coal, Sinter and Sodium Ferrite 160
(Co-rotation)
7.4 Coefficient of Restitution of Wheat, Millet and Coal (Counter- 160
rotation)
7.5 Particle Size Distribution versus % Undersize of Fly Ash A', 164
'B', C and 'D'
7.6 Particle Size Distribution versus % Undersize of Fly Ash 'E',
*F\ 'G' and 'H' 164
7.7 Variation of Frequency versus Particle Size for Fly Ash 'A', 'B' and 'C 165
7.8 Variation of Frequency versus Particle Size for Fly Ash 'D' and 'E' 165
7.9 Variation of Frequency versus Particle Size for Fly Ash'F and 'G' 166
7.10 Variation of Frequency versus Particle Size for Cement, P V C Powder
and Sand 166
7.11 Arch Tester 169
7.12 Tester for Measuring the Drained Angle of Repose 170
7.13 Arch Length versus Deaeration Time for Fly Ash 'A' 175
7.14 Arch Length versus Deaeration Time for Fly Ash 'C 175
7.15 Arch Length versus Deaeration Time for Cement 176
7.16 Arch Length versus Deaeration Time for Cement, Fly Ash 'A' and 'B' 176
7.17 Arch Length versus Deaeration Time for Fly Ash 'B', *C\ 'D', 'E'
and 'F 177
7.18 Arch Length versus Bed Height for Cement 177
7.19 Arch Length versus Particle Size Variation for the Powders Tested 178
7.20 Variation of Drained Angle of Repose with Bed Height for Cement 178
7.21 Variation of Drained Angle of Repose Versus Deaeration Time for the
xii
Fly Ash Tested 179
7.22 Variation of Drained Angle of Repose versus Deaeration Time for
Cement 179
7.23 Variation of Drained Angle of Repose versus Mean Particle Size 180
7.24 Powder Flow Functions 180
7.25 Tensile Tester 182
7.26 Tensile Strength versus Consolidation Force for Fly Ash 'A', 'B' and
C 183
7.27 Tensile Strength versus Consolidation Force for Fly Ash 'D', 'E' and
F 183
7.28 Tensile Strength versus Consolidation Force for Fly Ash 'A', 'B' and
'C with 15 minutes Deaeration 184
7.29 Tensile Strength versus Consolidation Force for Fly Ash 'D', 'E' and
F with 15 minutes Deaeration 184
7.30 Tensile Strength versus Voidage for Fly Ash A', 'B' and C 185
7.31 Tensile Strength versus Voidage for Fly Ash 'D', 'E' and F 185
7.32 Tensile Strength versus Bulk Density for the Fly Ash Tested 186
7.33 Tensile Strength versus Voidage for Fly ash 'A', 'B' and 'C with
15 minutes Deaeration 186
7.34 Tensile Strength versus Voidage for Fly Ash 'D', 'E' and F with
15 minutes Deaeration 187
7.35 Tensile Strength versus Bulk Density for the Fly Ash Tested with 15
minutes Deaeration 187
7.36 Tensile Strength versus Consolidation Force for Light Soda Ash,
Dense Soda Ash, P V C Powder and Castor Sugar 188
7.37 Tensile Strength versus Voidage for Light Soda Ash, Dense Soda Ash
and P V C Powder 188
7.38 Tensile Strength versus Consolidation Force for Fly Ash 'J' 189
7.39 Tensile Strength versus Consolidation Force for Fly Ash 'H', T and 'J' 189
7.40 Tensile Strength versus Consolidation Force for Cement 190
7.41 Wall Friction Rig 191
7.42 Frictional Force versus Aeration Air Pressure for Brown Rice 192
7.43 Frictional Force versus Aeration Air Pressure for White Rice 192
7.44 Frictional Force versus Aeration Air Pressure for Rice Flakes 193
7.45 Frictional Force versus Aeration Air Pressure for Millet 193
7.46 Frictional Force versus Aeration Air Pressure for Wheat 194
7.47 Frictional Force versus Aeration Air Pressure for Sand 194
xin
7.48 Frictional Force versus Aeration Air Pressure for Shirley Phosphate
7.49 Frictional Force versus Column Length for Brown Rice
7.50 Frictional Force versus Column Length for White Rice
7.51 Frictional Force versus Column Length for Rice Flakes
7.52 Frictional Force versus Column Length for Millet
7.53 Frictional Force versus Column Length for Wheat
7.54 Frictional Force versus Column Length for Sand
7.55 Shear Stress versus Air Pressure for Brown Rice
7.56 Shear Stress versus Air Pressure for White Rice
7.57 Shear Stress versus Air Pressure for Rice Flakes
7.58 Shear S tress versus Air Pressure for Millet ,
7.59 Shear Stress versus Air Pressure for Sand
7.60 Shear Stress versus Air Pressure for Shirley Phosphate
7.61 Aeration Air Pressure versus Wall Friction Factor uk for Brown Rice
7.62 Aeration Air Pressure versus Wall Friction Factor uk for White Rice
7.63 Aeration Air Pressure versus Wall Friction Factor uk for Rice Flakes
7.64 Aeration Air Pressure versus Wall Friction Factor uk for Millet
7.65 Aeration Air Pressure versus Wall Friction Factor uk for Wheat
7.66 Deaeration Tester
7.67 Pressure Variation During Filling; Permeable Base
7.68 Deaeration Time of Fly Ash A', F and 'G'; Permeable Base
7.69 Deaeration Behaviour of Fly Ash 'A', F and 'G'; Permeable Base
7.70 Pressure Variation During Filling; Impermeable Base
7.71 Deaeration Time of Fly Ash 'A', F and 'G'; Impermeable Base
7.72 Deaeration Time of Fly ash 'C ; Impermeable Base
7.73 Deaeration Behaviour of Fly ash A', F and 'G'; Impermeable Base
7.74 Deaeration of Fly Ash 'A', F and 'G'; Permeable Base
7.75 Deaeration of Fly Ash'E'; Permeable Base
7.76 Deaeration of Fly Ash A'and'G'; Permeable Base
7.77 A Filling-Deaeration Graph for Fly Ash 'A'; Impermeable Base
7.78 Fluidization Rig
7.79 Geldart's Fluidization Diagram Showing the Classification of Fly Ash
7.80 Fluidization Analysis of Fly Ash A'
7.81 Fluidization Analysis of Fly Ash 'C
7.82 Fluidization Analysis of Fly Ash A', "C and 'E
7.83 Fluidization Analysis of Alumina
7.84 Fluidization Analysis of Sand
7.85 Fluidization Analysis of P V C Powder
195
195
196
196
197
197
198
198
199
199
200
200
201
201
202
202
203
203
204
205
205
206
206
207
207
208
208
209
209
210
212
212
213
213
214
214
215
7.86 Deaeration of Fly Ash 'A' in Fluidization Rig 215
7.87 Deaeration of Fly Ash 'C in Fluidization Rig 216
7.88 Deaeration of Fly Ash *F in Fluidization Rig 216
7.89 Friction Loop 217
7.90 Exploded View of a Typical Pipeline Air Pressure Tapping Location 218
7.91 Transducer Air Pressure versus Distance from Blow Tank 229
7.92 Transducer Air Pressure versus Distance from Blow Tank 229
7.93 Transducer Air Pressure versus Distance from Blow Tank 230
7.94 Transducer Air Pressure versus Distance from Blow Tank 230
7.95 Transducer Air Pressure versus Distance from Blow Tank 231
7.96 Transducer Air Pressure versus Distance from Blow Tank 231
8.1 Variation of Specific Surface versus Mean Particle Size for the Fly Ash
Tested 237
8.2 Variation of Mean Particle Size versus Characteristic Dimensions of Fly
Ash 239
8.3 Variation of Average Characteristic Dimensions versus Mean Particle
Size 239
8.4 Variation of Mean Size versus % < 5.8 p m for Fly Ash Tested 240
8.5 Variation of Mean Size versus Particle Size Distribution Span for Fly
Ash Tested 240
8.6 Variation of Mean Size versus Average % < 5.8 p m and Particle Size
Distribution Span for the Fly Ash Tested 241
8.7 Bulk Density versus Major Consolidation Stress 242
8.8 Bulk Density versus Major Consolidation Stress 242
8.9 Compressibility Coefficient Variation versus Mean Particle Size 243
8.10 The Ratio of Packed to Loose Poured Bulk Density versus Particle Size
for the Fly Ash and Cement Tested 245
8.11 Variation of Mean Particle Size versus Hausner Ratio 245
8.12 Variation of Hausner Ratio versus Different Particle Size Span 246
8.13 Variation of Hausner Ratio versus Average Particle Size Distribution
Span 246
8.14 Voidage versus Mean Particle Size for the Fly Ash and Cement Tested 248
8.15 Variation of Ratio of Packed to Loose Poured Bulk Density versus
Drained Angle of Repose for the Powders Tested 251
8.16 Variation of Effective Angle of Friction from Experiment and Shear
Tester 253
8.17 Variation of Internal Angle of Friction from Experiment and Shear
Tester 254
XV
8.18 Flowability Index of Fly Ash and Cement versus Arch Length 255
8.19 Variation of Adhesion Force versus Consolidation for Fly Ash 256
8.20 Variation of Adhesion Force versus Consolidation for Fly Ash with
Deaeration 257
8.21 Wall Yield Loci for Fly Ash 'A', V and 'D' for Stainless Steel 259
8.22 Variation of Deaeration Time Constant versus Mean Particle Size for
Impermeable and Permeable Bases 261
8.23 Variation of Deaeration Time Constant versus Particle Size Distribution
Span for Impermeable and Permeable Bases 262
8.24 Variation of Dense Phase Voidage versus Particle Density 263
8.25 Deaeration Behaviour of Fly Ash 'A', 'C and 'E' 265
8.26 Variation of Permeability Factor versus Mean Size for Fly Ash Tested 266
8.27 Permeability of the Fly Ashes Tested 267
8.28 Variation of Permeability Coefficient versus Mean Particle Size for Fly
Ash Tested 268
8.29 Variation of Permeability Coefficient versus Pressure Gradient for Fly
Ash Tested 268
8.30 Variation of Permeability Coefficient a and Compressibility Coefficient
b versus Mean Particle Size for Fly Ash Tested 269
8.31 Factors Affecting Powder Flow Characteristics 273
8.32 Pneumatic Conveying Phase Diagram 276
8.33 Pneumatic Conveying Phase Diagram (Alternate View) 277
8.34 Schematic Presentation of the Variation of Cohesion and
Permeability with Particle Size 279
8.35 Schematic Presentation of the Variation of Cohesion and
Deaeration versus Particle Size 279
8.36 Variation of Reciprocal of Arch Length and Hausner Ratio with
Particle Size of Fly Ash Tested 280
8.37 Mechanical Interlocking 281
8.38 Schematic Representation of the Variation of Permeability versus
Mechanical Interlocking 281
8.39 Specific Examples of Powder Properties with respect to the
Proposed Powder Conveying Phase Diagram 284
8.40 Variation of Particle Velocity with Time for Cement 289
8.41 Variation of Particle Velocity with Time for Cement 289
8.42 Variation of Particle Velocity with Time for Wheat 289
8.43 Variation of Volumetric Air Flow Rate with Time for Cement 290
8.44 Variation of Mass Flow ratio with Solids Mass Flow Rate for Cement
and Wheat 290
8.45 Variation of Solids Flow Rate versus Air Mass Flow Rate 291
8.46 Variation of Experimental versus Predicted Solids Velocity for Sand 292
8.47 Variation of Particle Velocity versus Time 292
8.48 Variation of Particle Velocity versus Time 293
8.49 Variation of Slip Velocity versus (1- Voidage) for Sand 296
8.50 Variation of Superficial Air Velocity versus (1-Voidage) for Sand 296
8.51 Variation of Superficial Air Velocity versus Mass Flow Ratio for Sand 297
8.52 Variation of Pipeline Pressure Drop versus Air Mass Flow Rate for Sand 297
8.53 Variation of Mass Flow Ratio versus Initial Blow Tank Pressure for
Sand . 298
8.54 Solid-Air Ratio Variation with Initial Blow Tank Pressure for Wheat 300
8.55 Mass Flow Rate of Solids with Initial Blow Tank Pressure for Wheat 300
8.56 Solids Mass Flow Rate versus Air Mass Flow Rate of Wheat 302
8.57 Average Blow Tank Pressure versus Air Mass Flow Rate for Wheat 302
8.58 Pipeline Pressure Drop versus Air Mass Flow Rate for Wheat 303
9.1 Recommended Sequence of Powder Tests 307
A. 1 Settling Velocity in Still Air of Spherical Particles with Diameter d 332
A.2 Settling Velocity in Still Air of Spherical Particles with Diameter d 333
A.3 Moody Diagram 334
B.l Typical Slugging Diagram 356
C. 1 Calibration Plot 360
C.2 Typical Calibration Graph 361
C.3 Typical Calibration Graph 363
C.4 Concentration Graph 364
LIST OF PLATES
6.1 Control Panel 98
6.2 Blow Tank 101
6.3 Receiving Hopper 102
6.4 L o w Velocity Rig Blow Tank 104
6.5 H P 3721A Correlator connected to Tealgate T.200 Series Transducer 108
6.6 Fibre Optic Probe with H P 3721A Correlator 109
6.7 Fibre Optic Probe Located on Sight Glass 110
6.8 Chart Recorder Connected to the T.300 Concentration Meter 115
6.9 Data Acquisition System (DAS) 116
6.10 Wall Friction Rig 121
6.11 Coefficient of Restitution Rig 123
6.12
6.13
6.14
6.15
6.16
6.17
6.18
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
7.9
7.10
7.11
7.12
7.13
7.14
7.15
7.16
7.17
7.18
7.19
7.20
7.21
7.22
7.23
7.24
7.25
7.26
Beckman Pycnometer for Measuring Solid Density
Jenike Compressibility Tester
Malvern Particle Sizer
Jenike Direct Shear Tester
Ajax Tensile Tester
(A) Cohesion Arch Tester (B) Deaeration Tester
Fluidization Rig
SEM Photograph of Raw Sugar Grains (X= 14)
SEM Photograph of Raw Sugar Grains (X= 30)
SEM Photograph of Raw Sugar Grains (X= 144)
SEM Photograph of Raw Sugar Grains (X= 1440) -
SEM Photograph of Light Soda Ash (X= 162)
SEM Photograph of Light Soda Ash (X=780)
SEM Photograph of Dense Soda Ash (X= 180)
SEM Photograph of Dense Soda Ash (X=600)
SEM Photograph of Zinc Fume (X= 90)
SEM Photograph of Zinc Fume (X= 600)
SEM Photograph of Zinc Fume (X= 6000)
SEM Photograph of PVC Powder (X= 360)
SEM Photograph of PVC Powder (X= 1800)
SEM Photograph of Pulverised Coal - Tallawarra (X= 60)
SEM Photograph of Pulverised Coal -Tallawarra (X=600)
SEM Photograph of Pulverised Coal - Tallawarra (X= 2100)
SEM Photograph of Petroleum Coke (X= 12)
SEM Photograph of Petroleum Coke (X= 120)
SEM Photograph of Petroleum Coke (X=600)
SEM Photograph of Petroleum Coke (X= 3000a)
SEM Photograph of Petroleum Coke (X= 3000b)
SEM Photograph of Eraring Fly Ash (X= 1320)
SEM Photograph of Liddell Fly Ash (X= 1320)
SEM Photograph of Liddell Fly Ash (X= 6600)
SEM Photograph of Vales Point Fly Ash (X=468)
SEM Photograph of Vales Point Fly Ash (X= 6600)
125 125
129
129
132
134
137
139
140
140
141
141
142
142
143
143
144
144
145
145
146
146
147
147
148
148
149
149
150
150
151
151
152
LIST OF TABLES 1.1 Data for Pneumatic Conveying Systems for a Pipe Diameter of 100 m m 4
2.1 Blow Tank Characteristics and Operation - Literature Survey 15
2.2 Dense Phase Flow - Literature Survey 22
2.3 Powder Characteristics 26
2.4 Wall Friction - Literature Survey 30
3.1 Pneumatic Conveying Models - Literature Survey 40
3.2 Solids Velocity Measurement Techniques 43
3.3 Concentration Measurement 48
4.1 Coefficient of Restitution - Literature Survey 50
4.2 Bend Characteristics and Application 58
4.3 Bend Pressure-drop Factors for Use in Equn. (4.8) 60
4.4 Service Life of Long Radius Bends and Blind Tees Conveying
Zirconium Sodium 61
4.5 Variables affecting Attrition - 63
4.6 Assessment of Attrition 63
4.7 Types of Attrition Tests 64
4.8 Surface Roughness of Various Piping Materials 65
4.9 Piping Materials 66
5.1 List of Salient Powder Properties 69
5.2 Powder Properties 70
5.3 Mohs' Scale of Hardness 75
5.4 The General Relationships between Angle of Repose and the
Flowability of Materials 75
5.5 Particle Shape and Flow Characteristics 76
5.6 Angle of Repose - Literature Survey 77
5.7 Cohesion - Literature Survey 79
5.8 Tensile Strength - Literature Survey 82
5.9 Fluidization - Literature Survey 85
5.10 Deaeration - Literature Survey 87
7.1 Observations from Scanning Electron Microscope Photographs 153
7.2 Coefficient of Restitution 158
7.3 Particle Size Analyses 161
7.4 Size Analysis of Sand 161
7.5 Size Analysis of Brown Rice (I) 162
7.6 Size Analysis of Brown Rice (II) 163
7.7 Size Analysis of White Rice 163
7.8 Bulk Density 167
7.9 Loose Poured Bulk Density 167
7.10 Solids Density 168
7.11 Arch Length and Drained Angle of Repose of Cement, Fly Ash and
Sodium Ferrite 171
7.12 Instantaneous Yield Loci 181
7.13 Tensile Strength Versus Consolidation Stress 190
7.14 Deaeration Time Constant and Exponents 210
7.15 Air Pressure Channels 219
7.16 Transducer Locations 219
7.17 Material Flow Properties 219
7.18 Air Pressure Channels and Transducer Location 220
7.19 Conveying Characteristics - Cement 221
7.20 Conveying Characteristics - Wheat 222
7.21 Conveying Characteristics of Sand 223
7.22 Transducer Air Pressures 227
7.23 Bend Air Pressure 228
7.24 Data Channel Details 232
7.25 Pipeline Details 232
7.26 L o w Velocity Conveying Rig - Wheat 233
8.1 Effects of Different Lenses on Particle Size Distribution 238
8.2 Compressibility of Materials 243
8.3 Density Parameters 248
8.4 Wall Friction Tests of Fly Ash 258
8.5 Wall Friction Angles 259
8.6 Deaeration Factor 263
8.7 Collapse Air Velocity and Dense Phase Parameters 263
8.8 Permeability Factor 265
8.9 Ranking of Fly Ash Properties Based O n Bench Tests 271
8.10 Recommended Powder Property Bench Tests for Assessment of
Pneumatic Conveying Suitability 274
8.11 Factors Influencing the Mean Interparticle Spacing 281
8.12 Pressure Differential for Friction Loop 287
8.13 Air Velocity, Slip Velocity and Froude Number 294
8.14 Plug Velocity and Length 301
A.l Friction Factor 335
A.2 Bend Loss Coefficient for 90 Degrees Bends 337
C. 1 Typical Calibration Values 359
C.2 Calibration Results 360
1
CHAPTER 1 INTRODUCTION
1.1 INTRODUCTION:
In the last century, fans were used as primemovers to convey light powders and
dusts through pipes. The application of pneumatic conveying, on a large scale,
commenced in early as 1890. The necessary machines and controls were perfected in
several developmental stages in the process industries with automated installations. With
the development of fans, roots type blowers and rotary feed valves, pneumatic conveying
technology has developed quickly. By the end of the first war, a device called the Fuller-
Kinyon pump had been invented making it possible to convey materials like cement and
fly ash at higher concentrations than that possible using simple fan technology. This
was the birth of modern dense phase conveying.
Recently, there has been an increasing interest in dense phase and super dense
phase pneumatic conveying. The latter is defined as the conveying of powders by air or
gas along a pipe which is more or less filled with powders at one or more cross-sections.
The successful development of commercial conveying systems, during the 1960's like
the simple pressure pulse phase and bypass systems and the need to feed particles such as
coal or cracking catalyst into fiuidized beds, with the minimum amount of air, created
increased interest in dense phase systems.
1.2 PNEUMATIC CONVEYING SYSTEMS:
A pneumatic conveying system consists of four zones namely prime mover,
feeder, conveying and separation (Figure 1.1). A range of compressors, blowers and
fans are available as primemovers. For the design of a pneumatic conveying system, the
required gas flow rate and pressure should be determined. Feeding powders to the
conveying gas is critical. W h e n the powders are fed to the pipeline, they are initially
stationary and a large momentum change from the conveying gas is necessary to
accelerate the solids. Once accelerated, the powders convey in the pipeline and at the end
of the pipeline are separated from the conveying gas. Cyclone and bag filters are used to
separate the gas and powder to keep dust free. A typical pneumatic conveying system
layout with all the necessary components is shown in Figure 1.2. Conveying pressure
loss is dependent on many factors such as particle wall interaction, particle-particle
interaction, conveying gas, powder properties, moisture content, pipe roughness, etc.
2
1.3 FLOW PATTERNS; The design of a pneumatic conveying system strongly depends on powder
properties, the particle size distribution and the mass flow rate of gas and the solids. The
following table categorizes the various conveying systems [Bohnet, (1985)]. A phase
diagram for the pneumatic conveying system showing the flow patterns named in Table
1.1 is presented in Figure 1.3 .
SEPARATION
Cyclone
Bag Filter Cleaning;
Mechanical Reverse Jet
Prime Mover
CY Fan
<3> Blower
Compressor Reciprocating
Screw
Feeding, Mixing, Acceleration
\r Venturi
Rotary Valve
/I/I/I/I Scre*
Blow Tank
Conveying
Piping
JJ Bends
^Diverter
-JT- Coupling
Wear Resistance
Figure 1.1: Basic Components of Pneumatic Conveying Systems [ Chambers, 1987].
3
Storage
Hopper
Prime Mover
O O
^^^T^rr^f
Acceleration « r Z o n e j . 1.1.'j.i.».!. i.i.i
Coarse and Fine
Particles
Feeding Zone
Air/Solids Seperation
/fl*
\Q7
Discharge Hoppei
Conveqinq Pressure Loss.
end Flov
./ Y
>
Particle-| Vail Interaction
Particle- Particle
Interaction Variables "ohesive,Humidity, Electro-
static,Pipe Roughness
Bend Acceleration Zone
Stepped Pipe To Slov Conveying Velocity
jjmyy-y-y-'-y-yy-'Mm O t II I I » I I I I I I I I I I I I I I I I L I I ifc i i r r -
4—-Dense Phase Dilute Phase—f
ear and Degradation
Figure 1.2: Pneumatic Conveying System Layout [ Chambers, 1987].
4
Table 1.1: Data for pneumatic conveying systems for a pipe diameter of 100 m m .
Type of
conveying
Dilute phase
SUdingbed
Plug flow
Plug flow
with bypass
Plug flow
with controlled
secondary gas
Extrusion flow
Gas velocity
m/sec.
15-35
5-20
2-6
3-10
5-15
0.4-4
Solid to gas
velocity ratio
0.3 - 0.7
0.1 - 0.5
0.6 - 0.9
0.2 - 0.8
0.2 - 0.8
0.6 - 0.9
Particle
size
coarse
fine
coarse
fine
fine
fine
Solids mass
flow ratio
30
100
50-100
100-500
100-500
400-800
Typical flow patterns in a horizontal pipe are shown in Figure 1.4. Dilute phase
conveying is a fully suspended flow generated by large volumes of gas at high velocities.
The flow pattern in dense phase flow can vary from an unstable flow to a stable flow
depending on gas velocity, powder characteristics, flow rate, pipe roughness and
diameter. Flow patterns for dense phase conveying vary from conditions in which the
solids completely pack sections of the pipe and move as a continuous plug to conditions
where the solids on the bottom of the pipe move as a series of dunes with a dilute phase
layer of solids flowing above the dunes.
At high solid loading ratio conveying occurs as plugs of coarse powder with high
pressure gradient but low velocities. Obviously, the specific energy consumption required
for dense phase flow will be low, if fine powders are conveyed which are fiuidizable.
In dense phase flow proper, the material fills completely the pipeline cross
section. A reduction of the gas velocity less than a critical value to keep the particles in
suspension results in a non uniform distribution of powders over the conveying pipeline
cross section. This critical velocity is termed as the saltation velocity for horizontal
conveying and choking velocity for vertical conveying.
5
Loading of solids U.-W) '30
Dilute-phase conveying
1 10 100 m/s Gas velocity w —
Figure 1.3: Phase Diagram for Pneumatic Conveying of Solids [ Bohnet, 1985].
Homogeneous Flow 7*T^f+T^^Ti^^T^^*^^*T^
Immature Slug Flow
»______
Degenerate Homogeneous Flow
nrrrrr
^^i'^-Ai^iSirtr»^rtiiS«w*
Slug Flow
Immature Dune Flow Degenerate Slug Flow
^^T*T*^^^?*^^T*mT^.
\__£k&sm&ik
Dune Flow *r^^T*^T**?^^*T^^^Tm*^?
Ripple Flow
Degenerate Dune Flow Pipe Plugged
Figure 1.4: Flow Patterns in a Horizontal Pipe [ Wen, 1959].
6
1.4 TYPES OF DENSE PHASE PNEUMATIC CONVEYING SYSTEMS:
The various dense phase pneumatic conveying systems are characterized by the
system pressure and plug location along the pipeline. Variation of the plug location can be
controlled by parallel gas booster supply lines or gas bypass arrangements.
Gas bypass systems are employed for impermeable products, which tend to form
solid plugs when conveyed at low velocities. W h e n a plug forms in the conveying line,
the gas bypasses and reenters where the resistance of the plug is less than in the bypass
line. A long plug of material is thus divided into shorter plugs and material transport is
thus reestablished. In general, dense phase pneumatic conveying systems are categorized
into four types. These being:
1.4.1 CONTROLLED PLUG FORMATION:
The natural plug formation of coarse-grained powders is induced by generating
plugs into the conveying pipe, e.g. this natural plug formation process occurs when
conveying Wheat, Rice, Millet, etc.
1.4.2 CONTROLLED PLUG BREAK-UP; This method consists of localizing plugs in their initial stages and breaking them
up before a critical length is exceeded. A disadvantage of this system is the possibility of
powder penetrating into the internal or external by-pass lines.
1.4.3 SUSPENSION METHOD; Such systems exploit the high gas retention capacity of some powders. The gas
and solids are mixed at regular intervals to restore fluidization of the slow moving
powder.
1.4.4 C O M B I N A T I O N S :
Other systems combine the latter two methods. All practical systems can be
categorized into one of the four groups mentioned above. Harder et al. (1988a) provides
an excellent summary of dense phase pneumatic conveying system classification as
presented in Figure 1.6. Without the knowledge of the powder behaviour, it is not
possible to design dense phase conveying systems for reliable operation. The usual feeder
for dense phase conveying systems is the pressure vessel or blow tank. These feeders
are capable of achieving any required conveying pressure without uncontrolled gas losses
or leakage.
All conveying requirements may not be solved using dense phase conveying
systems and for this reason dilute phase conveying systems still have their field of
application. These fields include for example, conveying systems with frequently
changing solids or very cohesive powders, where high velocities are necessary to
overcome the interparticle forces.
Dense phase conveying methods
£ Dense phase conveying conventional
tf
JS£<r
Strand conveyance, dunes, conglobations
— •
e.g. CPAG.
5SW^S*SB*^<
I Dense phase conwying with stabilization
I Solids with high gas permeability / low gas holding capacity
Solids with tow gas permeability / high gas holding capacity
S.5CT
Conveyance with controlled slug production
e.g. Pulsecon/CPAG
^-F^F
Conveyance with controlled slug degradation
Suspension method of conveyance
Fluidstat
^mSO
Pneumosplit
Twistcon/CPAG
Turbuflow
____&(&
Uncontrolled slug degradation* suspen= sion effect
System Gattys
is*&3t£'
e.t.c.
Fluidschub
y
Figure 1.5: Classification of Dense Phase Pneumatic Conveying Systems
[Harderetal. (1988a)]
8
1.5 ADVANTAGES AND DISADVANTAGES OF PNEUMATIC CONVEYING SYSTEMS:
15.1 ADVANTAGES:
The advantages of pneumatic conveying, in general, include:
[ 1 ] Clean transportation of a large variety of powders;
[2] Relatively simple system;
[3] Flexibility in routing- it is possible to convey vertically or horizontally by the
simple addition of bends;
[4] Distribution of powder to different areas within a plant and the ability to pick up
powder from several locations;
[5] L o w maintenance and manpower costs;
[6] Multiple use - a single pipeline can be used for a variety of powders;
[7] Security where the pipeline is used to convey valuable powders;
[8] Lower initial cost, savings of bulk shipments, totally enclosed, less maintenance,
easy to automate.
1.5.2 DISADVANTAGES: Unfortunately, pneumatic conveying systems incur the following disadvantages:
[1] High operating pressure;
[2] High energy consumption;
[3] Possibility of complete pipeline blockage;
[4] Difficult to predict the nature of the flow;
[5] Wear and abrasion of system components;
[6] Conveying distance is presently limited to a few kilometers;
[7] Cost of transportation increases with the addition of bends;
[8] The allowable powder mass flow rate decreases with increasing conveying
length.
1.5.3 ADVANTAGES OF DENSE PHASE CONVEYING:
The notable advantages of dense phase conveying include:
[1] The energy required per kg. of solids and metre of conveyor length is less than
that of comparable dilute phase conveying.
[2] Smaller air powder separators are required
[3] The total pressure drop does not vary as widely with air flow rate, as it does for
dilute phase systems.
[4] System operation is more stable;
[5] The air velocities are generally in the range of (or less than) the choking or
saltation velocities in dilute phase conveying.
9
1.5.4 DISADVANTAGES; In c o m m o n with all pneumatic systems, dense phase conveying incurs the
following disadvantages:
[1] The problem of feeding large quantities of solids from an atmospheric
environment to a high pressure pipeline generally means that rotary valves are
not suitable.
[2] In general, single blowtanks are used to feed the solids which necessitates batch
mode operation.
[3] Not all powders which can be pneumatically conveyed are conveyed satisfactorily
in dense phase.
1.5.5 ADVANTAGES OF LOW VELOCITY CONVEYING:
The advantages of low velocity conveying or super dense phase conveying
include:
[1] L o w rates of pipeline wear and powder degradation;
[2] Minimal segregation of conveyed powder;
[3] Minimum conveying air requirements.
The need to minimize conveying air volume is highhghted as follows:
[1] Blower power requirements increase approximately as the cube of the air
velocity.
[2] Pipe erosion increases approximately as the cube of the air velocity.
[3] Powder degradation occurs at high velocity.
Hence, there is a clear trend, within the pneumatic conveying industry, towards
dense phase and low velocity conveying. This trend is consistent with the distinct
advantages of these systems. However, use of dilute phase system will be continued for
the reasons discussed earlier.
Due to the above features, pneumatic conveying is one of the fastest developing
methods for the transportation of bulk solids. This method is proving to be cheaper,
easier and more convenient than many other more conventional methods of transporting
bulk solids including belt conveyors and mechanical conveyors. Air is relatively cheap
and easy to obtain in large quantities. Furthermore, the escape of air contaminated with
dust particles usually causes only minimal environmental damage.
1.6 POWDER PROPERTIES:
The important properties of powders, governing pneumatic conveying
characteristic include,
1 0
[ 1 ] Particle Size and Distribution
[2] Particle Shape and Structure
[3] Bulk Density
[4] Particle Hardness
[5] Permeability
[6] De-aeration
[7] Floodability
[8] Corrosiveness
[9] Cohesiveness
[10] Explosibility
[11] Moisture Absorbancy
[12] Toxicity
[ 13] Angle of Repose
[14] Electrostatics
As can be seen the large number of powder variables causes pneumatic conveying
to be an extremely complex phenomenon. To partially overcome this complexity
improved knowledge of the interaction between powder properties and conveying
characteristics is sought. T o this end, the effects of a number of powder properties on
pneumatic conveying characteristics were selected for further examination. Actual details
of this examination are summarized in the following section.
1.7 THE OBJECTIVES OF THE RESEARCH: The broad aim of this thesis is to gain further insight into dense phase and super
dense phase pneumatic conveying processes, to improve the design procedures for the
conveying of various powders and to develop standard bench tests to assess material
pneumatic conveying characteristics.
Particular effort will be devoted to the latter since pneumatic conveying is not
fully understood in regard to the conveyability of a powder on the basis of properties
determined from bench tests. In regard to bench test development, this work studies in
detail the cohesive arch behaviour, measurement of tensile stress, fluidization and
deaeration properties, wall friction of aerated powders, surface characteristics, bulk
density, solid density, flow properties, particle size analysis and powder coefficient of
restitution. The properties so measured are then correlated with pneumatic powder flow
behaviour in dilute phase, dense phase and super dense phase flow systems.
11
The need to assess the foregoing properties to successfully design a pneumatic
conveying system is highlighted by the following. Firstly, wall friction is an important
factor contributing to the pressure drop in dense phase pneumatic conveying. Obviously,
the frictional properties of powders have an adverse effect in pneumatic conveying. As an
initial quantification of wall friction effects during pneumatic conveying, wall friction
measurements were evaluated under aerated conditions in a perspex tube by pushing
powders upwards at different column lengths.
Secondly, the internal friction angle, shearing cohesiveness and tensile strength of
powders are significant parameters during dense phase and super dense phase conveying.
Notably, in these modes, the creation and breakage of plugs depends on powder
cohesiveness. In fact, many problems, associated with powder handling, originate from
the influence of the cohesive forces on the flow behaviour of powders.
To assess the effect of cohesion, the arching dimension and hence cohesive
strength were evaluated in a purpose built Cohesive Arch Tester for various powders.
The cohesive strength so measured was then compared to that measured using the Jenike
Direct Shear Tester. To further elucidate powder cohesive properties, the tensile strength
was evaluated for various powders. This powder property was measured using an Ajax
Tensile Tester at different consolidation levels.
Thirdly, as previously indicated, powder fluidization characteristics, air
permeability and air retention characteristics of powders are important considerations in
pneumatic conveying. Since these properties are intimately related to the particle size
distribution, measurement of the same was evaluated using a Malvern Particle Sizer. The
particle size distribution, so measured, was also correlated to other powder characteristics
assessed in this investigation. The particle size distribution also governs its air
permeability and air retention characteristics. The latter properties were evaluated and
assessed by conducting measurements in fluidization columns and deaeration rigs.
Fourthly, the surface characteristics of powders are important in regard to powder
degradation and pipeline wear in pneumatic conveying systems. Furthermore, surface
characteristics contribute to the powder's internal and wall frictional behaviour. To obtain
knowledge of the powder surface characteristics Electron Scanning Microscope
observations were conducted. These examinations revealed a host of information
concerning expected powder behaviour.
1 2
Fifthly, bulk density, permeability and solid density bear an important influence
on the flow behaviour in pneumatic conveying systems. These important parameters
were determined using a Jenike Compressibility Tester, Jenike Permeability Tester and
Beckman Pycnometer, respectively.
Finally, the coefficient of restitution is very important in governing the flow of
granular materials during dilute phase flow. This coefficient was determined for several
particulate materials using a basic rotating disk technique. Since the coefficient of
restitution is of secondary importance in regard to dense phase conveying, minimal
discussion of this coefficient is presented.
Powder properties determined from bench tests provide convenient and rapid
assessment of a powder's flowability. This assessment is useful for ranking of different
powders and identification of the optimum mode of pneumatic conveying. A new phase
diagram is proposed incorporating powder properties such as cohesion, deaeration,
permeability and mechanical interlocking to indicate pneumatic conveying flow
behaviour.
In regard to powder flow behaviour in actual conveying systems, the following
was conducted. Actual system performance was determined using a suitably configured
pneumatic conveying pilot scale test system. This closed circuit system comprised a
Sturtevant blow tank feeding a maximum of 71 meters of 50 m m nominal bore tubing
discharging into a receiving hopper. Initially, a friction loop was installed to measure
system pressure drop. Subsequent improvements to the system included measurements of
powder velocity and concentration by use of Tealgate T.200 and 300 series transducers
[Beck etal. (1971,1982)].
In the latter phases of the performance tests, the effect of bend geometry on the
flow characteristics was examined. In particular, two different bends were used namely,
long radius and vortice elbow. This examination was followed by the development of a
fibre optic probe to measure powder velocity. Subsequent testing, using this velocity
probe was conducted at different air flow rates.
The results of the above investigation yield useful information in regard to
pneumatic conveying flow behaviour of powders in general, and for the prediction and
design of practical pneumatic conveying systems in particular.
13
CHAPTER 2 BLOW TANKS, DENSE PHASE FLOW
AND WALL FRICTION
2.1 BLOWTANK:
2.1.1 INTRODUCTION:
A blow tank is essentially a pressure vessel configured with powder inlet and
discharge ports and valves, a pressurization port and a vent port, refer Figure 2.1. The
basic operating cycle of a blow tank comprises filling, pressurizing and conveying.
During filling, the powder inlet valve is open and the discharge valve closed. In this
phase, powder feeds into the blow tank (usually by gravity from a feed hopper). W h e n
the blow tank is full, the inlet valve is closed allowing pressurization. Once pressurized
to the required system pressure, the discharge valve is opened to supply powder to the
conveying pipeline.
Blow tanks may also incorporate fluidization, conveying and secondary air flow
systems. There are two types of blow tanks, the top discharge and the bottom discharge
also known as the Fluxo and Cera type, respectively. Blow tanks are pressure vessels
which have to be designed in accordance with the pressure vessel code according to A S
1210 in Australia. Because they work under internal pressure, they require a certificate of
fitness at regular intervals to ensure safe operation. A summary of blow tank
characteristics and operation, as found in the literature, is presented in Table 2.1.
2.1.2
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
ADVANTAGES:
The advantages of blow tank feeders include:
High solids to gas ratios;
Small pipelines;
Small dust filtration systems;
N o moving parts;
Minimum powder degradation and segregation;
Simple control of flowrate;
Higher conveying capacities;
Long conveying distance possible;
Minimum bend and pipeline wear;
Can convey hot powders;
N o difficulty in feeding against adverse pressure gradient.
14
Vent line
Control valves
Feeding hopper
Inlet gate
From compressed air
supply
Level indicator
Blow tank
Conveying
ine
Figure 2.1: A Single Blow Tank System.
2.1.3 LIMITATIONS:
The limitations or disadvantages of blow tanks include,
[1] System conveying rate is limited by blow tank size;
[2] They operate in batch mode in most situations;
[3] Minimum product cooling (low gas to solid ratio);
[4] They are of high pressure design;
[5] Due to the use of high pressure, the conveying velocity increases with
distance; down the pipeline due to expansion of the compressed conveying gas.
Since the blow tank discharges product to the conveying line in batches, two blow
tanks may be used to operate in sequence, so that one is being recharged, while the other
is discharging.
15
TABLE 2.1 BLOW TANK CHARACTERISTICS AND OPERATION
- LITERATURE SURVEY
YEAR
1972
1978
1982
1982
1984
1985
AUTHOR
Rain
Shepherd
etal.
Tomita
etal.
Hitt
Lohrmann
etal.
McLean
COMMENTS
Reviewed c o m m o n configurations of blow tanks and their
fields of applications.
Reported the difficulty of dense phase conveying of wood
pulp due to hold up and blow hole formation. Conveyed
either by a higher initial pressure in the blow tank or
alternate pulsing of the inlet and discharge valves.
Measured the velocity profiles of non cohesive granular
materials at different flow rates in a top discharge blow j
tank. Found similarity between the gravity discharge of
solids from an orifice to that of the flow of solids in the
blow tank. Observed that the material flow was not affected
by the air flow except near the pipe inlet and is independent
of the pipeline pressure drop.
Applied the time derivative of the ideal gas law to the blow
tank discharge flow to calculate solids loading ratio, to
predict pressurization transient and conveying characteristics.
Reported experimental results of Group A powders using a
bottom discharge blow tank. Examined the influence of
initial blow tank pressurization, charging fraction, air flow
rate and total length by testing Lime, Portland Cement and
fly ash. Observed that Group A powders are good
candidates for dense phase conveying, are easily fluidizable
and retain fluidization air.
Analyzed blow tank design using the principles of gravity
flow bins taking into account the fluid pressure gradient.
16
1986
1987
1988b
McLean
Kennedy
etal.
Harder
etal.
Recommended critical dimensions and main design
considerations to determine the geometry of blow tank for
reliable flow.
Presented expressions of steady state blow tank
characteristics applicable to both Fluxo and Cera type blow
tanks conveying either cohesive or non-cohesive bulk solids.
Investigated the effects of different methods of air injection
on the performance of bottom distharge blow tanks. Found
that this can have a significant effect on the overall
performance of pneumatic conveying system.
Described criteria for blow tank design, various possible
arrangements and alternative feeders for dense and dilute
phase conveying.
2.1.4 PARALLEL ARRANGEMENT:
In this system, one blow tank discharges powder into the conveying pipe, while
the other receives powder from the hopper. Hence, by alternate sequencing of this cycle
continuous powder conveying is possible (Figure 2.2). W h e n installed with individual
feed hoppers, each blow tank may all handle the same material or they may alternately
handle different types of material.
2.1.5 SERIES ARRANGEMENT:
In the series arrangement, each blow tank is separated or isolated from each other
by isolation gates and each blow tank have their o w n venting and pressurizing
connections. The lower blow tank pressure is selected to be slightly higher than the
operating pressure to ensure a uniform gravity flow of powder from the blow tank to the
conveying pipe. The upper blow tank is alternately pressurized so that powder can be
discharged batchwise to the lower blow tank or it is vented so that powder can be charged
into it from the hopper. The bottom blow tank, however, conveys more or less
continuously (Figure 2.3).
Figure 2.2: Parallel Arrangement [ Reed, 1989 ].
Hopper
Pressure
balance
and vent"
S/enV line
Conveying line
/.ir supply
Figure 2.3: Series Arrangement [ Reed, (1989)].
18
2.1.6 C A P A C I T Y :
The approximate capacity of a blow tank system can be calculated from the
equation (Kraus, 1983),
8.156 x IO-4 ph V C = 1
P b (2.1)
where, C = capacity, tonnes / hr.,
Pb = loose poured bulk density, kg / m3
V = blow tank volume, m 3
t = total cycle time, sees.
The system capacity can be increased by selecting a series or parallel arrangement.
2.1.7 AIR REQUIREMENTS:
Air requirements depend upon the characteristics of the powder, the distance to
be conveyed and the diameter of the pipe line.
2.1.8 SECONDARY AIR:
Secondary or supplementary air is required to promote the powder flow in the
conveying pipe. As the slug moves through the conveying pipe, it tends to compact due
to the frictional forces or from the loss of air due to its leakage through the material.
When this happens, additional air can be introduced to break down the plug and to
promote flow.
2.1.9 VENTING:
Proper venting is important for smooth blow tank operation. If the blow tank is
not vented, a large adverse pressure gradient occurs, which prevents further material flow
into the blow tank. The existence of this adverse pressure gradient severely retards infill
flow rates of both cohesive and low density powders.
2.1.10 AERATION AND FLUIDIZATION: /
The flowability of powders can be increased by aeration. This is the injection of
air upwards through the powder. Low pressure differentials are sufficient for aeration. At
higher air velocity, the powder will be suspended and fluidized.
A common aeration device is the plenum chamber through which air can be passed
and supports a permeable or porous membrane (Figure 2.4). The plenum chamber is
bolted to the bottom of the hopper of the blow tank.
19
Figure 2.4: Aerated Blow Tank [ Reed, (1985)].
2.1.11 BLOW TANK PERFORMANCE CHARACTERISTICS: Jotaki et al. (1978) studied conveying characteristics of the Fluxo type of blow
tank, when conveying P V C powder and polyethylene pellets. The blow tank solids
mass flow rate is evaluated by knowledge of the superficial air velocity at the pipe inlet
and is independent of the pipeline pressure drop. They stated that the relation between
the solids mass flow rate and the superficial air velocity at the pipeline inlet can be taken
as a blow tank characteristic curve. Furthermore, secondary air in the pipeline has no
effect on this relation in so far as it only reduces the solids concentration. The discharge
characteristics of a fluxo blow tank for coarse granular solids are adequately described by
(McLean, 1986),
Ms = 0.988 ps 7T ( D - 1.9 d )25 g?5
where Ms = solids flow rate from blow tank, kg s"1;
ps = solid density of bulk solids, kg nr3;
d = diameter of solid particles, m;
ga = effective gravitational acceleration - 9.81 m s2;
D - inside diameter of blow tank delivery tube, m ;
u0 = superficial gas velocity through solids bed, m s- 1 ;
u0* = characteristic velocity of powder at channel outlet, m sr1.
u. - 1 (2.2)
20
2.2 S Y S T E M nFfiTQl^
The design of a pneumatic conveying system involves the specification of:
1. Route of the pipe;
2. Type of conveying system to be used (e.g. vacuum or positive pressure; low,
medium or high pressure; closed or open loop);
3. Details of individual components (feeding and discharge mechanism, valves,
cyclones, bag filters, types of bends, materials of construction);
4. Flow pattern in the pipe (dilute phase, dense phase or moving bed);
5. Pipe size;
6. Solid mass flow rate;
7. Air flow rate;
8. Horizontal and vertical distances of pipeline;
9. Particle density of the powder and its particle size distribution;
10. Overall pressure drop;
11. Determination of power consumption;
12. Air blower / compressor;
13. Determination of the critical velocity.
Generally, the design of pneumatic conveying systems is based on rules of thumb,
previous operating experience and know-how of specialist companies. A summary of the
design procedure for dilute phase conveying was recommended by Bandrowski et al.
(1981). The basic parameters required to effect optimal powder conveyance (minimum
power consumption, maximum throughput of solids and longest possible life of pipes
and equipment) should be determined in the design phase. The methods for increasing
the effectiveness of pneumatic conveying systems include achievement of the maximum
flow concentration and assurance conditions for its maintenance for the longest possible
time.
The important parameters are the capacity to be conveyed, the distance over which
the powder will be conveyed and the number of bends involved. Since bends create large
pressure drops, it is highly desirable to minimize their number. Furthermore, the
required velocity must be determined. The solids mass flow rate should be continuous to
minimize energy consumption.
2.3 DENSE PHASE AND SUPER DENSE PHASE FLOW :
Dense phase conveying occurs, when the conveying velocity is below the saltation
or choking velocity. In this phase, the mass flow ratio is high and significant pressure
21
fluctuations occur due to the existence of dunes and slugs. Dense phase conveying can be
used for powder and granular materials, but blockages may occur. Hence, the
conveying velocity is an important consideration.
Dense-phase conveying in the form of plugs is the most economical form of
conveying. In this mode of conveying, the pressure drop across a plug is approximately
proportional to its length, provided the particles forming the plugs do not exhibit
decreasing permeability with increasing consolidation. For such powders, discontinuous
phase conveying is feasible. In general, plugs of large, mono sized particles exhibit
linear pressure drops. For such powders, the permeability of the plug is insensitive to
bed consolidation.
2.3.1 CLASSIFICATION OF DENSE PHASE FLOW:
Types of dense phase conveying systems are discussed in Chapter 1. Dense
phase flow can be classified into continuous phase, where the powder moves by saltation
over a stationary or sliding bed and discontinuous phase, where powders move as slugs.
This classification can be further subdivided into pulse phase for granular materials and
plug phase for powders. In the latter case, powder plugs are essentially extruded
through the pipe.
2.3.1.1 CONTINUOUS SYSTEMS:
Non ..ohesive powders may be conveyed over short distances using continuous
phase flow systems. These systems usually consist of a conventional blow tank
supplying a standard pipeline arrangement. Hence, this mode is non optimal in solids
mass flow loading and tend to exhibit non stable flow characteristics. This flow type also
characterizes dilute phase systems operating non optimally. Hence, continuous phase
systems have limited practical significance.
2.3.1.2 DISCONTINUOUS-PHASE SYSTEMS:
Most systems use some form of air injection to fluidize the powder and prevent
wedging between particles. Some provision is made to break up long plugs, but plug
formation is not directly controlled. Plug-forming systems create small plugs at the feed
point. Other systems destroy plugs on formation and operate in dune phase.
In the pulse phase system, air knife is used to seperate plugs by introducing air
periodically. The advantage of this system is that the pressure drop, when conveying a
series of discrete plugs is less than the pressure differential required to transport a single
continuous compact material plug over the same conveying length. In the former case,
22
the pressure drop is equal to the summation of the pressure drop of the individual plugs
in the conveying pipeline. As long as the plugs are stable and do not join together and
block the pipe, discrete plug flow conveying is very efficient. This system can be used
for fine cohesive powders.
Due to high pressure losses in discrete dense phase powder plugs, air expands
significantly along the pipe and the conveying velocity increases. A s a result of this
increase in air velocity along the pipe, tensile forces are produced in the powder plugs,
which tends to tear them apart. T o prevent disintegration of the discrete powder plugs, a
valve or orifice must be located at the end of the conveying pipeline to provide a system
back pressure.
Air boosters positioned along the conveying pipeline sense the pressure at each
stage and adjust the pipeline pressure to convey granular materials smoothly and prevent
high back pressures. For very cohesive powders, a booster line may be added to pulse
phase system for continuous plug flow. Details of dense phase flow and super dense
flow systems is presented by Klinthworth et al. (1985). A literature survey summary
relating to dense phase flow is presented in Table 2.2.
TABLE 2.2 DENSE PHASE FLOW - LITERATURE SURVEY
1980
1981
1981
Konrad et al.
Chan et al.
Wilson
Evaluated the pressure drop required to move a plug in a
horizontal pipe. Using the packed bed theory, he predicted
an expression for the pressure drop for a cohesionless
material. For fine powders, he suggested that the pressure
drop across a plug varies exponentially with plug length.
Considered one dimensional plug flow. Examined stability
criteria by considering the axial interparticle stresses within
single plugs and the effect of wall friction.
Considered the effects of stress on deformation within a
plug. H e stated that this deformation effects the
permeability of the plug, the pressure gradient and the
stresses along its length.
23
1981
1981
1982
1982
1983
1983
1986a
1986b
1987
Klinzing et
al.
Tomita et al.
Tsuji et al.
Hitt et al.
Tsuji
Werner
Konrad
Konrad
Hauser et al.
Applied porous media and turbulent flow concepts to
extrusion flow. Suggested a permeability factor for low
gas velocities.
Observed wavelike slug motion of polyethylene pellets in a
horizontal pipeline. Found the pressure drop caused by the
wavelike slug motion is estimated by the Ergun eqn.
Observed plug conveying of coarse particles in a horizontal
pipe with secondary air injection. Studied the effects of
particle size and number of injection holes. Experimental
results deviated from Ergun's eqn.
Studied two models of slugging, shearing type and full bore
flow in a horizontal pipe and compared predictions with
experiments.
Reported that the pressure drop across a moving plug was
less than across a packed bed of the same particles. It was
noted that turbulence and vibration prevented wedging of
the conveyed powders.
Reported the influence of particle size distribution on dense
phase pneumatic conveying in vertical and horizontal pipes.
Studied the difference between the conveying of uniform
sized powders and powders having a wide size distribution.
Discussed the similarity of plug conveying of cohesive
powders to the arch formation theory of material flowing
out of a hopper (Jenike, 1967) neglecting the effect of
gravity and presented an excellent review of dense phase
pneumatic conveying.
Considered the effect of air compressibility on the pressure
drop in dense phase pneumatic conveying.
Observed that abrasion of agglomerated lectose can be
24
1987
1987
1988
1988a
Zheng
Borzone
etal
Aziz et al.
Harder et
al.
reduced in plug conveying by using a back pressure system
aimed at reducing the volume expansion of air.
Reported the relationship between pressure drop and plug
length to be an approximate linear relationship.
Suggested that the pressure drop was found to be
independent of the air flow rate and to vary linearly with
plug length. The plug velocity was independent of the plug
length and for vertical plug flow at low velocities,
gravitational forces are significant.
Indicated that the pressure drop variation is linear with plug
length. Found particle size distribution and cohesion govern
plug formation and stability. Proposed a pressure drop
model for plug flow and found wall shear stress was
important for controlling the flow in horizontal and inclined
pipes.
Discussed the effect of powder properties on dense phase
conveying and energy optimization with respect to
industrial applications.
2.4 POWDER PROPERTIES: The suitability of powders for discontinuous dense phase conveying (pulse, slug
and solid phase) depends on numerous physical powder properties. These are particle
size distribution, particle shape, hardness, compressibility, adhesiveness, cohesion, de
aeration, coefficient of friction and coefficient of restitution which also control, to a
large extent, the system pressure drop.
For super dense phase conveying, air permeability, air retention capability,
particle size distribution, density, wall friction, internal friction and product adhesion are
important powder considerations. Super dense phase pneumatic conveying exhibits
greater stability at high solids loading compared to that when conducted at low solids
loading. However, occasionally powder plugs consolidate to form immovable plugs.
25
W h e n powder permeable to air are transported, the conveying air is able to
penetrate into the plug of material, fluidizing it and so limit the plug formation process.
In addition, pressure compensation between the individual pockets of air and the slugs of
material is possible without the aid of bypass or parallel booster lines.
When conveying powders exhibiting low air retention characteristics, additional
facilities have to be provided to prevent the particles packing together and forming
immovable plugs. For such materials, bypass pipes m a y be used. These bypass pipes
increase the turbulence and mixing in the flowrate [ Klintworth et al. (1985)]. Bypass
pipes with regular connections can be used internal or external to the conveying pipeline
The need to prevent formation of plugs is particularly relevant for low air retentive
cohesive powders. Conveying of such powders, with limited overall pressure drops, is
possible by keeping the plug length short. It should be noted that the wall friction is also
reduced by preventing unlimited plug formation.
As the conveying velocity is reduced, the strand characteristics of continuous
dense phase conveying changes to moving dunes. These dunes m a y subsequently form
into plugs which will fill the pipe. During dune flow, efficiency is reduced due to
continual deceleration and reacceleration of the particles. Furthermore, wall friction
provides a major energy loss during plug conveying. In this mode of conveying, powder
properties and size distribution are important considerations.
The powder characteristics governing horizontal dense phase conveying system
characteristics are summarized in Table 2.3.
cn oo ON »-H >~'
• rH *->
C/5 C/5
o C-r
U HH H cc i — <
P-l W H <
P-5 <
a u Cri W
Q O DH I
CQ
J o OH
8 pj ^J
co O r--
5J w o OH
5 CO > — <
Q
[_H 2>
2
55
o w CO
W Q Or
ft O Q W
e
CO
U P CO >-H
0,
2 U Or
W § O OH
ti 3 o 55
13
o *-H tr <D > —f cd
e o H--»
3 1—1
00 cd rr.
3
o •i-H 4—*
ed N *3 • 1—<
r-
13
1 Hj-r-
X
W 3
o •d
c *- »
•a e "8 E 3 Cd N
•t-H
3
53
c '% o 53
I
ii
o 55
, , cd
E iy • * — »
X
w
*0 <U N •c 3 oo
ex,
o Hlr-I
N
IS DH
Si
c cd J3 C ed
o c o • rH Hr->t
3
5-
e -3 3
.5? «
.2 3
•si D 3 UH
C
DH
bJO 3
••3 B 1 trO j3 3 H
*e3
g
„^ cd
E ti r
X W
0 N
•c 3 00
1 00
O DH
3
T_S 3 cd
40 8 0 3
3 cd N ••a 3
tS >
•>-H 00 0) X!
6
•U cd 13 ti 00 00
B
1 O O
'rt
X 3 W r3
,^ cd
E 1) «---X W
T3 0) N
•c 3 00 00
>> > cd
r3
1)
• rH 00
cd
1 O
r3 t>0
r-}
3 ed N
•-3 • rH
3
E
(-1
a 3 3 0 .3
<
^^ ed
E a •*-» X W
1 6 <
00
1 O Di
>
_> J3 O O 0>
•a N
3 3
^ 0 q-! c«
3 4H
3 52 4--r
•i-H
4-r
3 l-H
M
^^ cd
E S3 •4-t
X W
T3 U N
•c 3 00 00 C-H
(D
-a • rH 3
3 O
55
-9 3 ed N
• 1 ^
T3 • rH
3 qH 3 O C Or
• rH flJ
•9 --fi fe 00 0 ^ -3 S 'ri-
1 « S 0 c3 '^ 3 ^
i B 3 -s 3 w
27
2.5 WALL FR1CTTON: 2.5.1 INTRODUCTION:
The wall friction is an important design parameter in pneumatic conveying. In
solids handling, it is usual to assume that the shearing of granular material along a surface
is similar to the solid body friction along a surface. Hence, it is usual to apply Amonton-
Coulomb's law. In investigations, concerning the frictional forces between a granular
material and a surface, most researchers assume that the friction force to be of the same
form as solid body friction and apply the Coulomb friction law to describe the same.
However, other researchers [Platonov et al. (1969)] consider the resistance to slip as a
combination of an adhesive force, which is independent of the normal pressure and a
shearing force proportional to the normal pressure. Wherein the characteristics of both
friction components would be dependent solely on the materials interacting at the surface.
The total resistance to sliding of powders against a boundary surface is a
combination of the force due to external and internal friction. Under the action of a load,
the powders change their density and this creates difficulty in formulating a theory of
frictional contact of powders and in calculation of the actual contact area on which
frictional force is realized.
2.5.2 THE FRICTION OF SMOOTH AND ROUGH PARTICLES:
For smooth particles the real area of contact may be less than that for the rough
particles. The frictional force F is the product of the shear strength x and the real contact
of area A,
F - t A (2.3)
For smooth elastic spheres, the magnitude of the contact surface area A at the wall
can be obtained from the Hertz equation,
? * —
( 1 2 , 2 V 1
1-V- 1 - V2 — T + Tr
(2.5) V c i z_ J
where R = radius of the sphere,
W = the normal force,
V; = the Poisson's ratio, and
Ej = Young's modulus for the sphere and the flat surface, respectively.
28
For rough particles, the real area of contact increases at least linearly with the
applied normal force. The problem of computing the actual magnitude of the contact area
is complicated and requires details of the surface topography. For instance, the frictional
force for rough particles is directly proportional to load.
Many studies have been done to evaluate the wall friction of powders under de
aerated conditions, but very little research has been undertaken to evaluate the frictional
coefficient variations under aerated conditions. The friction angles determined in shear
cell tests appropriate to hopper design [ Arnold et al. (1980) ] are not directly applicable
for powders pneumatically conveyed. A literature survey summary relating to wall
friction of powders is depicted in Table 2.4.
2.5.3 ANALYSIS:
Considering the fact that frictional resistance of the container walls offer support
to the material in the vertical direction (Figure 2.5) and assuming equilibrium of an
elemental strip of thickness dz; Figure 2.7, the following equation was derived for the
pressure drop variation,
P =J5_ z 4pk
Solution to this equation gives:
(2.7) l - e x p [ - ^ —
The distribution described by equation (2.7) is shown in Figure 2.6. The above
analysis assumes the following conditions:
1. The coefficient of friction p is constant;
2. The ratio of lateral pressure to vertical pressure k = Pr / P z is constant;
3. The pressure distribution over the surface of the piston is constant;
4. The tube bore is perfectly parallel
5. The grain is incompressible.
It therefore follows that the conveying force necessary to move a column of
material up a cylindrical tube can be evaluated by the following expression,
F = 77tD 3 f4ukL^
z = l 6 T I k L e x i \ — J -1 (2.8)
dz'
T
D! iliJHljljljlHJltJliY
Figure 2.5: Column of Material. Figure 2.6: Pressure Distribution.
if
Column of Material
*•••*•*"*••••••••••'•-•-•-'---*••-•-•-•-•-•-•-•---- •»-- • •-•---r>-.-.-w-"---*---'-i
V Piston
Figure 2.7: Force Analysis of a Column of Bulk Material.
[ Arnold et al. (1980) ].
30
TABLE 2.4 WALL FRICTION - LITERATURE SURVEY
YEAR
1958
1959
1960
1966
1969
AUTHOR
Barth
Muschel-
knautz
Zenz
Roberts
Platonov
etal.
COMMENTS
Reported the value of the coefficient of friction for cokes,
coal and for crushed Wheat based on experiments in
vertical pipes.
Obtained the value of the coefficient of friction by
measuring the rebound motion of solid particles from a
rotating disk surface, when dropped vertically.
Conducted experiments to measure solid-wall friction. The
bed of powders rested on a fixed piston. The pressure
exerted at the bottom of the powder bed was measured by
means of a pressure gauge. The frictional force between the
solids and tube wall was taken as the difference between the
pressure exerted on the powder bed, when moved upwards
and that when it was stationary. The experiments were
performed with and without powder aeration and tube
diameter was varied from 8.9 m m to 14.0 m m inner
diameter. The tube was raised by hand and the rate of
movement was recorded by high speed motion pictures.
Studied the forced flow of Millet in a 3.66 m. long and 44.5
m m . diameter perspex tube fitted with a perforated piston
and piston rod to force columns of material upwards
(Figure 2.8). Resistance strain gauges were used to
measure the conveying force. The upward movement of the
piston was effected by mechanical means. His results for
different column lengths are shown in Figure 2.9.
Developed a test rig for measuring the coefficient of
friction between a granular material and a solid surface.
31
1978 Rademacher
They found that the coefficient of friction was dependent
on the normal pressure of the granular material and
furthermore that the deviation from Amonton-Coulomb's
friction law varied also with normal pressure. They also
reported that the Amonton-Coulomb theory has not been
verified under experimental conditions, as it has not been
possible to identify and measure individual friction
components.
Their test rig is considered to simulate accurately the
real process of slip between a surface and a material.
Experimental investigations using granular materials
including Iron ore, Polysterene, Wheat, Glass bead and
Millet revealed that the coefficient of friction to shear for all
materials tested was not constant and depended
considerably on the normal pressure of the granular
material.
Observed that during the flow of coarse granular materials
along a surface essentially translational particle motion
occurs. H e reported that the dead load method for
measuring friction coefficients between a bulk solid and a
surface, suggested by Brubaker et al. (1965) and Platonov
et al. (1969), does not simulate the actual phenomenon
correctly. Such methods almost eliminate the rotational
movement of granules. H e observed such granular materials
microscopically and concluded that this process would
result in a flattened particle surface. Hence, the results
under dead load conditions can't be considered as a true
representation of the actual friction coefficient.
His test rig eliminated the deficiencies associated
with dead load methods. It consists of two parts: a fixed
vertical tube and a horizontal disc, which is driven by a
fixed variable speed motor, as shown in Figure 2.10. Since
the test rig provided consistent results, it became apparent
that the friction coefficients along the tube walls and on the
32
disc would follow by simple calculations. This evaluation
requires that the geometry of the granular mass, the angular
speed co and the moment required to drive the disc be
known. T o facilitate this evaluation, the original unit was
replaced by a vertical motor - gear box combination with
cushioned air bearings.
The experimental investigation of three different .
materials, namely, Rape-seed, Vetch and Millet using the
latter test equipment produced very consistent results. The
observed coefficient of friction between steel disc surface
and Millet seed compared favourably to that obtained using
the dead load method and other reported research work
[ Brubaker et al. (1965)]. This favourable comparision may
be due to the fact that the coefficient of friction between
Millet and the surface does not vary significantly with slip
velocity.
His method is useful for measuring the kinetic
coefficient of friction between granules and a surface.
However, the method is not suitable for testing cohesive
powders nor aerated powders. Further, it can't be used to
determine the wall friction parameter (fik) applicable to
powder column conveying.
1983 Thompson
etal.
Measured the wall friction coefficient of Wheat using the
test rig shown in Figure 2.11. In their test rig, a flexible
pressure diaphragm near the walls was used to exert a
known force on the grain mass to simulate the pressure;
which occur in a grain bin. T o determine the friction
coefficient for a given pressure, the force required to pull a
metal blade through the grain mass was measured. They
found that the coefficient of wall friction for Wheat on Steel
varies with moisture content, overburden pressure and
sliding velocity. For an increase of moisture content of
Wheat from 8 to 2 0 % , the coefficient of friction increased.
33
1984
1987
1988
Kano
Berkovich
Morikawa
However, it decreased as the overburden pressure increased
from 7 to 172 kPa.
Investigated the wall friction factor in a vibratory field using
a shear cell like test rig. A sample of Millet, packed in a
cubic acrylic vessel on an acrylic plate, was vibrated by
means of a vibrator and the wall friction factor was obtained
from knowledge of the horizontal force required to move
the vessel horizontally.
Reported methods to estimate wall frictional force of
powders based on the calculation of actual contact area. H e
found that under conditions of powder elastic and plastic
deformations at low pressure, the frictional force is close to
that predicted from the monomial Amonton's law. At high
pressures and with constant nominal contact area, the actual
contact area is completely saturated and the formula for the
frictional force is transformed into the binomial Coulomb's
law. H e also reported the effect of humidity on frictional
properties of powders. His experimental finding is
presented in Figure 2.12.
Reported that the aerated coefficient of wall friction was
independent of solid loading and Froude number defined by
pipe diameter and mean particle velocity for Lupin, C o m ,
Wheat, Polystyrene pellets, Steel balls, Glass beads in steel
pipes 5, 10 and 20 cm. inside diameter.
___^_^<l___7<y Scorn
^ p___*£___\_J__i_______c
se
Pressore Prop Across Co/___o
F/osv Meter
vppj'-t jkqo/eifbr
Figure 2.8: Forced Flow Apparatus [ Roberts, (1966) ].
80 -,
TIME. -- S
~ i —
10 ECS
I5j* COLUMN
I2> COLUMNi
8 COLUMN! 6" COLUMM 4?"- OLOMivj
15
Figure 2.9: Conveying Force Results for Millet [ Roberts, (1966) ].
Figure 2.10: Rademacher Wall Friction Tester.
(a) Original with Variable Transmission and Electric Motor in Fixed Position
(b) Modified with Measuring Arm [ Rademacher, (1978) ].
36
GALVANIZED STEEL BLADE
REMOVABLE SLIDE GATE
VERTICAL ROLLER OUDE ASSEMBLY
AIR INTAKE SYSTEM
RUBBER PR DIAPHRAGM
REMOVABLE -1 SLIDE
GATE
Figure 2.11: Front View of the Coefficient of Friction Test Rig
[ Thompson et al., (1983) ].
1, 2, 3, 4 - Relative Humidity: 10.3, 15.7, 18.7 28.7 %.
Figure 2.12: Variation of Frictional Force and the Normal Load for Brown Coal
[ Berkovich, (1987) ].
37
CHAPTER 3 PNEUMATIC CONVEYING SYSTEM
3.1 GAS-SOLID SUSPENSION:
The motion of a fluid may occur under the influence of the force of gravity or
pressure created by an air blower or fan. In pneumatic conveying systems, the energy
expended in flow must originate from the air stream (Clark, et al. 1952).
Evaluation of system pressure drop is paramount for the determination of power
requirements and specifications of prime movers. The total pressure drop consists of that
frictional losses due to gas flow alone and an additional pressure drop caused by the
presence of the solid particles.
Air friction is affected by the presence of solids because of the degree of
turbulence of the fluid and pressure at any point will be influenced by the presence of
solids. Furthermore, diminishing pipe cross section will be available for the air flow, if
the concentration of solids is high.
The additional pressure drop arises because energy is transferred from the air to
the particles. Firstly, to overcome the inertia and to accelerate the particles and secondly,
to compensate for energy losses occurring when particles collide with the wall or with
each other. Since the drag force on the particles is a function of their relative velocity in
the air stream, the rate of transfer of energy will be a maximum, when the particles have
to be accelerated from rest and will decrease, as the velocity of the particles increases.
The viscous forces perform friction work against the walls and in the formation
of a continuous velocity field in the flow, absorbs the flow's mechanical energy. The
motion of the fluid is therefore accompanied by dissipation of energy. Energy obtained
from the flow must be used to overcome those forces, which tend to force the solids to
the boundaries of the system. Such forces may include electrostatic attraction,
hydrodynamic wall interaction forces, gravity forces, etc.. If the suspension is flowing
vertically, the fluid drag on each particle must be greater than the force of gravity.
When a fluid flow acts upon a particle lying on the bottom of a pipe, three types
of particle motion are possible depending on its particle size and the mean flow speed.
38
These three types are rolling or sliding motion, separation from the flow suspension with
repeated movement in jumps or steady motion in the suspended state. These types of
motion characterize the mechanism of suspension and transfer of the particles. A n
excellent discussion of the flow type and phenomena in lean phase systems is presented
bySmoldyrev (1980).
In pneumatic conveying for fully developed flow in a horizontal pipe, the
additional pressure drop is essentially due to the collision between particles and the
surface of the pipewall. W h e n the moving particles collide with the wall, some of their
kinetic energy will be lost. This lost energy has to be offset by the gas in order to
maintain steady flow. Hence, the particles are conveyed colliding with the pipe wall and
assume various velocities. Large particles m a y loose only a part of their velocity at the
wall. Small particles m a y loose all their momentum through impact and adhesion with the
wall. Moreover, elastic collisions are expected for fine particles (Boothroyd, 1969). H e
concluded that inelasticity of impacts, rotation of particles of irregular shape, influence of
velocity gradient and other factors decrease the interaction of colliding particles.
The slip velocity is caused by velocity losses due to collisions of the particles with
the wall and sliding friction between the wall and particles at the points of contact.
Interaction of the translational movement of the fluid and the rotational movement of the
particle generate a Magnus force. The strength and the direction of this Magnus force is
determined by the conditions prevailing at the point of contact and can be given by:
Fm= 0.5pfVf d3u3p (3.1)
where d = particle diameter, (m)
pf = fluid density, (kg/m3)
V f = superficial fluid velocity, (m/sec.)
(jop - angular velocity of particle, (rad/sec.)
The strength of the Magnus force may be important in regard to the angle of
reflection and the shape of the particle trajectory.
A sufficiently robust and accurate model to calculate the pipeline pressure loss for
dilute phase pneumatic conveying systems is presented in Appendix A for completeness.
39
3.2 I N S T A B I L I T Y :
Flow instability in dense phase flow investigations were conducted by Myler et.
al. (1986). In this investigation the stability of pneumatic conveying systems in terms of
flow behaviour, choking and saltation and a linear solution to the unsteady force balance,
was examined. Subsequent observations and experimental data revealed that a wide range
of instabilities are possible in a pneumatic conveying system. For instance, as the gas
velocity decreases, the balance of forces occurs by a decrease in voidage. At a certain
instant, the drag force is insufficient to balance the force of gravity, friction and pressure
and instability occurs termed choking in vertical systems and saltation in horizontal
systems. Jones et al. (1978) compared various correlations for determining the saltation
velocity. They suggested the Rizk correlation defined thus,
U8Sait -CgD»5 R
AM (0.1) D„+ 1.96
(1.1 Dp+2.5) (3-2)
where D p = particle diameter in mm.,
Ugsait - Superficial gas velocity at saltation,
D t = Pipe diameter, to be the most accurate correlation.
The stability of a pneumatic conveying system can be described by the use of the
basic dynamic equations of the flow. The velocities of the gas and solid can be expressed
as a steady state component plus a fluctuation from that steady state that is,
U p - Up. + Op (3-3) U g - U g . + Ug (3.4)
where U p = Particle velocity,
U g = Superficial gas velocity,
Up- = Steady state particle velocity,
U g s - Steady state gas velocity,
Up = Particle velocity fluctuation,
u g= Gas velocity fluctuation.
In terms of flow fluctuations, if the fluctuating terms decay, then the flow is said
to be stable, whereas, if the fluctuating terms grow the flow is said to be unstable.
3.3 PNEUMATIC CONVEYING MODELS; A literature survey summary of pneumatic conveying models is presented in
Table 3.1.
40
TABLE 3.1 PNEUMATIC CONVEYING MODELS - LITERATURE
REVIEW
YEAR
1958
1959
1965
1978
1980
1980
AUTHOR
Barth
Muschelknautz
Julian et al.
Crowe et al.
Molerus
Wheeldon
etal.
COMMENTS
Proposed a model based on the forces acting on the
particles during lean phase pneumatic conveying. They used
a rotating disk on which particles could be dropped at
various speeds. In this way, very large angles of incidence
which occur during pneumatic conveying could be
simulated.
Suggested that for dilute phase conveying, the presence
of solids is reflected by modification of the local turbulence
in the gas phase. This effect causes an increase in turbulent
fluctuations, mixing length and eddy viscosity and frictional
pressure drop.
Developed a particle trajectory model based on treating the
particles as being equivalent to a gaseous phase. They also
developed an implicit quasi one dimensional numerical
formulation for two phase flow.
Derived the energy conservation law for particle motion in
pipeline. Dimensionless equations for the additional
pressure drop in the conveying of coarse and fine powders
are derived from the energy loss between the fluid and
particle. Contributions of particle / wall friction of sliding
particles and losses due to particle / wall and / or particle /
particle collisions are included in the total pressure drop
equation.
Analyzed the fundamental equation of motion by selecting
particle velocity data from the literature and examined their
influence on particle velocity and the additional pressure
41
1984
1984
1985
1986
1986
1986
Shen et al.
Michaelides
Tsuji et al.
Doss et al.
A d e w u m i et al.
Edwards
drop. H e has shown that the coefficient of restitution is the
most significant variable in predicting these parameters.
Modelled the particle collisions of rough, inelastic discs
by considering the geometry of particle trajectories before
and after collision, and computing statistical averages. A
strong dependence on volume fraction was found.
Presented a two-dimensional model based on a turbulence
model. H e considered the eddy viscosity taking into account
the Reynolds stress changing with both velocity and density
gradient. As a result of this work he calculated the Reynolds
stress according to the mixing length hypothesis.
Proposed a model for abnormal bouncing and found that the
particle flow predicted by his simulation agreed with
measurements regarding particle distribution, pressure drop
and particle velocities including angular velocities. Studied
also the effects of particle size, pipe diameter, particle
density, etc.
Formulated a model to consider wall friction for
multispecies. The expressions for the friction factor, to
simulate the effect of particle-wall interaction with a single
solid species, have been extended to model the wall shear
stress for multispecies solid-gas flows. This model can be
used to study the effect of particle-wall interactions on flow
characteristics.
Presented a two-dimensional steady state hydrodynamic
model for vertical pneumatic conveying. They considered
viscous dissipation in terms of the gas and particulate phase.
Developed a three dimensional computer model for dilute
phase pneumatic conveying in a circular pipe. This model
takes into account the particle-particle interaction and
particle-wall interactions.
42
3.4.1 PARTICLE VELOCITY:
Measurement of the particle velocity in pneumatic conveying is necessary for
evaluation of the optimum gas velocity for stable operation, system pressure drop and
the solids residence time [ Matsumoto et al. (1982)]. Unfortunately, direct particle
velocity measurement is difficult due to the complex gas-solid flow existing.
The particle velocity is governed by many factors such as air velocity, solid / air
ratio and type of solids. The distribution of the solids is not uniform and is changed by
the flow pattern. The complexity of this flow is highlighted by the particle-particle,
particle-gas and particle-wall interactions present. These individual interactions are
difficult to separate experimentally. Furthermore, knowledge of the particle velocity is an
important parameter for estimating energy requirements and frictional losses in pneumatic
conveying systems.
Boothroyd (1971) reviewed solid velocity measurement techniques. In general, he
concluded that measurement of the mass flow rates of gas and solid are simple and
accurate, but the direct measurement of solids velocity is somewhat difficult. The
different techniques for measuring solids velocity are summarized in Table 3.2.
3.4.2 INDIRECT METHODS:
The solids velocity can be determined from knowledge of the volumetric
concentration of solids in the pipe and volumetric flux of the solids. The latter quantity is
usually determined easily.
3.4.3 FIBRE OPTIC TECHNIQUE:
For the last 15 years, measurement of particle velocities using optical fibres has
been reported in the field of fluidization. This measuring method can be classified into
two types; one based on the correlation technique [ Oki et al. (1977)] and the other based
on the space filtering in which the frequency is proportional to the particle velocity
[Morikawa et al. (1986)]. Davies (1984) and Matsumoto et al. (1986) used photo-cells
based on the cross-correlation to determine particle velocities.
The method used by the author is likewise based on the correlation technique.
The fibre optic probe consists of a pair of bundles each consisting of small polymer
fibres. Within each bundle, three fibres are used to illuminate the flow stream and a
fourth fibre to detect the light signals reflected by the travelling particles. The detected
light signals are then cross-correlated to find the transit time (tm) between the two
43
t/3
W
•H
z K U rr]
H H Z W
C-H r - -
< Cd
><
H H
u o r-r
Q
O
*
C-r r-r
CQ H(J
H
w Z /*> C-H
e w (Sr
GO
W
o < Z < >
CO h—1
Q
W
a z < > Q <
W J Cu r — 1
u z 1—H
Pi CL.
CO
Q O
„ /*~N
ON
13 -— 0)
E CU rS
o '3 N c cd >
1 • rH
E cd 3 O
o • * - * •
oo
O o -3 r r .
•rH
PC o c o — J
o ti H—r
<u •a "cd
E cu H i — -
Pr,
T3 ti
o 3 O rH
a. on In
<u cd In •4—*
on cd
•a CU 00
3
i/i
U
CL,
In <U CU cd
£ CU >
. — H -_H
cu cd O
• rH
•s Or
NO r--ON
_2
o
o 1—1
E r - .
r_J
s CX 3 O
• rH r-»
cd 3
cu o 3 cd r -
1—
3 r-t
oo
£ o Cr.
<+-
o T3 3
_o "o Wj
3 •1-H
OO lfl cd O H
ci 3
• IF-H • * — '
cd
-3 cd 3
g 3 CU 3
"8 _3 •-I
<U
S
NO
r~ Os r**
"-3 •H—-
CO in r-
o rC
L5
2 o
r_-
"cd • 1-H
cu -U
ex -0
O 3
C
.2 r — r
e -O
"cd
cu
C/I
c .2 cd
cu o o Ir-—-t
r-l
cd T3 <U O <U -*—' CU
J8 3 cd
cu 3
o •3 cd
g
CN oc ON
r ^
"cd
CD
E o r3 H
d . r ^
3 CT -H
CU CJ 3 cd r-
cd
73
d cu £H '3
tr
O T3 CU >
o r3 CO 00 In
o r r
O CU r H
cu •a 3 O •3 cd
\3
.3 'o l-Cl
>-. JO
T3 CU OO 3 CU
_o 3 cd
o in
r ^
"o
'i Cu In CU
cu
g JL) rO cd
N •3 CU 3 Cfl cd
E
3
.2 cd 3
r->
3 O CU CU
• r r
o cd O
•rH
cd In 0) r3 • » - *
0) —-• cd 3
• rH
E n H
13 2
o oo CTN ^ ,
"cd r-»
<u In <U -•—1
st 2r CQ
"id O CU
9< IS>
1 3 c_r cu OH
r H
o cu s-o
00
cd -3 cu C/3
3 00
M "o • I-H H—r
3.
-a cd
O o — H
3 CU O oo CU
In
o r3 oo
o r3 OH -o CU — H
cd O U H—t
3 <U CU oo <u
C-H 00
o _3 OH
O 3
*Sr
3 * l-H
st 0) '>
o ^-t
00
O — J
CU
cu — r
CU
*a ln 0) • H
"B, • — H
~£ E o •H—'
o r3 DH
•a
3 cd 00
In
cu % 3
•8 r3
cu cu cd
eb 3
•r-.
-t >-r2 > ^ --P O
o
S 8 O 3 •rH 00
to p ti s: 3 ex
C-0 3
•l-H
00 00
cu -CX "cd 3
ho •—H
oo
, , cu •a 3
s <
ON NO ON
"cd — H
<u r-i CU CU
CQ
0) 4—1
_3 •3
o -t—-
T3 U — H
o • f i
in r - r
o 3
CU > •rH
3 U CX X cu J3
i
00 00
o In O >. X)
-a
1 oo cd ti
£ • * — *
• rH
CJ ^O
> C/3
.2 "o co
au cu
.-9 u cd CX cd
u 13 CJ
^^ NO
ON
Id — H
cu oo
<u 4—*
o
ON
00
a o m oo cd r3 CX
O -—-3 O
•rH r~-
o
g oo o
CU 00
• f-H O 3 rt
o -rr
<+n O
u
E •3 4—' • r-t
(A
fa u -3 +—I 3 td
o
-8 •5
1
oo 3 O r3
00
ON nn
^ Id -t-H
0) cd
-a 3 in cd s
O 3
o q-3 In
cu -3 st o CX
CU CJ 3 cd — H •-H
CJ
O
a 4-1
u r3 - H
3 U <U
st H--g H—' • - — 1
cd
ex
#-cn C50 Os
N • rH
CQ r.
cn X) ON
"3
oo
ex bO 3 • rH > O
E
oo 4-*
3 & 3 O i/i
In CU
o 3 X) OO
3 ed
fa
44
Mathur etal. (1983),
Klinzing
etal. (1987)
Jotaki etal. (1971)
Jodlowski (1976)
Required
special
viewing window,
average solid velocity
can't be determined.
Local solids velocity
can be
obtained.
Two photographs superimposed on same
photographic negative, velocity calculated
from
the displacement of the particle in the
negative, stroboscope used as a light
source, interval being determined by a
multivibrator.
Photographic
Stroboscopic Method
Konno etal. (1969),
Reddy etal. (1969),
Jodlowski (1976),
Jotakiet
al. (1971),
Tokar etal. (1983)
Required
special
viewing window,
can't be used for
small particles owin^
to rapid dispersion
of coloured particles.
cu 3 CT* •l-H
3 r3 CJ
Si ir "CL,
E co
Solids velocity measured by comparing
the progress of coloured granules, frame
by frame
against a metered
scale.
-8 •5 rt
<u
E cd
U <u 3
- i-H
U
Hariu etal. (1949)
Mehta etal. (1957),
Capes et al. (1973),
Ostrovskii
et
al. (1976)
Disturbance of flow,
time consuming.
cu 3 cr •rH
3 r3
o
a JO "ex
E • rH
CO
Valves
installed at two ends of the test
section to hold up particles.
After valves
closed, particles removed
and weighed.
Velocity determined from mass
flow rate
of solids, weight of solids and test section
length.
Quick Closing Valve
Technique
Riethmuller etal. (1973),
Birchenough et al. (1976),
Scott (1978), Lee et al.
(1978, 1982), Tsuji et al.
(1982), Davies (1984).
Required
special
viewing window,
expensive.
Can be used
for wide
range of velocities,
calibration not
required,
accurate
results for dilute
phase flow.
Particle intercepted by a laser beam
with
a shift in a frequency which
is then
related to particle velocity.
Laser Doppler
Velocimetry (LDV)
Hamid etal. (1975),
Howard (1976)
Stuchly etal. (1977)
Required
special
viewing window,
difficulty in
calibration.
Low cost, compact,
effective for dilute
phase flow.
Microwave irradiation used as the energy
source. A microwave
signal directed
into
the flow stream from a horn antenna,
particle velocity obtained from doppler
frequency shift of transmitted and reflected
signal.
00
•g X 4—»
<u
s ID
s rr
.§
46
detecting points and thus solids velocity can be calculated using the following
relationship.
Vs= h (3'5) where L = the distance between detecting points and tm the transit time.
The size of the probe must be sufficiently small to detect the reflected light signals
from the individual particles. More complete details of the fibre optic probe used in this
investigation are presented in Chapter 6.
The advantages and disadvantages of the fibre optic probe technique will now be
briefly discussed.
3.5.4.1 ADVANTAGES:
The advantages of the fibre optic probe include:
1. It can be used for both dilute and dense phase flows;
2. Low cost instrumentation;
3. The probe is external, readily moved to a desired position and does not induce
flow disturbances;
4. The strength of the reflected light signals can be calibrated to solids
concentration.
3.5.4.2 DISADVANTAGES:
The disadvantages of the fibre optic probe include:
1. Limited application depending on concentration;
2. Not applicable for measuring particle velocity at the top of pipes due to the
low concentration in this location;
3. Only surface particle velocity is measured;
4. Not applicable for flows where in a stationary layer forms on the inside of the
pipe.
3.5.4.3 CROSS-CORRELATION TECHNIQUE:
In the correlation technique, two sensors are used to convert the random
fluctuations of a physical property of the conveyed solids into stochastic voltage signals
X(t) and Y(t). These physical properties may be for instance temperature, permittivity,
conductivity, porosity or permeability variations in the flow as detected by suitable
sensors. The variations in flow properties used for cross correlation must be of random
stochastic structure. If the distance L between the sensors is not too large, both signals
47
are very similar. Under ideal conditions they are identical and shifted by the transit time
tm of the solids from the first sensor to the downstream sensor.
The transit time (tm) can be found by computing the cross-correlation function
over a time period T. The cross-correlation function is defined by,
R x y (0 = Y £ X (t - x) Y(t) dt (3.6)
where t = the adjustable time delay.
It has been proved [Beck et al. (1968)] that the value of the cross-correlation
function reaches a maximum, when the delay time (T) equals the transit time (tm). Hence,
the solids velocity can be easily calculated by using eqn. (3.5).
There are many advantages in the cross-correlation method for velocity
measurement. The principal advantage is that calibration of sensors is not required
because the time delay is measured with reference to a crystal controlled time standard in
the cross-correlator, secondly cross-correlation rejects the effect of spurious interference
on the signals.
Bitz (1983) designed a microprocessor based correlator and in combination with
an electrostatic sensor conducted experiments to measure velocity and mass flow rate of
Coal. Recent developments in large-scale integrated circuits and microprocessors enable
simple, fast and reliable cross correlators to be designed at a cost that is acceptable for
industrial use. B y use of cross-correlation velocity and flowmeters a wide range of
industrial and environmental measurement problems can be solved (Beck, 1981).
3.6 PARTICLE CONCENTRATION: Usually, the particle density ps of the conveyed solids is normally known and the
velocity v is calculated by one of the correlation methods. Hence, measurement of the
solids concentration C s remains. Optical, capacitive and radiometric methods have been
used for this measurement. Unfortunately, for very low concentrations capacitive and
radiometric sensors do not work properly.
The conveyed mass flow rate ms is,
m s = Ps C s A v (3.7)
The different techniques for measuring solids concentration are summarized in
Table 3.3.
48
H Z w w as CO
z o I-H
H OS H
Z w CJ z o u •
<-.
aa H
8 z a e a
w D 2 U
a • D co < §
^—v fr-OO ON >~-' -5-1
cn oo ON "So § X
U <-3 00
g A i—i
r--. ON T—1
"3 4-rr
(U r-r
o <u PQ
-8
CU
a 73
•a o cu
W
/r»\
VO O ON I-H v-' J ed 4->
<u • iH
a »o > o fa (/>
o /-s
cn i> ON 4—1 " - - r - .
. 73 4-i
(U oo CU
4*
Ov nH
73 4-4
cu ed 4-»
-3 ti
» ON"
"-t 0\
73 4-4
<u 3
-8
1 8
•a ed
73 CO I-H
ON NO ON I — I N w .
o 4-»
cd
co
o 8 3 O
3 • ST
.§ O <J
r3 ex
B. o J2 £
OS CO ON <-H
73 4-4
CU >-. ed
T3 3 O •rH
4-4
ed J3
u NO
r-« ON
i 5-O
i—»
8 O
••a S3 u 73 3 rH
o 3 .2 .3 3 3
1 <
^ oo ON ^—r
; ed 4-4
CU
CU
1 4-*
CO
> Q d.
J
rS CX
5 In ed J
/r-S
NO oo ON
c. . 73 4-4
CU ed
a o
1 •a
1 •rH b
49
CHAPTER 4 COEFFICIENT OF RESTITUTION,
BENDS AND WEAR
4.1 COEFFICIENT OF RESTITIJTION(C.O.R.):
The theory of impact is based on the impulse-momentum law for rigid bodies. For
perfectly elastic impact of two bodies, the law of conservation of mechanical energy can
be used to determine the final velocities. The resilience of a particle is defined as the
ability of the material to resist conversion of kinetic energy to other forms of energy on
impact
When the impact produces a permanent deformation, this relation can be replaced
by a coefficient of restitution, e, for the process. This coefficient describes the degree of
plasticity of the collision. Theory and experimental data indicate that a definite value for
the coefficient of restitution can't be assigned to the impact of bodies unless their size,
material and impact velocity are specified initially.
The coefficient of restitution is an important parameter in predicting particle
velocities and additional pressure drops in lean phase horizontal and vertical pneumatic
conveying. It is a measure of the kinetic energy exchange between the two bodies upon
impact and it gives a good indication of the particle-wall interaction. It is a measure of the
elasticity of collision. It is defined as the ratio of the relative normal velocity components
of the two colliding particles after impact to the normal velocity components before
impact.
e = (v'2n- v'ln) (4.1)
( V2n ~ Vln )
The loss of kinetic energy may be expressed by the following equation
(Konno etal. 1969),
E = -|mi(yJ-vf) + ^(cD?-cn|) ^
where m_ = mass of colliding body, kg.
Vj = velocity of colliding body before impact, m/s.
v2 = velocity of colliding body after impact, m/s.
50
coj = angular velocity of colliding body before impact, rad/sec.
o>2 = angular velocity of colliding body after impact, rad/sec.
I = mo m e n t of inertia of colliding body, kg-m2.
For completely elastic collisions (e=l) there is no kinetic energy loss, whereas for
completely inelastic impact (e=0) there is appreciable loss of kinetic energy. Hence, the
values of e=l arid e=0 denote the idealized concept of perfectly elastic and plastic
impact, respectively.
Neglecting the air drag on a falling particle, the coefficient of restitution is simply
the square root of the ratio of rebound height to initial height.
e = si-Rebound h. initial (4.3)
Because of the scatter in the height after impact due to the irregular shape of
particles, e-values must be averaged to obtain a representative value of the coefficient of
restitution. A literature survey summary declaring coefficient of restitution investigations
is presented in Table 4.1.
YEAR
1954
TABLE 4.1 COEFFICIENT OF RESTITUTION -
AUTHOR
Tillet
LITERATURE SURVEY
MATERIAL
TESTED
Steel balls
on Glass and
Plastic plates
COMMENTS
C.O.R.impacting on Glass-0.985,
Perspex- 0.95, Glycerol sextol
phthalate-0.94, Steel- 0.95; Time of
contact for 1/4 in. dia. Steel ball with
velocity of 0.9 m/sec, when
impacting Perspex - 73 x 10"" sec.,
Glass- 32 xlO'6 sec, Steel - 21 x
10"6 sec.. Typical variation with
temperature over the range 20° and
51
1957
1959
1960
1961
1970a
1970b
Adam
Muschel-
-knautz
Ranz et al.
Macre et al.
Matsumoto
etal.
Quartz and Lime
Granular
materials
Plastic,
Rubber, Lucite
Steel, Glass,
Poly styrene,
Phosphor
Bronze
Glass beads
90° C for an 1/8 in. diameter ball
impacting on 2 in. thick perspex,
refer Figure 4.1.
Studied particle trajectories with pipe
materials namely Glass, Rubber and
Lead by use of high speed motion
pictures. Figure 4.2 depicts the
observed particle trajectories showing
the impact and rebound angles.
Used a rotating disk technique onto
which the particles are allowed to
drop from a certain height (< 5 cm.).
The disc's material, particles (> 2
mm.) and speed (< 4 m/s.) were
changed and the rebound height,
torque and reflected angles measured.
Studied particle collision of shot gun
projected particles against a hard
surface at varying incident angles.
H e noted rebound trajectory scatter
due to impact friction and particle
shape.
Studied the effect of the coefficient
of restitution for various materials as
a function of height, refer Figure
4.3.
Simulated irregular bouncing model
of ellipsoid particles and found e =
0.97. Found from experiments using
photographic techniques that particles
flowing in the duct were rotating at
high speeds of 1000 r.p.s. or more.
52
1970
1976
1978
1978
1980
Maeda
etal.
Matsumoto
etal.
Tsuji et al.
Scott
Tabakoff
Polyvinyl
Chloride and
Polyethylene
Ellipsoid
particles
Coarse particles
Rubber
pellets
Quartz and
coal ash
Presented an alternate model based
on wall roughness.
Used photographic techniques to
study particle trajectories of both
150 n m mean size polyvinyl chloride
particles and 100 |im mean size
Polyethylene particles impacting flat
plates at 20° and 10°, found that
particle-particle collisions seldom
occur and if they collide with each
other the momentum losses are
negligibly small.
Simulated in a circular pipe in terms
of concentration distribution, particle
velocity, additional pressure drop and
frequency of particle collisions with
the pipe wall.
Calculated friction loss due to
collision of the particles with the
pipe wall using the impulsive
equations.
Used rotating disk technique, drop
height 100 m m and impact impart
Aluminium. Observed e= 0.8 and
f = 1.5 (±0.5).
Used a Laser Doppler Velocimeter
(L.D.V.) system, particles of 0.5 to
60 particles microns in size colliding
with Aluminium, Stainless Steel and
Titanium alloys plates. Measured
normal and tangential components.
Concluded that the coefficient of
53
1980
1981
1981
1982
1987
Brauer
Sabbaghian
Ottjes
Ottjes
Devas within
etal.
Steel
particle
Coal
Polypropylene
Steel and Rubber
Polypropylene,
Nylon and
Rubber
Sand
restitution for particles below 30
microns in size can only be measured
with a L.D.V. system.
Used photographic techniques to
observe Steel 6 m m . spheres
impacting on various wall materials,
refer Figure 4.4. Found that for the
twelve different wall materials tested,
the coefficient of restitution generally
decreases, with increasing impact
angle, refer Figure 4.5.
Studied effect of angle of impact,
particle size, particle concentration
and shape impacting on Stainless
Steel.
Developed a detector to measure
particle / wall collisions in a
pneumatic conveying rig and
expressed collision measurements
in terms of pressure loss.
Used a rotating disk technique, disk
diameter of 0.3 m. and speed of
2990 r.p.m., e= 0.6 with a standard
deviation of 0.2 for impact velocities
in the range of 1.2 - 2.5 m / s . ,
/
Used a sand-shot blasting machine (< 400 n m ) impact on a mild steel
plate, velocity 68 - 92 m / s., impact
angles 30° and 40°.
54
0-95
e
0-9
Temperature (°c)
Figure 4.1: Variation of the Coefficient of Restitution of Perspex with Temperature
[ Tillet, (1954) ].
100 m m
Materials:
Fbrticle /Pipewal Quartz /Glass
Lime A3 lass
Lime /Rubber
Quartz/Lead
Figure 4.2: Particle Trajectories for Quartz and Lime Impacting
Various Pipe Materials [ Adam, (1957) ].
10
c o ~ 0-9 in ff 0-8
.!§ 0 7 o
ri-
8 0 6
0-5
"•«---rc—o
--
-Q--Steel
-a- Q •-• • + GJQSS_t
V--... " ^ - r . .. Polystyrene
-© Phosphor bronze
--x-"* * — - . - „
12 18 2-, 30
Height of drop, (inches)
36
Figure 4.3: Variation of Coefficient of Restitution versus Impact Height
[ Macre, (1961) ].
-P- i
Figure 4.4: Test Rig for Particle / Wall Collision [ Brauer, (1980) ].
56
1.0
0.8
i 3* H CJ
? 0.6 ,4* .U QJ
O <J C
.g
a? 0 IS 30 iS 60 75 90
impac zngte a, ["}
Figure 4.5: Variation of Coefficient of Restitution versus Impact Angle
[ Brauer, (1980)].
4.2 BENDS:
4.2.1 I N T R O D U C T I O N :
Bend geometry has a strong influence on the performance of a pneumatic
conveying system. Space limitations usually make the use of bends essential in
pneumatic conveying. The usual method of calculating energy losses in bends for single
phase flow is to obtain a factor by which the diameter of the pipe is multiplied to obtain
an equivalent length of straight pipe. Due to convenience, this method has been
extrapolated to two phase flow.
In general, visual observation of powder flow in bends reveals that two types of
flow occurs: in the first powder slides around the outer radius of the bend at a slower
velocity than the conveying gas, whereas, in the second the powder makes a number of
collisions in traversing the bend. The particle trajectories between the particle-wall
impacts are sometimes reported as straight lines and frequently as distinct curves. The
flow pattern in bends is complicated by secondary flow of the conveying gas induced by
centrifugal effects. Twin eddies are formed in the radial plane and in combination with
the main flow creates a double spiral motion downstream. Deceleration and acceleration
of the powder occurs as well as segregation of the powders by the sliding and erosion of
the pipewall. Hence, the pressure loss across bends is larger than that across an
equivalent length of straight horizontal pipe.
57
4.2.2 TYPES OF BENPS; Bends in pneumatic conveying systems usually have the following forms.
1. Long radius bend - Most widely used
2. Short radius bend - Available as a standard
3. Blinded Tee bend - Excellent wear properties
4. Wear back bend - Commonly accepted method of reinforcing a bend
5. Impact bend - High pressure loss
6. Vortice Elbow - Lower pressure loss
A complete discussion of the flow mechanism in the various bend types,
geometries and their application is presented by Hilbert(l--83). A summary of this
discussion is presented in Table 4.2.
To reduce the extent of erosion in bends various wear prevention techniques are
employed. C o m m o n wear prevention techniques include bend lining materials, bends
with drop out boxes, etc.. Lining materials used to date include various refractory
materials, ceramics and epoxy resin mixed with high abrasion fillers. Various bend
geometries and types commonly used in pneumatic conveying systems are shown in
Figure 4.6.
4.2.3 PRESSURE DROP CORRELATIONS:
Ito (1960) reported a suitable correlation for the long radius bend as,
/2RBV>-9
— (4.4) A _ 0.248 a i -p-A PB V D
A C ., 2 r, 0.2
0.5 pf vf Re
where, a = 0.95 + 17.2
-1.96
(4.5) f2RB
I D Re = Reynolds number,
pf = density of gas, and
Vf = gas velocity.
Schuchart (1968) evaluated an experimental correlation on pressure losses in
bends as,
APB (2RBV115 ,.,.
= 210 —-=. (4.6) Apst K D
58
z o IM
H I—1
r-l
r-r
"rj
a z t/5
u ** H -*-> l-N Or
U • <
Pr "r
8 U Q Z W pa
.
•N Tf
w
00
|
C/5
<
e Or I
H oo O U
-y
•2 -T £
_ -2 r-> O H
r O iV
> H
e W '""'"*
o S w o
fl
£ H Q CQ
CO
9
i*
s g
i
1 -H
c
I u •S
"r3
Cr
4=
rH
8 u
BO >— r
^ _o
§ .fl
I ••8
.s
S •
00
•a e J8 co 9 «3 S t>0
rJ
CO
E co
J e»o 9
el
i (SO 9
73
1 S CO rS
60
Oi
s
J-l
.2? ie (SO
.S 60 Sj>
9 O •s CO
TJ
H •8
•a CO
S3 -i-i O
S <u 3 5 r) Jr -rO
2 § * 1 a o. •S e
CQ ••a <u g .S
£ _2
£ ,0
m •
es
c I J.
3 vfl S
6 J3 Cfl
-_0
•S o 53
J O XI -3 CO
1 9 CO
e
i "a & 5-
8.
.53
ca =3 S g .5
•hr--
s £ JO
I 1 2 .9
i
"§
1 8 £
rS PO
(50
c • 1—.
&0 (-0 2 T3
If 5) _3 CO
•a
9 .9 9 s CO
73
1 9 >
1
§ •a
•s rt-H
"I
s 13 c o
I a TJ .5
x: go _=
i* ^
| o
1
Sr
-2 w
1 1
59
Long Radius Bend
Wear Backed Long Radius Bend
^
Short Radius Bend
Blinded Tee
Impact Bend
Vortice Elbow
Figure 4.6: Examples of Bend Geometries [ Arnold, 1989 ].
60
where A p B = pressure drop in a bend conveying powders,
A Pst = pressure drop of solids of an equivalent length of straight pipe,
R B = radius of curvature of the bend, and
D = diameter of the pipe.
For fine particles, Muley et al. (1982) developed an experimental correlation as,
A Pbends 1_,7f2RBy0-64
— —_i3S7[—) (4.7) ^ rst. pipe >
Mason et al. (1973) data on the flow of fine particles 15, 40 and 70 micron in
diameter through long radius bends having a diameter ratio D B / D of 20 found that
existing bend pressure drop correlations overestimate the pressure drop.
Scott (1977) suggested a simple approach to evaluate the overall system pressure
by treating the flow as fully accelerated flow and then to add an appropriate additional
pressure drop for each bend. This extra pressure drop arises from the need to reaccelerate
the solid particles after they have been slowed down by the bend.
Alternatively, to estimate the bend pressure drop the concept of an equivalent pipe
length, using factors derived from experimental work can be used. In this procedure the
pressure drop is expressed as,
Apt>= 2 (48>
where ps = density of the gas / solids suspension,
kb = a coefficient,
Vf = gas velocity.
TABLE 4.3: BEND PRESSURE-DROP FACTORS FOR USE IN
EQUN. (4.8)
Bend ratio (= 2 x radius of bend/ diameter of pipe)
4
8
12
Bend pressure-drop factor kb
1.50
0.75
0.50
61
From the Table 4.3, it can be seen that, in general, the sharper the bend radius,
the higher the pressure loss. Unfortunately, the information presented in Table 4.3 is
particular to a specific experimental set up and hence application of the bend pressure
factor presented to other application situations is limited.
In another experimental investigation, Hilbert (1983) compared the service life
between blinded tees and long radius bends, when conveying zirconium sodium. His
findings are summarized in Table 4.4.
TABLE 4.4: SERVICE LIFE OF LONG RADIUS BENDS
AND BLIND TEES CONVEYING ZIRCONIUM SODIUM.
Bend Type
Long radius R/D = 8
Long radius R/D=12
Long radius R/D= 16
Long radius R/D= 24
Blinded Tee
Service Life (Hrs.)
8
14
15
26
487
4.3 W E A R A N D ABR A S I O N :
It is generally considered that wear in pneumatic conveying systems is largely
dependent on material, hardness, particle size and concentration. Furthermore, wear of
pneumatic conveying lines is aggravated by the following factors:
1. High abrasiveness of conveying material;
2. Inadequate selection of conveying equipment;
3. Poor pipeline design;
4. Excessive conveying velocity and
5. Inadequate pipeline installation.
To decrease the degree of abrasion in a pipeline, the following is recommended
(Stankovich, 1985):
1. Use dense-phase pneumatic conveying systems where possible and applicable.
2. Use the lowest possible conveying velocities consistent with transportation
limits. The need to operate conveying systems at the minimum transport velocity
is apparent from the following simple abrasive wear correlations,
Abrasion W = k v3 for straight pipelines (4.9)
62
and W = k- v 4 for fittings, (4.10)
where k and kx = coefficients related to the material conveyed and pipe
characteristics (material, hardness) and v = the conveying velocity.
3. Use continuous conveying (e.g. double blow tanks) whenever possible and
practical to eliminate the filling and pipeline purging phases of the conveying
cycle. It is found that most pipeline abrasion wear occurs during these phases of
the conveying cycle. In particular, it is usual to observe very high transient
transport velocities during these phases.
4. Use purpose designed abrasive-resistant pipes and fittings made of abrasive-
resistant iron and steel alloys, ceramics, with wear-backs, replaceable back bends
with longer ends to remove their joints from wear impact zones as much as
possible and practical.
5. Use larger size pipe and fittings at the pipeline terminal than that for the main
pipeline section to minimize the effect of the terminal velocities caused by air
expansion,
6. Whenever possible and practical, use a smaller pipe size to minimize the effect
of material pulsing and minimize flow instabilities.
7. Properly support all pipe joints and fittings to avoid vibrations and pipe
misalignment.
4.4 ATTRITION:
Particle attrition generally arises from mechanical forces, thermal forces, chemical
stress or pressure changes between the inside and outside of particles. The attrition is
caused by collisions between particles and collisions between particles and the pipe
walls. As the particle velocity increases, the degree of fragmentation of the particles also
increases. Attrition is manifested through particle size reduction and particle shape
deformation (Bridgewater, 1987).
First order kinetic formulations for this phenomenon have been reported in the
literature. The rate of loss of material in a certain size interval is proportional to the
amount of material in that size-interval. However, this is not generally true. G w y n
(1969) suggested a time dependent formulation for attrition to be,
W = k t m (4-n)
where W = the weight fraction attrited,
t = the time,
k = a constant and is a function of the initial particle size, and
m = an exponent.
63
H e found the value of m to be about 0.46 for the catalyst particles used in his
experimental investigation. Variables affecting particle attrition, different methods of
assessment for particle attrition and types of attrition tests are shown in Tables 4.5, 4.6
and 4.7 respectively (Bemrose et al., 1987).
TABLE 4.5: VARIABLES AFFECTING ATTRITION
Particle properties
Size
Shape
Surface
Porosity
Hardness
Cracks Micro structure
Environment properties
Time
Velocity
Pressure
Shear
Temperature
TABLE 4.6: ASSESSMENT OF ATTRITION
Individual particles
Particle shape
Particle size
Group of particles
Number concentration
Surface area
Particle size distribution
Indices:
Attrition index
Hardgrove index
W o r k index
Elutriation rate
Breakage and selection
functions
Bulk material
Settling density
Tap density
Flowability
Packing index
Angle of internal
friction
64
TABLE 4.7: TYPES OF ATTRITION TESTS
Single particle
(Fragmentation)
Crushing
Impact
Multi-particle
(Fragmentation
and abrasion)
Fluidizedbed
Shear cell
Rotating drum
Grindability
Vibration
Drop shatter
Paddle wheel
Enhanced sieving
Possible tests
Chemical reaction
Pressure change
Heating
Fluid Transport
From the various research work conducted on attrition, it can be concluded that
1. It increases if the particles are subjected to impact.
2. It is m a x i m u m at particle to surface impact angles of 30°-50° from the plane
of the surface.
3. It occurs mainly from breakage of the coarser particles.
4. The rate of attrition for spherical particles is approximately one-half of the
rate for nonspherical particles. In particular sharp, angular particles erode
more than do rounded particles [ Tilly (1969)].
The impaction of particles onto components of the pneumatic system results in
contamination and attrition of the conveyed material. Powder attrition can cause problems
due to changes in particle shape and particle size distribution which effect flow
characteristics. Plant operating difficulties are experienced because of the fines produced,
particularly with filtration equipment. In the latter equipment the filter cloth and screens
tend to block due to the high flow rate of fine powder. This increases the pressure drop
across the filter reducing the pressure drop available for conveying. These effects
combine to reduce the conveying rate. Therefore, it is essential that powder attrition be
minimized.
65
Properties of materials like internal angle of friction, particle size distribution,
shape, surface area, bulk density and minimum fluidizing velocity change due to
attrition. Loss of material occurs due to change in particle size which, in turn, generate a
dust pollution problem. Sometimes this fine material can be recovered and recycled by
agglomeration into large particles.
Attrition can be minimized by operation at minimum transport velocity, the use of
streamlining the system and elimination of unnecessary components and bends, smooth
internal surfaces and gradual change of internal cross sectional areas and the
maintenance of high system pressure.
4.5 PIPING:
The correct installation of piping is most important and where possible
misalignment should be avoided. Furthermore, the pipe should be free from blemishes
and be undented. Obviously, any irregularity in the piping will promote wear in that
particular area.
In dilute phase systems and dense phase systems it is normal to use standard
medium gauge or steam piping. The common methods for connecting these pipes include
welding, slip-on couplings and screwed flanges. Compression-type sleeve coupling
allow easy rotation to equalize wear. They are often butt-welded. However, although
convenient welded pipe joints are undesirable, especially if maintenance woi*. is
required on a particular section of pipe. Typical details of surface roughness and piping
materials are presented in Table 4.8 and Table 4.9, respectively.
TABLE 4.8: SURFACE ROUGHNESS OF VARIOUS PIPING MATERIAL
Material
Concrete
Cast Iron
Galvanized Iron
Commercial Steel
Wrought Iron
Drawn Tubing
Surface roughness m m .
0.3-3
0.26
0.15
0.045
0.045
0.0015
66
TABLE 4.9 - PIPING MATERIALS
PIPING M A T E R I A L
Seamless mild steel
Stainless steel
Carbon steel
Spun sand cast chrome iron alloy
Plastic
Glass
Carbon-based antistatic plastic
Rubber, various rubber
compounds
CHARACTERISTICS A N D
APPLICATIONS
Rust contamination, Commonly used except
in the food industry
Corrosion resistant material, Used for
chemically active material like food
substances, plastics, resins and similar soft
materials.
Suitable for inert materials, ductile,
weldable, cheap.
Used for ash handling
Suitable for food substances and/or
chemically corrosive materials. Less wear
compared to steel and rubber-lined pipes.
Disadvantages are high cost, problems of
static charge generation and inability to
withstand high temperature.
Chemical resistant, clean and transparent
Used essentially to combat the static
electricity problem.
Good abrasion and impact resistance, but the
disadvantage is a high frictional resistance;
Synthetic rubber has wear resistance
comparable to natural rubber. Used for
67
Pipe lined with ceramics
Pipe lined with Alumina oxide
Silicon carbide ceramics lined
Basalt lined pipe
Shot blasted Aluminium
Special pipe lined with
Asbestos-cement
Abrasion-resistant lining special
pipe
bends and flexible situations and for long
horizontal / vertical runs.
Developed essentially to provide long service
life, highly wear resistant.
Developed to withstand high temperature
and impact ceramics.
These pipes are suitable for fine, extremely
abrasive particles, however these pipes are
expensive.
High resistance to sliding wear and erosion.
Used in the plastic industry for good wear
characteristics and ehmination of electrostatic
charges due to high conductivity. Light
weight.
Ideal wear resistance, commonly used in the
cement industry.
Used for very abrasive conditions
68
CHAPTER 5 POWDER PROPERTIES
5.1 INTRODUCTION;
The following properties should be considered to ascertain or predict the flow of
powders in pneumatic conveying systems:
1. Particle Properties
2. Bulk or Particle Assembly Properties
3. Fluid Phase Interactions
4. Synergetic Properties
1.1
1.2
2.1
2.2
2.3
3.1
3.2
3.3
4.1
4.2
4.3
4.4
4.5
Solid
Surface
Assembly Characteristics
Interparticle Forces
Particle Property Distribution
Fluid Properties
Individual Particle
Interactions
Bulk Interactions
Flowability
Slugging andDuning
Segregation
Electrostatic
Explosibility
Obviously, these properties interact with each other in a very complex manner to
generate actual powder pneumatic conveying flow characteristics. Unfortunately, the full
description and analysis of the forestated powder properties is beyond the scope of this
work. In view of this difficulty, brief details of the salient powder properties effecting
pneumatic conveying flow characteristics is n o w presented. More complete details of the
powder properties and their assessment is presented in Appendix 'B'.
5.2 SALIENT POWDER PROPERTIES:
Table 5.1 provides a list of the salient powder properties effecting powder flow
and Table 5.2 presents the powder properties assessed in Appendix 'B'.
69
TABLE 5.1: SALIENT POWDER PROPERTIES
1. Individual
Particle
1.1 Solid
Hardness
Attrition
Degradation
Porosity
Size
Coefficient of -
Restitution
Elasticity
Abrasion
Combustibility
Fabric
Shape
Structure
1.2 Surface
Surface Energy
Surface -
Electrostatic
Surface Profile
Surface Area
Roughness
2. Bulk
2.1 Assembly
Characteristics
Packing
Porosity
Bulk Density
Compressibility
2.2 Interparticle Force
Internal & Wall Friction
Agglomeration
Contact Stresses
Shear Strength
Yield Strength
Cohesion
Angle of Repose
Tensile Strength
2.3 Particle Property -
Distributions
Flow Capillary -
Distribution
Distribution of Solids
Particle Size Distribution
Void Space Distribution
Contact No. Distribution
3. Fluid Interactions
3.1 Fluid Properties
Buoyancy Force
Viscosity
Surface Tension
Compressibility
3.2 Individual
Particle
Interactions
Drag Coefficient
Turbulence
3.3 Bulk Interactions
Permeability
Moisture Content
Flow Resistance
Deaeration
4. Synergetic
Properties
4.1 Flowability
4.2 Slugging &
Duning
4.3 Segregation
4.4 Electrostatic
4.5 Explosibility
70
Cfl
w HH
H Pr.
u C-r
o r-S Om
X w Q O cu 1/3
w -J CQ
1 i 5-
o c B p-l
(N CQ
r-H
H
_ o
1 a 5. trQ
c
.9 m
I B Q
.S e-<P o >
§ -a 1
B
(U • S
-- c« . •> <-> l-H r
S3, g
S CM
U
§ V. 3
Cr
(U
e o PH
!
rr § Sr, EM
cn CQ
« -a
CQ
eU
cd
a
>-0
CQ
&
s in
-ir-rl
C-H
rC
B
c/5
u
OH
fix *>
5P r
U '3
TZT
<+-i J-5
«- S K *-> oi o> S «- r?P B _ ->
1
Sr
^
l
I J?
n
J-
u
-a o
•s c«
.a i B O
S 2
R -S
B
.2
e 3
%
w
I CO
5
71
72
Depends on particle size, shape and effect of
consolidation.
i—1
. — i
CQ
i Indicates indirectly particle size distribution, powder
packing,
particle hardness, surface area.
Describes the variation of bulk
density
with consolidation.
Compressi
bility
pd
IT}
ti
•s
Influences flowability, packing and interaction with
fluid flows.
Describes the spatial arrangement and
orientation of the particle matter.
Difficult to measure.
1 CO
"ft T—1
PQ
|
Influence the moisture adsorption characteristics, the
relative density of particles and flow behaviour.
Intraparticle porosity describes the void
space within individual particles, whereas
interparticle porosity describes the void
space between particles.
i PL,
"fr i — i
CQ
•
Influences the particle density, interparticle porosity and
permeabihty.
Refers to the arrangement of particles
within
the powder.
(JO
B
OH
CQ
1
Determines
a powder's cohesive properties, extent of
adhesion of particles on
the pipe walls and
agglomeration
characteristics.
Caused by molecular, capillary, electrical
and coulomb forces. Difficult to isolate.
Interparticle
Forces
73
B.17
(a.l)
Table 5.4
and 5.6
Used
to indicate
the flowabiUty of a powder.
Varies with
the particle size, shape, degree of segregation and
aeration,moisture content, cohesion, intemal
friction and
compaction.
Describes the angle of incUnation of
the
powder
free surface when poured onto
a flat surface.
Angle of
Repose
oo 1—1
CQ
C O
i-H
ett
H Used
to measure the degree of cohesion and
agglomeration of powders.
Is the minimum force required to separate
a powder
bed and
is a fundamental
failure
property.
Tensile
Strength
/ - r \
CQ
Chapter
2.5
Friction angles are important in the design of powder
storage,handUng and transportation equipment Depends
on particle size, tensile strength, cohesion, shape, surface
properties and
bulk density.
Internal
friction determines
the shear
resistancewithin powder beds, whereas
waU f
riction determines
the shear
resistance between
the powder bed and
the container waUs.
Internal and
WaU Friction
Xi
< — 1
CQ
r-;
p
Determines
type of system, feeder type and dimensions
and need
for discharge aids in bins and hopper.
Paramount in regard to flowabiUty
of a powder.
Defined as
the molecular
attraction by
which particles of
a powder are held
together.
g >i-H
CM
P
o U
i—i
CQ
'
Used
for measuring
surface area of
the powder.
Strongly
influenced by porosity of the powder & particle size
distribution.
Describes the extent of fluid flow through
a powder
bed.
£ •a p Om
74
ON 1—1
PQ
Os wn rH x> c. H
CM
•B 9„ «- S-P N P 3 a 3 " rB P
*- <_. trO -B
1 8
be classified accon
to free-flowing and
3 •-*
Powders c
behaviour
5 6 T3 P
P CM
n of a settl
d like mas
w.
2 3 o
S a M H ed -r § Q bo l B ^
Sr ^ 3 •S € "2
B
1 1 E
rH
tN CQ
u r-i </"i p r Q C-
H >> <-> • rH
-B r O
"3 CM
o r H
V S3 bfl
B .3
I
g •B
e 43 TJ •+-1
O
§ rH
l cd • * - »
•a p
2 .S 3 ca -r
P B -8 g *-> p •a
i. CM
S
I P l "
to B •rH
> P 1
o p D CM C_
rB CX p CM
B •8 &
bO
.3 CM
6 .3 2 *-> p p r-T 3 _! 55 B
"8 1 fe 5-P 'S
"9 8 a P
75
TABLE 5.3: MOHS' SCALE OF HARDNESS
Mohs Scale
Hardness
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Material
Talc
Gypsum
Calcite
Fluorite
Apatite
Feldspar
Quartz
Topaz
Corundum
Diamond
Explanation
Very soft, can be powdered with the fingers
Moderately soft, can scratch lead
Can scratch a fingernail
Can scratch copper
Can scratch a knife blade with difficulty
Can scratch a knife blade
All products harder than
Feldspar will scratch glass
TABLE 5.4: THE GENERAL RELATIONSHIPS BETWEEN ANGLE OF REPOSE AND THE FLOWABILITY OF MATERIALS
[Carr, (1976)]
Angle of Repose
25-35°
25-35°
35-45°
35-45°
45-55°
55-65°
65-75°
Characteristic of Material
Very free-flowing; noncohesive; for granular material
Very floodable to floodable; for fine powders
Free-flowing; some cohesiveness; for granular material
Fluidizable powders; some cohesiveness
Non-free flowing; cohesive ,
Very non free-flowing; very cohesive /
Very, very, non free-flowing; very, very cohesive
TABLE 5.4: PARTICLE SHAPE AND FLOW CHARACTERISTICS
TERM
Acicular
Angular
CrystaUine
Dendritic
Fibrous
Flaky
Granular
Irregular
Nodular
Spherical
M E A N I N G
Needle shape
Sharp edged or having
rough polyhedral shape
Of geometric shape, freely
developed in a fluid medium
Having a branched
crystalline shape
Regularly or irregularly
threadlike
Platelike
strongly anisotropic
Having approximately
equi-dimensional,
but irregular shape
Lacking any symmetry
Having a round,
irregular shape
Globule shape
FLOW
CHARACTERISTICS
Poor- interlocking
Fair
Fair - poor
Poor - interlocking
Poor - interlocking
Poor - extreme interlocking
Good to fair
Poor
Fair
Good
TABLE 5.6 ANGLE OF REPOSE - LITERATURE SURVEY
YEAR
1952
1958
1960
1961
1967
1974
1976
AUTHOR
Dawes
Train
Zenz et al.
Brown
Bruff et al.
Fryman
Cartensen
etal.
C O M M E N T S
Reported angles of repose measured for cohesive powders
including coal and siUca sand using a fixed cone method.
Observed the angle to be 50° or more at a mean particle size near
100 [tm. Furthermore, observed variation of angle of repose with
tensile strength.
Used four methods to measure angle of repose namely fixed
height cone, fixed base cone, tilting table and rotating cyUnder.
Determined angle of repose of a wide range of different materials.
Reported that the variation of angle of repose for materials of
different particle size increased with increased cohesion.
Used a conical funnel of 0 cm. long to measure the poured angle
of repose of anthracite in various gas media. Observed that in
hydrogen, (low viscosity) the angle of repose was highest and
equal to 31°, in carbon dioxide (medium viscosity) the angle was
smaller, while in air (high viscosity) the angle was lowest. This
suggests that at higher gas viscosity, anthracite has a tendency to
spread, whereas at low gas viscosity, it settles rapidly.
Measured the poured angle of repose for potash peUets and
suggested to increase the angle by preventing rolling of the
particles in the top layer and use of a light spray of water to the
surface.
Reported a relation between particle size and angle of repose of
powders consisting of mono sized cohesive spherical particles.
The angle of repose was observed to decrease with increasing
1982
1984
1985
1985
1986
1990
Tuzun
etal.
Cheremi-
sinoff
etal.
Augwood
Kalson
etal.
Kraus
Geldart
particle diameter. H e related the angle of repose to the cohesion
force and the coefficient of internal friction.
Observed drained angle of repose for glass ballotini in flat bottom
bin in the range of 35°- 45°. Suggested that for simplified bin
design the angle of internal friction be replaced by the drained
angle of repose to evaluate the hopper half angle.
Reported that for monosized particles or particles with a narrow
size distribution, the drained and poured angles are approximately
the same, whereas, for powders with a wide size distribution, the
drained angle is higher than the poured angle.
Observed the effect of aeration and deaeration on the angle of
repose. Reported that deaerated powders exhibit angles of repose
higher than aerated powders.
Reported experimental measurements of the angle of repose as a
function of hopper angle and of the critical drainage angle for
several granular bulk solids in a wedge-shaped hopper.
Reported that the angle of repose is used for the design of bins
and hoppers and to select the type of flow inducers in a
pneumatic conveying system.
Developed a tester to measure the poured angle of repose of soda
ash to assess flowabiUty. Evaluated the effect of size and size
distribution on angle of repose.
79
TABLE 5.7 COHESION - LITERATURE SURVEY
YEAR
1957
1961
1963
1966
1969
1973
1974
AUTHOR
Langmaid
etal.
Jenike
Jenike et al.
Richards
Stepanoff
Wright
Ecknoff
etal.
COMMENTS
Performed experiments on wedge shaped and conical
hoppers with non-cohesive granular material. Presented
correlations for the critical hopper outlet width and diameter.
Found variation in results due to differing material shape.
Considered stability of a self-supporting cohesive arch of
unit thickness and stated that the force tending to break the
arch is due to the weight of the material within it. This is the
worst case for maintaining flow.
Extended the arch analysis to include the variation of
thickness of the arch.
Presented a summary of the equations for minimum
cohesive arch length of smooth and rough waU suggested
by various authors. Discussed saUent features of bunker
design.
Reported large arch lengths for cohesive powders.
Suggested scale factors for arching of the prototype and
model for the same cohesive powders.
Evaluated the Jenike design method by performing
experiments on iron ores using wedge shaped and conical
bunkers. Found that the Jenike method does not consider
impact fiUing.
Compared the minimum outlet slot width and the minimum
hopper wall slope for mass flow predicted by the Jenike
method to that observed in a silo with symmetrical wedge-
shaped hopper. Found that the Jenike method overdesigned
80
1975
1966
1967
1975
1982
1982
1983
1984
Jenike
Walker
Walker
Enstad
Borg
Molerus
Yamashiro
etal.
Geldart et al.
the critical hopper slope by 8-10° and the slot width from
0-100% depending on the extent of extrapolation of the
flow function.
Suggested the minimum outlet dimension required to
maintain flow from a mass flow hopper as a measure of
flowabiUty of powders.
Derived a force balance of the weight of material in the arch
relative to the shear stress in the material at its periphery.
Arching test results and predictions from his theory are
shown in Figure 5.1, whereas, his results and predictions
from Jenike's theory are shown in Figure 5.2.
Made an extensive investigation of critical hopper outlet
openings, took into account vertical pressure acting on the
arch and claims to have reduced overdesign, but still a
considerable difference was found between calculated and
experimentaUy observed outlet openings.
Calculated critical arching outlet diameters for many
powders of varying degrees of cohesiveness. Studied effect
of time consolidation, moisture, temperature, particle size
on critical arching diameter.
Reviewed flow behaviour of cohesive materials. Used a
centrifuge bunker to study critical outlet ciimension
requirements. Found that the initiation of flow depends on
previous consolidation.
Studied compressibility, fluidity and cohesion of single and
mixed powders using a KYT-1000 commercial tap density
meter.
Suggested that a smaU change in particle size and other
parameters which affect interparticle forces can transform a
81
1984
1985
1985
1987
1987
1988
1988
1990
Piepers et al.
Scott et al.
Novosad
etal.
Reinhold
etal.
Luqing et al.
Knight
etal.
Aziz et al.
BeU
free flowing powder into a cohesive one. This property was
assessed by measuring the ratio of tapped to aerated bulk
density.
Measured the cohesion constant of the powder using a
tilting bed technique, the results from which revealed that
the cohesion constant increases with increasing pressure
and dependence of pressure was observed with the
increasing bed expansion, adsorption of gas to the soUd
and the increasing elasticity modulus.
Studied the effect of moisture, clay content and chemical
composition on cohesive critical arch dimensions of a
steaming coal.
Developed an arch tester to measure critical outlet openings
in mass flow hoppers. Indicated that the overdesign in the
Jenike method is due to extrapolation of the flow function in
the design process.
Applied statistical powder theory to arching by calculating
arching probability.
Analyzed the pressure drop in horizontal plug pneumatic
conveying for both cohesive and non-cohesive powders.
Measured the powder cohesive strength of five different
powders ranging from slightly cohesive to highly cohesive
with a penetrometer and compared results with cohesive /
strength measurements obtained using a shear ceU. /
Observed fine cohesive coals conveying in half or full plug
forms depending on gas velocity and plug length.
Discussed the effect of cohesion, moisture content,
permeabiUty, air retention and other powder properties on
pneumatic conveying system design.
82
TABLE 5.8 TENSILE STRENGTH - LITERATURE SURVEY
YEAR
1952
1970
1964
1965
1973
1973
AUTHOR
Dawes
Rumpf
Ashton
etal.
Farely
etal.
Stainforth
etal.
Kocova et
al.
COMMENTS
Used a spUt-glass plate technique to measure the tensile
strengths of cohesive powders.
Suggested a model for tensile strength of soUds arising out
of the forces acting between individual particles in terms of
mean bonding force at contact points, void fraction and
particle diameter.
Developed a basic tensile tester driven by a motor with
constant speed with no tilting of the specimen. A probe is
attached that stops the motor after the ring has traveUed a
certain distance, refer Figure 5.3. This reduces the problem
of visually observing whether the sample has sheared or
not The tester was used in conjunction with the Jenike
Direct Shear Tester to examine, for a number of powders,
that the yield loci, at constant bulk density, follow the
proposed power law relationships with the applied
compressive stress. H e found that the tensUe strength
increases logarithmically with packing fraction.
Presented a design method to calculate slot outlet opening
for arching and rathoUng from data obtained by use of the
Tensile Tester and Jenike Direct Shear Tester. They
developed a yield locus equation and measured the tensile
strength of different plastic materials.
! Evaluated four constants to describe flowabiUty of powders
based on the Warren Spring Yield Locus equation (refer
Ashton et al. (1964)).
Conducted tensile and shear tests on narrow size fractions
of powders and mixtures of different size fractions.
83
1974
1975
1976
1978
1982
Turner et
al.
Molerus
Turner et
al.
Eckhoff et
al.
Yokoyama
etal.
Evaluated parameters for both types of powders to study
flow and failure properties.
Performed tensile strength experiments on dry small
spherical glass beads. Found that the particle-particle bond
strength for a cubic packing from R u m p f s model is
approximately 1 p:N. This suggested that the bond strength
developed by van der Waals forces, electrical forces and
capiUary forces were smaU compared to mechanical forces
like friction, interlocking and arching.
Assumed the stress force relation expressed by Rumpf only
appUes under isotropic or hydrostatic pressure conditions
and applied this conclusion to the analysis of the shearing
mechanism. Furthermore, he extended the analysis to
predict the yield locus of cohesive powders.
Measured tensile strength of limestone powder using the
Shinohara and Tanaka compaction ceU and the Warren
Spring Laboratory Tester. Presented the results in terms of
the void fraction and moisture content of the sample. Found
the tensile strength is dependent on the particular properties
of the tester.
Combined tensile strength data of powders and failure loci
from Jenike Shear Tester Cell tests. Reported that tensile
strength results are not sufficiently accurate and
considerable over-design results with the Jenike theory in
regard to arching.
Reported a dimensionless number expressing the ratio of
the cohesive force to the gravity force on a single particle
and related this number with the floodabiUty index evaluated
by Carr's method. This indirectly suggests that tensile
strength is related to particle size. Powder filling was
effected by a spatula and a 10 minutes deaeration time was
allowed.
84
1983
1984
1984
1986
1988
Chen
etal.
Tsubaki
Tsubaki
et. al.
Terasbita
etal.
Nikolakakis
etal.
Investigated the tensile strength of both single powders and
binary mixtures. Developed equations which relate tenstte
strength, particle size parameters and the composition of
binary mixtures.
Indicated that the relationship between tensile stress and
porosity should be a straight line on a serm-logarithmic
paper. Presented expressions for the tensile strength and
consolidating pressure in terms of porosity and provided an
excellent review of powder bed mechanics.
Proposed experimental equations to correlate the tensUe
strength of a powder bed measured by spUt-ceU methods
with powder bed porosity. Found that the pre-compressive
force at the interparticle contact point effects the tensile
strength more strongly than porosity.
Conducted flowability assessment of dry and wet fine coals
by evaluating powder properties namely intemal friction,
cohesion and tensile strength. Found flowabiUty can be
more readily and accurately assessed by tensile strength and
cohesion compared to the assessment using internal friction.
In particular, he measured the tensile strength by use of a
hanging-type cohesion tester. Assessed the flowability of
fly ash to be higher than that exhibited by fine coal. They
stated the lower values of internal friction factor and tensile
strength of fly ash m a y be due to the loss of fixed carbon
and volatile matter and the sphericity of fly ash particles
resulting from high-temperature combustion. H e found that
the tensUe strength of fly ash increased dramaticaUy with a
slight increase in water content. This highlights that the
flowabiUty of fly ash is effected by environmental humidity.
Studied the effect of particle shape and size on the powder
tensile strength and proposed correlations of tensUe strength
in terms of particle shape, particle size and packing
fraction.
85
TABLE 5.9 FLUIDIZATION - LITERATURE SURVEY
YEAR
1973
1979
1982
1982
1984
1984
1988
AUTHOR
Geldart
Dixon
Molerus
Obata et al.
Zenz
Rietema
Kretschmer
COMMENTS
Developed a classification system for the fluidization
properties and behaviour of particles, refer Figure 5.4.
Developed a slugging diagram for different pipe diameter
systems based on the Geldart's fluidization diagram.
Proposed a similar classification to that of Geldart.
Attributed the differences in behaviour between Group A, B
and C powders to the relative magnitude of adhesive forces
between the particles dominated by local deformations of
the contact areas. Treatment in terms of adhesive forces
constitutes a quantitative justification of Geldart's
boundaries. Deduced a combination of the variables
defining lines of similar slope working backwards
from Geldart's laboratory scale observations.
Reported a method of particle size distribution measurement
for binary and tertiary mixtures using information from a
fluidization curve.
Reviewed the classification suggested by Geldart in
relation to solids incipient fluidization velocity, solids
surface tension, solids viscosity and the powderiness
i versus granularity classification.
Presented Geldart's classification of A, B and C powders in
dimensionless form by the consideration of the parameters
of cohesion, gas viscosity and gravitational acceleration.
Reported experimental data on fluidization and correlated
this with dense phase pneumatic conveying characteristics.
86
1988 Clark et al.
Presented expression for flowability in terms of potential
and kinetic energy.
Suggested a numerical representation to Geldart's
classification of powders by assigning arbitrary
classification numbers and correlated with Geldart's
and Molerus powder classification.
»
87
TABLE 5.10 DEAERATION - LITERATURE SURVEY
YEAR
1953
1972
1973,
1976
1977
1980
AUTHOR
Diekman
etal.
Johanson
etal.
Sutton
etal.
Farley
Rietema,
etal.
COMMENTS
Observed that powders exhibiting smaU viscosity (or
permeabiUty) changes with deaeration display good flow
characteristics.
Conducted an analysis of powder deaeration based on
continuity of the gas and solids and the equilibrium of the
forces acting on the solid. They accounted for the variation
of powder permeabUity and bulk density with consoUdation,
gas compressibUity and waU friction. The model was
numericaUy solved using finite difference methods with the
results presented in dimensionless form.
Conducted deaeration tests by observing the collapse of a
fluidized powder column. In particular, column height with
time on coUapse was observed using video techniques.
Their experiment rig is depicted in Figure 5.5. B y extensive
experimental work, he classified powders according to
deaeration properties.
Observed the effect of deaeration on the strength of
powders. Stated that if the powder deaerates slowly, the
pressure distribution wiU be hydrostatic and decaying and
hence the containing bin walls should be designed for this /
pressure. Also, the strength of the powder tends to mcrjeiase
as it losses air.
Suggested that deaeration is slow, if the powder consists
of fine particles. O n ceasing aeration, any bubbles first
leave the bed while an expanded dense phase is left At the
bottom of the bed, the relative gas flow is zero. The bed
settling starts from the base and maintains a relative gas
88
1980
1980
1983
1984
1984
Murfitt
etal.
Abrahamsen
etal.
Dry etal.
Zenz
Piepers
etal.
velocity at the top, where the powder is stiU fluidized.
Henceforth, a packed bed of increasing thickness is created
from the base upwards, while the total bed height decreases
and continues decreasing until the deaeration is completed,
refer Figure 5.6.
Stated two drainage mechanism namely, double drainage
and single drainage. A cyUnder with an open top and a
permeable base fiUed with powder approximates to double
drainage. Hence, if the base has low fluid resistance or
permeabiUty, the permeable base oase corresponds to the
single drainage case of double height They suggested that
the rate of collapse of the powder in a fluidized bed is one
way of measuring the rate of deaeration.
Used the coUapse rate technique to predict the average
dense phase properties in bubbling beds of fine powders.
They also observed the effect of fines (< 45 p.m), bed
height, distributor detaUs on fluidization characteristics.
They observed that the average dense phase voidage of
Group A powder increases as the particle density and mean
particle size decreases. Likewise the voidage increases as
the fraction of fines < 45 p>m, gas viscosity (temperature)
and gas density (pressure) increases.
Reported bed collapse experiments using powders of
particle size 12-67 |J,m in a 140 cm. column and they found
that the dense phase voidage (for class A C powders)
depends on the extent of fines.
Reported a classification of powders based on their
deaeration times for pneumatic conveying.
Reported deaeration tests in which they calculated the height
of the dense phase, the dense phase gas velocity and the
bubble hold up. In their tests, the powder bed was fluidized
89
1984
1985
1985
Geldart
etal.
Tardos et al.
Kirby
for several minutes at a superficial gas velocity higher than
Ubp after which the air supply was suddenly shut off and
the bed height was found to decrease quickly to a certain
value because of the quick escape of bubbles. After this
phase, the deaeration was found to be much slower. They
conducted experiments at pressures up to 15 bar using N 2
and Ar. The time and bed height were recorded on
videofilm.
Reported the difference between cohesive (Group C) and
less cohesive (Group A ) powders by examining the
deaeration characteristics of powders. They observed that
by ignoring the data for the first two seconds, when the
bubbles in the bed escape to the top, less cohesive powders
deaerate at a constant rate. Whereas, the cohesive powders
deaerate faster for the first ten seconds and afterwards at a
slow rate. The powder mass then remains in a sUghdy
deaerated state for a considerable time with the pressure at
the bottom of the bed decreasing very slowly. In the case of
cohesive powder, the gas flows through cracks and
microvoids. W h e n moistioned, Group A powders exhibit a
deaeration curve sirmlar to Group C powders.
Observed that the pressure profiles remain approximately
the same shape throughout the deaeration process and
indicate a decreasing pressure gradient with depth below
the top surface. A typical filUng-deaeration curve for the
pressure variation at the base of the hopper versus time is
shown in Figure 5.7, for a maximum filling height of 15
cm.. The fiUing curve is not smooth because inflowing
powder impacts on existing settled powder. Also, some
deaeration takes place during the fiUing phase. This non
uniform characteristic may also be due to sUp stick waU
friction characteristics.
In regard to pressure profiles in collapsing bed, he reported
that maximum air pressure occurs at the base, when the
90
1985
1987
Geldart
Rathone,
etal.
hopper is impermeable and somewhere in the lower half for
a permeable base, refer Figure 5.8.
Conducted an extensive series of deaeration tests. A typical
coUapse curve for a Group A powder is shown in Figure
5.9. The time required for Group A powders to deaerate is
dependent not only on the coUapse rate, but also on the
height through which the bed surface has to faU. Cohesive
powders in Group C display a different coUapse rate as
shown in Figure 5.10. For these powders, the bed doesn't
bubble evenly and bed expansion is caused by the presence
of cavities and cracks of various inclinations. When the
gas supply is stopped, the large cracks close up rapidly and
further coUapse proceeds more slowly, as in the final
consoUdation stage of Group A powders. They found that
the bed height decreases linearly with the time for Group A
powders, whereas for Group C powders it decreases
exponentially.
Conducted a theoretical analysis of powder deaeration.
Good model experimental correlation was observed
particularly for Group A powders. However, Group C
powders deviated considerably due to channeUing.
91
Predicted from Walker theory using ring cell shear strengths
Experimental results
\Y\W\ 30 Experimental
\ \ \ \ \
15 Experimental
' / / /
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
% Moisture content
l.D.F coal
Figure 5.1: Critical Arching Diameters [ Walker, (1967) ].
36
34
32
30
~" 26
2 24
v
l~ •o 20 cn c 18
t 16 S 14
3 12 Z. 10
° 8 6
4
2
o Predicted from Jenike theory and Jenike shear celt
Experimental resuLts
© 30" No solution
t
\\\\f A 30 Experimental
/ / / / / ' >15 Experimental
// / 7 / /
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
•/. Moisture content IDF coal
Figure 5.2: Critical Arching Diameters [ Walker, (1967) ].
Figure 5.3: Tensile Tester [ Ashton et. al. (1964) ].
Key Group C properties
reported a Baerns
v Brekken et aJ
c de Jong et al
Group A properties reported
& Baerns
a Davies et a!
a Rietema
B de Jong et ai
D Godarti et ai
« Oltrogge
* Kehoe
» de Groot
» This wcrk
Group 8 properties reported
* de Grooi
• This work
oroep D properties reported
x. Mathur
20 50 100 200 500 1000
Mean particle size d.v (pm)
Figure 5.4: Geldart's Classification of Powders [ Geldart, (1973) ].
Fi«*4 sc«l«
PeralM-sidcd container l-IOJJ pifK)
P»wd*r
Sinttf«d plastic bos*
Paro(ltl--i-<4 Conta-Mf
(FV-poc b«U
Snt*r*4 -f aji pad
Figure 5.5: Deaeration Test Rig [ Sutton et. al. (1976) ].
(a) Before shutting of the air supply (b) After shutting of air supply
(c) Deaeration completed
Figure 5.6: Deaeration Experiment [ Rietema et. al. (1980) ].
94
D
5
E *4 a. O Q 3 ill
EC
r . 2
UJ
GC
"1
•i~- t - i — r 1-
M
f \
?4 \ 1 <5 1 1 1 1 1 1 1
p
t 1
• s l-fl-'l 1 • 1
1 T T T 1 1 1 1 1
ASSUMED PROFILE
-o- MEASURED PROFILE -
•
V
\
— • 1. ,—.-•— i 1 1 • 1 H T - T T T V O —r-l
20 40 60 80 100 120 140 TIME, sec
Figure 5.7: Filling-Deaeration Plot for Zyolite Powder
[ Tardos et. al. (1985) ].
Figure 5.8: Pressure Variation in a Hopper; Permeable and Impermeable Bases
[ Kirby, (1985) ].
95
I iSSetU
ing bed<K
Bubble escape stage
Hindered settling stoge; slope-Uc
l<0 H-U
Figure 5.9: Deaerated Bed Settling for Group A Powder [ Geldart et. al. (1985) ].
He
H.
t-0 tc
t<0 - l c
Figure 5.10: Deaerated Bed Settling for Group C Powder [ Geldart et. al. (1985) ].
96
CHAPTER 6 TEST EQUIPMENT AND
PROCEDURES
6.1 PNEUMATIC CONVEYING RIGS:
6.1.1 STURTEVANT PULSE PHASE RIG:
This rig consists of a 0.425 m 3 Sturtevant pulse phase blow tank with a
maximum safe working pressure of 350 kPa (Figure 6.1). In addition, blow tank top air,
fluidizing ring air and conveying air facilities are incorporated
- A n electro-pneumatic control cabinet housing all the necessary control equipment for
conveyor operation (Plate 6.1);
- 7 1 m . and 61 m. total effective length of 50 m m Schedule 40 (52 m m I.D.) conveyor
pipeline (Figure 6.2);
- 3.6 m vertical lift pipe section (located 5.4 m from the blow tank outlet);
- T w o blinded-tee bends connected to the vertical pipe. In later testing, the blinded-tee
bends were changed to long radius bends and then to vortice elbows (Figure 6.3);
The blowtank has a butterfly-type discharge valve mounted at the bottom of the
vessel whic^ is supported by four shear-beam-type load cells to measure the si <ply
solids mass flow rate (Plate 6.2). In addition, the receiving hopper, which is mounted
directly above the blow tank is supported by tension load cells to monitor the rate of
solids discharging from the end of the pipeline (Plate 6.3).
The air supply consists of an Atlas Copco Model GA308 rotary screw
compressor, an S M C Model LDP-150S refrigerated air dryer and two air receivers
totalling 7.8 m 3 volumetric capacity. The capacity of the compressor is 3.1 N m 3 min'1
(free air delivery) with a maximum pressure head of 800 kPa. Orifice plates with D and
D / 2 tappings (designed according to B.S. 1042 : Part 1 : 1964) measure air flow rates
during the conveying cycles.
The reverse-jet air filter consists of carbon-impregnated Goretex™ filter bags
earthed to the filter housing, a horizontal upstand fitted with an explosion relief panel, an
exhaust fan to provide a vacuum in the receiving hopper and the filter housing and a
discharge duct to provide venting of any explosion directly to atmosphere.
9 7
Material Inlet
Vent Air
Top Air
Fluidising Ring Air
Discharge Valve
Conveying
Figure 6.1: Configuration of Sturtevant Blow Tank
Plate 6.1: Control Panel
A - HOPPER METER
B - BLOW TANK DISCHARGE VALVE
C - HOPPER INLET VALVE
D - FLUIDIZING RING AIR VALVE
E - AIR REGULATOR
F - CONVEYING AIR VALVE
G - HOPPER CONTROL VALVE
H - VIBRATOR ACTIVATION SWITCH
99
100
\u VORTEX ELBOW
Blind Tee
Elbow
fr *' »-•
Short Radius
Lon^ Radius
Figure 6.3: Types of Bends
101
iai»^M^g-^#/^
Plate 6.2: Blow Tank
A - LOAD CELL
102
rtJto. ----1 -l •<WUj;^rA
•pr r-,F>- -1» utur UtiUUhi J J i] .^ i'jn /.Ml' !-Wi'R|l|
"... ^rSE- ' 1 '
iW*K.-'^>>'i«E-1.*.*^A*i "#""'
Plate 6.3: Receiving Hopper
A - HOPPER LOAD CELL
Material Inlet
Top Air
Aeration Air
Supplementary Air
Low Velocity Attachment
Pipeline
Figure 6.4: Low Velocity Rig Blow Tank
103
6.1.2 L O W VELOCITY RTf>
The test rig consists of the following components:
- 0.9 m 3 blow tank (700 kPa m a x i m u m safe working pressure) with low-velocity
attachment air to fluidize the solids as it is fed into the pipeline, refer Figure 6.4 and
Plate 6.4.
- mild steel conveying pipeline (L = 97.10 m and 51.3 m, D = 105 m m )
- six or eleven l m radius 90° bends;
-1 m 3 receiving silo;
- Atlas Copco Rotary-screw air compressor (3.1 m3min-1 free air delivery, 800 kPa
maximum pressure head) connected to a refrigerated air dryer and an air receiver.
A schematic layout of this test rig is shown in Figure 6.5.
The following conveying parameters were recorded using a portable HP data
acquisition system:
- blow tank top air pressure;
- two downstream intermediate pipeline air pressures;
- mass of solids discharged from the blow tank;
- mass of solids returned to the silo;
- air mass flowrate via orifice plate measurements.
6.2 VELOCITY MEASUREMENT:
6.2.1 TEALGATE T.20Q SERTES TRANSDUCERS; The Tealgate electro-dynamic T.200 series transducer is a continuous on-line
solids velocity measurement test unit. This unit is a solid state electronic device capable
of detecting the velocity of dry particulate material. There are three basic components to
the T.200 series system namely the electrode, the transducer and the display unit. The
schematic block diagram is shown in Figure 6.6.
Electrode: The electrode is a leakage field type which forms part of the pipe wall. A
small section of the pipe wall is insulated from the main body of the pipe thereby creating
an electrode which does not interfere with the particulate material flow. The electrode
forms a complete circumferential section of the pipe.
Transducer: In the electro-dynamic transducer, the principle employed is that of
electro-dynamic induced charge, by which a charged particle brought close to a surface
induces a charge on that surface. The change in charge which occurs when particles
move through the pipe is detected. A n input capacitor acts as a charge to voltage
converter. The A.C. components of the signal voltage are used for velocity measurement
utilizing the cross-correlation technique. The material conveyed must be dry, either
conducting or non-conducting and the transducer must be free from vibration.
104
Plate 6.4: Low Velocity Rig Blow Tank
105
106
A
r
L_
li
o u HH- Q)
CD -P O hO U hO CO CD U -P > 03 rH fl r-J O O O > O
J
•a
a CO
o on J-l PP S3 o 3 •a
ti
•g CO
CN
H *
rX)
it
107
Display unit: The display unit is housed in a self contained instrument case, which
requires a standard 240 V A.C. mains supply. The facia panel incorporates a power
supply switch and indicator, a 0 - 1 0 0 % analogue meter, a response switch, a range
selector switch, two B N C sockets and a 3.5 m m jack socket (Plate 6.5).
The T.200 transducers were separated by a distance of 70 mm, about three meters
downstream from the blow tank discharge valve and a HP3721A cross-correlator is used
to cross-correlate the transducer outputs. The product of the selected time scale and the
horizontal displacement from the y-axis to the most dominant peak of the correlogram is
the transit time (t^ of the flow between the transducers. Since, the distance L between
the transducers is known the solids velocity is given by,
_. L 70 x 10~3 _! . V s = — = m s (6.1;
6.2.2 FIBRE OPTIC PROBE; A fibre optic probe was developed to measure the velocity of the particulate flow
in the dilute phase conveyor system. This probe consisted of two bundles of polymer
fibres with each bundle comprising three emitting fibres and one receiving fibre. The two
fibre bundles were mounted so that the receiving fibres were 13.5 m m apart. The
reflected light signal, detected and transmitted by each receiving fibre was then converted
to a voltage and amplified in separate pin diode amplifier circuits (Figure 6.7). The
amplified voltage outputs from these circuits were then cross-correlated using a H P
3721A correlator to determine the transit time (tm) for the particle to travel the reference
distance. Since the reference distance was 13.5 m m , the solids velocity at the test section
is given by,
_. L 13.5 x 10"3 _i (fx ^ V s = — = m s (6.2J
TTl TTl
Details of the probe configured in the actual rig are shown in Plates 6.6 and 6.7.
The light source used was a Tungsten Quartz Halogen 12 V, 100 W L a m p with a
operating temperature range between-30 degree Celsius to +85 degree Celsius. A lens
was used to concentrate the light. The optical fibres are 2.25 m m . in overall diameter and
1 m m . core diameter. The numerical aperture (NA) is 0.47 for normal use and for low
attenuation, 150 dB / k m max.(600 nm).
«
108
Ijn^o^*
*
9
wm
•'•"•;'' ,' • ..
Plate 6.5: H P 3721A Correlator connected to the Tealgate
T.200 Series Transducer
109
A
&]&<&&ri
1 M D
1 * — 1
f7
•i
-
- f
1 wtbij • "ISH
MiumTmfflHwtwwvwmvv^^^ \
«
tfiStTVQ
IffJfcJS-fc.,-', ,v%p t ^ B
fl
ri
- ^ * -
/-—- 1
:Z±': f^J ^ r- r
1 B | IB ~ I
Plate 6.6: Fibre Optic Probe with HP 3721A Correlator
A - PROBE
B - HP 3721A CORRELATOR
C - AMPLIFIER
D - LIGHT UNIT
110
!>-=.'J. .
Plate 6.7: Fibre Optic Probe Located on Sight Glass
A - FIBRE OPTIC PROBE
B - SIGHT GLASS
C - MILD STEEL PIPE
D - FLEXIBLE COUPLING
Ill
1 Mil lMfl
PIN
DIODE
-15 VOLTS
+15
n> -15 10KO. o.l u f
h
10 KG
I
+15
C ^ -15 1
OUTPUT
Figure 6.7: Pin Diode Amplifier Circuit.
6.2.3 HEWLETT-PACKARD 3721A CORRELATOR:
The H P 3721A correlator is a digital instrument designed to compute and display
probability, auto-correlation and cross-correlation functions. The computed function is
displayed on the internal C R T using 100 points. The horizontal axis of the display is
scaled in millimeters. B y selecting a suitable sec / m m time scale from the correlator, a
correlogram can be obtained. The time scale can be varied from 1 pis to 1 s. In
particular, the H P 3721A correlator was connected to the T.200 Tealgate transducer or
fibre optic probe for solids velocity measurement. The front panel of the correlator
consists of various selection knobs. During the cross-correlation process, the function
knob was set to A delayed or B delayed cross-correlation and averaging function knob to
summation. The V 2 / cm. knob was set at a suitable value to suit the correlogram on the
display using A input and B input switches.
6.3 POWDER CONCENTRATION:
This was measured by using a T.300 Tealgate series transducer connected to a
display unit. There are three basic components in the system namely the sensor, the
transducer proper and the display unit.
6.3.1 Sensor: The sensor is a leakage field type device, which forms part of the pipe
wall. A small section of the pipe wall is insulated from the main body of the pipe, thereby
creating capacitance between the insulated ring and the remainder of the pipe, the sensor
forms a complete circumferential section of the pipe.
6.3.2 Transducer: The principle used is to measure the capacitance of the sensor with
the material being monitored as the dielectric. The sensor is one arm of a bridge network
1 1 2
and the amount of imbalance created by the presence of material is measured. If the
relative dielectric permeability of the material is known, the capacitance measured is
related to the solids concentration of the material. Figure 6.8 depicts a block schematic
of the T.300 transducer.
6.3.3 Display unit: It is housed in a self contained 30 x 42 H P module, which will
fit into a standard 19" sub-rack. It requires a 240 volt A.C. mains input. The facia panel
incorporates a flat analogue meter, three flow level indicators and two ten turn vernier
controls. The meter gives a continuous indication of the solids concentration on a 0-
100% scale. A chart recorder can be connected to measure the variation in solids
concentration.
The layout of the front panel is shown in Figure 6.9. Particular details are
declared in Figure 6.10 revealing the T.300 transducer board. The display meter was
calibrated to a chart recorder to obtain steady state readings. Details of the recorder set up
are revealed in Plate 6.8.
6.4. HEWLETT PACKARD 3497A DATA ACQUISITION SYSTEM:
The H P 3497A Data Acquisition System was used in combination with a H P 85B
computer and a Tektronics 4923 tape deck. The H P 3497A can datalog up to 20 channel
transducer inputs and can be used to measure various parameters such as voltage,
pressure, temperature, resistance and frequency (Plate 6.9). The H P 85B computer was
used to set up the required experiment parameters. These include; experiment date,
number of channels used by the H P 3497A, number of scans and time lapse between
each scan. By entering the maximum and minimum pressure in the conveying system to
the H P 85B, it is possible to obtain the calibration factors for each transducer. In addition
to this, the H P 85B can be used to check transducer responses. The data processed by the
H P 3497A and H P 85B were recorded to tape using the T E X 4923 tape deck. The
recorded data was transferred to the University's mainframe computer (UNIVAC) via a
Tektronics T E X 4010 terminal to obtain computer plots.
All the important conveying parameters such as blow tank top air pressure,
pipeline air pressure, supply / delivery mass of solids and supply air mass flow rate are
recorded with respect to cycle time using the Data Acquisition System. Typical transducer
input channels, recorded with respect to cycle time, included blow tank air pressure;
pipeline air pressure; upstream pipeline and differential air pressures and the mass of
material entering the receiving hopper and / or leaving the blow tank.
113
r -
CO
H u o o
•a
i o J-l PQ
00 vd
PH
114
r .
Q) •-,
5 ?' -5 .3
Q Uj -4
L. QJ -rl
5
9 Uj
QJ
2! CD
Q UJ >rj
u UJ
s ^
u QJ
Si
ll o OJ •rj - j
115
SW l
< CO
ro.
TP1
< CO
cn m o < TP2
< CM
o < TP3
3
« • _
o CN CO < u
CO LL.
0 0
oo CO
o oo
I I I •l5VI5V0VSig ' ' L_l
Figure 6.10: T.300 Transducer Board [ Tealgate Manual ]
Plate 6.8: Chart Recorder connected to the T.300 Concentration Meter
116
•ZI r~t Cl
o 13 o rl
A -55-*
-8-1-1-1=1=) ••MUl.
'J-SP
f)r*yk\W
flH S 0rrB|
C--- ^
___________
BJ
W\V
Plate 6.9: Data Acquisition System
A - HP 3497A Data Acquisition System
B - HP 85B Desk Top Calculator
C - TEKTRONICS 4923 Tape Deck
117
6.5 TEST PROCFXHTRFS FOR PNEUMATIC CONVEYING RIGS:
6.5.1 INTRODUCTION! The test procedures for the experiments involve the following phases: transducer
calibration, blow tank pressurization, material conveying and data logging. Before
commencing the experiments, the following checks are conducted;
1. All transducers are connected to the proper channels of the data acquisition system
(DAS);
2. T.200 system is properly connected to the correlator;
3. T.300 system is connected to the chart recorder.
Also, atmospheric pressure, temperature and relative humidity readings should be
recorded from the gauges provided in the laboratory.
6.5.2 CALIBRATIONi
Transducer calibration and T.300 system calibration should be done at the
beginning of each experimental session, refer Plate 6.1.
Transducer calibration:
1. Set the H P 85B in calibration mode and zero the H P 3054A Data Acquisition
System (DAS). Material should be in the hopper ( check meter reading A ) .
2. Open blow tank discharge valve (Switch B ) and close the hopper inlet valve
(Switch C ) .
3. Connect the pressure meter to the pipeline.
4. Pressurize the blow tank and the pipeline to 100-200 kPa (Switch D and air
regulator E). 5. W h e n the pressure reading is steady, read the pressure meter. This pressure is the
maximum pipeline pressure.
6. Depressurize the blow tank by opening the blow tank vent valve and read the
pressure meter. This pressure is the minimum pipeline pressure.
6. Enter the above selected maximum and minimum pressure into the H P 85B and
run the D A S . Get the print out of the calibration values from the H P 85B.
6.5.3 T ™n SERIES SYSTEM C^JJLBRAIIQNI In common with other instruments, this device requires calibration. Calibration of
this instrument is effected as follows. Initially, the display meter reading is set up to 0 %
with no flow in the conveying pipeline. Then, for a fully packed condition in the pipeline
the display meter is set to 100%. These display meter readings can be set by adjusting
the trimmer capacitor switch, which is fixed onto the transducer board.
118
The actual calibration operating procedure is as follows.
Open the conveying air valve (Switch F) to give a high air flow rate across the
T.300 transducer.
Close the conveying air valve and set the T.300 display reading to zero by
adjusting the trimmer capacitor switch.
Convey material with a low air flow rate to effect a solids blockage condition near
the T.300 transducer. This can be checked from the sight glass.
Set the T.300 display reading to 100% by adjusting the switch 1 or 2 and the
vernier scales.
Convey material into the hopper and calibrate the T.300 display reading to a chart
recorder to obtain the steady state concentration range.
5.4 OPERATION:
(a) Feed material into the blow tank:
Close the blow tank discharge valve and open the hopper inlet valve.
Open the blow tank inlet valve (Switch C) and allow material to flow into the
blow tank. W h e n the material stops flowing i.e. meter reading A steady, lift the
hopper (Switch G ) and vibrate (Switch H ) for about 30 seconds to allow the
remaining material to fall into the blow tank. Using the switch (G), put the hopper
down.
(b) Pressurize the blow tank:
Using switch (D), pressurize the blow tank.
Adjust the blow tank pressure to a required value by using the air regulators
(Switch E) for top ring and probe air system.
(c) Set the D A S to run:
Follow the instructions appearing on the H P 85B screen and enter all required
values and data required by the D A S .
Just before commencement of material conveying, start the data logging process.
Also start a stop watch to record the experimental time.
(d) Start conveying:
Open the conveying air valve (F) slowly and steadily.
Open the blow tank discharge valve to convey material.
Take correlogram readings and T.300 display meter readings.
(e) Correlogram measurement:
Just after the conveying cycle starts, take correlogram measurements from the H P
3721A correlator every 15 seconds timed with a stopwatch.
To get the correlogram into the display screen, press the m n button on the
correlator. Select a suitable time scale from the correlator and measure the
119
horizontal displacement (in m m . ) of the most dominant peak of the correlogram
from the Y-axis.
(0 Solids concentration measurement from the T.300 system:
Just after the conveying cycle starts, take the steady state solids concentration
measurement from the chart recorder.
T o run the chart recorder, select a suitable sweep rate setting on the chart recorder
and press the start button.
(g) Stop conveying:
After conveying all the material into the receiving hopper (conveying cycle
completed), depressurise the blow tank using the switch.
Close the conveying air valve gradually.
Close the blow tank discharge valve.
(h) Data transferring:
Follow the instruction appearing on the H P 85B screen and record the data onto a
tape using T E X 4923 tape deck.
Transfer the recorded data to the University's mainframe computer ( U N I V A C ) via
the Tektronix 4010 terminal for final processing and graphical output.
TEST PROCEDURE FOR LOW VELOCITY RIG:
The valves controlling the distribution of air to the blow tank (top air, aeration
and low velocity attachment air) were first set for each experiment.
The pressure regulator controlling the air supply to the blow tank was set at a
predetermined level.
Under these conditions the air supply to the vessel was actuated and conveying
commenced.
During this time, the various conveying parameters were recorded via the data
acquisition system.
In addition, the amount of material remaining in the blow tank was regularly
monitored so that the air supply could be turned off before the blow tank was
empty.
After the air supply was shut off, the material continued conveying as the residual
air pressure in the vessel and pipeline dissipated.
120
6.6.1 WALL FRTCTTOV RTrT;
The test rig was designed to measure the force required to convey a column of
material along a cylindrical perspex tube. By using a permeable piston, it was possible to
measure the variation of conveying force for both aerated and non-aerated conditions. A
schematic of the wall friction rig is shown in Plate 6.10.
The rig consists of a 100 mm inner bore perspex tube fixed to a vertical square
section steel frame. The perspex tube, in which the 99.7 m m diameter piston arrangement
travels is mounted vertically during operation. To facilitate the distribution of air over the
top of the piston, concentric grooves are used. Pressurized air supplied to the piston,
flows into the tube by permeating through a Vyon -D plate fitted to the top of the piston.
The piston may be used to convey the column of material along the pipe test section. The
movement of the piston and material column along the tube is effected by a screw-bar,
powered by a variable speed D C motor, which mns through a ballscrew nut. The motor is
supported on a guide roller mechanism, which allows vertical movement. This
arrangement causes linear motion of the piston whenever the reversible motor is actuated.
Rotational movement of the piston is prevented by a guide bar, which runs through a
linear bearing for the full length of the screw bar. The maximum linear speed attained by
the piston is 35.2 c m / min.
Two foam rubber piston rings are used to prevent powder loss down the side of
the piston, restrict the downward flow of air as well as to provide a smooth surface for
the piston travel. Initially, these foam rings were adequate when testing granular
materials. However, when testing fine materials, the foam rubber piston rings were
replaced by Teflon rings to minimize leakage of the material through the piston rings.
The whole arrangement of the tube, piston, screw bar and motor is pivoted about a
horizontal axis through the main frame. This pivot allows the tube to be rotated in a
vertical plane parallel to the main frame. This rotational arrangement assists the loading
and unloading of materials into the tube. A locating pin at the bottom end of the frame
and a bracket fitted to the ground locks the whole arrangement in the vertical position
during testing. T o minimize the electrostatic charge, a wire was wrapped around the
perspex tube.
To facilitate the measurement of the load acting on the piston, a load cell is
mounted between the piston and the back up plate fixed to the end of a screw bar. The
output from the load cell is connected to a chart recorder via a control unit. A platform is
fixed to the vertical frame so that loading and unloading of the material and calibration of
121
Plate 6.10: Wall Friction Rig
A - PERSPEX TUBE
B - D.C. MOTOR
C - CONTROL UNIT
D - CHART RECORDER
122
the load cell can be effected. The air supply is connected through a pressure regulator and
a rotameter to the distributor/piston by means of a flexible plastic tube.
6.6.2 OPERATION:
First the load cell was calibrated using known weights and recording the chart
recorder readings. The piston was then returned to the lowest position in the tube and the
force required to convey the piston alone in the tube recorded. The test material was then
loaded into the perspex tube. The column length of the material was recorded before
observing the force necessary to convey the column upwards with the motor actuated.
The observations are recorded during the piston movement over a selected test section of
the tube. The piston was then returned to its original position by reversing the motor.
Before commencing the next test, a selected air flow was supplied to the piston. The
supply air pressure and air flow rate were recorded before observing the upward
conveying force, when the motor was actuated. This procedure was repeated for selected
supply air pressures. The expansion of the powder bed was noted by measuring the
expanded heights for each supply pressure.
6.7 COEFFICIENT OF RESTITUTION RIG:
Basically the rig consists of:
- a rotating backing disk direct mounted on a high speed trunnion mounted D.C.
motor. The motor is trunnion mounted to allow tilting of the impact surface.
- a clamp mechanism to maintain the selected tilt angle during testing.
- a protactor to measure the tilt angle.
- a variable drop height granular material gravity feed arrangement
- a background grid of non reflective coating to measure the granular material rebound
height.
The actual test impact surface is changed by fixing disks of the test surface
material to the backing plate. In this test series, various granular material were caused to
impact selected rotating surfaces at preselected tilt angles and rotational speeds. The
rebound height of the material stream was recorded using video equipment and the results
analyzed by slow motion of the video cassette (Plate 6.11).
6.8.1.1 SOLID DENSITY:
Powder solids density was measured using a Beckman Model 930 air-
comparison pycnometer. This instrument measures the true particle volume of a powder
sample of known mass. The solids density, ps is the ratio of mass and particle volume.
This instrument consists basically of two chambers and two pistons (one measuring and
123
• •
=*£
-f .,«#>*•»-
H
T^vcc^c.^,yi:n%i^
**tfr^} *®&* f t M ,
Plate 6.11: Coefficient of Restitution Rig
A - D.C. MOTOR
B - WALL SURFACE
C - PROTACTOR
D - FEED ARRANGEMENT
F - BACKGROUND GRID
G - HANDLE
H - LOCKING DEVICE
124
one reference), a valve connecting the two chambers, a differential pressure indicator
and a digital counter calibrated for readings in cubic centimeters (Plate 6.12). The two
pistons are moved together to ensure no differential pressure between the chambers. The
distance, the measuring piston differs from its reference position before inserting the
powder sample is proportional to the volume being measured
6.8.1.2 TESTING PROCEDURE; - Close gauge valve.
- Rotate handwheels to counterclockwise extreme.
- Turn measuring handwheel clockwise until starting number is set on the counter which
is 108.68.
- Place sample of powder in cup, insert cup in compartment, lock sample cup in place
by pressing clamping lever down firmly.
- Wait 15 seconds, then close the coupling valve.
- Turn both handwheels simultaneously or alternately until reference handwheel rests
against stop. Keep pointer on scale during this operation.
- Wait 10 seconds, bring pointer to zero with measuring handwheel.
- Open coupling valve and read sample volume on counter directly in cubic centimeters.
6.8.2 LOOSE POURED BULK DENSITY:
This was measured by placing a sample of known mass in a measuring cylinder.
6.8.3.1 COMPRESSIBILITY TEST:
The bulk density of a powder was determined using a compressibility tester, the
arrangement of which is depicted in Plate 6.13 (A) and Figure 6.11. The tester consists
of a 63.5 m m diameter x 19 m m deep cell which is filled with a sample of powder. A lid
is placed on top of the sample. Loads are applied to the lid by means of a weight carrier
and the compression of the sample measured with a dial gauge. Knowledge of the mass
of the sample, the volume of the sample and the vertical loads applied allows the
relationship between bulk density (pb) and consolidation (a) to be determined. The
resulting variation can be presented using either a linear scale or logarithmic scales.
6.8.3.2 TESTING PROCEDURE:
Indicator calibration:
Place the gauge block in the cell. Place the cover over it and set indicator
holder on the cell. The indicator should read 0.75".
Filling of Base:
Spoon the material into the cell, taking care not to pack the material during the
filling process. Place the cover on the material. Clean the top surface of the cell with a
small brush.
1 2 5
Plate 6.12: Beckman Pycnometer for measuring Solid Density
Plate 6.13: Jenike Compressibilty and Permeability Tester.
A - Jenike Compressibilty Tester B - Jenike Permeability Tester
126
Compaction and measurement:
Place the indicator holder on the cell. Using the twisting bar, apply 30 twists (back
and forth) of about 10° amplitude. Remove the twisting bar and record the indicator
reading for a load of 0.12 kg. Place the combination weight hanger on the cover and
indicator holder on the cell over the weight hanger. Repeat previous steps six times by
adding the weight hanger equal to 0.5 kg; then by doubling the total weight (including
weight hanger) each time (i.e. 1, 2, 4, 8, 16 kilograms). The total load is equal to the
weight of the cover, the weight hanger and any weights placed on the weight hanger. A n
indicator reading is taken for each vertical load applied. Remove the indicator holder,
weights, weight hanger and cover. Weigh the cell and its contents. The net weight of the
powder sample is this figure less 243.4 grams (the weight of the base).
6.8.4.1 PERMEABILITY TESTER:
The tester consists of a cabinet and a stainless steel test cylinder, cover and dial
indicator with holder, refer Plate 6.13(B) and Figure 6.12. The powder is placed in the
cylinder and compacted to various increasing densities. The height of the sample is
measured with a dial indicator. Air is blown into the bottom of the cylinder to flow
through the sample. The pressure drop is set at a value slightly less than that which will
lift the powder. From knowledge of the mass of sample, a permeability versus
consolidation relationship can be determined.
6.8.4.2 TESTING PROCEDURE:
Filling of Cylinder:
Place the material in the test cylinder with a spoon, layer after layer. Scrape off the
excess material. Weigh the material before and after filling to determine the actual material
mass in the cylinder, refer Figure 6.12
Air Pressure:
Back off regulator R 2 by turning counter-clockwise. Open toggle valve V (gauge
Gi should read 11 psi). Determine the optimum air pressure P0 to be applied to the
cylinder full of material using P0 = W t / 80.6 where Wtis the net weight (in grams) of the
material in the test cylinder and P0 is in inches of water. Slowly increase the air pressure P
at the base of the test cylinder using regulator R 2 and gauge G2. Try to make P = P0.
Record the value of P which is finally used.
Packing and Measurement:
Place the cover over the material. A plastic hammer is used to pack the material.
Use sharp blows on both sides of the lower part of the cylinder. Measure the height using
the indicator after each packing. The height is measured with the cover on the sample. The
height of the material in the test cylinder is equal to the dial reading plus 3.00 inches.
After the indicator reading has been taken, set air pressure P and read the flow rate.
127
DIAL INDICATOR
INDICATOR HOLDER
COVER
"/////////////////////
SAMPLE
WEICHT CARRIER-
Se: air supply to 11 psig
First stage regulator
Air supply
Figure 6.11: Jenike Compressibility Tester [ Arnold et al. (1980)]
Air Pressure Gauge 0-)
G.
5, Ov ©
Regulator
Rotameters
f,
o o o
X
5
o o
X
F»
c •H e O u
O O O
^•s* dia. ~1 Test Cylinder
<
L- • Yjr'. Sample ' </
r/ .,'
1 r
g iv/-:?./;-!
i I.
cdifrJb->fioo
Figure 6.12: Jenike Permeability Tester
[Arnoldetal. (1980)]
128
6.9 PARTICLE SIZE MEASUREMENT.
6.9.1.1 SIEVE ANALYSTS-
Sieve analyses of various materials were conducted The equipment used included:
1. Selection of clean dry sieves of different sizes,
2. Endecotts (Soilcrete) mechanical test sieve shaker, and
3. Mettler P C 4400 top pan direct reading balance,
6.9.1.2 OPERATION:
The sieves were assembled in ascending order with a catch pan and lid having first
been weighted empty. A net weight of dried sample of powder was placed in the top sieve
before the lid was placed and top clamps of the sieve shaker were put to seal the sieve
assembly. The shaker was activated for 10 minutes, after which the sieves were
dismantled Their weights were recorded and the net weight of size fraction obtained.
6.9.2.1 PARTICLE SIZER:
Particle size distribution was measured using a Malvern 3600 E C particle sizer.
In the Malvern particle sizer, Fraunhofer diffraction is applied to determine the particle
size analysis. The sample is illuminated by a low-power laser (1 m W H e - N e laser). The
particles scatter some of this light at angles, which are characteristic of their sizes forming
a series of diffraction patterns, each consisting of concentric bright and dark rings. This
scattered light is collected by the Fourier optical system and focused onto a radial diode
array detector. The signal, which is derived from each detector element and which varies
according to the intensity of the light falling on it, is collected, amplified and digitized for
processing by the in built computer (Plate 6.14).
6.8.2.2 OPERATION:
The E A S Y - S I Z E R software has been designed for use with the Malvern particle
sizer. It consists of four special function keys, which normally performs a measurement.
These are:
Sample details F 2 Allows entry of sample details for annotation of prints.
Set zero F3 Measures the diffracted light with no sample present to
establish the base line.
Check sample F 4 Gives a dynamic display of the diffracted light and the
concentration sample loading with an indication of
acceptable values.
Measure sample F 5 Measures the diffracted tight from the sample, corrects for
and analyse the base line, analyses and presents the results.
129
Plate 6.14: Malvern Particle Sizer
Plate 6.15: Jenike Direct Shear Tester.
Shear Cell
B - Drive Mechanism
C - Load Tranducer Output
D - Chart Recorder
130
These keys make up the measurement sequence and are used in numerical order
for user interactive single measurements. W h e n changes to the optical system have been
made, it is necessary to check and adjust the optical alignment. This is achieved by
pressing the key "a" (for alignment). Access to the disc transfer system is effected by
pressing the key "d" (for disc). This allows the storage of data on the disc for later recall
and re-analysis. Access to the package M A S T E R - S I Z E R is possible by pressing the key
"m" to use the full range of M A S T E R - S I Z E R options.
6.10.1 JENIKE DIRECT SHFAR TESTER-
The main features of the Jenike Direct Shear Tester are as follows:
1. Circular shaped shear cells.
2. Normal loads are applied to the cell by means of gravity vertical loading.
3. The shearing action is by means of an electromechanical drive moving a load
sensing stem horizontally at a certain velocity.
4. The shear force is sensed by the load sensing stem and readings displayed on a
chart recorder (Plate 6.15).
5. T w o different size shear cells are used namely: for low pressure consolidation a
95.3 m m I.D.(cross sectional area of 1 /140 m 2 ) cell is used, whereas for high pressure
consolidation a 63.5 m m I.D. (cross sectional area of 1 / 315 m 2 ) cell is used.
Use of the Jenike Direct Shear Tester, is that of producing instantaneous yield loci
in order to determine the instantaneous flow function. The yield loci are determined for
different consolidation loads (usually 3 loci). Jenike Direct Shear testing was conducted
in accordance with the procedure declared in the text by Arnold et al. (1980).
6.10.2 Effective yield locus:
The straight line through the origin and tangent to the major Mohr circle of stress
is called the Effective Yield Locus. The angle 5 made by this line to the horizontal axis is
the effective angle of friction. For yield loci at different consolidation, 5 is approximately
constant or it m a y decrease with increasing consolidation.
6.10.3 Flow function:
The instantaneous yield loci are used to determine the bulk solids flow function.
The flow function is a plot of the unconfined yield stress oc versus the major
consolidating stress 0\. These two parameters are obtained from the relevant yield loci.
The flow function is dependent on several factors including duration of consolidation,
moisture content, particle size and distribution, mechanical vibration, etc.
131
6.11.1 TENSILE TESTER-
The tensile tester used was manufactured by Ajax Equipment (Bolton) Co. Ltd.,
England and is known as the Ajax-W.S.L. Tensile Tester. It consists of a split cylindrical
cell with one half fixed and attached to the main body of the machine, whilst the other
half is mounted on a pivoting block supported by low friction radial bearings. The
pivoting block is balanced by means of two opposing springs that are tensioned by
screwed spindles. The front spindle has a counter attached that is set to zero and the rear
spindle is adjusted until the two halves of the cell just touch, thus being the position of
null-balance (Plate 6.16).
6.11.2 O P E R A T I O N :
Choose two springs from the selection given and attach to the underside of the
tester. Adjust the front hand wheel until the counter is showing '000'. Turn the rear
wheel until the pivoted cell half just parts. Screw the clamping screw onto the top of the
tester to hold the cell firmly together.
Place the top ring over the cell. Powder is then placed in the cell until the level
reaches approximately halfway up the top ring. Place the compaction plunger on the ring
and apply four to five twists of about 30° each. Once the powder has been compacted,
the plunger and compaction ring are removed, with the excess powder being scraped off
level with the top of the cell. The sample is now ready to test.
The clamping screw is unlocked and a load applied to the pivoting block by means
of turning the screwed spindle which extends a spring attached to the block. Once the
shearing action has finished, indicated by a split forming in the prepared cell, the figure is
read from the counter on the spindle and the tensile stress is found by referring this
reading to the calibration graph supplied with the machine.
Three different test series were conducted. Firstly, a test series was conducted
immediately after sample preparation, a second series of testing was conducted
incorporating 15 minutes deaeration time. In the final test series, the tester cell was filled
using a screen vibrator. O n completion of testing, the powder sample tested is weighed.
This mass was then divided by the volume of the cell to obtain the sample bulk density.
For this determination a mass balance accurate to the nearest O.lg was used.
132
'•N.
Plate 6.16: Ajax Tensile Tester
A - SPLIT CYLINDRICAL CELL
B - CLAMPING SCREW
C - SPINDLES
D - COMPACTION RING
E - COMPACTION PLUNGER
F - COUNTER
G - SCREEN
133
6.12 C O H E S I O N A R C H T E S T E R :
The arch tester basically consists of squat perspex silo which height is 500 m m .
and side length and width are 250 mm., fitted with a novel oudet gate arrangement. The
latter slotted outlet can be operated by a hand drive chain mechanism that symmetrically
opens the oudet gates without disturbing the material. The maximum opening size of the
slot is 100 m m . A graduated scale is fitted to measure the arch length.
The silo was filled up by using a hand pouring or mechanical feed techniques.
Initial experiments were performed by hand pouring the powder, but later the silo was
filled up by raining the powder through a 3 m m . or 4 m m . aperture sieves to intersect the
flow stream to ensure uniform powder distribution and consolidation stresses. This
procedure was also adopted by Novosad et al. (1985). In the latter part of the testing,
powder was hand poured onto a conveyor belt discharging into the arch tester (Plate
6.17). Using this technique, the filling time was selected to be about 5 minutes.
A set deaeration time is allowed before opening the outlet. The recorded
deaeration time includes the time for filling and the deaeration time necessary to dissipate
the entrapped air. The filling time was varied between 0 and 28 minutes, followed by a
deaeration time proper, which varied between 0 and 62 minutes. Eckhoff et al. (1974)
also allowed a deaeration time of 30 minutes.
The drained angle of repose made by the remaining powder in the silo with the
horizontal was also measured. After discharge, the powder was collected in a receiving
bin fitted below the tester. O n complete discharge of the gravity activated material, the
drained angle of repose, the angles formed between the four flow channels boundary silo
wall interface and the horizontal were measured.
The advantages of the arch tester include:
1. Convenience of measurement;
2. Effect of deaeration could be observed;
3. L o w stress levels in powder bed.
Unfortunately, the tester exhibits the following disadvantages:
1. Actual consolidation level unknown;
2. Unknown internal friction;
3. Limited arch dimension;
4. Limited consolidation stresses.
Plate 6.17: (A) Cohesion Arch Tester
(B) Deaeration Tester
C - SQUAT PERSPEX SILO G - DEAERATION CYLINDER
D - CHAIN DRIVE MECHANISM H - PRESSURE TAPPING
E - RECEIVING BIN I - GRADUATED SCALE
J - STAND
K - CHART RECORDER
135
6.13 DEAERATION TESTER:
The deaeration tester consists of:
a perspex cylinder fitted with detachable permeable and impermeable bases;
a pressure transducer connected at the bottom for impermeable base testing
and at the middle of the cylinder for permeable base testing;
a graduation scale to measure the settling bed height;
a chart recorder to record the pressure variation during filling and deaeration.
The experimental time was recorded with a stop watch. The pressure transducer
outputs were calibrated by use of a pressurized tank. Controlled filling was effected by
directing the discharge from an elevated belt conveyor into the cylinder. In each test, the
conveyor was loaded with a known mass of powder (Plate 6.17). For all deaeration
experiments, the chart recorder speed was 6 cm. / min. The length of deaeration cylinder
was 94 cm. In all tests, the approximate filling time was noted.
Initially, in trial runs filling was effected slowly by pouring material on the
conveyor belt. For example for fly ash 'E' with a filling time of 6 minutes, no
observation for deaeration interstitial pressures were possible using a slow filling
procedure. W h e n fast filled, pressure peaks of magnitude between 5 kPa to 20 kPa were
observed.
6.14.1 FLUIDIZATION RIG:
The fluidization rig consists of:
- a plenum chamber base with a retaining assembly to house the 6 m m thick Porex™ gas
distributor;
- a permeable plastic (6 m m , 35 m m Porex™) gas distributor covered with a carbon-
impregnated Goretex™ filter material to prevent particulate penetration into the gas
distributor and to allow the discharge of excessive electrostatic charge;
- a 102 m m internal diameter pyrex vertical column (750 m m long) fitted with a
graduation scale for bed height measurement;
- the air line pressure is regulated using a Fairchild, Kendal Model 10 regulator (Max.
supply 700 kPa);
- the air flow is measured using Fischer-Porter rotameters series 10A3000 with the
following ranges;
Rotameter 1-1.47 litres min-1 (full scale)
Rotameter 2 - 7.372 litres min"1
Rotameter 3 - 22.653 liters min'1
Rotameter 4 - 86.083 liters min"1
136
- the rotameter operating pressure is measured using a Eicon Instruments SRI 200 kPa
(Max. 350 kPa) pressure gauge;
- the air is regulated using two flow control valves in parallel, viz; a 6 m m Parker and 13
m m Flutec valve;
- two Rosemount Model 1151 D P differential pressure transmitters (0 to 152 m m H 2 0
and 0 to 762 m m H 2 0 ) for direct measurement of the pressure drop across the bed via
Goretex™ protected pressure tappings;
A n illustration of the test facility is presented in Plate 6.18.
6.14.2 TEST PROCEDURE:
A test procedure is conducted as follows:
- Place the test powder of known weight into the test rig, usually between
1.5 to 3.0 kg.
- Bolt on the perspex cap with the Goretex™ filter.
- Switch on the power source for the differential pressure meters and connect the
pressure taps onto the test rig.
- Adjust the air regulator to read approximately 50 kPa.
- The air flow is regulated via the flow control valves and measured using rotameter 1,
2, 3 and 4, as appropriate.
- The air flow is increased to allow the material to be fully fluidized.
The following parameters were recorded for each sample.
- Height of material above the Porex™ gas distributor, H b (cm),
- Air pressure drop across the bed of material, Ap b ( m m H 2 0 ) ,
- Volumetric flow rate of air passing through the rotameter, Q f (cm3 s"1),
- Operating conditions of the rotameter (i.e. air pressure and temperature), and
atmospheric conditions (i.e. air pressure, temperature and relative humidity).
For each value of Qf, the corresponding value of mf was calculated using the
operating conditions of the rotameter. The superficial velocity of air, V f (cm s_1), leaving
the bed of material was also determined. The variation of the average air pressure
gradient, Ap b / H b ( m m H 2 0 cm"1), with respect to V f was plotted for each sample and
the resulting fluidization curve plotted.
137
"pl
mm
if i>-_
oj,.»«Mm
- . * . . < • . ; - • , ; v ......... -. , -
StK-Ss«l83
*
g£ 7$
."-. !V
9 r
vT "-•" ;ft ilk-* JE_5;
f •
Plate 6.18: Fluidization Rig
A - Plenum Chamber
B - Flow Rotameters
C - Flow Regulators
D - Differential Pressure Transmitter
138
CHAPTER 7 RESULTS
In this chapter, the observations and results obtained from the bench tests
conducted on the various powders tested and the observed flow characteristics in actual
pneumatic conveying rigs are discussed. In summary, the bench tests included;
[1] Scanning Electron Microscope (SEM) photographs of the pneumatically
conveyed powders at various magnifications, refer Section 7.1;
[2] Coefficient of restitution of various granular materials measured using a bench
scale tester based on the rotating disk technique, refer Section 7.2;
[3] Particle size analysis of various powders obtained by sieve analysis and the
Malvern Particle Sizer, refer Section 7.3;
[4] Loose poured bulk density and compressibility evaluated by a measuring
cylinder technique and Jenike Compressibility Tester, refer Section 7.4;
[5] Powder solids density as measured using the Beckman Pycnometer, refer
Section 7.5;
[6] Arch dimension and drained angle of repose of various powders measured using
the bench scale Arch Tester, refer Section 7.6;
[7] Flow function measured using the Jenike Direct Shear Tester, refer Section 7.7;
[8] Tensile strength measured using the Ajax W.S.L. Tensile Tester, refer Section
7.8;
[9] Wall friction behaviour of aerated powders evaluated from the bench scale Wall
Friction Test Rig, refer Section 7.9;
[10] Deaeration behaviour of various powders measured in both a permeable and an
impermeable base deaeration cylinder, refer Section 7.10;
[11] Fluidization and deaeration behaviour observed using a Fluidization Rig, refer
Section 7.11;
[12] Permeability measured using the Jenike Permeabihty Tester, refer Section 8.12;
In regard to observation of powder flow characteristics, the actual conveying rigs
used included;
(A) Sturtevant Pneumatic Conveying Rig:
- friction loop;
- particle velocity obtained from the T.200 Tealgate velocity sensor and a fibre
optic probe developed in combination with H P 3721A Correlator;
139
- particle concentration obtained from the T.300 Tealgate concentration sensor,
- long radius and vortice elbow bends.
(B) Low Velocity Rig:
- Conveying characteristics of Wheat at different air flow rates and pipeline
lengths.
The results and observations from this test work now follow:
7.1 SCANNING ELECTRON MICROSCOPE(SEM) PHOTOGRAPHS; The powders tested were selected to represent a range of different conveying
characteristics, particle shape and surface characteristics. Details of the observations are
presented in Table 7.1. For each powder examined, a number of S E M photographs
were taken at various magnification levels in each case to highlight paramount features.
Plate 7.1: S E M Photograph of R a w Sugar Grains (X = 14).
Plate 7.2: S E M Photograph of Raw Sugar Grains (X = 30).
Plate 7.3: S E M Photograph of Raw Sugar Grains (X = 144).
141
Plate 7.4: S E M Photograph of Raw Sugar Grains (X = 1440).
* »
. . ,
.:-A
***--_*
| 216 um|
?->
Plate 7.5: S E M Photograph of Light Soda Ash (X = 162).
Plate 7.6: SEM Photograph of Light Soda Ash (X = 780).
— ^
1
>
c -
& * -'•• 4r
1
f 13! ?pml ^ '
Plate 7.7: SEM Photograph of Dense Soda Ash (X = 180).
143
Plate 7.8: SEM Photograph of Dense Soda Ash (X = 600).
Plate 7.9: S E M Photograph of Zinc Fume (X = 90).
144
Plate 7.10: SEM Photograph of Zinc Fume (X = 600).
Plate 7.11: S E M Photograph of Zinc Fume (X = 6000).
145
W_t__^__\% y
_vTji££_'-'^-*** ___&/^__. t :__\
r *< H . ^ ' . •; \.\
• " \ • • _ \ - '
/ ' ^"
V 1
1
\ • " _
\ y^^y_^
' .AM'. r^*s^-* -'•'": A
V
in
1/ V J I F ^ t ?kr
, ' *•..-> i {^£^ ^^r^-y x i
|69u-n|
_____
V ii 1 /
Plate 7.12: SEM Photograph of PVC Powder (X = 36Q).
Plate 7.13: SEM Photograph of PVC Powder (X = 1800).
146
Plate 7.14: S E M Photograph of Pulverised Coal - Tallawarra (X = 60).
Plate 7.15: S E M Photograph of Pulverised Coal - Tallawarra (X = 600).
147
Plate 7.16: S E M Photograph of Pulverised Coal - Tallawarra (X = 2100).
Plate 7.17: S E M Photograph of Petroleum Coke (X = 12).
148
Plate 7.18: S E M Photograph of Petroleum Coke (X = 120).
Plate 7.19: S E M Photograph of Petroleum Coke (X = 600).
149
Plate 7.20: S E M Photograph of Petroleum Coke (X = 3000a).
Plate 7.21: S E M Photograph of Petroleum Coke (X = 3000b).
Plate 7.22: S E M Photograph of Eraring Fly Ash (X = 1320).
I
Plate 7.23: S E M Photograph of Liddell Fly Ash (X = 1320).
Plate 7.24: S E M Photograph of Liddell Fly Ash (X = 6600).
Plate 7.25: S E M Photograph of Vales Point Fly Ash (X = 468).
152
\
«'• '+•';•_
^ ... , r»*t-vW;^ " • > - • • v "<w • -
Plate 7.26: S E M Photograph of Vales Point Fly Ash (X = 6600).
153
1/3
S
o o H o X Qu
a o. O u O Cr-U
Z o as H U
w rJ
w O
z z z << U rt
o as tt.
co Z O hH
> OS U va CQ
o
w -J ca <
bO B
• — I
fr
O
U
Cfl
U
•_. cfl
-C
U
a B <_>
E E o u
o
> .s >> It
> B
o
~3
•8 § £ E o
8 _P bo V
£7 >
2 I,
I Cr
M B u 3 •a B O
•a
bo B U > B O u
B O
I-H
a •a
Q
-5 T3
5
- E S «-- H 0
I _3 Cfl
Cfl
I
E ~ S S 60 g
8
B°
fc
cfl
s S3
J "bO
<0
E o bO
Sf <o
E o oo
B
Cr Cfl
B O
•a a- .& UJ 0
____
B 3
I OO
ft § I z -8
_= oo
o B
=3 8.
Cfl
B
"6 § i
•s
I 00
.a cfl
§ -C bO 3
s
-o-'S, 8 .S a E M
J3 4)
bo u
K 3
%
V
T I
4> r—» -.
u P
i i OH 00
•a
I
.a oo
ra °
cfl 3
I I S3 o u c. •8
3
I I
s z
fa bO 3 cfl
£ 05
1—1
t-
Cfl B
&
CS f-
co t^
Tf fr
E
§ x w
r — I
3 M
g cfl 3
I >
I
IT)
bo J=
3 a
5" x"
1 E
3 bO
1 3
i cr > in es
c.
[r- 00
154
Highly cohesive,
ratholing tend
encies
in blow
tanks,
inter
locking
tendency
Prone
to
degradation
Small particles
not weU
cemented. At
higher magni
fication
particles
consist of fused
rhombus crystals
Individual
particles have
formed
agglomerates
Some sharp
edges and
corners
Vesicular
surface
Very rough,
fluffy
Considerable
particle
porosity,
At high
magnification
reveals the
extent of pore
volume
i 8 8
<3
1
7.9
7.10
7.11
Static electricity
iffects conveying,
tendency
to plug,
interlocking
tendency,
effective volume
greater than
actual particle
volu
me
No, hard
incompressible
particles
generaUy
individual
particles.
Some
dumbeUing
present
Individual
particles
A fractured, rough
fused subparticle
agglomerate
Rounding
of
edges and
comers
Very rough
and uneven
surface at
high mag
nification
Uneven,
irregular,
undulations,
cauhflower
appearance
Macroscopic
indentations
visible
o CO
Polyvinyl
chloride
C»vc)
7.12
7.13
Good
conveying
characteristics,
but definite
edges suggest
No,
individual
particles
Individual
particles
varying in
size
No agglomerates,
definite edges
from grinding
operation
Sharp edges
and comers
Fractured
-leated,
rough
surface at
Highly angu
lar
CO
3
1 1 O
Si
Pulverized
coal
Tallawarra
7.14
7.15
7.16
155
significant
abrasion
high mag
nification
Cleating and
flakiness
characteristics,
very a
brasive
Sharp edges,
prone to
extreme
degradation
Cracks/
fracture
2 : E ! bo
Highly
conchoidal
structure
Visible,
cleated,
uneven
surface
Angular,
very
irregu
lar
CO
3
1 1
•8 co
6
Petroleum
coke
7.17
7.18
7.19
7.20
7.21
Degradation
increases the
submicron
loading (HI
downstream
dust collection
facilities. This
fly
ash fluidizes
& conveys weU
Submicron
particles
dislodge from
the large
particle surface
during
pneumatic
conveying
Larger
particles
surrounded by
smaller
particles
Partial agglo
meration of
different sized
small particles
No edges
or comers
Smooth
and
sintered
surface,
Spherical
particles
co 3
I 1 I/O
Eraring fly
ash
CS CS
r-
Unsteady
conveying
Significant
due to
Micron and
submicron
Partial agglome
ration of small
&
No edges or
comers
Some
particles
Spherical
particles,
Low
permea
bility, signi-
2-22 nm
smaller in
Liddell fly
ash
7.23
7.24
156
-a Cfl
U
•a CM
5 a u
E %
1 •8
•8
•8 •r.
Cfl
E O
_ J _ - _
E bo
S bo
I Cfl
a
Cfl
bO 3
> a Cfl
O O u
s
r S 3 _> •ir M cfl ta U r-i
u o
a 2
8
I *-» O E
s O 5-O o 5r
ti
g
s bo
a Q
I
1
1 cfl
2 2 £ o bO
cf
v E Cfl
Cfl
cl
Cfl
a 2 o E o "bb cf
il & 3 rt 57 co
.S .s
Cfl CD
bo
3 a Cfl I 1
•a E E
CO
8
J. 1
<U r.
i a 3 E Cfl Cfl
Cfl
O ri,
•S
2 « •8 | co g .3 3 CrH Cfl
-a "S a
a 00
Cfl
U
•a
1 g1
§ a
g •a
£
.s i- i top O 3 •S o "53 co -3 <u «*- ta >"
o 8. «
1 3 H a
s 8 -5
| 3
5 ti
a o TT r—<
u
N
O
cs
<2 CO
O
-a >
Sr,
cs cs t"r fr
157
7.2 COEFFTCTFNT OF RFSTTTTTTTON;
The details and operating procedure for this rig are described in Section 6.6. A
schematic of this rig is presented in Figure 7.1. Observations and results from the
various experiments are declared in Table 7.2 and are depicted graphically in Figures
7.2, 7.3 and 7.4. In this test series, different wall surfaces namely, Mild Steel and
Stainless Steel were used. The wall surfaces were tilted, during impact, both in (
denoted by C O in Table 7.2) and counter ( denoted by C O U N T in Table 7.2 ) to the direction of rotation of motor.
INCIDENT PRRTICLES
REBOUND PRRTICLES L-/
FEEDER
REFERENCE
GRID —
\ \ : -
'!
DISK
WALL SURFRCE
FLEKIBLE SHAFT
HANDLE
a \
PROTRCTOR —
BRSE
D.C.
MOTOR
Figure 7.1: Coefficient of Restitution Rig.
158
TABLE 7.2 COEFFICIENT OF RESTITUTION
SR.
NO
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
MATERIAL
WHEAT
MILLET
MUNGBEAN
COAL
COEFFICIENT
OF RESTITU
TION e
0.628
0.618
0.635
0.655
0.675
0.665
0.674
0.674
0.585
0.611
0.648
0.569
0.647
0.551
0.55
0.57
0.59
0.56
0.57
0.59
0.60
0.63
0.623
0.64
0.574
0.589
0.61
0.62
0.656
0.569
STD.
DEVIATI
ON
0.159
0.192
0.147
0.174
0.161
0.217
0.195
0.202
0.174
0.164
0.196
0.161
0.191
0.17
0.17
0.15
0.15
0.18
0.15
0.14
0.178
0.177
0.155
0.146
0.169
0.089
0.093
0.071
0.076
0.109
ANGLE
OF
IMPACT
0
10
15
20
5
10
15
20
0
5
10
15
20
0
5
10
15
5
10
15
0
5
10
15
20
0
5
10
15
20
WALL
MATERIAL
MILD STEEL
STAINLESS
STEEL
MILD STEEL
S. S.
M. S.
DISK
R.P.M.
2632
2757
2649
2649
2655
2660
2660
2675
2606
2603
2652
2711
2146
2265
2611
2603
2620
2634
2146
2621
2730
2611
2615
DISK
TILT
CO
COUNT
COUNT
COUNT
CO
CO
159
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
COKE
SINTER
SODIUM-
FERRITE
SUGAR
PLASTIC
BALLS
0.589
0.695
0.645
0.692
0.529
0.467
0.489
0.539
0.536
0.573
0.53
0.574
0.548
0.509
0.579
0.531
0.7085
0.086
0.099
0.093
0.087
L 0.064
0.056
0.051
0.099
0.109
0.048
0.052
0.093
0.132
0.108
0.070
0.147
0.120
5
10
15
20
0
5
10
15
0
5
10
15
0
10
15
0
0
s. s.
s. s. s. s.
*
s. s.
M. S.
M. S.
2727
2738
2750
2652
2682
2725
2709
2774
2497
2670
2683
2672
2779
2649
2706
2740
COUNT
CO
COUNT
COUNT
z O F r-
00 LU CC LL
O Z LU O LL LU
LU
O O
U.bb-
0.64-
1 0.62-
0.60"
0.58-
0.56-l
0.54 -*
1 •
•
»
i
• D
•
8
D
... ,
• D
•
• •
Q
1
Q
•
B O
LEGEND WHEAT M.S. WHEAT S.S.
MILLET M.S. BEAN S.S.
Figure
0 10 20 30 ANGLE OF IMPACT, DEGRESS
7.2: Coefficient of Restitution of Wheat, Millet and Bean (Co-rotation).
160
z o
H t to LU CC Ul
o Ul
o o i i r
10 20 ANGLE OF IMPACT, DEGREES
30
Q
• D •
LEGEND
COAL M.S. COAL S.S. SINTER S.S. FERRITE S.S.
Figure 7.3: Coefficient of Restitution of Coal, Sinter and Sodium Ferrite (Co-rotation).
z O
Ul oc u.
o
LEGEND
• WHEAT M.S. • MILLET M.S. • COAL M.S.
UJ
u LL U-Ul
O O
T • r 0 10 20 ANGLE OF IMPACT, DEGREES Figure 7.4: Coefficient of Restitution of Wheat, Millet and Coal (Counter-rotation).
161
7.3 P A R T I C L E S I Z E ANAI.VSF.fi;
The particle size analyses of the various powders and granular materials tested are
declared in Tables 7.3 to 7.7. This information is presented graphically in Figures 7.5 to 7.10.
T A B L E 7.3 PARTICLE SIZE ANALYSES
Sr.
No.
1.1
.11
2.1
.11
3.
4.1
.11
5.1
6.1
.11
7.1
.11
.III
8.
9.
10.
Material
Fly ash 'A'
Fly ash 'B'
Fly ash 'C
Fly ash 'D'
Fly ash *E'
Fly ash F
Fly ash 'G*
Cement
Sand
P V C Powder
d50
Lim
16.9
15.1
15.0
14.3
9.2
10.0
11.8
5.4
13.8
14.0
14.9
13.9
13.8
18.3
209.5
149.6
d90
u m
56.7
64.8
65.8
61.9
30.3
64.5
84.0
27.1
64.5
65.4
83.5
75.0
74.8
55.1
364.6
221.7
dlO
Lim
4.8
4.7
4.1
3.9
3.1
3.2
3.0
2.0
3.5
3.6
2.8
3.2
3.2
5.2
105.4
123.5
D(4,3)
Lim
24.4
26.6
26.7
22.6
13.9
22.3
28.5
10.2
22.7
25.1
26.3
28.2
28.0
25.7
221.6
163.7
D(3,2)
Um
11.1
10.2
9.7
9.4
6.6
7.2
7.7
3.6
8.5
8.7
7.8
7.8
7.9
11.5
120.4
151.8
Specific
surface
sq.m / cc.
0.10
0.06
0.07
0.08
0.12
0.06
0.05
0.18
0.07
0.07
0.06
0.06
0.06
0.08
0.02
0.03
T A B L E 7.4 SIZE ANALYSIS O F S A N D
Mesh size,
U m
1000
850
710
500
425
Tare.g
440.00
423.00
421.4
406.9
386.4
Weight, g
447.1
428.4
432.8
453.0
440.5
Weight
fraction, g
7.1
5.4
11.4
46.1
54.1
%
1.42
1.08
2.28
9.21
10.8
300
212
1%
180
150
125
106
90
75
45
22.5
329.2
360.9
333.0
302.4
348.8
302.3
344.2
475.9
267.6
340.1
334.7
473.8
487.4
438.1
352.7
386.4
313.7
348.3
477.3
267.9
340.3
334.9
144.6
126.5
105.1
50.3
37.6
11.4
4.1
1.4
0.3
0.2
0.2
28.88
25.26
20.99
10.05
7.50
2.28
0.82
0.28
0.06
0.04
0.04
Surface to volume mean diameter = V ^ weight fraction ------ mean diameter
= 305 \im
TABLE 7.5 SIZE ANALYSIS OF B R O W N RICE (I)
Mesh size
mm
3.35
2.80
2.36
2.0
1.4
1.18
1.0
850 um
425 urn
Tare,g
493.1
489.4
427.1
412.7
444.6
389.6
386.2
423.1
334.6
Weight, g
493.4
492.6
1160.4
572.9
481.3
390.8
387.0
423.7
337.5
Weight
fraction, g
0.3
3.2
733.3
160.2
36.7
1.2
0.8
0.6
2.9
%
0.003
0.003
0.78
0.171
0.039
0.001
0.001
0.001
0.003
Surface to volume mean diameter = 1
weight fraction mean diameter
= 2.0 m m
TABLE 7.6 SIZE ANALYSIS OF B R O W N RICE (II)
Mesh size
mm
2.80
2.36
2.0
1.4
1.18
1.0
500LUT1
Tare, g
489.4
427.1
412.7
444.6
389.6
386.2
334.6
Weight, g
492.6
816.4
607.5
469.7
390.3
386.7
336.3
Weight
fraction, g
3.2
389.3
194.8
25.1
0.7
0.5
1.7
%
0.005
0.633
0.317
0.041
0.001
0.001
0.003
Surface to volume mean diameter = 1
weight fraction mean diameter
= 2.42 m m
TABLE 7.7 SIZE ANALYSIS OF WHITE RICE
Mesh size
mm
2.8
2.36
2.0
1.4
1.18
1.0
500um
Tare.g
489.6
427.1
412.7
444.6
389.6
386.2
334.2
Weight, g
492.3
878.4
633.0
454.9
391.0
387.4
340.0
Weight
fraction, g
2.9
451.3
220.3
10.3
1.4
1.2
5.8
%
0.004
0.651
0.318
0.015
0.002
0.002
0.008
Surface to volume mean diameter = 1
weight fraction mean diameter
= 2.49 m m
1
Ul N eo DC Ul
o Z 3 as
LEGEND FLY ASH -B- 'A'
-D- 'C
••- -D'
PARTICLE SIZE (nm)
Figure 7.5: Particle Size Distribution versus % Undersize of Fly Ash 'A', 'B', C and 'D'.
ui N
<7i cc ui Q
z 3
LEGEND FLY ASH
-O- '£<
•+• V HB- 'G'
-••CEMENT
PARTICLE SIZE (um)
Figure 7.6: Particle Size Distribution versus % Undersize of Fly Ash 'E, 'F, 'G' and Cement.
165
> •
o ui 3
O Ul CC
PARTICLE SIZE (um)
Figure 7.7: Variation of Frequency versus Particle Size of Fly Ash 'A', 'B* and 'C
> •
ui
o z Ul 3
o Ul DC
LEGEND FLY ASH
-a- 'D' •+- 'E'
PARTICLE SIZE ( m)
Figure 7.8: Variation of Frequency versus Particle Size of Fly Ash 'D* and 'E'.
166
>-
o z Ul 3 O
LEGEND
FLY ASH
-a- 'F •o-'G-
PARTICLE SIZE (um)
Figure 7.9: Variation of Frequency versus Particle Size of Fly Ash 'F and 'G\
>-
o z Ul 3
a ui DC LL
LEGEND
•o-CEMENT -#- PVC -B- SAND
10' 10' PARTICLE SIZE (um)
Figure 7.10: Variation of Frequency versus Particle Size of
Cement, PVC Powder and Sand.
7.4 B U L K DENSITY
The observed powder bulk density and loose poured bulk density properties are
summarized in Table 7.8 and 7.9, respectively.
167
TABLE 7.8 BULK DENSITY
Sr.
No.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Material
Fly ash 'A'
Fly ash "B'
Fly ash 'C
Fly ash 'D'
Fly ash 'E
Fly ash "F
Fly ash *G'
Cement
Wheat
White Rice
Brown Rice
Sand
Sugar
PVC Powder
Sodium Ferrite
Po
kg/m3
1032
1134
944
1088
975
1275
1294
1312
868
865
825
1502
883
649
1512
Oo
kPa
6.458
6.458
6.458
6.458
6.566
6.458
6.458
6.458
6.458
6.458
6.458
6.458
6.458
6.458
6.458
constant b
0.0417
0.0598
0.0554
0.049
0.057
0.0584
0.053
0.0557
0.0021
0.0018
0.0016
0.011
0.0023
0.0084
0.0314
correl.
coeffi.
0.99
0.98
0.74
0.95
0.99
0.98
0.98
0.99
0.99
0.99
1.00
0.98
0.98
0.99
0.99
TABLE 7.9 LOOSE POURED BULK DENSITY
Material
Fly ash 'A'
Fly ash *B'
Fly ash 'C
Fly ash 'D*
Fly ash *E'
Fly ash T
Loose
Poured
Bulk
Density,
kg/m3
769
750
520
703
617
965
Packed
to Loose
Poured
B.D.
kg/m3
1.342
1.512
1.815
1.548
1.58
1.321
Material
Cement
Sand
PVC Powder
Wheat
Brown Rice
White Rice
Loose
Poured
Bulk
Density,
kg/m3
950
1424
575
709
703
Packed
to Loose
Poured
B.D.
kg/m3
1.381
1.055
1.129
1.224
1.173
751 L152
168
7.5 SOLIDS DENSITY.
The observed particle solids density of the materials tested are summarized in Table
7.10.
Material
White Rice
Brown Rice
Riceflakes
Wheat
Millet
Castor Sugar
Light soda ash
Dense soda ash
PVC Powder
Sodium Ferrite
T A B L E 7.10
Solids Density,
kg/m3
1478
1579
1579
1380
1550
1500
2454
2508
1378
1147
SOLIDS DENSITY
Material
Fly ash 'A'
Fly ash 'B'
Fly ash 'C
Fly ash 'D'
Fly ash 'E*
Fly ash F
Fly ash *G'
Cement
Sand
Solids Density,
kg/m3
2130
2492
2195
2624
2536
2537
2391
3100
2735
7.6 ARCH LENGTH AND DRAINED ANGLE OF REPOSE:
The details and operating procedure for the Arch Tester shown in Figure 7.11
are provided in Section 6.12. This same tester was also used for measuring the drained
angle of repose as shown in Figure 7.12.
Table 7.11 summarize the observed critical arch dimensions as measured in the
Arch Tester. In addition, this Table represent a comparison of the predicted arclf
dimension assuming rough wall boundaries apply in the Jenike (1970) and Walker (1966)
analysis, refer Section B.17 (b). In this latter evaluation, the assumed powder flow
function information is presented in Table 7.13 and graphically in Figure 7.24.
Figures 7.13 to 7.15 represent the variation of arch length versus deaeration time
with different relative humidity for fly ash 'A', 'C and Cement, respectively. Figures
7.16 and 7.17 reveal the variation of arch length versus deaeration time for Cement and
fly ash, respectively. Figure 7.18 depicts the variation of bed height versus arch length
for Cement, whereas, Figure 7.19 represents the variation of mean particle size versus
arch length for the different powders tested in the Arch Tester.
169
Furthermore, the observed drained angle of repose at the four flow channel
boundaries are presented in Table 7.11. Figure 7.20 represents the variation of drained
angle of repose with bed height for Cement, whereas variation of drained angle of repose
versus deaeration time for fly ash and Cement are depicted in Figures 7.21 and 7.22,
respectively. Figure 7.23 represents the variation of mean particle size versus drained
angle of repose for the different powders tested.
C H A I N
DRIUE
DUST COUER
PERSPEK SILO
ROTATING H A N D L E
SCALE
SIGHT GLASS D I S C H A R G E BIN
Figure 7.11: Arch Tester.
170
POWDER
DRAINED ANGLE OF REPOSE
SIGHT GLASS
DUST COVER
PERSPEX SILO
SLIDE GATES
RECEIVING BIN
Figure 7.12: Tester for Measuring the Drained Angle of Repose.
171
a f-r**
a
Q
O C/3
a z <: X c/3 <C >« J Cs.
H Z W ri
u u fc-
o
H -TJ
O a. u ce: u. O u J z < Q
z < OS Q Q
Z <
s H Z w -J
B U as <
W -1 09 < H
>-co
2
z B O Z rU B U Or
B Or
Q UJ PD
C/3
W J a Z ^ i-r
u < ca _
H Z o g
B u SS 5!
<
Q 1
rJ
3! 2
r >
Q O B
D,
g
Q O B t -c/j
W r ^
z w 1—1
#
o § CN
P-H
CS
P—-1
1
O z 3 w r5 p— 1
1 P O z 1
rj
-J
B
O Oi
-J -J <
B o o 05
E E
o T3
60 1> T3
P s
e
c
S
rC ~
H E
^H ON NO CS NO ^H cs - < t — 1
CO 00 r* CD in r*
oo io o r- NO in Tt CO cs
o NO
o r-
CS NO NO m Tt m
z
u
cs cs
CO
o
in
r-
00 t~--t
o oo
o r-
oo -t
ON
'—'
00 in
in r-
m NO CO
o 00
o 00
m 00
o 00
00 cs
NO
H H
cs o
in r--
o m cs
r-00
o 00
m 00
o 00
Tt Tt
CO NO
in
in
r~
m CO ~*
o oo
o r--
o r-
o 00
cs CO
H
cs cs
ON ON
Tt r~
00 r--Tt
O 00
o 00
o r-
o 00
Tt CO
ON
NO
CS
Tt r~
in NO CO
m r-
o 00
o 00
o 00
NO cs
NO
I—1
CO
o
Tt r-r
o in cs
in 00
o 00
in oo
o 00
oo Tf
Ea
NO I — 1
1 — «
8
o in cs
o ON
o ON
o 00
o 00
Tt Tt
m Tt
O P-H
8
o m cs
o ON
o 00
o oo
O ON
oo
O ON
CO NO
NO
m
in CO —,
o 00
o ON
o r-
o r-
o 00
NO CO
in
3 O
B
£
NO
l-H
8
NO in
o >n cs
o 00
o 00
o 00
o
o Tt
m cs
NO r*
s
NO m
o in cs
o 00
o 00
o 00
o 00
oo Tt
in
cs
NO I—I
CS Tt
in NO m
o in cs
o 00
o 00
o I--
o r—
o in
Tt co
>
172
NO S-J NO
r~ 2 <"-
-* jn cs ON NS ON
O NO o
r- NO r--
in o in NO cs NO HH CS HH
O O O 00 00 oo
O O O 00 oo oo
O O O ON OO 00
O O O 00 00 00
cs oo cs TT Tt m
O >n oo CO CO CO
O 00
o o cs cs
s
« \o vo S JS « p~ p~ r» _] 2
r-t i-H i—H
ry. r . .» r-- in NO -o oo S 2 J-J r2
I-H NO NO NO CO
r-- NO NO NO r-»
m in in O O O NO NO NO CO co in HH HH HH (S| cs cs
o o O o in oo oo r-r oo oo
O O O O O 00 00 00 00 ON
O O O O ON NO ON ON
m O O O O
oo r- r oo ON
Tt Tt O O O HH CN co Tt r-r
O O O Tt Tt
O ^H rs in in NO
O O in o HH cs f- ON
£
NO NO NO NO Cr- Pr- f- t^
oo oo r-- cs oo oo oo ON
O O O O Pr- t^ P-" Pr-
in in in in NO NO NO NO
in o o 00 ON 00
o in ON 00
O O O ON ON 00
NO O 00 P^
O O O CS CO Tt ON NO
O O O CS H. CJ\ Tt
Tt in
in
cs
g
NO NO NO NO NO t~- P^ ^ Pr- ^
HH ON ON ON O P^ VO NO NO Pr-
NO V^ NO NO NO
in in in in in NO NO NO NO NO
O O O O O Pr 00 00 P- SO
O CO O S O p- oo oo r--
in o o O O P~ 00 O0 00 ON
O O O O O oo pr. 00 oo 00
00 00 Tt NO NO CS CS CO Tf Tt
NO NO Tt HH ON —1 CS Tt NO Pr
co r-r in cs O — CS Tf NO
NO co r~ NO r-i—1 H H i—1 r—c i-H
X
in m >n in
cs NO cs cs
in o CS Tf
O ON
— cs
in «-i H ^1
<
<
173
in m in in
f
NO O CO Tt
o in NO P-
P- CO Tt NO
CO CS
Os ON ON ON
in in in in
cs cs cs 00 cs co co cs
ITS OO CO O ^ <N H-
o o o
"" S r-l
r—1
in cs in >n OO 00 00 00
r-- o co co 00 00 OO 00
in in in P--' in oo NO co Tt in p~ in p- in in in
m o in in in in in in
O O O O in Pr- Pr (—
o o o o in NO P- P~
o o o o NO P^ Pr" P-
o o o o in r-- NO NO
Tf Tt Tf 00 -t Tt NO cs cs co cs co co co
P-. >n O O r- m co HH CS CO CO Tt NO
r~. r-_ co cs O oo in ^ <-J HH cs co co m
-— I-, -"* °° -** r~- °°
P-
00 O P- O P~ ON 00 00
r-t Tt ^H vo P-- Pr- 00 00
P- P- Pr-NO NO NO
in in o oo co in in co
m O O O pr- ON oo r~-
o o o o 00 ON Tt 00
O O O O in ON Tf 00
O O O O VO 00 00 pr-
00 O NO o in Pr- P~ P-
o cs oo (~, CS Tf P-
o S S °
O r >n r-. CS r-t r-t
y
174
o o OO 00
in ON NO 00 00
00 00 CO CO
O O pr- ON
O O ON P--
o o ON p-
o o oo oo
oo cs r- oo
in o CS NO
° 5
•n vo cs —
--> rQ vo ~
00 cs Tl" ON
in in NO NO NO NO
o O O ON
o o 00 oo
o o r-- ON
o o P- NO
o o NO 00
00 NO CO Tt
O Pr-CN CN
in o --I CN
m ^
Q
cs o o P- ON ON
O 00 00 CO CO CO
00 00 CO P- Pr- t~-
o o o cs m in
o o ON ON
o o —' ON H-
o o ON ON
O O ON ON
vo O 00 3 -> cir ;_; r-- oo oo
•a -a •c "C m m O O O
Tt H Tt
^ ^ ^ ' iS
CO O Tt -H -H CN
w
NO CS pr- CO NO r-r Tt NO
CS Tt r-l 00 in pr- in NO
P~ Pr- |-~ P-
O o o — CO o cs
o o o oo r-- in
o o o in oo NO
O O o NO NO 00
O O Q NO Tt NO
oo NO oo co co co co in
Tt in o O rl CS Tf VO
a- oo cs m ° H- co «n
in P- oo in
P-
NO NO in in
m vo
o o p- r-
o o NO NO
in o in r-~
in O NO r-
CN NO in co
in o H-l CO
^ £
rZ J
B 2 Q oi
175
FLY ASH 'A'
i ' r 20 40 60 Deaeration time, minutes
LEGEND
RELATIVE HUMIDTY H 55% • 59%
Figure 7.13: Arch Length versus Deaeration Time for Fly Ash 'A'.
FLY A ? H '<•?'
-, i 1 • I
20 40 60
Deaeration time, minutes
LEGEND
RELATIVE HUMIDITY
B 67% • 73%
Figure 7.14: Arch Length versus Deaeration Time for Fly Ash 'C.
176
/u -
60-
50-
40 -
30-
2\J -
<r
e
a • a
T
•
B
B
•
•
1
0
• B D
•
1
B
•
•
i
•
1
20 40 60
Deaeration time, min. 80
LEGEND
REL HUM.
•a 56% • 61% B 66% • 70%
Figure 7.15: Arch Length versus Deaeration Time for Cement
B • B
LEGEND Cement Fly ash *A' B'
Deaeration time, mins.
7.16: Arch Length versus Deaeration Time for Cement, Fly Ash 'A' and 'B'.
LEGEND Fly ash B 'C e 'D' B 'E' • F
- i — i — i — | — i -
10 20 30 40 50 60 70 Deaeration time, mins.
Figure 7.17: Arch Length versus Deaeration Time for Fly Ash
*B\ 'C, T>\ 'E' and 'F.
CEMENT
ou -
50-
40-
30-•
B
B
•
i
B
B
• 1
B
•
B
*
LEGEND EXP. B 1 • 2 B 3
100 200 300 400
Bed height, mm. 500
Figure 7.18: Arch Length versus Bed Height for Cement
178
120
E E
o> c © X <
Mean particle size, micron
Figure 7.19: Arch Length versus Particle Size Variation for the Powders Tested.
? •a
i o Q.
s r -
o e cn c a c
B4 -
82-
•
80-
78-
76-
74 i
EI
T
B B
B
l i
Q
•
•
Q
1
LEGEND
REL HUM
H 75% • 74% • 66%
100 200 300 400
Bed height, mm.
500
Figure 7.20: Variation of Drained Angle of Repose with Bed Height for Cement
179
100
8 &
S? TJ
8 o Q. & o -2 cn c •
80-
70-
60-
50
• B
B B B
B " • — — i > r-
20 40 —1 1
60 80
LEGEND FLY ASH B 'B' • 'C B 'D' • 'E' B V
Deaeration time, mins.
Figure 7.21: Variation of Drained Angle of Repose versus Deaeration Time for the
Fly Ash Tested.
t? •o
8" o a £ r —
O • c a
c
8 5
90-
80-
70-
0
o
B
e
•
• D BB a
DO • • B 1 • El •
# D DD
i • l • l
20 40 60
Deaeration time, mins.
«
8
LEGEND
REL. HUM.
a 56% • 61% B 66% • 70% B 74% a 75%
0
Figure 7.22: Variation of Drained Angle of Repose versus Deaeration Time for Cement
CO 0 e -. cn e TJ
o » o Q. 0 O) c a CO
e
100
100 Mean particle size, microns
Figure 7.23: Variation of Drained Angle of Repose versus Mean Particle Size.
7.7 F L O W FIJNCTTON:
The observed powder flow functions are summarized in Table 7.12.
Alternatively, these flow functions are plotted in Figure 7.24.
a a.
n cn a
T>
>N
TJ
C
c o o c z>
El
• B
• B •
A
LEGEND
CEMENT FLY ASH 'A'
•B' •C'
•D' •E' 'F
2 4 6 8
Major consolidating stress, kPa
Figure 7.24: Powder Flow Functions.
181
TABLE 7.12: INSTANTANEOUS YIELD LOCI DATA
Sr.
1.
2.
3.
4.
5.
6.
7.
Material
Cement
Fly ash 'A'
Fly ash •B'
Fly ash 'C
Fly ash 'D'
Fly ash E'
Fly ash "F
ai(kPa)
8.55
10.46
5.78
6.51
8.53
4.12
6.46
9.19
4.68
6.82
8.41
4.53
5.91
8.05
4.14
7.37
9.23
4.59
5.94
7.93
4.59
<-c(kPa)
2.21
2.51
1.14
3.2
3.64
1.80
1.55
2.34
1.10
2.29
2.48
1.85
2.61
2.92
2.07
5.06
6.92
2.96
3.82
4.72
2.96
5(degree
43.64
45.15
46.20
42.22
54.13
41.69
38.55
39.15
39.30
40.27
44.13
36.69
41.27
43.21
43.80
57.84
56.38
52.08
35.28
38.53
38.45
<|>(degre<
37.13
39.35
41.66
31.20
21.47
28.37
32.14
32.33
33.08
30.75
36.50
23.75
27.58
33.07
27.84
35.56
27.70
30.05
31.67
33.26
35.06
-) oi/cc
3.869
4.167
5.070
2.726
1.294
2.299
4.168
3.927
4.255
2.978
3.391
2.449
2.264
2.757
2.00
1.457
1.334
1.551
1.457
1.334
1.551
Slope of F.F.
0.293
0.464
0.31
0.195
0.222
0.923
0.6
182
7.8 TENSUE STRENGTH-
/ clamping screw
S= -T—V\±— ^JVV7/-.--V-V-\/V2. /•• $S-J\r„%^j.
•%-»•
-L
^pivot point
pivoting block
Figure 7.25: Tensile Tester
The details and operating procedure for this tester are described in Section 6.11
and Appendix B.2.4. A schematic of this tester is presented in Figure 7.25.
The observed tensile stress versus consolidation stress for the powders tested are
depicted in Figures 7.26 and 7.27. The tt nsiL tests of the same powders when tested
with a deaeration time of about 15 minutes are shown in Figures 7.28 and 7.29. The
observed voidage variation versus tensile strength of the various fly ash tested are shown
in Figures 7.30,7.31,7.33 and 7.34, whereas, the observed bulk density variation are
shown in Figures 7.32 and 7.35. The observed tensile stress versus consolidation stress
and voidage for light soda ash, dense soda ash, P V C powder and castor sugar are
depicted in Figures 7.36 and 7.37, respectively.
To observe the effect of the filling procedure on the measured tensile stress versus
consolidation stress, two different procedures were used. In particular, in one
experiment, a screen vibrator was used and in another experiment spoon filling of the cell
was used. In each experiment an approximate deaeration time of 15 minutes was allowed.
The comparison of filling procedure for fly ash 'J' is depicted in Figure 7.38, whereas,
tensile stress versus consolidation stress for fly ash 'H, T and 'J' are shown in Figures
7.39 and 7.40, respectively.
183
LEGEND FLY ASH B •
B
TV 'B' 'C
0 20 40 60 80 100 120
Consolidation Stress, kPa
Figure 7.26: Tensile Stress versus Consolidation Stress for Fly Ash 'A', 'B'
and C.
LEGEND FLY ASH
a •
B
•D'
'E' •F
0 20 40 60 80 100 120
Consolidation Stress, kPa
Figure 7.27: Tensile Stress versus Consolidation Stress for Fly Ash 'D, 'E
and 'F.
(B O.
Si
c
CO
a w§
? to "35 c
184
(B
a
CO
20 40 60 80 100 Consolidation Stress, kPa
LEGEND FLY ASH B 'A' • 'B' B 'C
120
Figure 7.28: Tensile Stress versus Consolidation Stress for Fly Ash 'A', 'B' and
'C with 15 minutes Deaeration.
a o. -t
j£
S> c £ 55 CO
c e
1 -
T ' I r
20 40 60 80 100 120 Consolidation Stress, kPa
LEGEND FLY ASH
a 'D' • 'E" B F
Figure 7.29: Tensile Stress versus Consolidation Stress for Fly ash 'D', 'E' and
'F with 15 minutes Deaeration.
185
a OL
0
"35 c
LEGEND FLY ASH • .A.
e 'B' B •<_'
Voidage
Figure 7.30: Tensile Stress versus Voidage for Fly Ash 'A', *B' and 'C.
CB
a.
c
LEGEND
FLY ASH
• 'D' • 'E" B T
0.7
Figure 7.31: Tensile Stress versus Voidage for Fly Ash 'D, *F and 'F.
186
8.
c e
\£V '
100 -
80-
60-
40-
20-
B
B
B
• 1 •
B
i
u
B fl
B
•
B e
B
1 1
e
0 • DI
» » D
0 • D B
•a B
D •
1 1 1 1 1
LEGEND
FLY ASH
B •
B • B D
'A' 'B' •c •D' •E" •F
600 700 800 900 1000 1100 1200
Bulk Density kg/m3
Figure 7.32: Tensile Stress versus Bulk Density for the Fly Ash Tested.
2. -C
• tt c tt
0.50 0.55 0.60
Voidage
0.65
LEGEND
FLY ASH
B TV • 'B" B 'C
0.70
Figure 7.33: Tensile Stress versus Voidage for Fly Ash 'A', 'B' and 'C with
15 minutes Deaeration.
1
CB Q.
CO • •35
c tt
0.8
LEGEND FLY ASH B 'D' • "E" B *F
Figure 7.34: Tensile Stress versus Voidage for Fly A s h 'D', 'E' and 'F with 15
minutes Deaeration.
CB
co
c tt
3-
2-
B
• • I B B a e ••
o
•n
03
LEGEND FLY ASH
a 'A' • 'B' B 'C • 'D' B 'E' D 'F'
700 800 900 1000 Bulk Density, kg/m3
1100
Figure 7.35: Tensile Stress versus Bulk Density for Fly A s h Tested with 15
minutes Deaeration.
188
OL
c
2-
20 40 60 80
Consolidation Stress, kPa 100
LEGEND
BLIGHT SODA ASH •DENSE SODA ASr B PVCPOWDER • CASTOR SUGAR
Figure 7.36: Tensile Stress versus Consolidation Stress for Light Soda Ash,
Dense Soda Ash, PVC Powder and Castor Sugar.
CB CL
!
55 tt tt c •
2-
0.3
e •
e
•
B
B
LEGEND
SODA ASH a LIGHT •DENSE B PVC POWDER
0.4 0.5
Voidage 0.6 0.7
Figurte 7.37: Tensile Stress versus Voidage for Light Soda Ash, Dense Soda Ash
and PVC Powder.
189
CL
55 o « c tt »-
-i 1 1 1 1 | 1 — r
20 40 60 80 100
Consolidation Stress, kPa
LEGEND
FLY ASH 'J' B NODEAERA. • SCREEN VIBR. B SPOON
120
Figure 7.38: Tensile Strength versus Consolidation Stress for Fly Ash 'J'.
s 55 o n c e
LEGEND
FLY ASH
B 'H' • T B 'J'
20 40 60 80 100
Consolidation Stress, kPa
Figure 7.39: Tensile Strength versus Consolidation Stress for Fly Ash
'H, T and T.
2.4
190
CEMENT
a a.
2.2-
-- 2.0-CO
n c
1.8-
1.6H—.—i—r -J i 1 1 i r-
0 20 40 60 80 100 Consolidation Strees, kPa
Figure 7.40: Tensile Strength versus Consolidation Stress for Cement
It should be noted that the observed tensile strength variations are plotted using a
standard Macintosh computer software package with the observations correlated to
exponential regression curve fits. The values of the empirical constants, so evaluated and
slope of the tensile strength curve are presented in Table 7.13.
TABLE 7.13: TENSILE STRESS VERSUS CONSOLIDATION
STRESS
Material
Fly ash 'A'
Fly ash 'B'
Fly ash C
Fly ash TV
Fly ash 'E'
Fly ash T
Cement
Exponent
0.0017
0.0029
-
0.0021
0.0025
0.0032
-
Slope
L 0.632
0.214
0.741
0.474
0.666
0.957
0.682
Material
Light soda ash
Dense soda ash
P V C powder
Castor sugar
Fly ash 'H'
Fly ash T
Fly ash T
Exponent
0.006
0.0019
0.0017
0.0023
0.0038
0.0046
0.0039
Slope
1.04
0.314
0.244
0.458
0.306
0.533
0.413
191
7.9 WALL FRTCTTON-
PRESSURE GAUGE
' & -
PRESSURE REGULATOR
COMPRESSED
AIR SUPPLY
X \ ROTAMETER
CONTROL UNIT WITH FORWARD/REVERSE SWITCH/ I
EK i • i *. i
y
.100 mm <t> PERSPEX TUBE
^PERSPEX DISTRIBUTOR
LOAD CELL i
SCREW BAR!
CONTROL; UNIT :
DC MOTOR
CHART
RECORDER
Figure 7.41: Wall Friction Rig
The details and operating procedure for this rig are described in Chapter 6.6. A
schematic of this rig is presented in Figure 6.39. The observed wall friction force versus
piston aeration air pressure for different column lengths are depicted in Figures 7.42 to
7.48, whereas variation of column length and frictional force at different aeration air
pressure are depicted in Figures 7.49 to 7.54.
Average shear stress which is the ratio of the frictional force to the contact area
versus aeration pressure are depicted in Figures 7.55 to 7.60. Using the analysis
presented in Section 2.5, the evaluated wall friction factor uk versus piston aeration air
pressure for different column lengths are depicted in Figures 7.61, 7.62, 7.63, 7.64 and
7.65 for Brown Rice, White Rice, Rice Flakes, Millet and Wheat, respectively.
BROWN RICE
CD
o
o LL
CO C
o r—
o
T 1 1 1 r 100 200 300 Air Pressure, kPa
LEGEND
COLUMN HEIGHT •
•
o «
80 mm. 120 mm. 160 mm. 200 mm.
400
Figure 7.42: Frictional Force versus Aeration Air Pressure for Brown Rice.
WHITE RICE
u
CO
c o o LL
LEGEND
COLUMN HEIGHT •
•
a •
80 mm 120 mm 160 mm 200 mm
100 200 300 400
Air Pressure, kPa
500
Figure 7.43: Frictional Force versus Aeration Air Pressure for White Rice.
RICE FLAKES
-r 100 200 300
Air Pressure, kPa
LEGEND COLUMN HEIGHT B • B •
80 mm. 120 mm. 160 mm. 200 mm.
400
Figure 7.44: Frictional Force versus Aeration Air Pressure for Rice Flakes.
MILLET
LEGEND COLUMN HEIGHT B e a •
80 mm. 120 mm. 160 mm. 200 mm.
100 200 300 400
Air Pressure, kPa 500
Figure 7.45: Frictional Force versus Aeration Air Pressure for Millet
194
100 200 300
Air Pressure, kPa
LEGEND
COLUMN HEIGHT B • B •
80 mm. 120 mm. 160 mm. 200 mm.
400
Figure 7.46: Frictional Force versus Aeration Air Pressure for Wheat.
SAND
300
200-
100-
T 100 200 Air Pressure, kPa
LEGEND COLUMN HEIGHT B 80 mm • 125 mm B 165 mm
300
Figure 7.47: Frictional Force versus Aeration Air Pressure for Sand.
195
o
CO
c o u
100 200 300
Air Pressure, kPa
LEGEND
COLUMN HEIGHT
B 55 mm • 80 mm
400
Figure 7.48: Frictional Force versus Aeration Air Pressure for Shirley Phosphate.
BROWN RICE
0)
o o LL CO C
o o 'Z LL
LEGEND AERATION PRESSURE B o • 113 B 197 • 281
kPa
• i i
100 150 200 250
Column Length, mm.
Figure 7.49: Frictional Force versus Column Length for Brown Rice.
WHITE RICE
-H. -
30-
20-
10-
§ e
B
B
I •
• i i i i i i
50 100 150 200 Column Length, mm.
LEGEND AERATION
PRESSURE B o
• 71 B 113 • 197 B 281 n 366
kPa
250
Figure 7.50: Frictional Force versus Column Length for White Rice.
RICE FLAKES
4U -
30-
20-
10-
0-
n
B
i
— 7 — 1 - 1
B
e D •
• i • • •
B
o
o
— T — | — 1 1
50 100 150 200
Column Length, mm.
LEGEND
AERATION PRESSURE
B 0 • 113 B 197 • 323
kPa
250
Figure 7.51: Frictional Force versus Column Length for Rice Flakes.
MILLET
LEGEND
AERATION
PRESSURE
B 0 • 113 B 197 • 323 B 408
kPa
• • •
150 200 250
Column Length, mm.
Figure 7.52: Frictional Force versus Column Length for Millet.
WHEAT
B
. -i | i i i
100 150 200 250 Column Length, mm.
LEGEND
AERATION PRESSURE B 0 • 70 B 113 • 197 B 281
kPa
Figure 7.53: Frictional Force versus Column Length for Wheat.
CD O
o LL
CO C
o o iZ LL
300
200-
SAND
100-
60 T • I ' I • I r
100 120 140 160 180
LEGEND
AERATION PRESSURE B o • 70 B 113 • 197 B 281 kPa
Column Length, mm.
Figure 7.54: Frictional Force versus Column Length for Sand.
CB Q. ._-(0 OT LU OC tr < LU X CO LU (3 < OC LU > <
n -in -
i
0.45-i i
0.40 -
i
•
l ' B
• B
O
a
B
• •
B
•
B
BROWN RICE
•
B
•
B
•
B B
• • O
B
0.35-f T T 100 200 300
Air Pressure, kPa
400
LEGEND
COLUMN HEIGHT B « B P
80 mm. 120 mm. 160 mm. 200 mm.
Figure 7.55: Shear Stress versus Air Pressure for Brown Rice.
199
WHITE RICE
CO OL
CO (A CD r .
tin
CO 0) JZ
(0 a> at to w 0) > <
u.o-
07-1
0.6-1
< 0.5-
0.4-
f B
P
i B
> •
•
B
B
•
•
B
B
•
•
B •
9 B •
B
•
B •
— r
B
P
B
•
B
• B
•
100 200 300
Air Pressure, kPa
LEGEND
COLUMN HEIGHT B •
B
•
80 mm. 120 mm. 160 mm. 200 mm.
400
Figure 7.56: Shear Stress versus Air Pressure for White Rice.
RICE FLAKES
CO
a. -C
<n </> 0
</5 CO 0) JC
Ui Q>
at co _. 0) > <
U.b-;
1
i
1 0.4-
0.3 J
0.2-
>
> 5 o B
B
• B
•
T
B
P B
•
ft
B
•
1"
Q
B
•
G
m P
l
B
B
•
Q
B
•
r
100 200 300
Air Pressure, kPa
LEGEND
COLUMN HEIGHT B
•
B
•
80 mm. 120 mm. 160 mm. 200 mm.
400
Figure 7.57: Shear Stress versus Air Pressure for Rice Flakes.
200
co a. -C CO
n CD i-
55 CO
o £ CO 0)
cs CO i_
0) > <
1.0-
0.8-
0.6-
0.4-
0.2-
0.0-
I
B
B
; : : B H
-" 1
B
• • B
B
• B
i
B
•
1
B
• • B
— r
B
Q B
i i »
r r 1
100 200 300 400
Air Pressure, kPa
LEGEND
COLUMN HEIGHT B
•
B
•
80 mm. 120 mm. 160 mm. 200 mm.
500
Figure 7.58: Shear Stress versus Air Pressure for Millet.
SANP
CO D. -e co co 0) tm
<0
CO o x: Ui 0 at CO -. 0 > <
10
8-
6-
2-\
0
B
1 • 1
100 200 Air Pressure, kPa
•
B
B
a •
a
•
B
LEGEND
COLUMN HEIGHT
B 80 mm. • 125 mm. a 165 mm.
300
Figure 7.59: Shear Stress versus Air Pressure for Sand.
201
SHIRLEY PHOSPHATE
CO
CL -C
CO 0
CO V JC
CO 0> O) CO tm
0 >
4-
3-
2-
B
• B . • •
T — I *-
100 200 300 Air Pressure, kPa
LEGEND COLUMN HEIGHT B 55 mm. • 80 mm.
400
Figure 7.60: Shear Stress versus Air Pressure for Shirley Phosphate.
BROWN RICE
mm
0.08-
0.06-
i
0.04-
l
0.02-
0.00-1
1
1
8
•
B
B
•
•
T r " —
8 •
B •
1
e a
•
i
•
B
LEGEND
COLUMN HEIGHT
B •
B
•
80 mm 120 mm 160 mm 200 mm
100 200 300 Air Pressure, kPa
400
Figure 7.61: Aeration Air Pressure versus Wall Friction Factor \ik for Brown Rice.
202
-C
U.f-
0.3-
0.2-
0.1 -
I
5
B
•
9
B
•
9
B
P
B
—r
B
•
8
B
# •
8 8 — i * > '
a
• B
- f — • 100 200 300
Air Pressure, kPa 400
LEGEND COLUMN HEIGHT B
•
a •
80 mm. 120 mm. 160 mm. 200 mm.
500
Figure 7.62: Aeration Air Pressure versus Wall Friction Factor uk for White Rice.
RICE FLAKES
It,
LEGEND
COLUMN LENGTH
B •
B
O
80 mm. 120 mm. 160 mm. 200 mm.
200
Air Pressure, kPa
400
Figure 7.63: Aeration Air Pressure versus Wall Friction Factor uk for Rice Flakes.
203
mm
0.5
0.4-
0.3-
0.2
0.1 -
0.0 I
B B
P P •
B • 100 200 300
Air Pressure, kPa
LEGEND COLUMN LENGTH
B 80 mm. • 120 mm. B 160 mm. • 200 mm.
400
Figure 7.64: Aeration Air Pressure versus Wall Friction Factor |ik for Millet.
WHEAT
LEGEND
COLUMN HEIGHT B
•
B
O
80 mm. 120 mm. 160 mm. 200 mm.
200 300
Air Pressure, kPa
400
Figure 7.65: Aeration Air Pressure versus Wall Friction Factor u\k for Wheat
7.10 DEAERATTONr
SCALE
PRESSURE TRANSDUCER (PERMEABLE BASE
DEEAERATION CYLINDER
T CHART RECORDER
Z E R O CONDITIONING UNIT STAND
PRESSURE TRANSDUCER (IMPERMEABLE BASE)
Figure 7.66: Deaeration Tester.
The details and operating procedure for this test and test rig are described in
Chapter 6.13. Figure 7.65 depicts a typical pressure variations during filling of fly ash
'A', 'F and 'G' for a permeable base. The subsequent respective, deaeration
characteristics of the powders tested in Figure 7.67 are depicted in Figure 7.68, when
tested with a permeable base, whereas Figure 7.67 depicts the combined filling and
deaeration pressure variations.
In comparison, the filling and deaeration pressure variations of the fly ash when
tested in the deaeration cylinder with a impermeable base are depicted in Figures 7.70,
7.71, 7.72 and 7.73. The observed bed height versus deaeration time variation for the
fly ash studied when tested with permeable and impermeable bases are shown in Figures
7.74, 7.75 and 7.76.
205
PERMEABLE BASE
ou -
40-
30-
20-
10-
0 - B
D
J_
D •
B
B •
B
a
B
—r-
a
B
B
B
1
10 20 Filling Time, sec.
LEGEND
FLY ASH
B 'A' • F B 'G'
30
Figure 7.67: Pressure Variation During Filling; Permeable Base.
PERMEABLE BASE
T 100
Deaeration Time, sec.
LEGEND FLY ASH
-B- TV •P- F •a- 'G'
200
Figure 7.68: Deaeration Time of Fly Ash 'A', 'F and 'G'; Permeable Base.
PERMEABLE BASF
CO 0.
0 m.
3 CO CO
a>
T 100
Deaeration Time, sec.
LEGEND
FLY ASH
-a- 'A' -p- F -a- 'G
200
Figure 7.69: Deaeration Behaviour of Fly Ash 'A', 'F and 'G'; Permeable Base.
IMPERMEABLE BASE
co 0.
3 CO (0 0
DV -
40 -
30-
20-
•
10-
D
t
n
D •
B
D •
•
B
•
1
B
D
B
•
F
i •
10 20
Filling Time, sec.
LEGEND FLY ASH
B TV • F B 'G'
30
Figure 7.70: Pressure Variation During Filling; Impermeable Base.
IMPERMEABLE BASE
co a.
3 CO (0 0
FLY ASH
-B--•-
*
•A'
F •G'
-i | i 1 1 | i — i r-
0 20 40 60 80 100 120
Deaeration Time, sec.
Figure 7.71: Deaeration Time of Fly Ash'A','F and'G'; Impermeable Base.
FLY ASH 'C
co a -C
3 CO CO 0
T
100 Deaeration Time, sec.
200
Figure 7.72: Deaeration Time of Fly Ash 'C; Impermeable Base.
CO Q.
0 m.
3 CO w 0 km
0.
50 -i
4 0 -
3 0 -
20-
10-
0 -1
C
wr\.
*T \|ID
IB ^^^><^^k",*SL_
r • , - i |
» 100 20
LEGEND FLY ASH
-B- 'A'
-»- F •»• 'G'
0
Deaeration Time, sec.
Figure 7.73: Deaeration Behaviour of Fly Ash 'A', 'F and 'G'; Impermeable Base.
PERMEABLE BASE
E E
JZ OJ 0
z TJ 0 OQ
LEGEND FLY ASH
-a- 'A' •P- 'F -B-'G'
0 100 200 300 400
Deaeration Time, sec.
Figure 7.74: Deaeration of Fly Ash 'A', 'F and 'G'; Permeable Base.
E E
x
o
z TJ 0 OQ
40 60 80 100 120 140
Deaeration Time, sec. 160
Figure 7.75: Deaeration of Fly Ash 'E; Permeable Base.
IMPERMEABLE BASE
E E
"0 X TJ 0
m
100 200
Deaeration Time, sec.
LEGEND
FLY ASH
-B- 'A' -«- 'GV ••- 'G2'
300
Figure 7.76: Deaeration of Fly Ash *A' and 'G'; Impermeable Base.
210
A typical filling-deaeration graph obtained from the Deaeration Tester is revealed
in Figure 7.77 for fly ash 'A*.
/
mU m\ V 1 J
f t 1 £: £--
• • - • *
•
, *~_,_» :
- - t —
' L"
' !. :i
V
N , j .
• i
; , i-
, i
t
--i ^ t
••-.
'.
i
!
V " • « .
1 -
s
— 1
J 1
[
V j ] •->
1 1 1
t~-\ 1
— i —
— [ —
i
-1
•
. -- <
.r=f—
r - 1 i 1
( '
" . -*. — » ~-1
I
— 1 — . 1
f — 1 f -- 1 -—
_j. _l t ( 1
1 j
. .... .
4
1
— -
—--
_ ... .
Figure 7.77: A FiUing-deaeration Graph for Fly Ash 'A'; Impermeable base.
Fly ash deaeration behaviour with time are correlated with exponential regression
curve fits (y = a* 10*). The time constants, exponents a, b and regression coefficients
are presented in Table 7.14.
TABLE 7.14: DEAERATION TIME CONSTANT AND EXPONENTS
Fly ash 'A'
Fly ash 'F
B y ash 'G'
M A TERIA^ EXPONENTS a, band REGRESSION
COEFFICIENTS
PERM. BASE IMPERM. BASE
a b R a b _R_
28.10
23.50
14.88
0.0054
0.0043
0.0019
0.95
0.95
0.96
38.90
30.92
32.28
0.0096
0.0121
0.0079
0.99
0.98
0.97
TIME CONSTANT,
Seconds
PERM. IMPERM.
BASE BASE
88
j60_
65
_33.
16_
23
211
7.11 FLUIDIZATION AND DFAFR ATTnjy;
Transmitter,
©0
Plenum Chamber
/: -10mm Dia. Holes
jbnnna-r-_Perspex •**!• il'-^^Flanges
.102mm 1.0. Pyrex Tube
Perspex Flange 102mm 1.0. Steel Pipe Gas Distributor
Assembly
Pressure Irri Meter — H U J
Air Supply
Figure 7.78: Fluidization Rig. [Wypych etal. (1987)]
The details and operating procedure for this rig are described in Chapter
6.14. A schematic of this rig is presented in Figure 7.78. The location of the fly
ash tested when plotted on the Geldart diagram are as shown in Figure 7.79.
Fluidization analysis of the powders tested are depicted in Figures 7.80 to
7.85, whereas, Figures 7.86 to 7.88 depict the deaeration behaviour of the fly ash
tested in fluidization rig.
1 10 100 1000
Mean particle size, micron
10000
Figure 7.79: Geldart Fluidization Diagram Showing
the Classification of Fly Ash.
60
FLY ASH A'
50 -
40 -
30 -
20-
10-
0 -T-""? 2 4 6
Superficial Velocity, cm/sec.
Figure 7.80: Fluidization Analysis of Fly Ash 'A'.
FLY ASH 'F E o o CM
X E E c •o CD
O
m m.
3 n n 0 2 4 6
Superficial Velocity, cm/sec.
Figure 7.81: Fluidization Analysis of Fly Ash 'E'.
E o o CM
z E E c 0 TJ CO
o 3 .) <A 0
2 4 6 SUPERFICIAL VELOCITY, CM/S.
Figure 7.82: Fluidization Analysis of Fly Ash 'A', C and 'E*.
214
8-
6-
4-
2-
u (
^ • — M - M M
r J
•* • .
) 1 2
Superficial Velocity, cm/s.
Figure 7.83: Fluidization Analysis of Alumina.
SANP
Superficial Velocity, cm/s.
Figure 7.84: Fluidization Analysis of Sand.
PVC POWDER
Superficial Velocity, cm/s.
Figure 7.85: Fluidization Analysis of P V C Powder.
600
500 -?
400-
300
DEAERATION TIME, SECS.
Figure 7.86: Deaeration of Fly Ash *A' in Fluidization Rig.
510
500-
490-
480-
470
460 — I 1 \ 1 1 —
20 40 60
DEAERATION TIME, SECS.
LEGEND
EXP. NO. B 1 • 2 B 3 • 4 B 5
80
Figure 7.87: Deaeration of Fly Ash 'C in Fluidization Rig.
510-IT
500-
490-
480-
470
LEGEND EXP. NO. B 1 • 2 B 3 • 4
20 40 60
DEAERATION TIME, SECS.
80
Figure 7.88: Deaeration of Fly Ash 'F in Fluidization Rig.
217
7.12 PNEUMATIC CONVEYING FLOW CHARArTERTSTTrs-
(A. STURTEVANT PNFTTMATIC CONVEYING RTOr
FRICTION LOOP-
LEGEnD
Channel Ilo.
JJ 3 --c*
-ilo-
O — Pressure Transducer
1.91 m.
-o-
0.4 m.
1.41 m.
o-1
1.41 m.
11
0.4 m.
Figure 7.89: Friction Loop.
A schematic of this test rig is depicted in Figure 7.89. This rig incorporates seven
air pressure tapping locations. Six of these pressure tappings were used in the friction
loop as shown in Figure 7.89. In these tapping locations, pressure transducers were
installed with the transducer output recorded using a Data Acquisition System.
The pressure tapping details are shown in Figure 7.90, whereas the Data
Acquisition Channels details are summarized in Table 7.15. The transducer locations,
measured from the blow tank, are declared in Table 7.16.
The materials tested in the friction loop were cement and Wheat The flow
properties of these materials are declared in Table 7.17.
218
Pressure Transducer
Retaining Screw
Porex Disc
Quick-Connect Coupling
1/4" BSPT Thread
O-Ring
1/4" BSP Socket
52mm I.D. Pipeline
Figure 7.90: Exploded View of a Typical Pipeline Air Pressure Tapping Location.
TABLE 7.15 - AIR PRESSURE CHANNELS
Channel
number
0
1
2
3
4
5
6
7
8
9
10
11
Nam e of channels
Blow tank pressure
First pipeline pressure
First friction loop pressure
Second friction loop pressure
Third friction loop pressure
Fourth friction loop pressure
Fifth friction loop pressure
Air supply pressure (orifice plate)
Differential pressure
Silo load cells
Blow tank load cells
Sixth friction loop pressure
TABLE 7.16 - TRANSDUCER LOCATIONS
Transducer
First transducer
First friction loop transducer
Second friction loop transducer
Third friction loop transducer
Fourth friction loop transducer
Fifth friction loop transducer
Sixth friction loop transducer
Downstream
location, m.
0.15 m.
4.68 m.
6.06 m.
7.45 m.
10.17 m.
11.55 m.
12.94 m.
TABLE 7.17 - MATERIALS FLOW PROPERTIES
Material
Cement
Wheat
Solid density,
kg/m 3
3100
1380
Loose poured
bulk density,
kg/m 3
950
864
d50,
|j,m
30
313
220
A summary of the test results are summarized in Tables 7.19 and 7.20. In this test
series, the particle velocity was measured by using a Tealgate T.200 series transducer in
combination with a Hewlett Packard Correlator, refer Chapter 6.2.
For the next test series, the air pressure channels and transducer locations details
are shown in Table 7.18, whereas Tables 7.21 and 7.22 summarize the results of
conveying characteristics of sand. The flow properties of sand are declared in Sections
7.3 to 7.5.
For experiment No. 1 to 49, the Tealgate T.200 series transducer was used in
combination with a Hewlett Packard Correlator, whereas for experiment No. 50 to 82, a
purpose built fibre optic velocity probe was used, refer Section 6.2.2. The variation of
transducer air pressure for sand versus distance from blow tank are shown in Figures
7.91 to 7.98.
TABLE 7.18 - AIR PRESSURE CHANNELS A N D TRANSDUCER
LOCATIONS
Channel
number
0
1
2
3
4
5
6
7
8
9
10
11
12
Name of channels
Blow tank pressure
First pipeline pressure
Second pipeline pressure
Third pipeline pressure
Fourth pipeline pressure
Fifth pipeline pressure
Sixth pipeline pressure
Air supply pressure (orifice plate)
Differential pressure
Silo load cells
Blow tank load cells
Seventh pipeline pressure
Eighth pipeline pressure
Transducer
No.
0
1
2
3
4
5
6
7 _
9
10
11
12
Downstream
location, m.
0
0.15
3.51
4.22
6.44
14.76
18.08
-
.
.
-
25.37
29.62
221
H Z w 2 w ,.
c« U HH
HH
P-J w
z ><
Ti
o
•
9S •
w 0Q
r-j
5*
e.
Sa <u 3 CA CA
6, •s O
a
1 c
1
jr Q
cm
*
a
<*-T
a
#*
6
HI CA
•3 -
6 I—I
8 .£3
u
»—i
1—1
VO
un
Tt
cn
cs
•
o
.£ o g a 1 a
8 CiO
00
l-CA -^ DO
M
6 Z
o cs Tt
1
1
OO
o 1—1
o ,—« 1-H
cn *""!
vo 1-H 1—1
00 CN 1—1
00 cs r-H
1
o cs cn
cs cs o d
Tt r-l
r~r
d
cs
un un cn
i
i
o oo
00 oo
00 oo
o ON
vo ON
1
—m
«t cn —m , — < © d
00 un cn d
cn
un cs Tt
</-> vo
m VO
VO vo
© »•
cs r-
un i--
vo f*
VO r
i
vo d cn
m »-< o d
00
o Tt d
Tt
1—1
"t
CS . — i
i—i
cs 1—1
1—1
cs 1-H
r-l
CS cs 1—t
cn cs 1—1
o cn ^
un cn i—i
un cn r-l
ON
o un d
00 »—i
CS
un cn o d
VO un r-d
un
un cs cs
un vo
un vo
un vo
© 00
cn 00
VO 00
p» C\
r-r ON
1—4
ON
Tt
00
cn cn
cs cn o d
i—i
oo © i-H
00
8 cn
Tt un
Tt un
Tt un
» •
un
oo VO
un ir
es ON r>
un 00
cn vo Tt Tt
> d Tt
00 —m © d
un Tt r-d
cs —-1
8 cs
0\
1-H
Os
cn Os
cs 1-H
1—1
cs 1—1
'"H
CS CS i-H
t-» cs T—1
VO cn »—i
Tt OO
O VO
vo ON 1—1
cn vo O d
ON Tt CS i-H
cn i—i
8 cs
w> r»
un t--
un » > •
o Os
o Os
un ON
un o i—i
o 1-H
1-H
i-H
00
cs r
cs cn 't
cs cs o d
vo cn ON
d
Tt r—1
o oo CS
o oo
t Os
CS r-
cs Os
CS ON
8 i—i
c-o i—i
CS T—1
1—1
un
s vo'
cn un Tt
i-H
CS o d
Tt ON
d
un r—1
un cs cs
o Os
o ON
8
o 1 — I
1-H
CS
i-H
i-H
CS 1—1
r-cs i i—i
o cn '""'
00 ON
r-Tt
t-; ON i—l
i—l
vO o d
vo ON , — i
1-H
VO
222
<
W X
•
C/5
U NH
H C/3
NH r-l
w H U <<
X u CJ
z HH
irr]
z o u * • e cs •
i-
W rJ
H
=6 O ca .Ss <
r-4
U
ei
a r
CA CA
S OH
•s CA
CJ P
•a
rH lj
c Or
*
a
r 0?
a
fr 14
J2 -0. a
CJ
a
1 li Cr
r-5
u
s-. *»-H
8 13
CJ
CA
^^ bO M <j <t> Q_
00 M
JO z
t/3
Si
i-H H
vo
un
Tt
cn
cs
1-H
©
CJ <L> CA ---.
a
Tt Tt »-r—1
OO »>• Tt cs un
un © 4 ©
CS
un cn cs
© i-H
cn
cs un
Tt un
vo un
8
cs vo
3
un r>
un oo
cs un un vo
oo r-r. Tt cs
un o ©
o cn CS i-H
cs cs
i-H
Tt r-; oo i-H
un cs cn
vo un
r-un
8
8
oo VO
cn
oo r-
cn 00
ON
© Tt un
un © d r» i—i
Tt 1-H
cn cs
r-un i-H
© cn
un cn cn
cs un
Tt un
vo un
©
cs r-»
© oo
un oo
© ON
un un cs 00
vo cn un 1-H
un cn vo © d un »>> ON ©
un cs
ON VO cn i-H
m
© cn cn
© r-
cs r-
Tt i>
VO 00
oo oo
© ON
un © <-H
© l-H 1-H
cn cn vo © cs
3 © ©
i-H
cs cn 1-H
i-H
cn
Tt r-r
cs' cs
un i-H
cn
© r-~
cs t-T
Tt r~
vo oo
oo oo
© ON
un © i-H
© 1-H 1-H
VO CS
oo i-H
00
vo © ©
Os ---cs 1-H
CS cn
un Is-vq cn cs
8 cn
s
3
VO
r-
VO
r-
oo lT-r
OO
© ON
8 —m
Tt c-© un cs
Tt un o © Tt VO cn 1-H
Tt cn
r-cn cn cn cs
©
cn
vo VO
OO
vo
©
r-
cn oo
un oo
un oo
un Os
VO
© i-H
S Tt un cs
Tt Tt © ©
oo i-H
i-H
1-H
un cn
223
Q Z < 73 r-.
o u HH
H HH
W H O
< K U O z ><
>
z 0 u 1-H
<s •
r--
<
3 sf; u m P 0
u. < Cfl
P s
§ 9 » '
r\ ^
3. ° s B 0 & H ° rH [JJ
1 s s K 2 ffl a < 2 u P w
P *
_\ $ rU
Hi U rj S
0 H
C-. ^
5 1 < J PM
2 CD P
!
a w Cfl
P u, <
o
1 s
3 Cfl
a n 8
1 o Fi
I 2 ^
l
r-r** y — * *
si
a 3
t-JQ •r-4
Sf
8 "-2. J?
B I H 2
RS
UJ 2
q d
90 CN
<s 00
<s <s
ON t-> ON
CN
ao
O
CN
CN
i
cn
l>
CN
» ao (N * •
O >n -*
H H
CN
t-vO
o o ci
NO
r-CN
TI
CN
*t ON
CI
v£
Tl
ao O o
as Tt Os
©
00 CN
o\
1
(N
H
o ON t—l
1
Tl
t-r—l
CN CN
t» CN O
o d
CN
Tl
00 Tl SO
TT
as Tl
d
NO 00
o o
- H
~-* ON
d
o oo CN
CN
i
Tl NO
"""
c*> CN
r-Tl
o d
Tl Tl Tl
ON
o
CN
Tl
NO
Tl O
O
t m CN
—"
00
1
(*>
NO
NO
1
o Tl
~*
Tt CN
Tl fr CN o o
d
Tl Tl Cl
d CN
CN 00
Tl CN
CN NA Tl
Tl O
d
i-—. CN
—'
• *
CN
O Tf
1 !'
Cl
NO
CN
NO
CN
"* q <*
© Tl r-l
Tl
CN
oo Tl NO © ©
d
Tl t -(r-
d CN
00 rM, SO
d CN
ON
NO
Tl
"" d
NO ON
r-d
ao CN
00 NO NO
Tl
NO Tl ON
CN t
Tl
r-"H
NO
<N
00
<N
NO
o «*•
T) Tf NO
r-Tl 00 cs
en •<T
Tl
rs ""'
» fN
224
Tl
8 d
23.632
33.414
3 vo
Tl
o d
oo NO
d
H-1 NO CO
Tt r-t 1
in
f—1
SO CO NO C-;
co Tt
<n CN HH
a
s rH
© d
r-ON Tt
d CN
NO Tl CO CN
NO
r-l
r~ 1 o d
CO CO CN -m
r-l CN CO
CN i
CO
s Tl NO H
rH
CO
CO Tt
o V"l H H
CN
ON
8 d
NO ON
r-
a ON Tt CO
s
CN 1—1
r-l
8 i-H
d
oo CO f-H
1—1
1—i
CN
co
00 CO
1
CO
CN NO t-
0C CV
s oc CC
c T
c c
3 i-m
O
d
CN
m CN i — l
ON H
r» in in
NO
CO H
(H
o d
ON
r--co
CO
CO
o 1
CO
00 Tl
q w-i rH
r-> Tl 1 00
• cs N 1—1
j 00 N. CO
- -3 1 CN H i-H
5 r2 o co
I-H
© CO
8 >o
in NC i —
o<
OC
c c
c c
s HH
o d
NO NO ON"
rH
Tt Tt ON
ro CN
o CN i-H
CO ON
O
d
00 00 rH rH
CO 1-H 1
CO
HH H-
c N£ 1—
r» > C*-
< t-
1 c
. c 1 T H. r-
Nl C 1 c
CO
CN HH
CO
oc o SC VC
1 T T
' C 1 T)
3 C • v
1 T 0 c
CN
.-^ O
d
CO
r-m NO
Tt
o r-; —m CN
O N
CO H
00
o d
CO
cs 1—1
r H
NO in
i
CO
oc rn
T Tl T
? a > t-
i T
r TI
3 C
o J!
NO Tt 1
CO
r-•— o NC
V
1
1
•
? s H r
1 NJ
0 c
Tl HH
o d
Tt HH Tl Tl
NO
m t-r
00
CO
o 00
o d
1-H
CO
rH CO
CO
c-V"
r-' NC * i —
3 C 1 •/
D r o c
NO HH
o d
CO
r-NO Tt
NO
r--NO Tl i-^
o 00
Tl
r--o d
CO in CO — H
-1 es CO
c •— t-V 1-™
3 1
0
o c
Tt HH
O
d
00 NO
cs Tt
CN Tl Tt Tl r-l
CN
r--
C-r
o o d
Tt Tl i—i
1-H
cN i
CO
CN
i Tt t r-
c 1/ H
o c o c
r-r i—i
o d
H H
»-H
CC
CO
HH
Tt Tl CS CN
oo CN r^
Os SO
o d
NO 00 00
d
H CN CO
r-i-^
Tt
OC NC T
r-i—
3 C 1 u H r
r c o -
0.009
0.014
0.015
0.019
0.014
0.018
HH Tt ON TN CO C^ 00 r- t>- ON 1-" HH Tt O Tt Tt ON Tl t> SO r-I col —" wi HH rH HH H H I ^H HH
ON HH co r—| CO CN ro NO NO -ri TI oo TI ON Tt en oo q w-i oo oo cs" t-r oo fS i—' H-t HH HH HH
00 ON ON CN CN CN ni r- oo CN Tt" ON HH HH I—1 CN H H
r~ i-- vo <-> r- oo
o o q q q q d d d d d d
SO T * N O r— Tt C3N
r-- CN I-* o * rr 0O CO ON Tl ON NO.
o HH' d —< d --
CN Q CN O O TJ-; rf T? CO CO CO CO
SO © 00 O vo ON - ON O Tt £ M-
1 ' Tl Tt Tt CN col CN
m r~ ON Tt CN cs i oo oo co co cs 3
rS vg T I r-# CN co oo so Tt* cs -* so
, _ -H H rH H i—1
00 ! CO CO 00
co co ON <n OS ON t— 00 O St 00 SO CS CN Tt. ,* r^ _J H SO CO 5 CO CO CO CO Tt
3 m vn >n O <£> TI
o cs cs cs r- £ •> -t i~\ ^-i *—- *** ^ ^
-, ^ cs co 3 wj so f| Tt Tt Tt| 't Tf Tt
225
© ©
i—i
ON
© r-l i-H
CN 00 © 00
r-cs ON I—I
r-r
r-©
d
CO O N
ON
©
oo cs Tt
m so mm ON —m
Pr 1 — t
m mm
co Tt
«n CN 1-H
TT
mm
© CO
ON CN
i
CO
CN NO NO
r» 00
co 00
wo 00 CN
Tl CN
00 Tt
©
q mm
m SO CO
00 Tl
i
CO
1
00 CN mm
© Tt
w-> wo rH
TP
O CN
© ©
00 ON Tt CN mm
ON
co CN
r-wo SO
d <N
© ©
3
3 CO
CN Tt 1
CN
8 ^ " r |
rH Tt in CO
© i-H
© •n
00
o ©
C3N 00 ON
CN
00 Tt"
ON
CO
00 mm
CO NO
m © d
cs
8 1—*.
NO CN CO
Tl Tl
i
CO
ON Tl Tt
as ©
CN © NO CN
m Tl
1-H
wo
CO
© ©
CO CN CO ©
00 Tt mm CO
oo m o ©
NO CN 00 i-H
SO CN CO
o 1
CN
co SO
vo
CN
co Tt
wo
Tl Tl mm
CN Tl
Tt CN ©
©
m m CN i-H
Tl i-H
00 Tt i-H
CN
Tt* CN
i-H
r-© ©
cs t-;
00 CN CO
CO 1-H 1
CN
00
r-CO 00 i-H
CN
in* CO
Tl
r-
co Tl
Tl CN 1
CN
r-
8 CN
w-i
r-r
r--© Tt © Tt
© 00 i-H
X
Tl ©
Tl CO l
CO
Tl CN
Tl
SO Tt WO
SO Tt 1
CN
Tl Tl 1-H
00 Tl
Tl
© CO
© ©
ON 00 mm
WO fr
so
© i-H
© ©'
r-r CN i-m CS* CN
CO CN ON ON
Tt
ON
t> mm
Tl ON
cS ©
Tl
CN*
CS CO
CO 1
CN
WO
r--
co NO
CN Tt mm © ©
CN Tt Tt
wo mm
T) CO SO • * "
1-H
q w-i mm
in 00 ©
d
NO
CN
Tl Tt 1
CO
Tl SO
3
r-© Tl 00
Tt
©
ON
3
Tt CO
1
CO
© Tl
S
Tt CN O
d
ON mm CN* 1-H
Tt mm
©
NO
r-* CN
00 1-H
s d
Tl
o mm
CN CO
CN
i
CN
© Tl
s
CO ON ©
OO i-H
CO
Tl CN •
Tt
r-Tl
© r--
Tt
cs o ©'
Tl CO NO CN
00 r-r
©
ON
O
oo" es
CO NO ©
d
ON NO t-; mm
© CO
mm\
w-i 1
CN
wo SO CO
r--
oo oo 00
es
Tl CN mm
r-r
Tt
© ©
8 Tt Tt
Tt O N
Tl
r~"
O N
wi mm
Tt t-
o ©
CN 00 HH 1—1
CN CO 1
CO
r-00 ON
00
1
Tl CN mm
CN I--
CN I—I
CO
wo"
mm mm
©
8 mm
o Tt
CO
C-T.
00 Tl 00
i
WO
cs
CO t—
CO HH
© ©
Tt
r-© O N
CN O N
CO
Tl
r--1—1
Tt 00
o ©
Tt i-H
ON CO
i
CN
Tl CO
d mm
© in mm
?
rr--l
o ©
oo CO
Tt" I-H
r-
8
CO ON CO
co' 1-H
00 © ©
r--©
3 Tt. 00
1
© Tl i-H
Tl C--
ON
8 ©
NO 00
cs i-H i-H
i-H
r-co ON
CS
S d co
3 ©
d
Tl CO ON i-H
8 i
CN
CO
co Tt.
r-
o wo mm
£
WO 1-H
© ©
i o
f-H
©
00 ON
CO
Ti
ON NO
© ©
CO
i-H
© 1
CN
oo Tt.
r--
1
wo »
CO mm
© d
CO
oo ON CN
i-H
O N
CN CO
i§
CN 00 © ©
r-Wl O N
©
i-H
WO CO
CN
ON CO
©* Tt
Tl
00
r--
CN mm
! ©
d
Tt
w-i
oo mm
CN Tf"
00 CO CO CO
CN 00
© ©
!
^-1
1
CN
ON
oo I-H
ON
SO CO
wo © Tt
o 00
ON
r-
o mm
© ©
CO CO Tt
CO CN 00 00
ON
s Tt
i-H
NO
© ©
r-wo 00
©'
CO CN
i
Tt
Tt
r-00 C--
00 CN ON
SO* CN
WO CN
O oo
Tl
© ©
WO 00 ON CN*
es mm
CO
WO NO ON 00
NO
© ©
wo 1—(
1-H
© CO
SO CN
CO
8 r-. 00*
J5
wo cs
00
1-H
CN © ©
1-H
Tt
co mm mm
00
Tt"
CN ON
Tt
r-© ©
CN
Tt i—t
© CO
1-H 1—t
1
CO
SN © ON
00 00 CO
wo CO
CN 00
227
TABLE 7.22: TRANSDUCER AIR PRESSURES, kPa
EXP.
NO.
21.
22.
23.
24.
25.
26.
28.
29.
30.
31.
33.
35.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
49.
50.
51.
52.
53.
57.
58.
62.
63.
CHANNEL NO.
0 1 2 3 4 5 6 11 12
110
120
113
105
105
120
80
105
95
112
100
95
105
105
90
80
75
86
102
85
84
130
155
152
100
124
146
129
107
117
110
101
101
115
72
102
90
107
96
90
103
101
84
75
65
80
76
98
81
113
82
90
126
90
148
148
98
120
136
125
120
130
120
115
115
125
88
110
105
120
105
100
110
107
90
78
80
95
82
110
110
90
82
120
85
140
138
91
112
127
123
85
100
85
75
75
85
50
70
65
75
65
60
70
70
53
50
.
55
48
70
70
70
50
112
133
90
110
125
121
65
75
70
60
60
68
47
40
55
57
55
50
60
60
50
45
40
48
44
60
60
62
42
50
i 75
50
90
I 87
56
70
80
78
62
70
65
56
56
65
45
38
52
55
52
48
58
58
48
42
38
45
42
55
55
60
40
1 45
68
! 45
80
! 80
53
67
75
70
56
60
55
60
60
60
40
50
45
52
50
42
50
50
40
38
35
40
38
50
48
55
35
40
62
40
70
70
50
60
.
68
46
50
46
42
42
46
31
40
36
42
38
32
38
39
32
28
.
-
25
.
32
42
28
34
48
34
50
55
36
46
56
48
42
46
39
38
38
42
29
37
34
38
34
30
35
36
28
26
.
-
29
-
28
40
26
28
42
26
40
50
33
42
48
42
228
64.
66.
67.
70.
71.
72.
73.
74.
75.
76.
77.
78.
79.
80.
81.
82.
115
100
110
72.5
_
87.1
100
130
115
132
132
123
115
70
92
105
112
90
106
70
100
85
97
122
112
129
128
120
111
68.6
86.6
98.3
107
85
97
66
97
80
89
113
105
120
120
110
106
64
80
90
105
83
95
64
65
78
87
111
103
118
117
106
105
61
78
88
70
50
62
42
.,
50
56
70
63
75
75
66
! 65
40
50
56
65
47
56
38
_
47
52
66
60
70
70
60
i 60
37
46
54
60
43
52
34
33
44
48
62
55
65
65
55
56
34
44
50
46
34
40
28
29
.
36
48
42
52
52
46
44
_
34
38
40
30
34
23
.
33
44
38
44
44
40
40
_
28
32
PARTICLE CONCENTRATION: This was calculated by using eqn. (C.5), refer Appendix C with the evaluated
values declared in Table 7.21. Also, a typical solids concentration variation during
experiments obtained from the Tealgate T.300 concentration sensor are depicted in
Appendix C.
BENDS: The pressure drop obtained using two different bends, namely long radius and
vortice elbow are shown in Table 7.23. As evident in Table 7.22, air pressure channels
No. 11 and No. 12 for experiments 24 to 82 are for vortice elbow bend.
TABLE 7.23: BEND AIR PRESSURE, kPa
Exp. No.
51
56
20
21
22
23
Channel No. 11
48
34
38
46
50
46
Channel No. 12
40
29
33
42
46
42
Type of Bend
Long Radius
Long Radius
Long Radius
Vortice
Vortice
Vortice
CB 0-
a M 0 0
0 O a •o
n c a I-
140
120-.
100
80-
60-
40
20
f
10 —r-20 J 30
LEGEND Exp. No.
B 21 • 24 a 29
Distance from blow tank, m.
Figure 7.91: Transducer Air Pressure versus Distance from Blow Tank.
a a. mm
3 0 0 0
140
o 3 TJ M C a
120
100-
80-
60-
40
20
3
* B P
B P
—r~ 10
5 I
LEGEND Exp. No. B 35 • 40 • 74
0 10 20 30
Distance from blow tank, m.
Figure 7.92: Transducer Air Pressure versus Distance from Blow Tank.
0 Q. -c
3 0 0 0
O O 3 •o 0 c 0
uu -
80-•
1
60-
40-
1
]
r
1 P
a B
I •
I Q
D
1
B B 1
E B
10 20
Distance from blow tank, m. 30
LEGEND Exp. No. B 28 • 41 B 47
Figure 7.95: Transducer Air Pressure versus Distance from Blow Tank.
0 CL -C
3 at 0 0
100
o o 3 •o 0 C 0
LEGEND Exp. No.
B 42 • 72 B 82
Distance from blow tank, m.
Figure 7.96: Transducer Air Pressure versus Distance from Blow Tank.
h 1
- 1 — 10 20 30
LEGEND Exp. No. B 26 e 46 B 79
Distance from blow tank, m.
Figure 7.93: Transducer Air Pressure versus Distance from Blow Tank.
\&J -
100 -
•
80-
60-
40-
C
1 B
i u
B
'' - 8 B
i • i
10 20
Distance from blow tank, m.
El 41 B
3
LEGEND Exp. No. B 71 e 73 B 81
0
Figure 7.94: Transducer Air Pressure versus Distance from Blow Tank.
232
(B) LOW VELOCITY CONVFYTNC- pTrrT
The relevant test conditions for this test series including data channel details and
pipeline details are declared in Tables 7.24 and 7.25, respectivelty. A summary of the
test results for the low velocity conveying rig are shown in Table 7.26.
TABLE 7.24: DATA CHANNEL DETAILS
CHANNEL
NOS.
0
1
2
7
8
9
10
NAME OF CHANNELS
Blow tank top air pressure
First pipeline air pressure
Second pipeline air pressure
Orifice plate upstream static pressure
Differential pressure !
Blow tank load cell
Silo load cell
TABLE 7.25: PIPELINE DETAILS
Series
No.
A
B
Experiment
Experiment 1 to 20
Experiment 21 to 30
Horizontal pipeline length,
m.
First loop Second loop
3.11
3.11
87.51
41.71
Vertical
pipeline,
length, m.
6.48
6.48
Bend Details
Number Equivalent
length, m.
11
7
44
28
233
< f-i ts
a I-N
o z w >• o u
o rj
•> 1
O
NO
cs •
-i a <
-? &
CO (Zi
ft.
Br <
ti
"3 £
• ;
Ui
s
:.
in
s
OT 6
' i ,
a
00 <
s
H W CO
g 3 ffl
OS CO
-
E
5F
* 6
"§ r-i
c/a
""-" bC M
m " 5
« ^
8
CO CO
a s i «
H
1
CN r-S
u I-H
-ei U
o
d 6
i ti
CO o
HH
in BO r-i
o ri CO
E-r •
ffl
V) M
r > r
« 2a
B CO CO
a i-H
00
8 d
8
8 l-H
H H
^
CS l-H
l-H
o Tt
uo d
NO r-©
ON
© ©
o m H H
o cs 1-H
cs
o r-
NO CO ^H
o <n
© 00 o
Tf l-H
1—1
s; in Tt r-_ r-* Tt
m m w-> d
CO ON ON
d
H H
cs o d
-t 00 NO cs" cs
8 cs
8
CO
CO l-H i-H
NO r--l-H
o OS
o IO m
00 Tf 1-; ^
CO CO cs
i H
cs IO C-* CO
o
8 cs"
cs l-H
ON d
CO •rt
cs o d
ON NO mm
m'
o in cs
© 00 l-H
Tf
o CO
8 cs
CO cs cs
o cs Tf
CO TI
CS
o io <N
cs cs CO Tt' cs
l-H
IO ON d
CO o 00 d
CO CO
© d
Tf t-; V*
8 CO
o cs cs
\n
%
cs TJ-CS
3 cs
cs CO CO
uo o ON cs"
Tt 00 r--cs CO oo Tt NO cs
00 cs —<
Tf
p mm:
oo CO
o d
m CO CO
*n
© in co
o r cs
NO
o ON I-H
o •>•
CS
3t ON CS
in mm CO
NO ON CO Tf*
00 Tf CO
CS ON CO Tf"
NO o ON d
3 d
•n s d
NO Tf co in*
8 Tf
8 CO
r-
o CO HH
o F-H
CS
© CS cs
o ON Tf
m i—i
00 1—1
s cs
in r--cs Tf"
in ON Tf d
l-H
r~ Tf
d
CO CO
o d
NO CO CO
vd
o •n Tf
o <N CS
00
g
o CO 1—1
© Tf cs
o ON Tf
m l-H
ON 1—1
OO in cs
NO ON oq in"
l-H
f-d
m cs m d
CO CO
o d
NO CO NO P-
8 m
s cs
ON
O t-r l-H
o m cs
m NO CS
NO CO Tf
CO •00 ON
cs"
Tf Tf CO
cs
oo o CO f r
m l-H
OO d
cs ON NO
d
s d
cs in 00 f-
o m m
8 cs
o
o ON
CO NO CS
NO 00 cs
o l-H
Tf
1
1
CO CO CO
cs r-r r--d
•n NO m d
Tf CS
3 d
in in CO 00
g NO
O ON CS
l-H
in mm NO
CS in CO
~*
ro CS
r--vo CO
o d
o in NO
8 cs
cs
o in
in cs cs
sn CO cs
o 00 Tf
1—1
CO tr-
^
oo CO cs
in fr-
CO r—t
^m ON NO d
•n ON Tf
d
NO CO
p d
in FT; NO
o •n cs
CO
r-1—1 p d
1 cs
o Tf
Tf
o CO 1—1
o Tj-
CO in
o
s
cs
s CO ro
^m t-r
»n d
r-00 f-
d
CO cs © d
CO t-r
ON © CO
8 cs
8 i — <
in
© Tf mm
CO
CS
r--mm
CS
8 Tf
00 00
cs co m
CO
o ON mm
CO © CS
1-H 00
co ©
d
cs ON 00 1-H
8 CO
©
VO l-H
©
NO CS
cs
S
m NO CO
oo l-H
CS
tr
io NO
©
fr
©
s ©
as vo P--in l-H
© m
8 co
P-l-H
© m l-H
o CO
cs
co cs
in vo Tf
NO
8 00 mm
CO
00 ©
Tt 00 NO
d
oo CO
© ©
m
00*
s
o •n cs
00
© CO
Tt ON I-H
8 cs
Tf ON
NO
m cs ON CO
P-
m CO
cs"
cs P-; l-H
d
Tf P-•t ON
©
m in
© in cs
ON l-H
NO
s d
1
s cs
© cs
tr
io
m cs
cs CO mm
l-H
3 ON
•n
VO ON
00 CO CO
ON NO ©
CS © ON
©
f-cs ©
d
cs oo
d CO
© m
© Tf
l-H
CS
p-p-
•-t
»n
8 mm
© CO VO
m Pr
p-cs'
Tf
VO CO 1-H
m
•n i-H
NO CO NO I-H
CS CO
© ©'
p-VO iH
cs' cs
8 cs
© 00 l-H
cs cs
© ON CO
CO
cs in Tf
8 Tf
Tf
cs © ©
© m cs
© cs
CO
cs
© ON
© 00 l-H
P--00 l-H
in NO Tt
3. P-; CO
8 Tt
© CO in mm
CO CS
•n ©'
NO
•n
d
p~ CO
© o"
in ON
8 CO
8 CS
Tf
cs
© cs Tf
00
CO
© m Tf
p--p-cs © ©
© m CO
1
in cs
© 00 Tf
°" cs_ Tf"
CO ON Tf
© CO
©
d
8
§
SO CS
© 00 Tt
CO cs VO CO
CO
oo Tt
CO
© ©'
© •n Tf
8 cs
r-cs
o ON
© r-r
© 00 mm
00 VO
m
CO ON CO
cU in
p-
VO r--I-H
5 ©
fr
©
r~ CO
© ©
CO CO
oq cs
8 in
8 cs
i 00 ! CS
in CO
© ©
© m in
8 cs
ON
cs
8
m 00
8 mm
© in in
NO mm Tf Tf"
in
in
NO NO ON Tf"
(N
8 ©
ON
<N NO ©
cs
s ©
NO ON
r-> cs* cs
§
8 cs
o CO
235
CHAPTER 8
DISCUSSION
8.1 SCANNING ELECTRON MICROSCOPE:
S E M photographs revealed important powder features including particle size, particle
porosity, surface roughness, edge description, the existence of agglomerates, interparticle
contact details, etc.. For example, from S E M photographs observations, the sharp and
angular edges of raw sugar grains, dense soda ash, zinc fume and pulverized coal suggest
that such materials may cause excessive wear and damage to pipes, blow tank and filtration
equipment. In comparison, the large rough surfaces of P V C powder suggest this powder is
prone to interlocking during dense phase conveying. In regard to fly ash, the significant
degradation observed during conveying (Wypych, 1986) is consistent with the existence of
loosely agglomerated macro particles. Hence, this information is vital for explaining both
directly and indirectly unusual flow behaviour, poor system performance, blockage, wear
and erosion characteristics. This information related to particle characteristics is summarized
in Chapter 7, Table 7.1.
8.2 COEFFICIENT OF RESTITUTION:
Figure 7.2 depicts the observed variation of the angle of impact versus coefficient of
restitution for Wheat impacting on Mild Steel and Stainless Steel wall surfaces, Millet on Mild
Steel and Bean seed on a Stainless Steel wall surface. The observed range of the coefficient
of restitution, 0.55 to 0.65, is typical for these particle wall contacts (Ottjes, 1982).
It should be noted that both Bean and Millet seeds are spherical granular particulates.
It was found that Millet has a lower coefficient of restitution compared to that for both the
spherical Beans and Wheat. This trend was attributed to the smaller particle size of Millet. In
this series of tests, the coefficient of restitution was measured with the disc tilted in the
direction of rotation.
In Figure 7.3, which depicts the variation of coefficient of restitution tilted in the
flow direction, the observed range was 0.44 to 0.65. For coal, the coefficient of restitution is
higher, when impacting on Mild steel (Initial drop height = 10.6 cm.) compared to that
when impacting on Stainless Steel. For Sinter and Sodium Ferrite, the observed coefficient
of restitution ranged from 0.52 to 0.58.
236
In Figure 7.4, the coefficient of restitution was measured with the impact disc tilted
against the direction of rotation. In this configuration, the coefficient of restitution ranged
between 0.56 to 0.69. A n examination of this Figure clearly reveals that Millet has a lower
coefficient of restitution compared to that observed for Wheat and Coal.
The coefficient of restitution for Coke impacting on Stainless Steel was found to be
0.54, Sugar on Mild Steel 0.53 and plastic balls on Mild Steel 0.71 which is relatively high
compared to the other materials tested. The latter coefficient of restitution was the highest
observed coefficient of restitution.
It was found that the coefficient of restitution was lower on Stainless Steel compared
to that for Mild Steel. The latter suggests that for a given system a lower pressure drop will
result when using Stainless Steel pipe compared to that for a Mild Steel pipe. However, this
operation advantage must be compared with the additional capital cost associated with the use
of Stainless Steel pipes. Obviously, for food standard systems the use of Stainless Steel
pipework is mandatory.
The use of Stainless Steel may also be warranted for materials exhibiting high
coefficients of restitution and hence high system pressure drops when conveying in dilute
phase in Mild'Steel pipe systems. In these situations, the lower wall friction characteristics of
Stainless Steel may also be an advanatage.
As stated, the aforesaid pressure difference will be more significant for dilute phase
systems. This implies, in turn, that knowledge of the coefficient of restitution is more
important in dilute phase systems, in regard to explaining unusual flow behaviour, high
system pressure drops or in systems operating at low flow rate.
8.3 PARTICLE SIZE ANALYSIS; From an examination of Table 7.3, which presents the size characteristics of the fly
ash and cement tested, it is apparent that fly ash 'E' and 'C display the smaller Sauterpaean
diameter D(3, 2) and the mean diameter D(4, 3) derived from the volume distribution
compared to the other fly ash tested. In particular, fly ash 'E' has a mean Sauter diameter
of 10.2 |im, whereas the next finest, fly ash 'C, has a mean Sauter diameter of 13.9 ,am.
The mean diameter for fly ash 'E' and 'C are 10.2 and 13.9 |J.m. Furthermore, Table 7.3
reveals that fly ash 'E' has the lowest mean particle size of 5.4 |im. In comparison, the
mean particle size of fly ash 'A' is 16.9 |im and cement 18.3 |im.
237
Fly ash 'E' and ' C have the higher specific surface of 0.18 sq. m / cc and 0.12
sq. m / c c , whereas Sand and P V C powder have lower specific surface areas of 0.02 and
0.03 sq. m / c c , respectively. Figure 8.1 depicts the variation of specific surface versus
m e a n particle size for the fly ash tested for all measurement runs. It can be seen from this
Figure that as the m e a n particle size increases, the specific surface decreases.
o ti
VI
ft
u L 3
(0 ti u ft
o-(0
I I — I I I — I — I I I I I I — I I I — I I I — I -
6 8 10 12 14 16 18
Mean Particle Size d50. "im
Figure 8.1: Variation of Specific Surface versus M e a n Particle Size for the Fly Ash
Tested.
The particle size analysis of Sand, Brown Rice and White Rice, as declared in
Tables 7.4 to 7.7, reveal that these materials are granular materials somewhat coarser than
fly ash samples and cement powders.
Figures 7.5 and 7.6 which compare the particle size distributions of all materials
tested, indicate that all fly ash tested possess particle size distributions with similar slope
over the d6o- d4 0 range with fly ash 'E' displaying a slightly steeper slope. This
tendency suggests that the formation process of the various fly ash powders tested were
similar.
The above trends are reinforced in Figures 7.7 to 7.10, which depict the log
normal distribution plotted as a relative percentage frequency distribution using a
logarithmic scale for particle size of the powders tested.
238
TABLE 8.1 EFFECTS OF DIFFERENT LENSES ON PARTICLE SIZE DISTRIBUTION
Material
Cement
Fly ash 'E'
Fly ash 'D'
Fly ash 'D'
Fly ash 'D'
Fly ash 'G'
Fly ash'G'
Fly ash'G'
Fly ash 'F
Fly ash 'F
Fly ash 'C
Fly ash 'B'
Fly ash *B'
Fly ash 'A'
Fly ash W
Obscu
ration
0.1752
0.1867
0.1726
0.4830
0.2017
0.1599
0.1685
0.2335
0.1795
0.156
0.302
0.1323
0.1184
0.1941
0.2026
Focal
Length
'mm)
100
63
300
100
100
300
100
100
100
100
100
100
100
100
63
Geomertic
Volume
Median
Dia. (mm)
18.3
5.4
11.8
8.8
10.0
14.9
13.8
13.9
14.0
13.8
9.2
14.3
15.0
15.1
16.9
Volume
Concen-
-tration
0.0052
0.0018
0.0037
0.0094
0.0037
0.0037
0.0033
0.0048
0.004
0.0034
0.0054
0.0032
0.0028
0.0051
0.006
Beam
Length
14.3
14.3
14.3
14.3
14.3
14.3
14.3
14.3
14.3
13.8
9.2
14.3
14.3
14.3
14.3
Distri
bution
Width
(Span)
2.7
4.6
6.9
6.5
6.1
5.4
5.2
5.2
4.4
4.4
3.0
4.1
4.1
4.0
3.1
Log
Error
3.74
2.46
2.08
2.68
2.57
2.84
2.66
2.67
2.78
2.52
2.97
2.96
2.91
3.19
3.41
Proper selection of the Malvern particle sizer lens is important. In particular,
particles outside the range of the lens, lead to erroneous results. Table 8.1 shows
variation of mean particle size with respect to different lenses and the effects of sample
concentration on the mean size.
Figure 8.2 shows the variation of mean particle size versus characteristic
dimensions for the fly ash with different measurement runs. Figure 8.3 shows the
variation of average characteristic dimensions of each fly ash versus mean particle size.
From both Figures, it is revealed that fly ash 'E' and 'C have the lowest characteristic
dimensions of the fly ash samples tested.
239
100
80
.2 8 ** =i •2 . 60 H L. e ft e * * i ^
ti w *• _- 40 L * w
* G u a
20 H
'E'
'D'
I!
'D'
G
D O
T* >B> >A'
'G* D
ID 'A'
V «w t -
•"—r
B D(v,0.9)
• D(4,3)
• D(3,2)
• D(v,0.1)
• D(v,0.5)
4 6 8 10 12 14 16 18 20 Mean Size, d50, |im
Figure 8.2: Variation of Mean Particle Size versus Characteristic Dimensions of Fly
Ash.
>r* H
*- _± L e ft e •* ' o w
*"- ? rr S
u a
OM ~
60 -
40 -
?0 -
o -
'E' •
•
T"»T "•!'—!—
•c
1 0
I ' ' i
•
•D'
•
• D O i r-T-
•a*
T' 'B*
• • •
•••
i i i i i
•
'A'
•
• • o l ' '
14 6 8 10 12 Mean Size, d50, |im
16 18
LEGEND
•
•
n o •
D(v,0.9)
0(4,3) D(3,2)
D(v,0.1) D(v,0.5)
Figure 8.3: Variation of Average Characteristic Dimensions versus Mean Particle Size.
Figures 8.4 and 8.5 reveal the variation of mean size versus % < 5.8 u m and
particle size distribution span, respectively for all measurement runs. The particle size
distribution span measures the spread between the 10% and 90% points of the cumulative
undersize distribution, scaled in terms of the 50% point and defined as the ratio of
D(v,0.9) - D (v,0.1)/D(v,0.5) pm. Figure 8.6 reveals the variation of mean particle size
versus average % < 5.8 pm and particle size distribution span. It is revealed from these
Figures that fly ash _* has the highest % < 5.8 pm as compared to other fly ash,
whereas fly ash 'D' has the highest particle size distribution span.
240
60
50 -
09 10 30 -v
tf 20 -
10 -
B' -A'
— i I i I i I i I i I » l •
4 6 8 10 12 14 16 18 D(v,0.5), |im
Figure 8.4: Variation of Mean Size versus % < 5.8 pm for Fly Ash Tested.
a. (0
5 -
4 -
3 -
- I — • 1 1 1 • I • 1 — • -
8 10 12 14 16 18
D(v,0.5), p m
Figure 8.5: Variation of Mean Size versus Particle Size Distribution for Fly Ash Tested.
241
LEGEND
<3 % < 5.8ipm
• PSD.span
Figure 8.6: Variation of Mean Size versus Average % < 5.8 p m and Particle Size Distribution Span for the Fly Ash Tested.
The above Figures confirm clearly identifiable property differences between the
powders tested. In some cases differences in the numerical values for the properties
approaching an order of magnitude were apparent. These differences are mainly due to
differences in the particle size distributions and are strongly dependent on mean particle
size.
8.4 PACKED BULK DENSITY, LOOSE POURED BULK DENSITY, COMPRESSIBILITY:
Figures 8.7 and 8.8 depict the variation of bulk density with major consolidation
stress for the fly ash and cement tested. This testing revealed that fly ash 'C and 'E' had
the lower bulk density among the fine powders tested. Furthermore, it is evident from
Table 7.8 that the bulk density empirical coefficients ranged from 649 to 1512 kg/m3 and
0.0016 to 0.0598 for p0and b, respectively. The empirical coefficient b is highest for fly
ash 'B' (0.0598) followed closely, in turn, by that for Light Soda ash, fly ash 'F, 'E'
and 'C (0.0554).
Such high numerical values for the coefficient b suggest that these powders are
highly compressible. In comparison, low values of b were observed for materials such
as Wheat, Rice and Millet. This clearly indicates that these materials are relatively
incompressible. All fly ash and cement have a higher value of b compared to other
granular materials. Unfortunately, Figure 8.9 which reveals the variation of b versus
mean particle size dso, reveals no apparent trend.
60
50 -
CO IO
40 - 'E'
v 30 H tf
20 H
10
-6
-5
I • • l • • I • • l l ' ' I ' -4 6 8 10 12 14 16 18
D(V,0.5), pm
e « Ok Vt
M 8
-3
242
1300
Major Consolidation Stress (kPa)
Figure 8.7: Bulk Density versus Major Consolidation Stress.
1600
1400
1200-
1000-
800 0 10 20
Major Consolidation Stress (kPa)
LEGEND FLY ASH
B E • F n G • CEMENT
Figure 8.8: Bulk Density versus Major Consolidation Stress.
243
ft
e ti
VI Yl ft L
a E e u
u.ui -
0.06 -
0.05 -
U.\J*V
'E'
1
"C"
'D1
•
•F' 'B' •
•
'G' •
CEMENT
•A'
10 20 Mean particle size, microns
Figure 8.9: Compressibility Coefficient Variation versus Mean Particle Size.
TABLE 8.2 COMPRESSIBILITY OF MATERIALS.
Material
Fly ash 'A'
Fly ash 'B'
Fly ash 'C
Fly ash 'D'
Fly ash 'E'
Fly ash 'F
Compressibility, %
25
34
45
35
37
24
Material
Cement
Sand
P V C powder
Wheat
Brown Rice
White Rice
Compressibility, %
28
5
11
18
15
13
The compressibility, as the difference between the packed bulk density and the
loose poured bulk density, divided by the packed bulk density times 100, are shown in
Table 8.2. It can be seen that compressibility values of Sand, P V C powder, Brown Rice,
White Rice varies in between 5 to 15 percent and having free flow powder
characteristicks. For cement, fly ash 'A' and fly ash 'F the compressibility values range
from 24 to 28 percentage, whereas for fly ash 'B\ fly ash 'D\ fly ash 'E' the range is
from 34 to 37 and for fly ash 'C it has a value of 45.
244
From Table 7.9, it is evident that the ratio of packed to loose poured bulk density is
highest for fly ash 'C compared to the other fly ash tested. The loose poured bulk density of
the powders tested is the average of three observations. This value is closely followed by
that for fly ash 'E', 'D', 'B', 'A', cement, fly ash 'F, Wheat, Brown Rice, White Rice,
P V C powder and Sand, in that order. This ratio is a good indication of cohesion and
fluidization characteristics, with a higher value indicating greater cohesiveness and hence
decreasing flowability. In particular, Geldart et al. (1984) reported the ratio of tapped and
aerated bulk density is less than 1.25 for group A, from 1.25 to 1.4 for group A C and greater
than 1.4 for group C powders. The ratio of the packed and loose poured bulk density is also
known as the Hausner ratio.
Figure 8.10 which depicts the variation of the Hausner ratio (ratio of packed to loose
poured bulk density) with dso, indicates this ratio increases with decreasing particle size. A
closer examination of this Figure suggests that the Hausner ratio is relatively high for fly ash
'C. This high numerical value is consistent with the higher cohesiveness exhibited by this fly
ash. The same also suggests that fly ash 'C will display greater tendencies for flow
fluctuations and flow difficulties from blow tank especially in dense phase systems. The
exponential trend correlation for this Figure is y = 2.2877 x"0-173 where x = dso-
Figure 8.11 shows the variation of mean particle size versus all Hausner ratio data
obtained from experiments with 1 % error bars. Figure 8.12 shows the variation of Hausner
ratio versus particle size span for the fly ash tested, whereas Figure 8.13 shows the variation
of Hausner ratio versus average particle size distribution span. It is revealed from Figure 8.13
that fly ash 'C has the highest Hausner ratio with least span, whereas fly ash 'D' has
highest particle size span and an intermediate Hausner ratio.
M
o 5 >. o "n *- c
o o • - - _ *
o — a 3 a. .a O 3 ~ 2 a a K Mean particle size, microns
Figure 8.10: The Ratio of Packed to Loose Poured Bulk Density versus
Particle Size for the Fly Ash and Cement Tested.
1.9
1.8-
« 1-7 H
1.6
o r->
(0
OC
co « 1-5" X 1.4 H
1.3
'E'
9
•c
•D'
i •B'
i-+ - 1 — i — | —
6 8 Mean Particle Size, microns
! — i — r — i — i — • — i •—
10 12 14 16 18
Figure 8.11: Variation of Mean Particle Size versus Hausner Ratio.
CO
a oo
/ -1
6-
5 -
4 -
3-
2-
'A' B
B
• 1 -
El
•
'D'
'E1
,B. •
•
• i • i — * — 1 ' 1 1.3 1.4 1.5 1.6 1.7 1.8 1.9
H. R.
Figure 8.12: Variation of the Hausner Ratio versus Particle Size Span.
i.a -
1.8-
1.7-
1.6-
1.5-
1.4-
1 .o "1 2
•c B
' 1 3
TV •
•
'B'
•
i
4
'E'
B
'F B ' 1
5 1 1
6
'D' EI
7
PSD, Span
Figure 8.13: Variation of Hausner Ratio versus Average Particle
Size Distribution Span for the Fly Ash Tested.
247
8.5 S O L I D S D E N S I T Y -
From Table 7.10, it is evident that the solid density observed for the materials
tested ranged from 1147 to 2735 kg/m3. This range is described more fully in Table 8.3
which depicts the density and density related values for the powders tested.
From bulk and particle densities, the voidage e can be calculated. Apparent
specific volume V s is the bulk volume of powder of unit weight, which is the inverse of
bulk density. Bulkiness is the bulk volume of powders, which is the inverse of packing
density. Greater bulkiness reveals less flowability, whereas less bulkiness require less
storage. Void ratio is the ratio of the void volume to the net volume of powders.
The equations for the above parameters are shown at the top of the following
Table 8.3. This Table reveals that significant variations occur between the numerical
values of density and density related values for apparently similar powders. These
significant numerical differences, for the various parameters, reflect complex differences
in powder packing arrangement and particle shape. The same may also indicate test
deviations.
Fly ash 'E' having greatest bulkiness (2.604), exhibits less flowability, whereas
Wheat with lowest bulkiness (1.59) requires less free volume. Furthermore, it is revealed
from Table 8.3 that fly ash 'E' has the highest voidage and the lowest packing density,
whereas Wheat has the lowest voidage and the highest packing density.
An initial implication of the practical significance of these vast numerical
difference is the implied differences in blow tank volume system requirements. A further
significant variation is the variation of voidage versus mean particle size dso which is
revealed in Figure 8.14. The exponential trend correlation for this Figure is y = 0.77618 -
o. 143 where x = dso.
It is observed that fly ash 'G' and cement revealed greatest variation from the
observed trend as voidage is strongly effected by particle size distribution and packing
mechanisms.
248
TABLE 8.3 DENSITY PARAMETERS
Material
Fly ash 'A'
Fly ash 'B*
Fly ash C
Fly ash 'D'
Fly ash *E'
Fly ash T
Fly ash 'G'
Cement
Sand
P V C powder
Sodium Ferrite
Wheat
White Rice
Brown Rice
Sugar
Po
kg/m3
1032
1134
944
1088
975
1275
1294
1312
1502
649
1512
868
865
825
883
Voidage
e =
(Ps - Pb)/ Pi
0.515
0.545
0.57
0.585
0.616
0.497
0.459
0.577
0.451
0.529
0.411
0.371
0.415
0.478
0.411
Packing
density
<J)p=l-e
0.485
0.455
0.43
0.415
0.384
0.503
0.541
0.423
0.549
0.471
0.589
0.629
0.585
0.522
0.589
Specific
volume
V s= 1/Pb
9.689 x IO"4
8.818 x IO"4
10.593 x IO"4
9.191 x 10-4
10.256 x IO"4
7.843 x IO"4
7.843 x IO"4
7.621 x IO"4
6.657 x IO"4
15.408 x IO"4
6.613 x IO"4
11.52x IO"4
11.56x IO"4
12.121 x IO'4
11.325x10-4
Bulki
ness
<t>b=l/<t>p
2.062
2.198
2.326
2.41
2.604
1.988
1.988
2.364
1.821
2.123
1.698
1.59
1.709
1.916
1.698
Void ratio
(|)v=e/ 1-e
1.062
1.198
1.325
1.409
1.604
0.988
0.988
1.364
0.821
1.123
0.698
0.59
0.709
0.916
0.698
ffi c n co
o >
u.i -
0.6-
0.5-
0.4-
\ E'
'C* 'D* B
CEfvENT ,B. Q
r - T ^ E l
-F Q 'A'
'G' B
10 20
Mean particle size, microns
Figure 8.14: Voidage versus Mean Particle Size for the Fly Ash and Cement Tested.
249
8-6 COHESIVE ARCH LF-NOTH AND DRATNED ANGLE OF RF.POSF.. 8.6.1 COHESIVE ARCH l.RNOTH;
Since the ratio of the silo width to maximum outlet opening span of the Arch Tester is
2.5 : 1, plane strain conditions are assumed to apply during gravity discharge (Figure 7.11).
Hence, the Arch Tester can be used to measure the plane flow outlet span necessary to attain
reliable gravity discharge in a funnel flow channel. This span is commonly known as the
critical cohesive plane flow arching dimension for rough walls (Jenike, 1970). Unfortunately
for very cohesive powders, arching prevents realistic measurement of this arching dimension
and obviously, the drained angle of repose.
From the results, it was revealed that, in general the Jenike and Walker methods give
arch lengths much higher than those observed in the experiments. But for fly ash 'C and
'E', the difference between the experimental arch lengths and that predicted from the Jenike
and Walker methods were much less.
The Arch Tester may give the lower bound on the critical arch opening, whereas the
Jenike method the upper bound. The overdesign of over 1 0 0 % associated with Jenike's
method may be useful to account for initial filling conditions and other factors like increase in
moisture content. The correct determination of the flow function from shear strength
measurement with the Jenike shear cell m a y be an important factor generating this
overdesign.
The arch length for fly ash 'A' was tested on two days having different relative
humidity, namely 5 5 % and 59%, refer Figure 7.13. A n examination of this Figure reveals
that the arch length increased significantly with increasing relative humidity and the arch
length increased as the deaeration time including filling was increased. This observation is
consistent with that of Molerus et al. (1982).
Further evidence for the increase in cohesive arch length with increasing relative
humidity was clearly apparent from the arch tests conducted on fly ash 'C and cement, refer
Figure 7.14 and 7.15, respectively. This may be due to capillary condensation of water in the
void between two particles resulting in a component additional to the van der Waals attraction
(Visser, 1989). Thus, the environmental relative humidity strongly influences cohesive arch
length and hence cohesive properties of powders during transport.
250
From the Table 7.11, it is evident that for fly ash, cement and Sodium Ferrite, the
arch length increased with increasing deaeration time. It was observed that fly ash 'F
becomes cohesive after considerable deaeration. In fact, the cohesive arch formed was so
strong that no flow occurred and extended some 30 m m high across the outlet. Since, fly
ash 'E' exhibited the smallest mean particle size, the observed trend in the arch length
suggested that as the mean particle size decreases, the cohesive arch length increases.
Fly ash 'E' and 'C have higher arch length than other fly ash and cement, refer
Figures 7.16 and 7.17. Figure 7.19 depicts the variation of mean particle size versus arch
length for the fly ash tested. This Figure reveals that as the mean particle size dso decreases,
the arch length increases. This trend is consistent with that observed by Borg (1982).
The relatively large cohesion force observed for fly ash 'E' deviates from the
cohesive strength trend predicted by Molerus et al. (1982). This deviation highlights that the
particle size distribution and packing in addition to the mean particle size significantly
controls a powder's cohesion. Hence, fly ash 'E' with the smallest particle size and largest
% -10 p m material displays the highest cohesion of the powders tested.
Fly ash 'B' was found to be relatively free flowing compared to the other fly ash. In
comparison, fly ash 'C was very cohesive. In fact, the powder tends to form vertical flow
channels after long deaeration times. Furthermore, this powder tended to form arch across
the tester outlet. In particular after considerable deaeration, measurement of the cohesive arch
length was impossible due to the occurrence of cohesive arch.
In addition, testing with the Arch Tester indicated that the arch length was dependent
on the depth of material in the tester. This trend is evident in Figure 7.18 as observed for
cement.
Unfortunately, results from the cohesive arch tester proved to be highly sensitive to
test conditions including ambient relative humidity, filling procedure and extent of deaeration.
Hence, at best the Arch Tester should only be used for indicating the cohesiveness of various
powders or to indicate the relative differences in cohesiveness between seemingly similar
powders.
251
8.6.2 DRAINED ANGLE OF REPOSE: Figure 7.20 depicts the variation of drained angle of repose versus bed height for
cement at differing relative humidity. The trend that the angle of repose increases with
increasing storage time is clearly evident in Figure 7.21 which depicts the variation of drained
angle of repose versus deaeration time for various fly ash. These effects compound when
both the deaeration time and relative humidty increases as shown in Figrue 7.22 for cement.
A further interesting trend is evident in Figure 7.23 which depicts the variation of
particle size versus drained angle of repose for the powders tested. This Figure reveals that
the drained angle of repose increases as the mean particle size dso decreases. This is
consistent with the trend reported by Brown (1961).
-00-i
90- B 'E' ,
^—'•'""ETC-
80- ^*^*^
• BT^^^
70 - B B 'B*
CEMENT
60 H i 1 • i « I • I • I ' — 1.4 1.5 1.6 1.7 1.8 1.9
Ratio of packed to loose poured bulk density
Figure 8.15: Variation of Ratio of Packed to Loose Poured Bulk Density versus Drained Angle of Repose for the Powders Tested.
An increasing trend is evident from Figure 8.15, which reveals the variation of
drained angle of repose versus ratio of packed to loose poured bulk density of powders
tested. This trend is consistent with that observed by Grey et al. (1968).
It was observed that the drained angle of repose increased with deaeration time. For
fly ash 'D', the average front angle of repose was 65°, back angle of repose was 75° for 20
minutes filling and deaeration time, whereas for 27 minutes filling and deaeration time, the
average front angle of repose was 70° and back angle of repose was 85°. In this case, filling
was conducted by mechanical means.
° S
C TO n
• r
£ o. a «>
252
As noted from Figure 7.21 generally, the drained angle of repose increased with
increasing deaeration time. Furthermore, the experimental work also suggested in a
qualitative matter, that the drained angle of repose increases with increasing powder
cohesion. This trend is consistent with the trend predicted by Molerus et al. (1982). Hence, a
small change in fine particle content results in measurable change in cohesiveness. In
particular, fly ash 'E' was found to be the most cohesive of the powders tested. For this
powder, of mean particle size 5.4 p m , the angle of repose for all flow channel boundaries
was 90°. Particularly, in one test a drained angle of repose of 110° was noted. Testing was
conducted on two days at relative humidity of 7 3 % and 7 8 % , respectively. For all
observations, recorded angles of repose in excess of 90° were noted. After significant
deaeration, determination of drained angle of repose was not possible due to the tendency of
the powder to arch.
One powder, cement, however deviated from the general strength versus particle size
trend. It is considered that the hygroscopic nature of cement caused this powder to display
higher drained angle of repose compared to the observed general trend. This tendency to
absorb moisture, obviously, increased the powder's cohesiveness. This, in turn, suggests
that the drained angle of repose displayed by hygroscopic materials is dependent on test
relative humidity. Therefore, for hygroscopic powders the relative humidity must be
controlled and specified and be representative of actual plant conditions. This clearly suggests
that cement must be pneumatically conveyed with dry air.
Although variations with drained angle of repose were noted for the various powders,
no consistent trend was observed. For this reason, the angle of repose at best is a crude
simple indicator of a powder's cohesiveness.
8.7 FLOW FUNCTION: Here, cohesion is indicated by the intercept formed by the powder flow function and
the unconfined yield stress axis. It should be noted that the flow function, also called the
failure function, can be used to characterize the flowability of powders (Jenike, 1970) since
it is a property of the bulk material and its degree of compaction.
The flow function of powders were determined in a Jenike Direct Shear
various consolidation loads. This testing revealed that fly ash 'E' to be the most
2 5 3
and cement to be least cohesive of the powders tested. The other fly ash exhibited
intermediate cohesive strength as revealed in Figure 7.25 and Table 7.12.
A further characteristic of the flow function is its slope. In general, an increasing flow
function slope indicates decreasing stability and reduced suitability for pneumatic conveying.
From Table 7.15, it is evident that fly ash 'E' has the highest slope of 0.923. Furthermore,
powders exhibiting steep flow functions have a greater strength and ability to support an arch
and strong tendency to form plugs during pneumatic conveying [Thompson, (1984)]. These
plugs will possess considerable strength and be hard to dislodge due to the consolidation
effect caused by the action of the significant body forces present during the plug formation
process.
The trend between effective angle of friction 6 observed from the Arch Tester and 8
from shear tester is good, as shown in Fig. 8.16. However, the trend between internal angle
of friction 0 observed from Arch Tester and (J) from the shear tester is not as consistent, refer
Fig. 8.17. This is expected because the actual powder yield locus must take into account the
cohesion C of the powder.
100 -•
E 90-_> E Q)
S* 80-LU
E 2 70-
60 -30 40 50 60
From Shear Tester
Figure 8.16: Variation of Effective Angle of Friction from Experiment and Shear Tester
254
c tt E " u a. x LU E o
i i i i i i i i i i i
28 30 32 34 36 38 40
From Shear Tester
Figure 8.17: Variation of Internal Angle of Friction from Experiment and Shear Tester
In regard to the correlation between the flowability index suggested by Tsunakawa et
al. (1988), defined as the ratio of the unconfined yield strength to the bulk density, and
observed arch length of fly ash and cement minimum significance was observed, refer Figure
8.18. However, it should be noted that cement and fly ash 'C deviate the greatest from the
apparent trend. Fly ash 'E' has the highest flowability index.
E E
cn c tt
u <
120
100 -
0.1 0.2 0.3 0.4
Flowability Index
0.5 0.6
Figure 8.18: Flowability Index of Fly Ash and Cement versus Arch Length.
255
8.8 TENSILE STRENGTH;
The observed tensile strength versus consolidation variations lbr the powders tested
are depicted in Figures 7.26 and 7.27. In comparison the observed strengths, when subject
to a deaeration time of about 15 minutes, are shown in Figures 7.28 and 7.29. A n
examination of the tensile strength variations depicted in these Figures suggest that the 'y-
axis' intercept indicates the cohesiveness of a powder. Noting this for the fly ash tested, fly
ash 'E' exhibits the greatest strength whilst fly ash 'F the least. Relative ranking of the
intermediate fly ash is also clearly evident. Furthermore, the powders exhibiting tensile
strength variations of high slope display significant strength, when consolidated.
The observed voidage variation versus tensile strength of the various fly ash tested are
shown in Figures 7.30 and 7.31 for instantaneous conditions and Figures 7.34 and 7.35 for
a deaeration time of about 15 minutes. A n examination of these figures clearly highlight that
the tensile strength decreases as the voidage increases. Here, fly ash 'E' and 'C of smaller
mean particle size, display high voidage, yet fly ash 'E' exhibits the highest tensile strength.
The observed tensile strength versus bulk density variations for the fly ash tested are
depicted in Figure 7.32 and Figure 7.33 for instantaneous conditions and when subject to a
deaeration time of 15 minutes, respectively. A s a comparison, the observed tensile strength
versus consolidation for light soda ash, dense soda ash, P V C powder and castor sugar are
depicted in Figure 7.36, whereas the voidage variations are depicted in Figure 7.37. It
should be noted that the relatively high tensile strength of castor sugar evident in these
Figures was due to the presence of absorbed moisture. Furthermore, P V C powder exhibited
minimal strength with dense soda ash and light soda ash displaying intermediate tensile
strengths.
The effect of the filling procedure on the measured tensile strength is revealed in
Figure 7.37. In particular, both a screen vibrator and spoon cell filling procedure was used.
From an examination of Figure 7.38, it is apparent that at high consolidation, the observed
tensile strength is the same for each filling procedure. Whereas, at low consolidation the
observed tensile strength is relatively sensitive to the filling procedure used. The tensile
strength is small at lower consolidation as the particles are not closely packed consistent with
their low bulk density. Furthermore, at low consolidation the screen filled values are higher
than the spooned values. This observation confirms that of Yokoyama et al. (1982) who
likewise have shown the importance of filling procedure on the tensile strength observations.
256
O n identifying the suitability of the screen vibrator procedure, fly ash 'H', T
and 'J' were tested using this fill procedure incorporating the necessary consolidation and
twists. These small samples of fly ash were received from Queensland Electricity
Commission. The results are depicted in Figure 7.39. From an examination of this
Figure, it is evident that fly ash T exhibits the highest tensile strength and fly ash 'H
has the lowest.
The range of consolidation stresses for tensile strength test was 11.31 kPa to
107.647 kPa, whereas the tensile strength measured range was 0.672 kPa to 4.56 kPa.
The highest tensile strength of 4.56 kPa was measured with fly ash 'H\ Moreover, the
range of voidage was 0.0483 with P V C powder to 0.72 with fly ash 'E\ Also, the bulk
density ranged between 1311 kg/m3 for P V C powder to 733 kg/m3 for fly ash *E\
The adhesion forces for fly ash for instantaneous conditions and with 15 minutes
deaeration are determined from Rumpfs (1970) equation which can be revealed in
Figures 8.19 and 8.20. Adhesion force increases with consolidation force which is in
agreement with the findings of Tsubaki et al. (1984). It is also revealed that fly ash 'E'
has the lowest adhesion force. Unfortunately in both Figures 8.19 and 8.20 extrapolation
to zero consolidation force is difficult. From Figure 8.20, it is revealed that with fly ash
'B' and 'D' at higher consolidation adhesion force decreased.
Oi
6
o L e
tt
•a <
BUU ~
600 -
400 -
200 -
0 - -r—
— •
—f I
— m -
— , — - V
-m-
— r —
_ •
" I ' 1
•
1 1
LEGENC
FLY ASH
•
D
• —
D
)
A B C D E F
0 10 20 30 40 50 60
Consolidation, N
Figure 8.19: Variation of Adhesion Force versus Consolidation for Fly Ash.
257
LEGEND
FLY ASH — B — A — * — B — n — D —P— E — • — F
0 10 20 30 40 50 60 Consolidation Force, N
Figure 8.20: Variation of Adhesion Force versus Consolidation for Fly Ash with Deaeration
The foregoing examination suggests that the tensile tester provides the most evident
distinction between various yet seemingly similar powders. Hence, in view of the convenient
use and clearly evident ranking of powder properties, this test should provide useful
information regarding rapid assessment of a powder's cohesiveness.
8.9 WALL FRICTION:
The necessary calibration procedure and data for this test rig are presented in
Appendix 'C. O n using this information Figures 7.42 to 7.48 result. These figures
summarize the observed wall friction force variations for the various granular materials
tested under differing conditions of the slug length ( 80 to 200 mm.) and applied slug
aeration air pressure in the range of 0 to 400 kPa. A n examination of these figures reveal
that, in general, the frictional force increases with increasing column height. This is in
agreement with the finding of Roberts (1966).
Furthermore, for Millet and Rice Flakes, at high air pressure, a rapid decrease in
frictional force was noted, whereas, for Brown Rice, White Rice and Wheat a small
decrease in the frictional force occurred. This suggests that super dense phase flow or plug
flow will exhibit relatively low wall friction retardation effects. This reconfirms the
favourable flow characteristics of this flow mode.
o. I
o
tt u
-
o CO tt X •a
<
258
It was revealed that under the same conditions of air pressure and column height,
White Rice has higher frictional force as compared to Brown Rice, refer Figures 7.42 and
Fig. 7.43. It should be noted that the frictional force for Wheat is slightly higher-than that
observed for Millet. This may be due to a particle shape difference between the two grains, in
particular the Millet is spherical whereas Wheat is more angular. Also, at higher air
pressures the frictional force decreased for Millet, whereas for Wheat a slight decrease was
observed. This further confirms the favourable spherical particle shape of Millet
For sand, an opposite trend was observed. In particular at higher air pressure, the
frictional force increased. This increase m a y be due to particular powder characteristics
notably particle abrasiveness and angularitiy and test rig or procedure shortcomings.
The subsequent Figures 7.49 to 7.54 depict the variation of frictional force versus
column length for the materials tested, whereas, Figures 7.55 to 7.60 depict the average
shear stress versus aeration pressure. O n the other hand, Figures 7.61 to 7.65 depict the
evaluated wall friction factor uk for the respective materials tested. These figures reveal the
value of pk decreases generally with both increasing air pressure and column height. It is
interesting to note that this value is higher for Millet and Brown Rice as compared to that for
Wheat and White Rice, respectively.
W h e n performing wall friction tests using fine materials, the following results are
observed (Table 8.4). Furthermore, it was noted that fly ash 'A' and 'D' tended to form
plugs at air pressure of 1 and 13.78 kPa, respectively.
TABLE 8.4: WALL FRICTION TESTS OF FLY ASH
SR. NO.
1.
2.
3.
MATERIAL
FLY ASH 'A'
FLY ASH 'D'
FLY ASH *F
AIR PRESSURE
kPa
1
0
69.9
13.78
0
69.9
110
FRICTIONAL
FORCE, N
29.43
14.225
47.088
63.078
13.244
329.468
355.286
COLUMN
LENGTH, mm
60
80
80
130
80
80
80
uk
0.79
_
0.53
0.27
0.322
_
-
259
Wall friction angles of fly ash using Stainless Steel as a wall surface were
evaluated using the Jenike Direct Shear Tester (Table 8.5). The details of the tester and
operating procedure are provided by Arnold et al. (1980). The observed wall yield loci
using this test procdeure of fly ash are shown in Figure 8.21. A n examination of this
Figure suggests that the wall yield loci are generally convex from above. A similar trend
has been reported for cohesive powders by Roberts et al. (1984).
LEGEND FLY ASH
—B 'A' » 'D'
—• "F
0 10 20 30 Normal Stress, kPa
Figure 8.21: Wall Yield Loci for Fly Ash 'A', 'F and 'D' on Stainless Steel.
TABLE 8.5: WALL FRICTION
Material
Fly ash'A'
Fly ash 'D'
Fly ash *F
ANGLES
Angle of wall
friction
(degrees)
39
37
37
Unfortunately, the attainment of useful information regarding wall friction for
fine materials was marred by test difficulties. These difficulties included the tendency for
some powders to form plugs and separate from the piston. Whereas in other cases
powder plugs formed and broke up again with increasing air pressure. Hence, at best the
wall friction rig can only provide a crude assessment of the extent of wall friction between
a powder plug and a cylindrical wall surface.
GL -i
Vt VI 4 k. *r
(0 L * -fi
CO
2 6 0
provide a crude assessment of the extent of wall friction between a powder plug and a
cylindrical wall surface.
In view of these problems, it was considered that the familiar Jenike Direct Shear
Tester provides the best and most useful indication of a powder's wall friction characteristics,
refer Table 8.5. This latter tester obviously does not provide any indication of a powder's
plug formation tendencies.
The rig, does however, provide useful observations of plug formation of powders
and frictional forces of granular materials. The results obtained indicate the necessary
frictional force required to convey a plug of granular material in super dense phase pneumatic
conveying. This information ranges for different granular materials and column height,
which indirectly indicate pressure drop in low velocity conveying.
8.10 DEAERATION:
Pressure variations during filling and deaeration for fly ash 'A', 'F and 'G', for a
permeable base are shown in Figures 7.67 to 7.69. A n examination of Figures 7.68 and 7.69
suggest that fly ash 'A' has the most air retentive characteristics of the powders tested. In
comparison, the filling and deaeration pressure variations of the powders tested with the
impermeable base are depicted in Figures 7.70 to 7.72. From Figures 7.69 and 7.73, it is
observed that with fly ash, the maximum interstitial pressures ranged from 20 to 23 kPa for
the permeable base and from 42 to 44 kPa for the impermeable base.
The decay of the deaeration graph is more rapid, included intermittent bed dropping
and greater oscillations during filling when using a deaeration tester with an impermeable
base as compared to that when using a permeable base. Moreover, the peak of filling and
deaeration transition is sharp with an impermeable base, whereas it is smooth and non
distinct with a permeable base. This suggests that it is best to determine the deaeration
characteristics of powders using an impermeble base.
Figures 7.71 and 7.73 reconfirm that fly ash 'A' exhibits the strongest long term air
retention characteristics. O n the other hand, fly ash 'F and 'G' display relatively rapid
deaeration. Figure 7.72 reveals the deaeration pressure variation for fly ash 'C. From Table
7.14, it can be seen that the exponent obtained from the exponential curve fitting of data
points is lowest for fly ash 'G' under conditions of both permeable and impermeable bases.
261
Also, the deaeration time constant is highest for fly ash A', intermediate for fly ash 'G' and
lowest for fly ash 'F when tested using permeable and impermeable bases.
In regard to powder deaeration, the time constant obtained from decay curve of
interstitial pressure or bed height versus time indicates whether a powder is air retentive or
will rapidly deaerate. In particular, powders exhibiting large numerical values for the time
constant retain air for a significant time. These powders with slow deaeration rate retains
interstitial air maintaining separation of the powder plug. This allows easy conveying in
pipelines without interparticle interlockings. In comparison, powders with small numerical
values for the time constant deaerate rapidly.
Figure 8.22 reveals the variation of deaeration time constant versus mean particle size
of fly ash tested in a deaeration tester for impermeable and permeable bases. It reveals that the
deaeration time constant increases with mean particle size. Figure 8.23 reveals the variation
of deaeration time constant versus particle size span for impermeable and permeable bases.
It reveals that fly ash 'G' and 'F have higher particle size distribution span as compared to
fly ash 'A'.
o jz tt
O *-a a l_ «H
a c e o a a
B •
LEGEND PERMEABLE IMPERMEABLE BASE
13 14 15 16 17 D(v,0.5), pm
Figure 8.22: Variation of Deaeration Time Constant versus Mean Particle Size for Impermeable and Permeable Bases.
Figures 7.74,7.75 and 7.76 depict the bed height variation versus deaeration time for
fly ash tested under conditions of both permeable and impermeable bases. Initially, the bed
262
level falls rapidly as the bubbles leave the bed. Then the bed level falls slowly at a constant
rate. This characteristic is consistent with that of cohesive powders [ Geldart et al. (1984)].
Figure 7.77 reveals a typical filling-deaeration graph for fly ash 'A' with an
impermeable base. The filling pressure trace is not smooth suggesting the filling process
takes place in a series of steps. This observed trend is consistent with that observed by
Tardos et al. (1985).
tt E w — o 1- tt
C 0 «-r tr- C
-. *H 0) 0) CO c tt o Q O
100n
80-
•
60 -•
40 -. •
20-
0 -3 4 5 6
Particle Size Distribution Span, p m
Figure 8.23: Variation of Deaeration Time Constant versus Particle Size Distribution Span for Impermeable and Permeable Bases.
The deaeration parameters evaluated from a plot of bed height versus time include the
intercept U_, known as the dense phase bed height and the dense phase deaeration time. The
former corresponds to the y intercept obtained by drawing a tangent to the deaeration graph,
whereas the latter is the x axis intercept of the same tangent line. Bubble hold up, which is
defined as the fraction bubble volume / bed volume, may be evaluated from the ratio of the
difference in actual bed height and dense phase bed height to the actual bed height.
Deaeration parameters including deaeration factor, collapse rate air velocity, dense
phase voidage, dense phase bed height, dense phase deaeration time and bubble hold up for
the fly ash tested are declared in Tables 8.6 and 8.7. Variation of dense phase voidage
versus particle density is revealed in Figure 8.24. It is noted that the dense phase voidage is
higher when using a permeable base compared to that observed when using an impermeable
•
•
LEGEND
PERMEABLE IMPERMEABLE
BASE
2 6 3
base. Moreover, the dense phase deaeration time and bubble hold up for fly ash 'E' with
permeable base are the lowest.
« cn a o > co a x 0. tt CO
-tt
a
u.ou -
0.59 -
0.58-
0.57-
0.56-
0.55-1
El
•
i 1 r-
•
•
1 1 1 r
LEGEND
B PERMEABLE • IMPERMEABLE BASE
2100 2200 2300 2400
Particle Density, kg/m3
2500 2600
Figure 8.24: Variation of Dense Phase Voidage versus Particle Density.
TABLE 8.6: DEAERATION FACTOR Material
Fly ash 'A'
Fly ash 'C
Fly ash 'E'
Fly ash 'F
Fly ash 'G'
Deaeration Factor (mbar.s/m.)
Permeable Base
12170
-
_
11539
14037
Impermeable Base
7091
1891
.
4762
7460
Deaeration Factor / Particle Density
Permeable Base
3.33
0.86
0.643
1.88
5.87
Impermeable Base
5.71
.
_
4.55
3.14
TABLE 8.7: COLLAPSE AIR VELOCITY AND DENSE PHASE
Material
Base
Fly ash 'A'
PARAMETERS
Collapse air
velocity, cm /s
P.B.
0.086
LB.
0.09
Dense Phase
voic
P.B.
0.595
age
LB.
0.582
Dense Phase
Bed Ht. mm.
P.B.
80
LB.
83
Dense Phase
Deaeration
time, sec.
P.B.
350
LB.
250
Bubble
Hold up
P.B.
0.05
LB.
0.02
264
Fly ash'E
Fly ash 'F
Fly ash'G'
0.077
0.063
0.055
-
-
0.07
0.643
0.553
0.564
-
-
0.554
72
74
71
.
-
71
195
340
350
.
-
346
0.01
0.03
0.04
.
.
0.03
8.11 FLUIDIZATION AND DEAERATION: Figures 7.80 and 7.81 depict the fluidization behaviour of fly ash 'A' and 'E' for
different experimental runs, whereas Figure 7.82 depicts the comparison of fluidization
behaviour of fly ash 'A', 'C and 'E', respectively. A n examination of this Figure
suggests that fly ash 'E' and 'C exhibit more severe channelling than that exhibited by
fly ash 'A'.
During fluidization testing of fly ash 'A', a plug of 170 mm.; fly ash 'E' a plug of
175 m m formed due to the presence of strong interparticle forces and small mass of
particles. The high initial pressure gradient in fluidization may be due to the presence of
segregated fines at the top of column. In particular, the behaviour of fly ash 'A' and *E'
was typical of that for Group C powders which exhibit cohesive tendencies. With
increased air flow, the air opens the channels that extend from the air distributor to the
surface. If channels are not formed, the whole bed will lift as a plug.
Figures 7.83, 7.84 and 7.85 depict the fluidization characteristics of Alumina,
Sand and P V C powder, respectively. These powders are free flowing as compared to fly
ash, hence they can fluidize easily. The minimum fluidization velocity for Sand, Alumina
and P V C powders are 5, 1.22 and 1.37 cm/sec, respectively.
Figures 7.86 to 7.88 represent the deaeration behaviour of fly ash in the
fluidization rig. Some fly ash were tested a number of times for successive deaeration and
refluidization. It can be seen that in all experiments by repeating the deaeration on the
same sample the bed height variation decreased and the powder exhibits decreasing air
retentive properties. This trend confirms the existence of fines entrainment in the
fluidization air emitting the bed and the tendency for the fines to segregate at the top of the
column.
The implication of this finding is that the fluidization testing should be rapidly
conducted on a single undisturbed sample. Moreover, if the particle bed consists of a
wide particle size range, fluidization testing may not be practical or may be prone to error.
265
E E
X
g LU X Q LU m
550
500
450 -
400 -
350
LEGEND FLY ASH
a 'A' • 'C n 'E'
40 60
DEAERATION TIME, Sees. 80
Figure 8.25: Deaeration Behaviour of Fly Ash 'A', C and 'E'.
Figure 8.25 depicts the deaeration behaviour of fly ash 'A', C and 'E' observed in
the fluidization rig. It is clearly evident that fly ash 'E' and 'C have deaerated slowly due to
their small particle size as compared to fly ash 'A. The exponential trend correlations for this
Figure are y = 543.91 x"8-443, 516.53 x"1-546 and 514.79 x'1-365 for the fly ash 'A', 'C* and
'E' respectively. This Figure also reveals that fly ash 'E' has deaerated slightly slower than
fly ash 'C.
The following Table 8.8 depicts the permeability factor obtained from the fluidization
tests. Based on the Mainwaring et al. (1987) classification, all powders can be conveyed in
either dense phase moving bed type flow or can't be conveyed in dense phase at all, whereas
the same classification system suggests that fly ash 'E' can be conveyed in plug type mode of
dense phase conveying.
TABLE 8.8: PERMEABILITY FACTOR
Sr. NOJ
1.
2.
3.
4.
5.
6.
Powder
Sand
Fly ash 'A'
Fly ash C
Fly ash 'E
Alumina
PVC powder
Permeability factor, m2bar/s
0.67
0.61
0.86
2.96
0.52
0.71
266
Figure 8.26 shows the variation of permeability factor versus mean particle size
for fly ash 'A', *C and 'E' tested in the fluidization rig. It reveals that fly ash 'C and
'E' have higher permeability factors compared to fly ash 'A'.
Vt •
-
_a -% CM
E L
e
E L tt Q.
3 -
2 -
1 -
n -
tr
'E'
1 1 1 1
'C
i I • • 1 • • 1 •
'A' Dl
• | i i | — i — i — |
6 8 10 12 14 16 18
D(v,0.5), p m
Figure 8.26: Variation of Permeability Factor versus Mean Size for Fly Ash Tested.
Jones et al. (1989) revealed that the powders having high values of permeability
factor will exhibit poor air retention characteristics. This trend was observed with fly ash
'E\
Although the concept of fluidization is straightforward, the measurement of the
fluidization characteristics of cohesive powders, powders possessing a wide particle size
distribution or powders possessing ultrafines is difficult. For this reason the description,
interpretation, reporting and subsequent classification of fluidization characteristics of
such powders should be viewed with caution. Noting these difficulties various
techniques have been attempted to overcome the same. These techniques include poured
fluidization testing, vibrated and mechanical agitated fluidization columns. Unfortunately,
the success of these alternate procedures has been limited [ Geldart et al. (1984)].
Similar problems also occur with cohesive powders or powders with wide
particle size consists during quantification of deaeration characteristics. Notably
channelling of cohesive powder beds causes such powder beds to collapse at extremely
high rates, whereas the loss of fines and ultrafine particles from some powder beds
causes an incorrect assessment of the deaeration constant.
267
With these difficulties aside the fluidization and deaeration characteristics. of
powders are vital parameters for classifying the flow characteristics of powders especially
in regard to dense phase pneumatic conveying. This classification has been successfully
effected by Mainwaring et al. (1987).
8.12: PERMEABILITY:
The permeability of the fly ashes were determined using a Jenike Permeability
Tester as described in Chapter 6.8. The results are shown in Figure 8.27 as follows. It
reveals that fly ash 'C and 'B' exhibits the highest and lowest permeability coefficient,
respectively of the fly ashes tested.
£
vt
E -tt CL
1.0 1.2 1.4 1.6 Major Consolidation Stress, kPa
1.8
LEGEND FLY ASH • -A-
• 'B' D 'C P 'D' • 'F
Figure 8.27: Permeability of the Fly Ashes Tested.
Figure 8.28 reveals the variation of permeability coefficient versus mean particle
size for the fly ashes tested. Figure 8.29 reveals the variation of permeability coefficient
versus pressure gradient evaluated from fluidization rig for the fly ashes tested. It reveals
that the pressure gradient for fly ash 'E' and 'C are lower than fly ash 'A'. Figure 8.30
reveals the variation of both the permeability coefficient a and compressibility coefficient
b versus the mean particle size for the fly ashes tested.
268
10'
10
10
-4
1 IO - 6
o 10
o 10
-7 ^
10 -9
10 10
1
•j
1
1
1
•1
• •E' B
' 1 '
•c D
1 • 1
'D' •
1
•F'
'B' El
i
'A' B
—\ r •
8 10 12 14
D(v,0.5), p m 16 18
Figure 8.28: Variation of Permeability Coefficient versus Mean Particle Size for Fly Ash Tested.
r- 10
(0
c
E
o u
o u
a Q.
-3
10 -4
10'
10 -6
— 10
10
E 10
-7 ; i
"8 \
"9 •
10 10
El 'A'
• 'C
• 'E'
1000 dp/dl, mbar/m
2000
Figure 8.29: Variation of Permeability Coefficient versus Pressure Gradient for Fly Ash Tested.
269
10 -J
1 "
a
.1 -
• w • • i • i • i • i • — \ — • — i — i — |
4 6 8 10 12 14 16 18 D(v,0.5), Lim
Figure 8.30: Variation of Permeability Coefficient a and Compressibility Coefficient b versus Mean Particle Size for Fly Ash Tested.
8.13: RANKING OF POWDER PROPERTIES-
The observed ranking of powder properties appropriate to pneumatic conveying
determined from bench top tests, is presented in Table 8.9.
The arch length and drained angle of repose ranking is determined from
observations with maximum deaeration time. Unfortunately, clear ranking of the cohesive
powders using these bench tests is difficult. However, relative comparison of
cohesiveness can be made for seemingly similar powders. It is revealed from the table
that fly ash 'E' and 'C have higher arch lengths and drained angle of repose as compared
with other fly ash. Fly ash 'D' and 'B' have high ranks for drained angle of repose as
compared with arch length. Likewise, fly ash 'F' has higher rank for arch length in
comparision with the drained angle of repose.
Ranking from tensile strength and slope indicate that fly ash 'C\ 'A' and 'D' have
the same rank in terms of both parameters. Fly ash 'F' has lowest rank in tensile
strength, but exhibits the highest flow function slope. Fly ash 'E' has highest rank of
tensile strength, but third rank of slope.
Ranking from flow function and slope indicate that fly ash 'E' has the highest
rank. Ranking from deaeration tests in fluidization column is not easy as similar
deaeration behaviour was observed for fly ash 'E' and 'C\ However, from the results of
the purpose built deaeration tester, clear ranking was possible for fly ash 'A', 'F and
'G'.
'B'
'F' • •
270
Clear ranking is possible with the Jenike Direct Shear Tester. Clear ranking was
not evident from the permeability tests since during testing of cohesive fly ash 'E' plugs
formed.
In regard to a particular material this Table suggest that mean particle size is an
accurate first basis indicator for ranking. Other characteristic dimension numbers derived
from particle size distribution also give clear ranking.
Out of eighteen columns of the table, twelve columns indicate highest rank of fly
ash 'E', followed by fly ash 'C indicated by six columns. Similarly, lowest rank of fly
ash 'A' is indicated by six columns out of thirteen columns.
Simple bench top tests like mean particle size, bulk density, Hausner ratio and
tensile strength are easy to perform and clear ranking of similar powders can be
evaluated, whereas rough and first indications of cohesion can be evaluated from the
bench top tests such as arch length and the drained angle of repose. Wall friction
properties can be evaluated from aerated piston tester or shear cell. Deaeration behaviour
may be evaluated from fluidization column or purpose built tester.
Actual pneumatic conveying flow characteristics of the fly ashes have shown
good dense phase conveyability [ Arnold et al. (1986) ]. All fly ash have more irregular
flow characteristics and higher pressure drop, tendency to block and fluctuation in
pressure drop except fly ash 'F.
It is important to know whether a powder is suitable for conveying. Some
powders are difficult to convey, whereas some m a y have unstable flow characteristics.
After conveying, a large range and number of powders, adverse powders should be
identified. The reasons for the poor system performance of those powders should be
understood.
Simple non cohesive powders can be conveyed pneumatically in either plug or
moving bed depending upon the powder's deaeration and permeability characteristics.
Increasing cohesion results in greater instabilities during pneumatic conveying and more
attention should be given to pipeline details e.g. bends, flanges, pipe internal smoothness
and blow tank design.
271
H H H X u z w CQ
z o r r -
< CQ
W l-H
H C-5 H OH O Oi Dm X CO <
5H
fa
fa
O O
z l-H
z
• • ON 00*
fa rj
CQ < H
2
3 3 E i
ca ii
S.
73
*-J*i
•a 1
r--•a *—
o X
T3
JrP C-H
T3 r-3 O
Oj r-r
fa
«
•X-o
c 3 Iii pi
6 E 0)
C ii
H -s c *53
HC
1
c •8
N
C o -rH •4-*-
• r -
o CM
O -"I
T3
in 00
X in oo
C-H
fr • i-H
r O
fr • H H C/5
CJ <0
OJ 00
3 • *-< C/3
C
Q
i-H
S, o CO •5 bf)
CO
0) t-J
c < t-0 Cr
rJ
•rH
T3 "HH
3
5
r -
a c
!r! 0.
-o 3
N •HH
CO
Q
CQ
O C L
CO
H
«+H
o
•5
/—s
3
"3
a D. O CO
U m o p.
B
w
w
<
<
w
w
w
w
p
u
ffl
w
w
-^
w
w
w
y
y
fa
P
Q
Q
U
<
fa
w
fa
y
Q
?<
y
y
y
y
<
<
P
_ * •
y
fa
Q
fa
w
Q
<
u
<
w
ffl
p
fa
'
1
1
ffl
u
fa
u
«
«
y
p
fa
ffl
<
<
«
p
i
•
i
fa
«
PQ
«
y
<
p
ffl
ffl
p
p
p
fa
<
i
i
•
i
<
<
<
Q
<
fa
<
<
y
ffl
fa
•
ffl
272
Location of aeration points is important. Obviously, unstable flow and blockages
are relavent when transporting powders with considerable cohesion. For such cohesive
powders, pneumatic conveying is possible by applying vibration or using by pass, pulse
dense phase and controlled slug techniques. Thus, cohesive powders can be maintained
in a fluidized state, offsetting to some extent, the deaeration of the powder. If aeration can
be effected, difficulties associated with pneumatic conveying of cohesive powders can be
reduced.
With incresed cohesion, only dilute phase is possible. Standard pneumatic
conveying systems are generally not suitable with extreme cohesion. For such materials,
reliable and consistent feeding is difficult. Hence, cohesion is an important powder
property when selecting a powder's optimal pneumatic conveying mode.
At present, flow assessment of powders is evaluated from pilot scale plant tests.
From these tests, pressure drop, air flow rate, solids flow rate and particle velocity are
obtained. However, information m a y be obtained from bench tests regarding selection of
conveying phase and system components. The powder property evaluation from bench
tests is very important to identify adverse flow behaviour before running a full scale plant
in view of a possible failure. Emphasis should be placed on powder properties from
bench top tests and correlations to determine conveyability in dilute, dense and super
dense phase pneumatic conveying. In view of the significance of the powder properties
examined the important powder property bench tests and their ranking to assess
pneumatic conveying suitability are shown in Table 8.10, whereas factors affecting
powder flow characteristics are shown in Figure 8.31.
gravity assist smooth surface air retentive
characteristics low compressibility constant permeability mono sized particles spherical
POSITIV E EFFECTS
CONSOLIDATION
NEGAT
rougl
VE EFFECTS
i surface pressure gradient
cohesiveness wall friction acceleration force gravity high compressibility sensitive permeability degradation wide particle size
distribution
PERMEABILITY
Figure 8.31: Factors affecting Powder Flow Characteristics.
274
TABLE 8.10: RECOMMENDED POWDER PROPERTY BENCH TESTS
FOR ASSESSMENT OF PNEUMATIC CONVEYING SUITABILITY.
Property
Mean Particle Size
Mean Particle Density
Particle Shape
Basic Chemical
Composition
Loose Poured and
Packed Bulk Density
Extent of cohesion
due to moisture content,
due to particle size
distribution and individual
powder properties f*
L o w Cohesion Powders
Fluidization *
Deaeration *
Permeability *
Slugging *
Particle Size Distribution
Safety and System
Properties
Crude Assessment
Visual
Estimate
Visual or assumed
Specified
Simple Test
Hand Squeeze Test
Moisture Test
Accurate Assessment
Particle Size Analysis, Sieving
Laser Particle Size Analyser
Pycnometer
Jenike Compressibility Tester
Moisture Test
(a) Ranking (b) System Design
(i) Hausner Slope and
Ratio intercept
of the yield loci
(ii) Angle of (c) Fundamental
Repose Knowledge
(iii) Arch (i) Tensile Tester
Tester (ii) Cohesion Tester
(iv) Tensile Tester
Fluidization Column
Deaeration Tester
Vibrated Fluidization column
Permeability Tester
Develop suitable test
Particle Size Analyses, Sieving
Laser Particle Size Analyser
275
Toxicity
Contamination Tolerance
Radioactivity
Wear Considerations
Hardness
Abrasiveness
Dustiness (Dust collector
sizing)
Particle Shape (for Wear
and Mechanical Inter
locking considerations)
Tendency for Mechanical
interlocking
Tendency for Degradation
Tendency for Segregation
Tendency for Agglomerate
Removal from Dust
Filter Media
M o h s Hardness Scale
Pilot Tests
Optical
Visual
Pilot Tests
Pilot Tests
Pilot Tests
Vickers Micro Hardness Scale
Particle Size Analysis
Scanning Electron Microscope
(SEM)
SEM
Pilot Tests and SEM
Particle Size Analysis
Pilot Tests
Wall Friction Test
I*
* Observe extent of segregation, stratification and fines elutriation from bed,
tendency to slug, channelling and bed collapse during testing.
* Tests should only be conducted on powders of low cohesiveness.
276
8.14 A NEW PHASE DIAGRAM FOR PNEUMATIC CONVEYING OF
POWDERS:
8.14.1 INTRODUCTION:
Powders are in two forms dry and moist. Depending on their internal properties,
powders will be either free flowing or cohesive. A free flowing and dry powder can be
conveyed successfully in a pneumatic conveying system, whereas moist and cohesive
powders are difficult to convey. In particular, cohesive powders may stick or clog in
pneumatic conveying system components.
This suggests that cohesion be included in a phase diagram for pneumatic
conveying. For fine powders, the effect of permeability and deaeration is well known
(Mainwaring et al. 1987, Jones et al. 1989) and have already been incorporated into a
phase diagram for pneumatic conveying. The present investigation suggests that this
phase diagram is incomplete and should be supplemented by the inclusion of cohesion for
fine powders and the effects of mechanical interlocking for coarse materials or materials
possessing high permeability and rapid deaeration. The resultant proposed complete
phase diagram is presented in Figure 8.32, whereas Figure 8.33 shows the phase
diagram from another orientation.
Mechanical
Interlocking t N o Flow or
Unstable
Reliable Moving Bed Flow
Reliable
Plug Flow
Dilute Phase Conveying
Deaeration Permeability
Figure 8.32: Pneumatic Conveying Phase Diagram.
277
A n examination of this Figure suggests that for fine low permeability materials,
cohesion, deaeration and permeability should receive primary attention. If the extent of
deaeration decreases, the powder can be easily moved in the form of a moving bed flow.
Unfortunately for extremely fine low permeability powders, as shown in the phase
diagram, excessive cohesive forces exceed the otherwise slow deaeration and low
permeability. Powders in this zone can only be conveyed by dilute phase conveying if at
all.
Whereas, for coarse materials, permeability and mechanical interlocking, in
particular, should be considered. For these materials, the high permeability generates
minimum air flow resistance and hence minimum flow body forces. The high
permeability is also associated with low air retention and rapid deaeration characteristics.
For these materials, mechanical interlocking causes flow difficulties. The high values of
permeability suggests that these materials can be conveyed in slug type flow or super
dense phase pneumatic conveying.
Cohesion
i
Deaeration
Mechanical
Interlocking
1 Large particles strong mechanical interlocking
Permeability
Figure 8.33: Pneumatic Conveying Phase diagram (Alternate View).
8.14.2 EFFECT OF COHESION ON THE PHASE DIAGRAM POWDER
PROPERTIES:
8.14.2.1 Permeability:
Permeability is a function of the overall packing, the size of void channels and the
shape of the voids. In some situations, large particles act as a filter membrane during the
flow mechanism. This causes the permeability to decrease. This action is compounded by
the tendency for the slip velocity of coarse particles to be high and of fine particles to be
278
low. However, if sufficient air flow can percolate the bed, bed expansion of the air inside
the voids will keep the powder bed in tension and hence counteract the effect of cohesion.
In view of the above trends, the following can be declared.
(i) High permeability;
Powders with high permeability are conveyed easily but rapidly deaerate. For
these powders, considerable powder expansion due to air expansion, percolation of air
into the powder plug act to reduce the frictional forces and prevent formation of immobile
powder slugs. However, formation of immobile slugs is promoted by the decrease in the
mean particle separation due to wall friction and the pressure driving force. This suggests
that coarse particles can be conveyed in slug mode and super dense phase pneumatic
conveying provided the wall friction is minimal.
(ii) Extreme permeability:
For very coarse particles, insufficient motive forces are generated by the fluid
drag effect relative to weight and frictional forces. Hence, these materials are difficult to
convey. For these very coarse particles, blockages due to mechanical interlocking is very
common.
(iii) L o w permeability:
Coarse powders with low permeability are difficult to convey using slug mode
conveying. However, fine low permeability powders exhibiting low deaeration rate can
be easily conveyed using fluidized moving bed technique. Furthermore, increasing fine
decreases permeability and increase cohesion. The low air permeability demands that
short lengths be used. Insufficient air flow and air expansion keep the powder bed from
compacting during dense phase fluidized bed conveying. In general, fine powders with
low permeability will exhibit high cohesion. Hence, such powders may be more difficult
to convey. Figure 8.34 shows the variation of the reciprocal of cohesion and
permeability with particle size.
(iv) Constant permeability:
If a powder exhibits constant permeability, no blockages are expected since the
mean particle separation will increase during conveying.
279
1/Cohesion and
Permeability
Particle size, d
Figure 8.34: Schematic Presentation of the Variation of Cohesion and Permeability with Particle Size.
8.14.2.2 Deaeration:
With cohesive powders the deaeration rate is fast initially then exponential,
whereas for free flowing powders deaeration rate is fast. However, with less cohesive
powders, deaeration rate is slow and linear generally. Such powders will be easy to
convey in the form of a moving bed. O n the other hand, the effect of cohesion is to cause
strongly cohesive powders to deaerate rapidly due to the formation of capillaries and
cracks within the bed. For these cohesive powders with high de-aeration rate a by-pass
system is used for conveying.
Reciprocal of Cohesion Uariation
1/Cohesion
and Deaeration
Resulting Optimal Trend
— I / d2
Particle size, d
Figure 8.35: Schematic Representation of the Variation of Cohesion and Deaeration versus Particle Size.
280
Figure 8.35 shows the variation of reciprocal of cohesion and deaeration versus
particle size, whereas the variation of reciprocal of arch length and Hausner ratio with
particle size of the fly ashes tested are depicted in Figure 8.36.
In regard to flow characterization, powders having a wide particle size distribution
generally exhibit segregation effects and higher cohesion. Fine particles with a wide
particle size distribution have a poor air retention capacity and are not suitable for super
dense phase pneumatic conveying.
e
o L <
Vl *
e u
0.03
0.02 -
0.01 -
0.00
-tl
X w e o VI * -fi
e u 8 10 12 14
Particle Size, p m
Figure 8.36: Variation of Reciprocal of Arch Length and Hausner Ratio with Particle Size of Fly Ash Tested.
8.14.2.3 Mechanical Interlocking:
Noting that the air pressure gradient and air flow drag provide the motive forces
in pneumatic conveying. This suggests that for extremely permeable powders mechanical
interlocking, refer Figure 8.37 becomes the predominant factor causing slug formation or
the occurrence of a pipe blockage. For these powders any factor which causes the
interparticle spacing to decrease m a y result in a pipe blockage or require the use of a slug
creation or slug length control techniques to maintain flow. Factors which m a y cause the
interparticle spacing to decrease include wall friction, pipe irregularities, bends, etc. The
parameters controlling the mean interparticle spacing are summarized in Table 8.11.
Figure 8.38 shows the variation of permeability versus the expected extent of
mechanical interlocking. In particular, this Figure reveals that large particles exhibiting
high permeability have a strong tendency for mechanical interlocking and hence are
difficult to convey in pneumatic conveying.
281
O 0 Q QZ5.0
00(7 0
MECHANICAL INTERLOCKING
Figure 8.37: Mechanical Interlocking.
TABLE 8.11: FACTORS INFLUENCING THE MEAN INTERPARTICLE SPACING
Increasing
L o w cohesion
Gas expansion
Gas viscosity
Fluid turbulence
High permeability
Smooth surfaces
Uniform particle size
L o w consolidation
Insensitive properties
Decreasing
Strong cohesion
Long slug length
High bulk density
Wall friction
L o w permeability
Rough surfaces
Wide particle size
High consolidation
Sensitive properties
Mechanical Interlocking
Resulting Optimal Trend
Mechanical
Interlocking
Permeability
Figure 8.38: Schematic Representation of the Variation of Permeability versus Mechanical interlocking
282
8.14.3 INFLUENCES ON COHESION-
The following powder properties, which influence cohesion/ flowability, are
briefly described as follows:.
1 • Moisture Content: A slight increase of moisture to a dry powder can transform it
into a cohesive powder e.g. the addition of as little as 0.6 % moisture to dry sand will
change free flowing dry sand into a cohesive material.
2. Hygroscopicity: Powders having a hygroscopic nature tend to be very cohesive
especially if they absorb moisture from the conveying air. Hence, hygroscopic powders
are difficult to convey pneumatically. To prevent this absorption, it is recommended that
dry conveying air and a closed system should be used.
3. Agglomeration: Agglomeration of particles results in larger particles, changes in
shape and porosity.
4. Thermoplastics: Powders which are thermoplastics may be extremely cohesive
during flow due to the softening effects associated with thermal, friction or impact
stresses during flow.
8.14.4 TESTS TO ASSESS COHESION:
The following bench tests can be used to assess cohesion / flowability
characteristics of powders which can be divided into two categories, namely direct and
indirect tests.
(A) Direct Tests:
1. Hand Squeeze Test:
This is a simple qualitative test for flowability/ cohesion assessment of powders.
In this crude test, a powder sample is squeezed by hand to observe whether the sample
divides easily or remains as a lump.
2. Angle of Repose:
This simple test provides an qualitative and partial indication of cohesion /
flowability. Unfortunately, for cohesive powders, test results are highly variable with the
result that is is difficult to rank measurements.
More quantitative methods/tests to assess cohesion now follow:
3. Arch Tester:
The minimum width required to assume flow from a mass or plane flow hopper/
bin reveals a direct measure of flowability/cohesion of powders and other bulk solids. By
the use of this rig, it is convenient to compare seemingly similar cohesive powders.
283
4. Direct Shear Tests:
Cohesion can be assessed from the instantaneous yield loci obtained from shear
tests. Simple testers like the Uniaxial Compression Tester can also be used for relative
measurements. One advantage of the information gained from shear tests is that, in
particular, that from the Jenike Shear Tester, the same information can be used in the
geometric design of blow tanks and storage vessels. Here, the relevant parameters are the
slope of the flow function and its intercept with the unconfined yield strength axis. Here,
the relevant parameters are the slope of the flow function and its intecept with the
unconfined yield stress axis.
5. Tensile Tester:
Tensile strength can be assessed from the various tensile strength testers
depending on the direction of pull with respect to the applied consolidation force. In
particular in the split cell testers, the sample is subject to tensile forces perpendicular to
the direction of consolidation. In comparison, other testers exert tensile forces on the
powder in the same direction as that of the applied consolidation force.
(B) Indirect Tests:
1. Compressibility Constant (b): One of the indirect tests to assess cohesion is
the slope of compressibility versus consolidation stress variation plotted on a log-log
basis. This slope is referred to as the powder's compressibility (b).
2. Slope of F l o w Function: A steep flow function slope indicates high powder
strength sensitivity with increasing consolidation.
3. Hausner Ratio: This is a simple and rapid test for evaluation of a powder's
cohesion / flowability for seemingly similar powders based on bulk density
measurements. The use of this ratio of the packed to loose poured bulk density to indicate
a powder's cohesiveness was first suggested by Geldart et al. (1984).
4. Wall friction: Based on the simple assumption that cohesive powders exhibit large
wall friction angles. It is suggested that wall friction properties evaluated from an
aerated piston tester or shear cell be used to assess the extent of cohesion.
8.15 TEST PTFFTCITI TIES:
8.15.1 Aicliiii&i
For very cohesive powders, arching occurs across the outlet of the tester and
renders measurement of the arch length impossible.
8.15.2 Deaeration:
Difficulties experienced during the conduction of the deaeration tests include
powder lining the inside of the tube wall. Furthermore, since the top surface of the
powder bed level falls rapidly, measurement of the bed height using a video camera is
284
recommended. However, in a large scale deaeration test, visual bed height measurement
may be possible.
8.15.3 Fluidization:
For cohesive Group C powders, fluidization is difficult. During testing,
channelling, slugging and lifting as a plug occur. For fluidization testing of cohesive
powders, it is suggested that the column be closed with a sealed porous end creating
high resistance to prevent fine and submicron particles leaving the bed. For extremely
cohesive powders fluidization testing of powders is not practical nor meaningful.
8.15.4 Cohesion:
Some low permeability powders may rapidly deaerate due to cohesion. The
measurement of the permeability of such powders is difficult to measure due to strong
cohesive forces. Such powders may locate in zone T in the phase diagram (Figure
8.39). But they are expected to be located in moving bed flow zone as shown '2' in the
diagram.
Figure 8.39: Specific Examples of Powder Properties with respect to the Proposed Powder Conveying Phase Diagram.
8.15.5 Permeability:
T w o main problems associated with the conduction of the Jenike Permeability
Test result from the effect of cohesion. In particular, strongly cohesive powders will
form a plug and extrude from the tester on application of the pressure differential.
Whereas powders with low cohesion may exhibit segregation effects at low
consolidation. This segregation may result in fines leaving the powder bed. One
alternative procedure, to partially overcome' these problems, is to evaluate a powder's
285
nominal permeability from the fluidization column. Unfortunately, such a evaluation does
not provide knowledge of the permeability variation with consolidation.
8.16 EFFECT ON CONVEVINO-
8.16.1 Particle Shape;
Particle shape and surface characteristics of powders can be examined with a
Scanning Electron Microscope. Intuition suggests that irregular, sharp and angular
shaped particles are associated with high pressure drops. Furthermore, fragile and brittle
powders exhibit increasing fines percentage which, in turn, results in increasing
cohesion and decreasing permeability. Likewise, a fibrous powder is difficult to control
in a blow tank system. Powders having considerable flakiness and hence strong
tendency to mechanical interlocking are also difficult to convey.
Obviously, particle shape significantly controls the wear of compressor
components (in vacuum systems), rotary feeders and venturi feeders. In particular, sharp
edged hard particles rapidly wear the same, whereas soft spherical particles effect reduced
wear on system components directly exposed to the particle stream.
8.16.2 Air Compressibility:
The pressurized air will expand as the air pressure decreases along the conveying
pipe. This expansion imposes tensile stress in the flowing powder. The existence of
tensile forces in the flowing powder implies increasing mean interparticle distance. This
increasing interparticle distance generally promotes stable flow. The maintenance of
stable flow by the generation of tensile forces in the powder bed is the basis of by pass
secondary air techniques. However, it should be noted that secondary air can generate
compressive forces, if the by pass line becomes blocked downstream and the material
conveyed is cohesive or exhibits high wall friction and low permeability.
Air expansion along the pipeline causes an increase of superficial air velocity
which results in more energy consumption and wear of system components. For short
pipelines with low pressure drops, air can be considered as an incompressible fluid but
for long pipelines due to air expansion and increase of superficial velocity, it is c o m m o n
to design the pipeline with increasing pipeline diameters to control velocity near the end
of pipeline.
286
8.17: OTHER CONSIDERATIONS:
1. Toxicity: For toxic powders, precautions should be taken in the design of system
components and strict control is required for conveying operation. A vacuum and closed
system is required for toxic powders to prevent leakage to the atmoshphere.
2. Combustibility: Closed system should be used for powders having
combustibility property. Alternatively, inert gas conveying is necessary. Inert gases
include nitrogen, combustion gas C O 2 suitably cooled and dehumidified.
3. Hardness: Harder particles have more flowability. In general, hard incompressible
particles exhibit high internal friction angles and wall friction angles.
4. W e a r : Large diameter pipes should be used near the end of conveying line. The
conveying velocity should be low to reduce wear. Harder particles increases wear.
5. DegradLaliojoi Powders which degrade during flow effectively become fine and
hence more cohesive as they are conveyed. To prevent breakage of particles, low velocity
conveying and minimum number of bends should be used. Sharp changes and
discontinuity in pipeline or rough sections should be avoided.
6. Electrostatics: A n electrostatic powder usually exhibit unexpectedly high
cohesion and wall friction due to the occurrence of localized zones of oppositely charged
particles or wall boundaries. These powders may stick or clog in blow tanks, pipeline
and filters. Static eliminators provided in cyclone and bag-filters prevent build up of
powder. A static inhibitor coating can be used. For conveying of plastics, pipeline should
be grounded to earth.
6. Hygroscopicity: Dry conveying air should be used to convey hygroscopic
powders. These powders will cake and create problems in conveying.
287
8-18 PNEUMATIC CONVEYING CHARACTERISTICS:
The experiments were performed to obtain the steady state operating conditions. The
conveying characteristics results for cement and Wheat are tabluated in Tables 7.19 and
7.20, respectively. The air flow valve setting was used in the range of 0.05 - 0.09 kg/sec. It
was revealed, from experiments No. 21 to 25, refer Table 7.20 that as the air flow rate
increases, the powder velocity also increases. The solids flow rate was evaluated from the
slope of the load cell response graph.
The pressure differentials of a friction loop for cement and Wheat conveying
upwards are shown in Table 8.12. Furthermore, in this series of experiments, air pressure
fluctuating and unsteady flow were observed.
TABLE 8.12: PRESSURE DIFFERENTIALS FOR FRICTION
LOOP
Exp.
No.
5 <
12
13
14
15
16
Pressure differential
conveying upwards, kPa
Material: cement
8
18
10
5
8
11
Exp.
No.
22
23
25
31
34
35
Pressure differential
conveying upwards, kPa
Material: Wheat
4
7
10
4
4
2
Particle velocity was measured experimentally by using a Tealgate 200 series
transducer in combination with a Hewlett Packard Correlator as described in Section 6.2. The
transducer was located some 3 m. from the blow tank so that suspension flow is fully
developed. The particle velocity was obtained during the conveying cycle from the cross
correlation signal appearing on a Hewlett Packard Correlator screen before averaging out the
observations noted during the conveying cycle. The correlator time scale selected was 333
ps / m m .
Typical variations of the particle velocity during a conveying cycle for cement and
Wheat are graphically revealed in Figures 8.40 to 8.42. It is revealed from these Figures that
288
particle velocity variations are periodic cyclic oscillations. Particle velocity variation during
the conveying cycle is an indirect indication of the extent of steady state flow.
The variation of volumetric air flow rate with time for cement is shown in Figure
8.43. Figure 8.44 shows the variation of solids flow rate versus air mass flow rate for
cement and Wheat. The solids flow rate increases with increasing air flow rate. It can be
revealed that cement can be conveyed with less air mass flow rate as compared with Wheat.
The solids mass flow rate and mass flow ratio variations for cement and Wheat are
shown in Figure 8.44. In particular, the air mass flow rate for cement and Wheat was in the
range of 0.011 - 0.63 and 0.05 -0.0635 kg/sec, respectively. It can be seen that cement for
the flow conditions observed has higher mass flow ratio as compared to that for Wheat.
u <_
u 0 >
CO
O-
Time, Sec.
Figure 8.40: Variation of Particle Velocity with Time for Cement.
Time, Sec.
Figure 8.41: Variation of Particle Velocity with Time for Cement.
LEGEND Exp. No.
- a — 21 -P 22 - B — 23 -p 25
Time, Sec.
ure 8.42: Variation of Particle Velocity with Time for Wheat.
400
LEGEND Exp. No.
-B 31 — • — 32 - a — 34 —0 35
gure 8.43: Variation of Volumetric Air Flow Rate with Time for Cement.
3U -
40 i
30 -
20-
•
P
• • l i •
•
•
•
I • • 1
• •
•
1 ' l
P
B _ _
• • QD
i i | i i
LEGEND Material
• Wheat
• Cement
0.2 0.4 0.6 0.8 1.0 1.2 1.4
Solids Mass Flow Rate, kg/sec
Figure 8.44: Variation of Mass Flow Ratio with Solids Mass Flow Rate for Cement and Wheat.
291
CO
CC
5 o
(0
32 o w
LEGEND EI Cement • Wheat
-i 1 r-
0.01 0.02 0.03 0.04 0.05 0.06 0.07
Air Mass Flow Rate, kg/s
Figure 8.45: Variation of Solids Flow Rate versus Air Mass Flow Rate.
Tables 7.21 and 7.22, refer Chapter 7 summarize the conveying characteristics of
sand. At higher air flow rates, as can be seen from experiments 25 to 29, 34, 35, 41, 46, 47
powder velocity increases and with medium air flow rate such as in experiments 30 to 33,45,
79 and low air flow rate, experiments 43, 44, 51, 52, 71, 80, 81, lower particle velocity was
observerd. '
The variation of particle velocity obtained experimentally and predicted for low,
m e d i u m and high air flow rate is depicted in Figure 8.46. The predicted velocity was
calculated from the equation shown in Appendix C. From an examination of Figure 8.46, it
is apparent that the actual observed particle velocity deviated increasingly from the predicted
velocity with increasing air flow rate.
Cycle time is effected by blow tank pressurization which is revealed in experiments
29, 30, 34 to 37 and 39 with a blow tank pressure of 150 kPa. Here, the experiment cycle
time average w a s 3 min., 32 sec, whereas at lower blow tank pressure of 125 kPa,
experiments 41, 42, 43, 47 the average cycle time was 4 min. and 6 sec.
Figure 8.47 reveals the particle velocity variations with time during experiments 54,
53 and 77 at an initial blow tank pressures of 180, 175 and 175 kPa, respectively, whereas
Figure 8.48 reveals the particle velocity variation with time during experiments 74,75 and 76
at the initial blow tank pressure of 150 kPa.
2 9 2
The air mass flow rate as per valve setting for both Figures were 0.09, 0.085 and
0.0635 kg/sec, respectively. A higher particle velocity with increased air mass flow rate
trend is generally evident from Figures 8.47 and 8.48 even though the Figures indicate
considerable scatter.
o 0) (0
u o >
CO T3
o CO
•o
30
20 -
10 -
LEGEND
AIR FLOW
• HIGH • MEDIUM
a L£W
5 10 15
Experimental Solids Velocity, m/sec.
Figure 8.46: Variation of Experimental versus Predicted Solids Velocity for Sand.
u 9) 0)
O O
ti
>
o
(0 Q.
LEGEND
Exp. No.
-v— 54 - • — 53
-fl— 77
0 20 40 60 80 100 120 140
Time, Sec.
Figure 8.47: Variation of Particle Velocity versus Time.
293
o
-2 E
Ss mm
o o "3 > 73 "«H
a Q.
<-U -
10-
C
• B
)
B •
• B
• B
i | i i
50
•
D
• B
B B
i i | i
100
•
D
•
LEGEND
EXP. No.
B 74 • 75 B 76 '
150 Time, Sec.
Figure 8.48: Variation of Particle Velocity versus Time.
Transducer air pressure variation versus distance from blow tank are shown in
Figures 7.91 to 7.96. Details of transducer distances from blow tank are shown in Table
7.18. Figure 7.91 and 7.92 depict the variation of air pressure with initial blow tank pressure
of 150 kPa, whereas Figure 7.93 depicts with 175 kPa of high air flow rate. Figures 7.94,
7.95 and 7.96 depict the variation of air pressure at the initial blow tank pressure of 125 kPa
with low, high and medium air flow rate, respectively.
Table 8.13 presents the calculated values of superficial air velocity, actual air
velocity, voidage, slip velocity and Froude number derived from Table 7.21. Also, the
friction factor, calculated using the Konno et al. (1969) correlation, is included to indicate the
considerable range of test conditions.
To understand the variation of flow parameters with voidage during flow various
phase diagrams were prepared. In particular, Figure 8.49 depicts the variation of slip velocity
versus ( 1 - voidage ) for three different but almost constant solids mass flow rates. With
low solids mass flow rate, it is revealed that slip velocity decreases with (1 - voidage),
whereas with a high solids flow rate slip velocity increases. This trend is similar to that
reported by Klinzing et al. (1986).
294
T A B L E 8.13:
Exp.
No.
23
24
25
26
28
29
30
31
33
35
37
38
39
40
41
42
43
44
45
46
47
49
50
51
52
53
58
Superficial air
velocity
Vf rn/sec.
33.18
40.51
40.51
44.369
40.51
41.282
39.353
38.969
35.881
31.251
30.865
28.936
25.85
26.621
28.628
28.55
25.464
6.559
27.779
32.409
29.708
-
27.393
21.721
22.377
27.393
-
AIR VELOCITY, SLIP VELOCITY AND FROUDE
NUMBER
Material: Sand
E
0.843
0.788
0.791
0.863
0.882
0.788
0.804
0.763
0.796
0.775
0.775
0.767
0.801
0.847
0.849
0.772
0.832
0.741
0.831
0.721
0.829
0.828
0.748
0.824
0.686
0.704
0.734
Vf
m / sec.
39.359
51.409
51.214
51.413
45.93
52.388
48.946
51.072
45.077
40.324
39.826
37.726
32.272
31.43
33.72
36.982
30.606
8.852
33.428
44.95
35.836
-
36.622
26.36
32.62
38.911
-
Slip velocity
m / sec.
27.079
34.963
34.998
35.745
28.286
35.879
37.184
36.014
29.066
22.294
23.073
22.013
18.06
37.269
14.935
20.295
16.067
-3.882
22.206
28.608
16.671
-
25.22
15.414
20.554
26.073 —
Froude
number
46.456
56.714
56.714
62.116
56.714
57.795
55.094
54.556
50.233
43.751
43.211
40.51
50.685
37.269
40.079
39.97
35.649
9.183
38.891
45.372
41.591
-
38.35
30.409
31.328
38.35
'
Friction
factor
0.0066
0.0049
0.005
0.0052
0.0046
0.0049
0.0069
0.0054
0.0051
0.0045
0.0048
0.0052
0.0057
0.0046
0.0043
0.0049
0.0056
0.0064
0.0072
0.005
0.0042
-
0.0071
0.0074
0.0067
0.0063
295
62
63
64
66
67
70
71
72
73
74
75
76
77
78
79
80
81
82
11.767
26.814
32.794
28.55
23.843
-
24.306
28.55
-
32.409
30.865
24.692
26.621
31.637
31.637
23.535
23.535
28.55
0.675
0.614
0.78
-
0.706
0.839
0.696
0.797
0.828
0.745
0.816
0.667
0.817
0.835
0.812
0.853
0.801
0.757
17.433
43.671
42.044
-
33.772
-
34.922
35.822
-
43.502
37.825
37.019
32.584
37.889
38.962
27.591
29.382
37.715
-
-
-
-
-
27.557
26.835
-
33.367
29.341
29.586
25.103
-
29.773
19.717
20.673
28.621
16.474
37.539
45.911
39.97
33.38
-
34.028
39.97
-
45.372
43.211
34.568
37.269
44.292
44.292
32.949
32.949
39.97
.
-
-
-
-
0.0111
0.0091
0.0095
0.008
0.0096
0.0109
0.0108
0.0109
0.0088
0.0103
0.0093
0.0089
Furthermore, Figures 8.50 and 8.51 reveal the variation of superficial air velocity
versus (1- voidage) and mass flow ratio, respectively. The scatter in the results depicted
in Figures 8.50 to 8.51 appears to depend on many parameters including particle
diameter, density, shape, air velocity, particle-wall interactions, pipe diameter, etc..
Figure 8.52 shows the variation of pipeline pressure drop versus air mass flow rate at
initial blow tank pressures of 125, 150 and 175 kPa, respectively. Pipeline pressure drop
was evaluated by taking first pipeline air pressure differential alone, which can be
approximated as the experimental pipeline pressure drop [ Arnold et al., (1986)]. This series
of experiments indicated that with increasing initial blow tank pressure, an increase of
pipeline pressure drop results. However, no clear trend was evident.
Figure 8.53 shows the variation of mass flow ratio versus initial blow tank pressures
of 120-125 and 170-175 kPa. It is suggested, from the limited observations, that the mass
flow ratio increases with an increase in initial blow tank pressure.
29
-HJ -
30-
•
20-
10 i
•
• 1
• •
•
I i i 1 i
nn
• i • i
LEGEND
SOLIDS MASS FLOWRATE
B 0.857 - 0.886 • 0.957 - 0.984 ""- 1.214-1.234
kg/sec
0.14 0.20 0.22
(1 - Voidage)
Figure 8.49: Variation of Slip Velocity versus (1 - Voidage) for Sand.
DU ~
40 -
30-
20 -
B
Q
1 1
•
•
•
T — ' " ' 1 '
DB
• I • • —
0.14 0.16 0.18
(1 - Voidage )
0.20
LEGEND
SOLIDS MASS FLOWRATE
B 0.857-0.886 • 0.957-0.984 B 1.214-1.234
kg/sec
0.22
Figure 8.50: Variation of Air Superficial Velocity versus (1 - Voidage) for Sand.
1
u ti in
u o ii
>
ra o
0) Q.
cn
LEGEND
SOLIDS MASS
FLOWRATE
•0.857-0.886
-•—0.957-0.984 -fl—1.214 - 1.234
kg/sec
Mass Flow Ratio
Figure 8.51: Variation of Superficial Air Velocity versus Mass Flow Ratio for
Sand.
(0
a -C
a. o
-to (0 ai
0)
"3 a.
ca H-
o
ure8
140
120 -
100 -
LEGEND
Initial Blow
Tank Pressure
B 125
• 150 fl 175
kPa
0.06 0.07 0.08 0.09 0.10 0.11 0.12
Air Mass Flow Rate, kg/sec.
52: Variation of Pipeline Pressure Drop versus Air Mass Flow Rate for Sand.
298
0 r H
a CC 3 0 LU
n co S
26-
24-•
22-• .
20-.
18-,
16-
14-
12-110 120 130 140 150 160 170 180
Initial Blow Tank Pressure, kPa
Figure 8.53: Variation of Mass Flow Ratio versus Initial Blow Tank Pressure for Sand.
8.18.1 BEND PRESSURE DIFFERENTIAL OBSERVATIONS:
As stated in Section 7, two bends types were examined namely long radius and
vortice elbow, refer Table 7.23. For experiments 51, 56 and 20, the observed pressure
differential across the long radius bend was 8, 5 and 5 kPa, respectively. In comparison,
for the vortice elbow the pressure differential was 4 kPa as revealed in experiments 21,22
and 23.
This observation is consistent with that of Paulson et al. (1983) who also reported
that vortice elbow bends give slightly less pressure drop than long radius bends.This increase
may be due to the considerable pipe length in the latter. Obviously, the pressure drop in a
radius bend will depend upon the ratio of the bend diameter to the pipe diameter, surface
roughness and actual pipe length around the bend.
8.18.2 CONCENTRATION:
From the concentration variation observed in experiments 24 and 26, the
concentration peaks were higher for experiment 26 as compared to 24 due to the higher initial
blow tank pressure and air mass flow rate. For experiments 28, 29 and 30, it was observed
that the powder concentration was highest for experiment 30. Here, the blow tank pressure
was 150 kPa and air flow rate 39.94 m3/hour.
B B
->—r
•
*
LEGEND Initial Blow
Tank Pressure
B 120-125 • 170-175
kPa
2 9 9
Surprisingly, a lower concentration was observed in experiment 29 even though the
blow tank pressure was 150 kPa. However, the air flow rate was higher at 43.3 m3/hour.
This higher air flow rate results in more dilute flow compared to that for experiment 30. In
comparison, in experiment 28 the blow tank pressure was lower at 125 kPa and the solids
concentration was less.
In experiment 32, the blow tank pressure was 130 kPa, average air flow rate was
34.8 m3/hour. For this experiment, the concentration was observed to exhibit very uneven
peaks which, in turn, indicates the flow to be unsteady. For experiment 33, the blow tank
pressure was 140 kPa and average air flow rate 37.6 m3/hour. In this case, higher
concentration peaks were observed.
In experiment 36, higher concentration was observed compared to experiment 34 and
35, under relatively similar experimental conditions. However, in experiment 39, the
powder concentration was higher compared to experiment 40 and lowest for experiment 37.
In experiment 39, unstable flow was observed due to the high mass flow ratio.
In experiment 42, the concentration was higher compared to that in experiment 41.
This higher concentration may be due to the higher solid to air mass flow ratio of 17.9 as
compared to* 11.8 with experiment 41. In experiment 48, high concentration and unsteady
flow was observed compared to that for experiment 47. The initial blow tank pressure was
the same in both experiments. These trends are probably due to the reduced air flow rate.
8.18.3 LOW VELOCITY CONVEYING:
From Table 7.26, refer Chapter 7, it is evident that the increase in air supply
pressure results in a higher air mass flow rate. The range of mass flow ratio in this test series
was from 13 to 53. Obviously, an increase in air pressure and blow tank pressure results in
increased tonnage of material conveyed.
Figure 8.54 reveals the variation of solid to air ratio versus initial blow tank pressure,
whereas Figure 8.55 reveals the variation of mass flow rate of solids versus initial blow tank
pressure for Wheat. In particular, the observed range was from 120-200 and 200 - 300 kPa
under both low and high pressure conditions. It is evident from Figure 8.54 that at the higher
initial blow tank pressure i.e. 200 - 300 kPa, the solid-air ratio was less than that observed
when the initial blow tank pressure range was 120 - 300 kPa. Unfortunately, no clear trend
300
was evident for the variation in mass flow rate of solids versus with initial blow tank
pressure, refer Figure 8.55.
cc
<
o cn
ou -
40-
30-
20-
10 -
B
B
•
B H •
— , — ,. r ,,.
•
•
•
1
•
•
LEGEND
INITIAL BLOW
TANK PRESSURE
B 120-200 • 200-300
kPa
100 200 300
Initial Blow Tank Pressure, kPa
400
Figure 8.54: Solid-Air Ratio Variation with Initial Blow Tank Pressure for Wheat.
cfl
O
m*
co 0C
o
c/> its CO
LEGEND
INITIAL BLOW
TANK PRESSURE
B 120-200 • 200-300
kPa
100 200 300
Initial Blow Tank Pressure, kPa
400
Figure 8.55: Mass Flow Rate of Solids with Initial Blow Tank Pressure for Wheat.
Furthermore, from Table 7.26, it is revealed a reduced solids mass flow rate was
observed at the higher set pressure for experiments 5 and 8. These experiments were
performed at constant blow tank pressure (220 kPa) but with set pressures of 300 and 450
kPa, respectively This indicates that higher set pressure results in a reduced solids flow rate.
301
It should be noted that the set pressure provides the necessary aeration of material in pipeline
to be conveyed at low velocity.
Further information concerning the flow behaviour was gained by visual observation
of the plug flow behaviour. This observation was made using the sight glass located along
the low velocity conveying rig. This also facilitated measurement of the time taken for each
plug to pass through the sight glass. The observations are tabulated in Table 8.14. From
Table 8.14, it is evident that the plug velocity during low velocity conveying ranged between
0.66 - 1.02 m / sec. This velocity range is much lower than the observed dilute phase
conveying range of 7 -19 m/sec, refer Table 7.21.
TABLE 8.14: PLUG VELOCITY AND LENGTH
Material: Wheat
Experiment
No.
22
23
25
26
27
28
29
30
Plug Velocity
m / sec.
0.8
1.02
0.8
1.0
0.7
1.02
0.7
0.7
No. of
observations
13
15
6
10
9
8
3
5
Plug Length
cm.
11.07
12.8
10.0
11.5
12.25
13.17
13.25
13.25
No. of
observations
6
9
2
2 I 4
3
4
4
These observations are consistent with the fact that Wheat locates in Group B of the
Dixon diagram and it is a good candidate for low velocity dense phase conveying with strong
axisymmetric plugs. In this super dense phase conveying, Wheat was transported at
extremely high levels of volumetric concentration which are not possible in conventional
pneumatic conveying systems. In this mode, air pushes the plugs and percolates through
them. Also, towards the end of the conveying cycle high velocity purging was not present. It
is interesting to note that the conveying cycle is able to be stopped and restarted at any time.
This can be achieved due to the low velocities that are used during the conveying cycle and
the high permeability of the slug consist.
302
Figure 8.56 shows the variation of solids mass flow rate versus air mass flow rate for
two different test series (A and B) with two pipeline lengths, namely 97 and 51.3 m,
respectively, whereas Figure 8.57 shows the variation of average blowtank pressure versus
air mass flow rate. Full test details are presented in Table 7.25. In particular, a higher
solids mass flow rate and a lower average blowtank pressure were observed with the smaller
pipeline length test series.
*H
CO
CC
5 o loco co co
S CO •v
o CO
3 -
2 -
1 0.01 0.02 0.03
Air Mass Flow Rate, kg/sec.
LEGEND Test Series
B A • B
Figure 8.56: Solids Mass Flow Rate versus Air Mass Flow for Wheat.
ra
a -i
— -CO CO
-t
c ca
3 o CO © cn a »-0)
> <
300
200 -
100 0.02 0.03 0.04
mf, kg/s
LEGEND
Test Series
B A • B
0.05
Figure 8.57: Average Blow Tank Pressure versus Air Mass Flow Rate for Wheat.
303
Figure 8.58 shows the variation of pipeline pressure drop versus air mass flow rate
for solids mass flow rate in the range 0.47 - 0.69 and 0.9 - 1.01 kg / sec. Here, the pipeline
pressure drop was approximated by taking air pressure measurements near the blow tank
outlet. Unfortunately, no clear trend was evident in this Figure.
CO
O.
a. o
CO
co cu
0)
c 0)
a.
300
200 -
100 0.02 0.03 0.04
Air Mass Flow Rate, kg/sec.
0.05
LEGEND
Solids Mass
Flow Rate
B 0.471 - 0.692 • 0.902-1.014
kg/sec.
Figure 8.58: Pipeline Pressure Drop versus Air Mass Flow Rate for Wheat.
304
CHAPTER 9 CONCLUSIONS
9.1 GENERAL CONCLUSIONS:
General conclusions drawn from the investigations include:
[1] Scanning Electron Microscope observations are a very useful tool for revealing
vital information concerning powder properties particularly size, distribution, surface
characteristics, surface roughness, shape, porosity, pore shape, ease of degradation, the
presence of sharp or cutting edges promoting erosion, presence of fine or ultra fine
particles, fine particle agglomerates, effective transport volume of particles, etc. Hence,
information from Electron Microscope provides a good insight into and understanding of
powder flow characteristics during pneumatic conveying. As this information provides
fundamental details of powder properties, it is recommended that when additional
knowledge of powder pneumatic conveying flow behaviour is required, the powder
should be examined under a Scanning Electron Microscope.
[21 In pneumatic conveying, particle density, bulk density, compressibility,
permeability and particle size are important parameters. Knowledge of these powder
properties is essential for predicting pneumatic conveying behaviour. These are simple
and rapid powder properties to evaluate and compare flowability of seemingly similar
powders. Furthermore, particle size plays a significant role in determining a powder's
cohesiveness, fluidization and deaeration characteristics.
[3] The mean particle size and deaeration time are very important parameters
influencing the magnitude of the drained angle of repose. In general, for the powders
tested the drained angle of repose increased as the powder mean particle size decreased.
Also, the drained angle of repose increased as the deaeration and fill time increased.
Furthermore, the angle of repose exhibited by hygroscopic powders is sensitive to the
test ambient relative humidity conditions. In particular, the drained angle of repose
increases as the test environment relative humidity increases.
305
[4] For hygroscopic powders, pneumatic conveying should be effected by using dry
conveying air or gas.
[5] Usually, powders displaying high cohesive strength exhibit flow problems. On the
other hand, powders displaying minimal cohesive strength present few difficulties. Powders
which are cohesive also display a tendency to form ratholes in blow tanks during discharge.
Cohesive powders m a y stick in discharge hoppers, feeders and conveying pipelines.
Strongly cohesive powders usually exhibit plug behaviour during pneumatic conveying.
[6] For cohesive powders, the deaeration rate is fast initially and exponential. Deaeration
behaviour indicates the air retentive properties of powder wjuch indicates the ease or
otherwise of pneumatic conveying by slug or plug flow. Generally, powders with long
deaeration times are good candidates for moving bed flow pneumatic conveying.
[7] In general, on the basis of powder properties determined from bench tests, the
flowability of powders in pneumatic conveying systems can be predicted as follows:
high cohesion, rough particle surface, wall friction, increasing particle density,
increasing particle size, high coefficients of restitution, high drained angle of repose are
material parameters which act as flow resistances in pneumatic conveying.
slow deaeration, smooth particle surface, low packed bulk density are material
parameters which promote flow in moving bed flow.
high permeability, smooth particle surface, incompressible, consolidation stress,
insensitive properties promote conveyance by low velocity conveying.
[8] For powders exhibiting intermediate and extreme values of cohesion, wall friction,
particle size, particle density and permeability; prime mover, blow tank, aeration device/s
(primary, secondary and tertiary (along pipe)), air addition location and method (e.g.
annular), pipe details (number of bends, bend radii, steps, divergence, joints, etc.) and
discharge arrangement characteristics are paramount in determining pneumatic conveying
characteristics.
[9] Pneumatic conveying of powders is complex due to the interaction of a vast number
of individual powder properties. Assessment of pneumatic conveying characteristics can be
306
effected by performing pilot scale tests on pneumatic conveying rigs with the
observations and results suitably scaled up for actual plant conditions. However, for
dense phase and super dense phase pneumatic conveying emphasis should be placed on
powder properties determined from bench tests. These tests should include the specified
tests listed in Table 8.10. This Table also indicates the order of priority for conduction of
the test based on findings from this investigation. This ranking is given with the aim to
determine and identify as accurately and quickly as possible whether a given material will
be suitable for conveying or not. If conveyable, the tests should also be used as a guide
to select the most suitable mode of conveying. The implications of the test results are also
declared in Figure 9.1.
9.2 SPECIFIC CONCLUSIONS:
Specific conclusions drawn from this investigation include:
[1] The ratio of packed to loose poured bulk density or Hausner ratio is an relatively
simple bench test to compare cohesion of seemingly similar powders. The
compressibility of finer materials can be distinquished easily from packed bulk density
test.
[21 Measurement of characteristic dimensions from a particle sizer based on the
principle of Fraunhofer diffraction can indicate indirectly pneumatic conveying flow
behaviour of powders.
[3] Generally, the Jenike and Walker methods provide arch length higher than from
the Cohesive Arch Tester experiments.
[4] The Cohesive Arch Tester provides convenient comparative assessment or
ranking of a powder's cohesive strength. In this tester, significant cohesiveness is
indicated by the tendency of a powder to arch in the tester. Powders possessing such
cohesiveness are usually difficult to convey pneumatically.
[5] The Tensile Tester also conveniently reveals the ranking of powders in regard to
cohesion compared to that measured using the Direct Shear Tester. The latter requires
considerable experience to operate. To attain uniform porosity with minimum operator
care, filling of the tensile tester with a screen vibrator is recommended.
[6] For extremely cohesive powders, it is difficult to test the drained angle of repose.
This suggests that the drained angle of repose is an useful yet crude indicator of the
Mean Particle Size
Mean Particle Density
I Estimate of Particle Shape
Basic Chemical Composition
I Loose Poured and Packed Bulk Density
Lou- Intermediate
Dense Phase Conueying Possible
1
Extreme
Dilute Phase
Ewtent of Cohesion Unsuitable
Moisture Content
Low Intermediate
Dense Phase Conueying Possible
High
Dilute Phase
Increasing % M. C. causes increasing instabilities during flow or special mode of conueging is required e.g. extrusion flow
Unsuitable
Cohesion due to Particle Size Distribution and
Indiuidual Particle Properties
I Low Intermediate
[Dense Phase Conueying Possible
*,
High
Dilute Phase
307
Unsuitable
Improued Knowledge of Particle Size Distribution
Select mode
of conueying
according to
Mainwaring
and Jones Phase Diagram
I
308
Fluidization
I Deaeration
I Permeability
I Slugging
Plug Wall Friction
Paramount Safety and System Properties
Toxicity Contamination Tolerance Combustability and/or
Explosiueness
Radioactiuity Hygroscopicity
Hardness and Particle Shape Rbrasiueness
r Elastic Properties Extreme Particle Shape Fluffy, Flaky and Stringy Particle Shape Extreme Particle Density Swelling Tendency Extreme LLVall Friction
These tests are
not meaningful
for powders with
intermediate
and high extents
of cohesion
Unusual Powder
Properties
Thermoplasticity
Pjpzosoftening
Thermal Softening
Thixotropic Softening
Equiualent to increasing cohesion and wall friction
Figure 9.1: Recommended Sequence of Powder Tests.
309
flowability. Hence, flowability or cohesion of powders can be assessed in regard to
reliable pneumatic conveying characteristics.
[7J In view of the observed rapid gain in strength of fine powders with deaeration
time, minimum deaeration times should be effected in pneumatic conveying systems to
minimize powder strength. Hence, short cycle times are advantageous in pneumatic
conveying systems handling fine cohesive powders.
[83 It is recommended that the mechanized fill method be used for assessing
deaeration characteristics of powders having segregation tendencies. The fill rate should
be as fast as possible and the actual rate should be determined by a trial and error
procedure.
[9] Deaeration cylinders with permeable bases should not be used for assessing
deaeration characteristics since a more complex deaeration pressure variation results.
This variation is difficult to describe empirically.
[ 10] In this investigation, various fly ash of small particle size classified as Group C
powders in Geldart's classification, were found to exhibit poor fluidization
characteristics, whereas, Sand, Alumina and P V C powder were fluidized easily. Due to
cohesive effects, no clear minimum fluidization velocity were observed for the fly ash
samples tested.
[11] The determination of the fluidization properties of powders with a wide particle
size consist is difficult to test due to segregation effects. Elutriation of fines from the bed
and stratification of different particle size fractions result in changing powder
characterisitcs.
[12] The Jenike Direct Shear Tester can be used to evaluate both the powders internal
and wall friction characteristics, whereas the wall friction rig indicates a powder's plug
formation tendencies and plug wall friction characteristics for both aerated and deaerated
conditions.
[13] In general, the frictional force in the wall friction rig increases with increasing
column height, whereas the wall friction parameter pk decreases with both increasing air
pressure and column height.
310
[14] The developed fibre optic probe was found to be accurate and convenient for
measuring the powder velocity in actual lean phase pneumatic conveying systems.
However, it is only suitable for granular materials or powders which do not coat the
probe view ports. Obviously, the necessity of the view ports severely restricts the use of
this device. However, this m a y not be a significant disadvantatge as an increasing
number of pneumatic conveying systems are being installed with sight glass sections for
operational control.
[15] Due to difficulties encountered in calibrating the Tealgate T.300 concentration
meter, inaccurate results were obtained. Due to this reason, the device was only suitable
for indicating whether the flow was unstable or steady during each pneumatic conveying
cycle.
[16] A higher particle velocity is evident with increased mass flow rate as observed
with pneumatic conveying of sand.
[17] During the experimental phase, higher pressure drops were observed in the
vortice elbow bend compared to that in long radius bends, when conveying sand at
solids loading upto 30.
[18] In low velocity conveying of Wheat, an increase in air supply pressure and blow
tank pressure results in more tonnage of material conveyed.
[19] The proposed phase diagram of powder properties is a very important tool to
evaluate and predict the general material flow behaviour in dense phase and super dense
phase pneumatic conveying. B y assessing important powder properties like cohesion,
deaeration, permeability, mechanical interlocking and particle size, flow behaviour of
powders in pneumatic conveying can be predicted. This information is summarized in
Figure 9.1. However, there are limitations in general application of this phase diagram as
not all the powder properties affecting the flow behaviour are included.
[20] Powder properties evaluation from bench tests which are both simple and
convenient indicate fundamental and individual powder properties and allow ranking of
the same. In particular, the properties presented in Table 8.10 should be considered fully
before a system design is contemplated.
[21] Various bench tests provide qualitative and quantitative evaluation of powder
cohesiveness. Depending upon the requirements, selection of suitable bench tests should
311
be made. Qualitative tests are simple and convenient but for accurate measurement,
quantitative bench tests should be used. After conducting a series of bench tests for
different similar powders, mean ranking of cohesiveness should be made to indicate the
relative cohesion. Hence, for ranking of powders, a compromise should be made
between convenient qualitative and accurate quantitative bench tests selection. This
compromise is summarized in Table 8.10.
[22] The various powder properties determined from bench tests provide convenient
and rapid assessment of a powder's flowability, refer Figure 9.1 and Table 8.10. This
assessment is particularly useful for rapid relative flowability evaluation and ranking of
different powders. The powder properties determined from bench tests yield useful
information in regard to pneumatic conveying flow behaviour of powders in general, and
for the prediction and design of practical pneumatic conveying systems in particular.
9.3 SUGGESTIONS FOR FUTURE WORK:
[1] The Arch Tester should be modified by the installation of pressure tappings on
both sides 50 m m above the bottom. These pressure tappings should be of similar design
to those used on the deaeration cylinder with Vyon D low resistance plugs to prevent
powder ingress into the same. These pressure tappings should, in turn, be connected to
pressure transducers to measure interstitial air pressure during filling and deaeration of
the powder before the conduction of arch measurements.
[2] The maximum outlet opening dimension of the Arch Tester should be increased
as the present 100 m m severely restricts the assessment of powder cohesiveness. In
particular with very cohesive fly ash 'E' of arch length more than 100 m m , actual
measurement was not possible with this Tester.
[3] Additional arching tests should be conducted on various powders under
differing bed heights and different ambient test conditions.
[4] Determination of a powder's tensile strength should be effected utilizing a
differential velocity split cell linear tester. Modified Tensile Testers in which both sides of
the split cell are movable and which incorporate at least four sensitive load cells should be
used to extend the measuring range of the powder bed porosity and the tensile strength.
This suggestion is made to overcome the deficiencies of the Tensile Tester used in this
work.
312
[5] Tensile tests should be performed on powders consisting of different mean size
and actual distribution. These parameters were not considered in the present work.
[6] Cohesion and Tensile strength tests should be performed under controlled
atmosphere conditions wherein temperature and relative humidity can be varied as these
powder properties are strongly affected by moisture content and to a lesser extent by
temperature.
[7] Cohesion and Tensile strength tests should be performed at high pressure to
evaluate the effect of air pressure on cohesion. This high pressure testing should aim to
evaluate conditions in actual pneumatic conveying systems.
[8] Likewise, techniques should be developed to measure powder permeability under
pressurized conditions. The selected test pressure should be consistent with the actual
blow tank and pipeline operation pressures. This information would be extremely useful
in the design of high pressure dense phase pneumatic systems.
[9] Cohesion and Tensile strength tests should be performed by using other Cohesion
and Tensile testers including the Ajax Cohesion Tester.
[10] The unconfined yield strength should be evaluated by performing tests on the
annular shear tester to cover large shear displacements. This tester has a constant area of
shear and flow properties can be evaluated after repeated failure of the same sample. The
observations so obtained should be compared to those assessed using the Jenike Direct
Shear Tester.
[11] The unconfined yield strength should be evaluated by performing uniaxial
compaction tests. The evaluation of this test is simple compared to using the Jenike
Direct Shear Tester as the unconfined yield strength is determined by simply applying a
compressive vertical load on the compacted specimen.
[12] The present wall friction rig should be modified to measure wall friction for
slugging materials by observing the characteristics of the slug motion. The actual slug
motion could be observed by sonic, optic, laser or electrical techniques. The pressure
necessary to achieve constant velocity for slug movement should also be obtained from
experiments. This pressure drop will be useful for evaluating the pressure drop in dense
phase pneumatic conveying systems.
313
[13] The wall friction rig should be replaced by a ring torque measuring apparatus
wherein the ring is immersed at the top of a suitably fluidized powder bed. Also, the
effect of different shaped rings on the frictional force and torque should be observed.
[ 14] Wall friction tests should be performed on other wall materials including Mild
Steel, Polished Stainless Steel by the replacement of the existing Perspex tube with tube
constructed of the relevant material.
[15] Deaeration tests should be conducted using Group A powders and compared with
that of Group C powder deaeration behaviour for differing bed heights to evaluate the
effect of both consolidation and cohesion.
[16] Deaeration tests should be conducted using a video camera to observe accurate
bed height and other dense phase parameters. With a larger size of deaeration column,
rapid bed height observations would be possible without loss of accuracy.
[17] Fluidization of cohesive powders should be effected by the use of mechanical
stirrers or vibrators as the conduction of this test for typical practical powders is difficult.
[18] To consider the effect of particle size distribution in a Geldart's diagram, the
mean particle size should be replaced by the material characteristic dimension such as
particle size distribution span.
[19] Compressibility tests on powders should be conducted at different deaeration
times to observe the significance of deaeration on the compressibility and hence on the
flow properties. This information should then, be used to predict the pneumatic
conveying characteristics of powders.
[20] The fibre optic probe developed for velocity measurement in this work should be
utilized to measure powder concentration in combination with the Hewlett Packard
Correlator and Spectrum Analyzer as well as to study the flow patterns in pneumatic
conveying of powders.
[21] The fibre optic probe should be utilized to measure the rebound particle motion on
the coefficient of restitution rig. In this work, the rebound particle motion was measured
by use of video equipment.
314
[22] The fibre optic probe should be located on various positions of sight glass for
velocity measurement such as in vertical and inclined directions on horizontal pipeline,
on vertical pipeline and after bends. Furthermore, it should be located after the
accleration zone immediately downstream of each system discontinuity.
[23] A data base of powder properties should be developed to assist pneumatic
conveying selection and design. This data base of powder pneumatic conveying
characteristics should be prepared incorporating a large range and number of powders.
M a n y powder properties can be characterized and appreciated by an examination of the
powder's surface, particle shape and form based on S E M photographs. Furthermore,
powder exhibiting adverse flow characteristics and reasons for poor system performance
should be identified. Greater importance should be given to powder properties
determined from simple bench tests like particle size, Hausner ratio (ratio of packed to
loose poured bulk density), compressibility, particle size, tensile strength, deaeration to
determine suitability and ranking of powder properties. This data base should also be
used in a regression analysis to ascertain the significance both qualified and quantified,
of the various powder properties. This regression should also be used to indicate a
powder's suitability or otherwise for lean phase, dense phase and super dense phase
pneumatic conveying. Information so gained should also be used to confirm the proposed
phase diagram for pneumatic conveying.
315
BIBLIOGRAPHY
Abrahamsen A. R. and Geldart D., "Behaviour of Gas-Fluidized Beds of Fine Powders Part II. Voidage of the Dense Phase in Bubbling Beds", Powder Technol , Vol. 26, (1980), 47-55.
Adam O., "Feststoffbeladene Luffstromung Hoher Geschwindigkeit", Chemie-Ing. leduw Vol. 3, (1957), No. 3, 151-158. ^uauns-n^
Adewumi M.A. and Arastoopour, 'Two-Dimensional Steady State Hydrodynamics of Gas-Solids Flow in Vertical Pneumatic Conveying Systems", Powder Technol., Vol. 48, (1986), 67-74.
Arnold P.C., "Design of Pneumatic Conveying Systems Requirement for a System Approach", Short Course Notes on Design and Operation of Pneumatic Conveying Systems", Uni. of Wollongong, (1989).
Arnold P.C., McLean A.G. and Roberts A.W., "Bulk Solids: Storage and Handling, Tunra Ltd., Uni. of Newcastle, New South Wales, Australia, (1980).
Arnold P.C., McLean A.G. and Wypych P. W., "Dense Phase Pneumatic Conveying of Pulverised Coal and Fly Ash", N E R D D P Project Report No. 626, (1986).
Arundel P.A., Bibb S.D. and Boothroyd R.G., "Dispersed Density Distribution and Extent of Agglomeration in a Polydisperse Fine Particle Suspension Flowing Turbulently Upwards in a Duct", Powder Technol, Vol. 4,(1970/71), 302-312.
Ashton A. D., Farley R. and Valentin F. H. H., "An Improved Apparatus for Measuring the Tensile Strength of Powders", J. Sci. Instrum., Vol. 41,(1964), 763-765.
Augwood D.R., 'The Flowability of Alumina Reduction Plant Ores", EFCE Publication Series No. 49, Reliable Flow of Particulate Solids, Proceedings, Bergen, Norway, (1985).
Aziz Z. B. and Klinzing G.E., "Plug Flow Transport of Cohesive Coal: Horizontal and Inclined Flows", Powder Technol., Vol. 55, (1988), 97-105.
Bandrowski J. and Kaczmarzyk G., "Some Aspects of the Operation and Design of Vertical Pneumatic Conveying", Powder Technol., Vol. 28, (1981), 25-33.
Barth W., "Stromungsvorgange beim Transport von Festteilchen und Flussigkeitsteilchen in Gasen", Chemie. Ing. Tech., Vol. 30, (1958), No. 5, 171-180.
Beck C. M., Henry R. M., Lowe B. T. and Plaskowski A., "Instrumentation for Velocity Measurements and Control of Solids in a Pneumatic Conveyor", Pneumatech 1, (1982), Session 4, 1-12.
Beck M. S., "Recent Developments and the Future of Cross-correlation Flowmeters", Int. Conf. on Advances in Flow Measurement Technique. (1981). 241-252.
316
Beck M.S., Drane J., Plaskowski A. and Wainwright N., "Particle Velocity and Mass
269W277e a S U r e m e n t m n e U m a t i c Conveyon". Powder Technnl Vol. 2, (1968 / 69),
Beck M. S., Hobson J.H. and Mendies P.J.,"Mass Flow and Solids Velocity Measurement in Pneumatic Conveyors," Pneumotransport l; (1971), Paper D3, 37-47.
Bell J. M., "Material Characterization and its Importance to Pneumatic Conveying System Design", Bulk Solids Handling,Vol. 2, (1990), No. 3, 213-221.
Bemrose CR. and and Bridgewater J., "A Review of Attrition and Attrition Test Methods", Powder Technol., Vol. 49, (1987), 97-126.
Berkovich I. I., "Wall Friction of Powders", Jl. of Powder and Bulk Solids Technol , Vol. 11, (1987), No. 2, 1-5.
Birchenough A. and Mason J.S., "Local Particle Velocity Measurements with a Laser Anemometer in an Upward Flowing Gas-Solid Suspension", Powder Technol , Vol.
Bitz G., "Measurement of Velocity and Mass Flow Rate of Pneumatically Conveyed Solids by use of a Microprocessor-based Semiparallel Correlator", Int. Conf. on the use of Micros in Fluid Engg^ London., (1983), 45-53.
Boeck Th., "Mass Flow Measuring System for Pneumatically Conveyed Solids", Bulk Solids Handling, Vol. 7,(1987), No. 4, 539-545.
Bohnet M., "Advances in the Design of Pneumatic Conveyors", Int. Chem. Fngg , Vol. 25, (1985), No. 3, 387-405.
Boothroyd R.G., "Flowing Gas-Solids Suspensions", Chapman and Hall, London, (1971).
Borg L. Ter, "Evaluation of Shear Test Results on Bulk Solids in Chemical Industry", Gex.Cliem-.Jing., Vol. 5, (1982), 59-63.
Borzone L. A. and Klinzing G. E., "Dense-Phase Transport: Vertical Plug Flow", Powder Technol., Vol. 53, (1987), 273-283.
Brauer H., "Report on Investigations on Particle Movement in Straight Horizontal Tubes, Particle/Wall Collision and Erosion of Tubes and Tube Bends", Jl. of Powder and Bulk Solids Technol., Vol. 4, (1980), 2/3, 3-12.
Brewster B.S. and Seader J.D., "Nonradioactive Tagging Method of Measuring Particle Velocity in Pneumatic Transport", Jl. of AIChE, Vol. 26, (1980), 325-327.
Bridgewater J., "Attrition of Bulk Particulate Solids", Tribology in Particulate Technology, Edited by Briscoe B. J. and Adams M . J., Adam Hilger, Bristol and Philadelphia (1987).
Brisco B. J., Pope L. and Adams M. J., "The Influence of Particle Surface Topography on the Interfacial Friction of Powders", Powtech 85, Inst. Chem. Eng. Symposium Series No. 91, Birmingham, U. K., (1985), 197-212.
Brubaker J. E. and Pos J., "Determining Static Coefficients of Friction of Grains on Structural Surfaces", Trans. ASAE, (1965), 53-55.
317
Bruff W . and Jenike A. W., "A Silo for Ground Anthracite", Powder Technol Vol 1 (1967 / 68), 252-256. — -9v---.ui., vui. i,
Brown R.L., "Powders in Industry", Soc.Chem.Ind.. (1961), Monograph No. 14, 150-166.
Capes CE. and Nakamura K., "Vertical Pneumatic Conveying: An Experimental Study with Particles in the Intermediate and Turbulent Flow Regimes", The Canadian Jl. of ChgnkEngg^ Vol. 51, (1973), 31-38.
Carr J. F. and Walker D. M., "An Annular Shear Cell for Granular Materials", Powder Technol.. Vol. 1, (1967), 369-373.
Carr R. L., "Powder and Granule Properties and Mechanics", Gas-Solids Handling in Process Industries, J.M. Marchello, et al., Ed., Marcel Dekker Inc., New York, (1976), Ch. 2, 13-88.
Carstensen J.T. and Ping-Ching Chan, "Relation between Particle Size and Repose Angles of Powders", Powder Technol.. Vol. 15, (1976), 129-131.
Chambers A. J., "Pneumatic Conveying Calculations", Second Int. Conf. on Bulk Materials Storage Handling and Transportation. Wollongong. (1986), 49-52.
Chambers A. J., "Introduction to Pneumatic Conveying", Short Course Notes on Design and Operation of Pneumatic Conveying Systems, Uni. of Wollongong, (1989).
Chan S. M., Remple D., Shook C A. and Esmail M. N., "A One Dimensional Model of Plug Flow Pneumatic Conveying", Can. J. of Chem. Engg.. Vol. 60, (1982), 581-588.
Chan S. Y., Pilpel N. and Cheng D. C -H, "Tensile Strength of Single Powders and Binary Mixtures", Powder Technol.. Vol. 34, (1983), 173-189.
Chattopadyay A., Rao M. and Parameswaran M., "Dense Phase Pneumatic Conveying System for Fine Materials", Bulk Solids Handling. Vol. 1, (1989), No.l, 11-17.
Cheremisinoff N.P. and Cheremisinoff P.N., "Hydrodynamics of Gas-Solids Fluidization", Gulf Publishing Co., Houston (1984), 5-7.
Clark R. H., Charles D.E., Richardson J.F. and Newitt D.M.," Pneumatic Conveying : Part I: The Pressure Drop during Horizontal Conveying", Trans. Instn. Chem Engrs.. 30, (1952), 209-224.
Clark N. N., Van Egmond J. W. and Turton R., "A Numerical Representation of Geldart's Classification", Powder Technol.. Vol. 56, (1988), 225-226.
Crowe C. T. and Sharma M. P., "A Novel Physico-Computational Model for Quasi-One Dimensional Gas-Particle Flow", Jl. of Fluids Eng.. Vol. 100, (1978), 343-349.
Davies R. M., "Sensing of Gas and Particle Velocity by Transit Correlation", Uni. of New South Wales, Report, FMT/7, (1984).
Dawes J.G., "Dispersion of Dust Deposits by Blasts of Air, Part I, Safety in Mines Res. Estab., Res. Rep. 36, Sheffield, Ministry of Fuel and Powder, SMRE, (1952).
Devaswithin A. and Pitchumani B., "An Approach to Study the Particle Degradation During Impact", Bulk Solids Handling. Vol. 7, (1987), No.4, 547-550.
318
Diekman R. and Forsythe W. L., "Laboratory Prediction of Flow Properties of Fluidized Solids", Indust. and Engg. Chem., Vol. 45, (1953), No. 6, 1174-1180.
Dixon G., " How Different Powders Behave ?", Bulk-Storage Movement Control, Vol. 5,(1979), 81-87.
Doss E.D. and Shrinivasan, "Modelling of Wall friction for Multispecies Solid-Gas Flows", Trans, of the A S M E , Vol 108, (1986), 487.
Dry R. J., Judd M.R. and Shingles T., 'Two-Phase Theory and Fine Powders", Eowder Technoi^Vol. 34, (1983), 213-223.
Eckhoff R. K. and Leversen P. G., "A Further Contribution to the Evaluation of the
Jenike Method for Design Mass Flow Hoppers", Powder Technol., Vol. 10, (1974), 51-58.
Eckhoff R. K. and Leversen P. G., 'The Comparisons of Tensile Strength Data of Powders and Failure Loci from Jenike Shear Cell Tests", Powder Technol., Vol. 19, (1978), 115-118.
Edwards D., "A Computer Simulation of the Flow of Air and Particles through a Horizontal Pipe", Con, on the Flow of Particulate Solids, Bergen, Norway (1986).
Enstad G., "On the Theory of Arching in Mass Flow Hoppers", Chem. Engg. Sci., Vol. 30, (1975), 1273-1283.
Farley R., 'The Significance of the Flow Properties of Bulk Solids to the Designers and Users of Storage Hoppers and Associated Equipment", Second Int. Conf. on Bulk Solids Storage Handling and Flow, Nottingham, (1977), 1-14.
Farely R., "Problems Associated with Storatge Hoppers", Trans. Instn. Chem. Engrs., Vol. 43, (1965), 193-198.
Flain R. J., "Pneumatic Conveying: How the System is Matched to the Materials", Pjaoces-LJingng Nov., (1972), 88-90.
Flatt W., "Low Velocity Pneumatic Conveying of Granulated and Pulverized Products -Criteria for Selecting the Optimal Pneumatic Conveying System", Pneumotransport 5, (1980), Paper J2, 409-418.
Frydman S., "The Angle of Repose of Potash Pellets", Powder Technol., Vol. 10, (1974), 9-12.
Geldart D., " Types of Gas Fluidization ", Powder Technol., Vol. 7, (1973), 285-292.
Geldart D., Harnby N. and Wong A.C, "Fluidization of Cohesive Powders", Eowder TechnoU Vol. 37, (1984), 25-37.
Geldart D., Mallet M. F.and Rolfe N., "Assessing the Flowability of Powders Using Angle ftf R-T/----V' PowHerHandl. and Proc. Vol. 2, (1990), No. 4, 341-345.
Geldart D. and Wong A.C A., "Fluidization of Powders Showing Degrees of Cohesiveness-II. Experiments on Rates of De-aeration", Chem. Engg. Sci., Vol. 40, (1985), 653-661.
Grey R. O. and Beddow J. K., "On the Hausner Ratio and its Relationship to Some Properties of Metal Powders", Powder Technol., Vol. 2, (1968 / 69), 323-326.
319
Gwyn J.E., "On the Particle Size Distribution Function and the Attrition of Cracking Catalysts", AIChEJ, 15, (1969), 35-39.
Hamid A. and Stuchly S. S., "Microwave Doppler-Effect Flow Monitor", IF.FF Trans. on Industr. Electro, and Control Instrument., Vnl IECI-22, No.2, (1975), 224-228.
Harder J., Hilgraf P. and Zimmermann W., "Optimization of Dense Phase Pressure Vessel Conveying with respect to Industrial Applications - Part I", Bulk Solids Handling, Vol. 8, (1988a), No.2, 205-209.
Harder J., Hilgraf P. and Zimmermann W., "Optimization of Dense Phase Pressure Vessel Conveying with respect to Industrial Applications - Part II", Bulk Solids Handling, Vol. 8, (1988b), No.3, 205-209.
Hariu O.H. and Molstad M.C, "Pressure Drop in Vertical Tubes in Transport of Solids by Gases", Ind. Eng. Chem., Vol. 41, (1949), 1148-1160.
Harny N., Hawkins A. E. and Vandame D., "The Use of Bulk Density Determinations as a Means of Typifying the Flow Characteristics of Loosely Compacted Powders under Conditions of Variable Humidity", Adv. in Particulate Technology Symp., (1987), Manchester.
Hauser G. and Sommer K., "Plug Flow Type Pneumatic Conveying of Abrasive Products in Food Indusry", Pneumatech 3, (1987), 199-219.
Hilbert J. D., "Making the Turn Alternatives in Pneumatic Conveying Pipeline Bends", Int. Conf. Powder and Bulk Solids Handling and Processing, (1983), 298-305.
Hitt R.J., Reed A.R. and Mason J.S.,"The Application of the Time Derivative of the Ideal Gas Law to the Blowtank", Pneumatech 1, (1982), Session 2.
Hours R.M. and Chen C.P.," Application of Radioactive Tracers and B rays Absorption Techniques to the Measurement of Solid Particles Velocity and Space Concentration in a Two-Phase Air-Solid Flow at High Mass Rate", Pneumotransport 3, (1976), Paper B3, 21-35.
Howard A. V., "A Microwave Technique for Monitoring the Mass Flow Rate of Pneumatically Transported Solids", Pneumotransport 3, (1976), Paper S3, 17-22.
Irons G. A. and Chang J. S., "Particle Fraction and Velocity Measurements in Gas-Powder Streams by Capacitance Transducers ", Int. J. Multiphase Flow, Vol. 9, (1983), 289-297.
Ito H., "Pressure Losses in Smooth Pipe Bends", Trans. ASME, Vol. 82D, (1960), 131-143.
Jenike A. W., "A Measure of Flowability for Powders and Other Bulk Solids", Eowder TecliiiQUVol. 11, (1975), 89-90.
Jenike A. W., "Gravity Flow of Bulk Solids," Bull. 108, Engineering Experiment Station, University of Utah, October, 1961.
Jenike A. W., "Gravity Flow of Bulk Solids," Bull. 123, Engineering Experiment Station, University of Utah, 1970.
Jenike A.W. and Leser T., "A Flow-No Flow Critierion in the Gravity Flow of Powders in Conveying Channels", Int. Congress on Rheology, Part 3, (1963), 125-141.
3 2 0
Jimbo G. and Hatano S., "Experimental Investigation of the Tensile Strength of a Powder Bed by Split Cell Type Tensile Testers", Particle Charact.. Vol. 1, (1984), 133-
Jodlowski C,"Study of Minimum Transport Velocities for Upward Flow in Vertical Pipes". Pneumotransport 3. (1976), Paper D2, 15-32.
Johanson J. R. and Jenike A. W., "The Effect of the Gaseous Phase on Pressures in a Cylindrical Silo", Powder Technol.. Vol 5, (1971/72), 133-145.
Jones M.G., Mill D. and Mason J. S., "A Product Classification for the Design of Pneumatic Conveying Systems", 14th Annual Powder and Bulk Solids Conf. (1989),
Jones P. J. and Leung L. S., "Estimation of Saltation Velocity in Horizontal Pneumatic Conveying - A Comparison of Published Correlations", Pneumotransport 4. (1978), Paper Cl. *
Jotaki A. W. and Tomita Y., "Solids Velocities and Pressure Drops in a Horizontal Pneumatic Conveying System", Pneumotransport 1. (1971), Paper B3.
Jotaki T. and Tomita Y., "Characterisitics of a Blow Tank Solids Conveyor and its Operating Points Working on Pipe Lines", Pneumotransport 4. (1978), Paper D4, 51-
Julian F. M. and Dukler A. E., "An Eddy Viscosity Model for Friction in Gas-Solids Flow", AIChE Jl.. Vol. 11, (1965), No. 5, 853-858.
Kalson P. A. and Resnick W., "Angles of Repose and Drainage for Granular Materials in a Wedge-shaped Hopper", Powder Technol.. Vol. 43, (1985), 113-116.
Kano T., "Reduction of Power Consumption in Pneumatic Conveying of Granular Materials Through a Pipeline", Kona. No. 2, (1984), 24-33.
Kennedy O.C, Wypych P. W. and Arnold P. C, "The Effect of Blow Tank Air Injection on Pneumatic Conveying Performance", Pneumatech 3. (1987), 267-280.
Kirby J.M., "Deaeration of Powders in Hoppers: A Simple Linear Theory Including Filling", Powder Technol..Vol. 44, (1985), 69-75.
Klintworth J. and Marcus R. D., "A Review of Low-Velocity Pneumatic Conveying Systems", Bulk Solids Handling. Vol. 5, No.4, (1985), 747-753.
Klinzing G.E. and Mathur M.P.," The Dense and Extrusion Flow Regime in Gas-Solid Transport", Can. J. Chem. Eng.. Vol. 59, (1981), 590-594.
Klinzing G.E. and Mathur M.P.,"Comparison of Recent Pneumatic Transport Data Using Two-Phase Diagrams", Jl. of Pipelines. Vol. 5, (1986), 197-202.
Klinzing G. E., Rohatgi N. D., Zaltash A. and Myler C A., "Pneumatic Transport - A Review", Powder Technol.. Vol. 51, (1987), 135-149.
Kocova S. and Pilpel N., "The Failure Properties of Some Simple and Complex Powders and the Significance of their Yield Locus Parameters", Powder Technol.. Vol. 8, (1973), 33-55.
Konno H. and Satio S., "Pneumatic Conveying of Solids through Straight Pipes ", __ Chem. Eng. Japan.. Vol. 2, (1969), 211-217.
321
Konrad K., "Dense Phase Pneumatic Conveying: A Review", Powder Technol., Vol. 49, (1986a), 1-35.
Konrad K., "Dense Phase Pneumatic Conveying through Long Pipelines: Effect of Significantly Compressible Air Flow on Pressure Drop", Powder Technol., Vol. 48, (1986b), 193-203.
Konrad K., Harrison D., Nedderman R. M. and Davidson J. F., "Prediction of the Pressure Drop for Horizontal Dense Phase Pneumatic Conveying of Particles", Pneumotransport 5,(1980), Paper F. 1.
Knight P. C. and Johnson S. H., "Measurement of Powder Cohesive Strength with a Penetration Test", Powder Technol., Vol. 54, (1988), 279-283.
Kraus M. N., "Pneumatic and Hydraulic Conveying of Solids", Marcel Dekker, Inc., New York (1983).
Kraus M. N., "Pneumatic Conveying Systems", Chem. Eng., (1986), Oct. 13, 50-61.
Kretschmer H., "Dense Phase Pneumatic Conveying - A Low Wear Technology for Metering Pulverized Bulk Materials", Mechanics of Particulate Solids Pneumatic and Hydraulic Conveying, Mixing, Nurnberg, (1986), 311-337.
Langmaid R. N., "Arch Formation in a Non-Cohesive Granular Material", Jl. oflnstit. of_Eu.el,(1957), 166-172.
Lee S. L. and Srinivansan J., "Measurement of Local Size and Velocity Probability Density Distributions in Two-Phase Suspension Flows by Laser -Doppler Technique, Int. J. Multiphase Flow, Vol. 4, (1978), 141-155.
Lee S. L. and Srinivansan J., "An LDA Technique for in situ Simultaneous Velocity and Size Measurement of Large Spherical Particles in a Two-Phase Suspension Flow", InLl. Multiphase Flow, Vol. 8, (1982), 45-57.
Lippert A., "Pneumatic Conveying at High Solids Concentration", Cheme-Ing. Techn., Vol. 38, No. 3, (1966), 350-355.
Lohrmann P. C and Marcus R. D., "Minimum Energy Pneumatic Conveying II - Dense Phase", Jl. of Pipelines, Vol. 4, (1984), 123-129.
Luqing Z., Yaojin Z. and Xinzhang P., "Analysis of the Pressure Drop for Horizontal Plug Pneumatic Conveying of Particles", Bulk Solids Handling, Vol. 7, (1987), No. 6, 859-864.
Macrae J. C. and Gray W. A., "Significance of the Properties of Materials in the Packing of Real Spherical Particles", Brit. J. Appl. Phys., Vol. 12, (1961), No.4, 164-172.
McLean A. G., "Blow Tank Design", Bulk Solids Handling, Vol. 5, (1985), No.l, 213-218.
McLean A. G., Private Communication, (1986).
Macrae J. C. and Gray W. A., "Significance of the Properties of Materials in the Packing of Real Spherical Particles", Brit. J. Appl. Phys., Vol. 12, (1961), No. 4, 164-172.
322
Maeda M., Ikai S. and Ukon A., "Pneumatic Conveying of Solids: Part 2, On Slip Velocities and Interaction between Wall and Particles in Vertical Lines ", Bull. JSME. Vol. 17, (1970), 768-775.
Main waring N. J. and Reed A. R., "Permeability and Air Retention Characteristics of Bulk Solid Materials in Relation to Modes of Non-suspension Dense Phase Pneumatic Conveying", Bulk Solids Handling. Vol. 7, (1987), No. 3, 415-425.
Mason J. S. and Smith B. V., "Pressure Drop and Flow Behaviour for the Pneumatic Transport of Fine Particles around 90 degrees Bends", Pneumotransport 2. (1973), Paper A2.
Masuda H. and Iinoya K., "In-Stream Measurements of Powder Flow Rate in Gas-Solid Pipe Flow", Int. Svmp. In Stream Measurement of Particulate Solid Particles. Vol. 2, (1978), Paper C7.
Matsumoto S., Sato H., Suzuki M. and Maeda S., "Prediction*and Stability Analysis of Choking in Vertical Pneuamtic Conveying", J. of Chem. Engg. of Japan. Vol. 15, No. 6, (1982), 440-445.
Matsumoto S. and Saito S., "On the Mechanism of Suspension of Particles in Horizontal Pneumatic Conveying: Monto Carlo Simulation Based on the Irregular Bouncing Model", J. of Chem. Engg. of Japan. Vol.3. ri970a1. No. 1. 83-91.
Matsumoto S. and Saito S., "Monte Carlo Simulation of Horizontal Pneumatic Conveying Based on the Rough Wall Model", J. of Chem. Engg. of Japan. Vol. 3, (1970b), No. 2, 223-230.
Matsumoto S., Harakawa H. and Ohtani S., "Solid Particle Velocity in Vertical Gaseous Suspension Flows", Int. J. Multiphase flows. Vol.12, (1986), No.3, 445-458.
Matsumoto S., Saito S. and Maeda S.,"Simulation of Gas-Solid Two-Phase Flow in Horizontal Pipe", J. of Chem. Engg. of Japan. Vol. 9, No. 1, (1976), 23-27.
Mathur M. P. and Klinzing G. E., "Measurement of Particle and Slip Velocities in Coal / Gas System", Fluidization and Fluid Particle Systems: Theories and Applications, Edited by T. M. Knowlton, 222, Vol. 79, (1983), 60-65.
Mehta N.C, Smith J.M. and Comings E.W., "Pressure Drop in Air-Solid Flow Systems", Ind. Eng. Chem.. Vol. 49, (1957), 986-992.
Michaelides E. E., "A Model for the Flow of Solid Particles in Gases", Int. J. Multiphase Flow. Vol. 10, (1984), No. 1, 61-77.
Molerus O., "Theory of Yield of Cohesive Powders", Powder Technol.. Vol.12, (1975), 259-275.
Molerus O., "Interpretation of Geldart's type A, B, C and D powders by taking into account interparticle cohesion forces", Powder Technol. Vol. 33, (1982), 81-87.
Molerus O., "Description of Pneumatic Conveying". Int. Chem. Eng.. Vol. 20, (1980), No. 1, 7-18.
Molerus O., "Invited Review - Flow Behaviour of Cohesive Materials", Chem. Eng. Commun.. Vol.15. (1982), 257-289.
Molerus O. and Nywlt M., "The Influence of the Fine Particle Content on the Flow Behaviour of Bulk Materials", Powder Technol.. Vol. 37, (1984), 145-154.
323
Morikawa Y. and Tanaka T.,"An Evaluation of the Wall Friction in Vertical Air-Solid Two-Phase Flow", Bulk Solids Handling. Vol. 8, (1988), 69-74.
Morikawa Y., Tsuji Y. and Tanaka T., "Measurements of Horizontal Air-Solid Two-Phase Flow Using an Optical Fibre Probe", Bull. J.S.M.E.. Vol. 29, (1986), No. 249,
Muley S. R., Mathur M. P. and Klinzing G. E., "Analysis of Dense Phase Fine Coal Pneumatic Systems", Research Report, Pittsburgh Energy Technology Centre, U.S.A., No. D O E / PETC / TR-82 / 6, 1-23, (1982).
Murfitt P. G. and Bransby P. L., "Deaeration of Powders in Hoppers", Powder TechnoL Vol. 27, (1980), 149-162.
Muschelknautz E. "Theoretische und experimentelle Untersuchungen uber die Druckverlust pneumatischer Fordereleitungen unter besondefer Berucksichtigung des Einflusses von Gutreibung und Gutgewicht", V.D.I. Forsch. Heft 476, Dusseldorf, (1959).
Myler C.A., Zaltash A. and Klinzing G.E., "Gas-Solid Transport in a 0.0508 m Pipe at Various Inclinations with and without Electrostatics II: Stability", J. of Powder & Bulk Solids Technol.. Vol. 10, (1983), No. 3, 13-17.
Nedderman R. M., "Permissible Values of the Shear Index", Powder Technol.. Vol. 19, (1978), 287-288.
Nikolakakis I. and Pilpel N., "Effects of Particle Shape and Size on the Tensile Strengths of Powders", Powder Technol.. Vol. 56, (1988), 95-103.
Novosad J. and Majzlik, "An Arching Theory taking into Account the Vertical Pressure on the Arch", Proc. of Int. Symp. on Reliable Flow of Particulate Solids. Bergen. Norway. (1985).
Obata E., Watanabe H. and Endo N., "Measurement of Size and Size Distribution of Particles by Fluidization", Jl. of Chem. Eng. of Japan. Vol. 15, (1982), No.l, 23-28.
Oki K., Walawender W.P. and Fan L. T., "The Measurement of Local Velocity of Solid Particles", Powder Technol.. Vol. 18, (1977), 171-178.
Ostrovskii G.M., Isakov V. P. and Sokolov V. N., "Investigation of Local Velocities of Particles of Materials under Conditions of Pneumatic Conveying", Jl. of Appl. Chem. of the U.S.S.R.. Vol. 49, No. 8, (1976), 1795-1798.
Ottjes J.A., " A Measuring System for Particle Impact Velocities and Application in a Pneumatic Transport Line ", Chem. Eng. Sci.. Vol. 36, (1981), 1337-1347.
Ottjes J.A., "Pressure Losses in Pneumatic Transport", Ph. D. Thesis, Delft University of Technology, (1982).
Ottjes J.A., Meeuse G.C. and van Kuijik G.J.L., " Particle Velocity and Pressure Drop in Horizontal and Vertical Pipes ", Pneumotransport 3. (1976), 109-120.
Paulson J. I. and Hess L. W., "Vortice Chambered Elbows - A Laboratory Study in Comparison with Long Radius Elbows", Proce. of the Tech. Program, Int. Powder and Bulk Solids Handling and Processing, Atlanta, Georgia (1983).
324
Piepers H. W., Cottaar E.J.E., Verkooijen A.H. M . and Rietema K., "Effects of Pressure and Type of Gas on Particle-particle Interaction and the Consequences for Gas-solid Fluidization Behaviour", Powder Technol., Vol. 37, (1984), 55-70.
Platonov P. and Poltorak V., "Investigation of Shear of a Granular Material along a Bordering Surface," Powder Technol., Vol. 3, (1969/70), 361-363.
Rademacher F. J. C, "Accurate Measurement of the Kinetic Coefficient of Friction between a Surface and a Granular Mass", Powder Technol., Vol. 19, (1978), 65-77.
Ranz W. E., Talandis G. R. and Gutterman B., "Mechanics of Particle Bounce", AIChE L, Vol. 6, No. 1, (1960), 124-127.
Rathone T., Nedderman R. M. and Davidson F., "Aeration, Deaeration and Flooding of Fine Particles", Chem. Engg. Sci., Vol. 42, (1987), 725-736.
Ravenet J., "Flow of Cohesive Powder Products", Sec. Int. Conf. on Design of Silos for Strength and Flow", U.K., Vol. 1, (1983), 243-259.
Reddy K.V.S. and Pei D.C, " Particle Dynamics in Solid-gas Flow in a Vertical Pipe ", Ind. Eng. Chem. Fundanu, Vol. 8, (1969), 490-497.
Reed A. R., "Components of Pneumatic Conveying Systems," Short Course Notes on Pneumatic Conveying of Bulk Solids, Uni. of Wollongong, (1985).
Reed A. R., "Operation and Control of Blow Tank Systems", Short Course Notes on Design and Operation of Pneumatic Conveying Systems, Uni. of Wollongong, (1989).
Reinhold M. and Sinkwitz H., "The Calculation of Bridging Probability - A Result of the Changeover to a Statistical Powder Theory", Jl. of Powder and Bulk Solids Technol., Vol. 11, (1987), No. 2,7-10.
Richards J. C, "The Storage and Recovery of Particulate Solids", A Report on Working Party of Inst, of Chem. Engrs., U.K., (1966), Ch. 8, 95-182.
Rietema K., "Powders, What are they ?", Powder Technol., Vol. 37, (1984), 5-23.
Rietema K., Boonstra J., Schenk G. and Verkooyen A.H.M., "The Interaction between Gas and Dispersed Solids and its Effect on Handling Properties of Solids", European Symp. Powder Technol., Amsterdam. (1980), 981-996.
Riethmuller M. L. and Ginoux J. J.," The Application of a Laser Doppler Velocimeter to the Velocity Measurement of Solid Particles Pneumatically Transported", Pneumotransport 2, Paper D3t (1973), 3-21.
Roberts A. W., "Developments in Materials Handling Research at Wollongong University College", Institution of Engineers, Australia, Illawara Group, Engineering Conference, (1966), 2nd paper.
Roberts A. W., Ooms M. and Scott O. J., "Surface Friction and Wear in the Storage, Gravity Flow and Handling of Bulk Solids", Trans. Aust. Inst. Mech. Eng., (1984), C355, 123.
Rossetti S. J., "Concepts and Criteria for Gas-Solids Flow", Handbook of Fluids in Motion, Edited by Nicholas P. Cheremisinoff and Ramesh Gupta, Ann Arbor, 635-652, (1983).
325
Rumpf H., "Zur Theorie der Zugfestigkeit von Agglomeraten bei Kraftubertragung an Kontakpunkten", Chemie-Ing.-Techn.. Vol. 42, (1970), 538-540.
Sabbaghian M. and Pourmohamadian H., "Surface Erosion by Impingement of Aerodynamically Entrained Lignite Particles", Jl. of Powder and Bulk Solids Technol.. Vol. 5, (1981), 31-36.
Schuchart P., "Pressure Losses in Gas-Solid Flow in Bends and Curves", Chemie. Ing. Technol.. Vol. 40, (1968), 1060.
Scott A. M., "Pneumatic Conveyor Design - Art or Science", Powtech '77, (1977), 10-17.
Scott A.M., "The Influence of Particle Properties on the Pressure Drop during the Pneumatic Transport of Granular Materials", Pneumotransport 4. (1978), Paper D6.
Scott O. J. and Graham C. G., "The Influence of Moisture, Clay Content and Chemical Composition on the Handleability of a Steming Coal", Proc. of Int. Svmp. On Reliable Flow of Particulate Solids. Bergen, Norway, (1985).
Seville J. P. K., "Particle Cohesion in Gas / Solid Fluidisation" Tribology in Particulate Technology, Edited by Briscoe B. J. and Adams M. J., Adam Hilger, 173-199, (1987).
Shen H. H. and Ackermann N. L., "Constitutive Equations for a Simple Shear Flow of a Disk Shaped Granular Mixture," Int. J. of Eng. Sci.. Vol. 22, (1984), No. 7, 829-843.
Shepherd N.G. and Dickson A. J.,"Dense Phase Conveying of Wood Pulp", Pneumotransport 4. (1978), Paper D5, 61-72.
Smoldyrev A. Y„ "Pipeline Transport: Principles of Design", Terraaspace Inc., U.S.A., (1980).
Soo S. L., "A Review on Electrical Effects in Pneumatic Conveying", Pneumotransport JL (1971), Paper Rl.
Stanley-Wood N.G., Lllewellyn G.J., and Taylor A. "On Stream Particle Size, Concentration and Velocity Measurements by Auto-correlation of Scattered Laser Light", Powder Technol.. Vol. 29, (1981), 217-223.
Stainforth P.T. and Berry R.E.R., "A General Flowability Index for Powders", Powder Technol.. Vol. 8, (1973), 243-251.
Stegmaiker W., "Zur Berechung der Horizontalen Pneumatischen Forderung Feinkorniger Stoffe", Forriern und Heben. Vol. 28, (1978), 363-66.
Stemerding S., "The Pneumatic Transport of Cracking Catalyst in Vertical Risers", Chem. Eng. Sci.. Vol. 17, (1962), 599-608.
Stankovich, (1985), Materials Handling Handbook, Edited by Kulwile R. A., (1985), Ch. 8, 1172-74.
Stepanoff A. J., "Arching, Bridging, or Doming", Chapter 6, Gravity Flow of Bulk
Solids and Transport of Solids in Suspension. Wiley-Interscience, New York, (1969).
326
Stuchly S. S., Sabir M. S. and Hamid A., "Advances in Monitoring of Velocities and Densities of Particulates using Microwave Doppler Effect", IEEE Trans, on Instrument and Measurement, Vol. IM-26, No.l, (1977), 21-24.
Sutton H. M., "Characterization of Powder Surfaces", Ch. 3, Edited by Parfitt et al Academic Press, U. K, (1976).
Sutton H. M. and Richmond R. A., "Improving the Storage Conditions of Fine Powders by Aeration", Trans. Inst. Chem. Engrs., Vol. 51, (1973), 97-104.
Svarovsky L., "Powder Testing Guide", British Materials Handling Board, (1987).,
Tabakoff W., "Experimental Methods of Determining Particle Restitution Coefficients", Symp. on Polyphase Flow and Transport Technol., San Francisco, Edited by R. A. Bajura, N.Y., A S M E , 203-210, (1980).
Tardos G. I., Mazzone D. and Pfeffer R., Degani D. and Nir A., "Unsteady Flow of a Compressible Gas through Consolidated Porous Media - Application to Deaeration of Hoppers", Powder Technol., Vol. 41,(1985), 135-146.
Tealgate T.300 Concentration Meter Manual.
Terashita K., Miyanami K., Konishi T. and Furubayashi K., "Flowability Assessment of Fine Coals based on Dynamic Properties", Kona, No. 4, (1986), 35-45.
Thorn R, Beck M. S. and Green R. G., "Non-intrusive Methods of Velocity Measurement in Pneumatic Conveying", J. Phys. E: Sci. Instrum., Vol. 15, (1982), 1131-1139.
Thomson F. M., "Storage of Particulate Solids", Handbook of Powder Science and Technology, Editoried by Fayed M. E. and Otten L., Van Nostrand Reinhold, NY, 365, (1984).
Thompson S. A. and Ross I. J., "Compressibility and Frictional Coefficients of Wheat", Ijaiis^ofJhe^ASAE, (1983), 1171 -1180.
Tillet J. P. A., "A Study of the Impact on Spheres of Plates", Proc. Phys. Soc, London, 67, (1954), 677-688.
Tilly G. P., "Erosion Caused by Airborne Particles", Wear, Vol. 14, (1969), 63.
Tokar I. Y. and Orekhov A. P., "Determination of Slip Factors in a Two-Phase Flow in Pneumatic Transport of Free-flowing Materials", The Soviet Chem. Indust., Vol. 15, (1983), No. 6, 748-751.
Tomita Y., Jotaki T. and Hayashi H., "Wavelike Motion of Particulate Slugs in a Horizontal Pipe", -nt I Multiphase Flow. Vol. 7, (1981), 151-166.
Tomita Y., Jotaki T., Makimoto S. and Fukushima K., "Similarity of Granular Flow in a Blow Tank Solids Conveyor", Powder Technol, Vol. 32, (1982), 1-8.
Train D., "Some Aspects of the Property of the Angle of Repose of Powders", L-PJiam-Ehamac^ 10, (Suppl.), (1958), (Dec), 127T- 135T, 143T-144T.
Tsubaki J., "A Review of the Investigations into Powder Bed Mechanics based on a Microscopic View in Japan", Kona, No.2, (1984), 78-86.
327
Tsubaki J. and Jimbo G., "Theoretical Analysis of the Tensile Strength of a Powder Bed", Powder Technol.. Vol. 37, (1984), 219-227.
^ 0 ^ ( 1 9 8 3 ^ ^ Conveying in Japan", Bulk Solid. Hanrflinp
Tsuji Y. and Morikawa Y., "Computer Simulation for the Pneumatic Transport in Pipes with Bends", Pneumotransport 4. (1978), Paper Bl, 1-15.
Tsuji Y. and Morikawa Y., "Plug Conveying of Coarse Particles in a Horizontal Pipe with Secondary Air Injection", Int. J. Multiphase Flow, Vol. 8, No. 6, (1982), 657-667.
Tsuji Y. and Morikawa Y., "LDV Measurement of an Air-Solid Two Phase Flow in a Horizontal Pipe", J. Fluid Mech.. No. 120, (1982), 385.
Tsuji Y., Oshima T. and Morikawa Y., "Numerical Simulation of Pneumatic Conveying in a Horizontal Pipe." Kona. No. 3, (1985), 38-51.
Tsunakawa H., Kunni D., Takagi F., Sugita M., Tamura T. and Haze H., "Comparing the Flow Properties of Bulk Solids by Tri-axial Shear, Unconfined Yield and Direct Shear Tests", Kona. No. 6, (1988), 15-22.
Turner G. A. and Balasubramanian M., "Investigations of the Contributions to the Tensile Strength of Weak Particulate Masses", Powder Technol.. Vol. 10, (1974), 121-Xr^ / •
Turner G. A., Balasubramanian M. and Otten L., "The Tensile Stength of Moist Limestone Powder Measurements by Different Apparatuses", Powder Technol.. Vol. 15, (1976), 97-105.
Tuzun U. and Nedderman R . M., "An Investigation of the Flow Boundary During Steady-State Discharge from a Funnel-flow Bunker", Powder Technol.. Vol. 31, (1982),
Yamashiro M. and Yuasa Y., "Experimental Study Relative to the Cascading Angle of Powder Solids", Jl. of Powder and Bulk Solids Technol.. Vol. 34, (1982), No. 1, 1-9.
Yamashiro M. and Yuasa Y., "An Experimental Study on the Relationships Between Compressibility, Fluidity and Cohesion of Powder Solids at Small Tapping Numbers", Powder Technol Vol. 34, (1983), 225-231.
Yang W. C, "A Correlation for Solid Friction Factor in Pneumatic Conveying", AIChE -LVol. 24, (1978), 548. ~ ^ ~ ^
Yokoyama T., Fujii K. and Yokoyama T., "Measurement of the Tensile Strength of a Powder Bed by a Swing Method Measuring Instrument", Powder Technol.. Vol. 32, (1982), 55-62.
Yuasa Y. and Yamashiro M., "Study of the Bulk Density of Powder Solids", Powder Technol.. Vol. 36, (1983), No. 2, 267-270.
Van Zuilichem D.J. and de Swart J.G., "Slip-velocity Measurements by a Radiotracer-Technique in Vertical Conveying Systems", Pneumotransport 2. (1973), Paper D4, 33-46.
Visser J., "An Invited Review - Van der Waals and Other Cohesive Forces Affecting Powder Fluidization", Powder Technol.. Vol. 58, (1989), 1-10.
328
Walker D. M., "An Approximate Theory for Pressures and Arching in Hoppers", Chem, Engg^Science, Vol. 21, (1966), 975-997.
Walker D. M., "A Basis for Bunker Design", Powder Technol., Vol. 1, (1967), 228-236.
Walker D. M. and Blanchard M. H., "Pressures in Experimental Coal Hoppers", Chem-Engg. Sci., Vol. 22, (1967), 1713-1745.
Weber M., "Correlation Analyses in the Design of Pneumatic Transport Plant", Bulk Solids Handling, Vol. 2, (1982), No. 2, 231-233.
Werner D., "Influence of Particle Size Distribution During Pneumatic Dense Phase Conveying in Vertical and Horizontal Pipes", Bulk Solids Handling, Vol. 3, (1983), No. 2, 351-359.
Wen C Y. and Simons H.P., "Flow Characteristics in Horizontal Fluidized Solids Transport", AIChE J., Vol. 5, (1959), 263.
Wen C. Y., "Flow Characteristics in Solids-Gas Transportation Systems," U.S. Dept. of the Interior, Bureau of Mines, Pennsylvania, (1959), IC 1314, 62-72.
Wheeldon J. M. and Williams J. C, "The Analysis of Equilibrium Particle Velocity and Additional Pressure Drop Data for Granular Material from Horizontal and Vertical Light Phase Conveyors with Particular Reference to the Fundamental Equations of Motion and Additional Pressure Drop", Pneumotransport 5, (1980), Paper B3, 121-154.
Wilson K. C, "Analysis of Slip of a Particulate Mass in a Horizontal Pipe", Bulk Solids Handling, Vol. 1, (1981), No. 2, 67-71.
Wolf E. F. and von Hohenleiten H. L., "Experimental Study of the Flow of Coal in Chutes at Riverside Generating Station", Trans. ASME., Vol. 67, (1945), 585-589.
Wood P. A., "Fundamentals of Bulk Solids Flow", IEA Coal Research, London, Report No. ICTIS/TR31,(1986).
Wright H. "An Evaluation of the Jenike Bunker Design Method", Trans. ASMF, Jl. of
Wypych P. W., Private Communication, (1986).
Wypych P. W. and Arnold P. C, "Classification and Prediction of Fly Ash Handling Characteristics for Dense-Phase and Long Distance Pneumatic Transportation," TIZ-Fachberichate, Vol. Ill, No. 11, (1987), 753-761.
Zenz F. A., "Conveyability and Powder Classification", Proc. of 9th Annual Powder & Bulk Solids Conf., (1984), 63-81.
Zenz F. A. and Othmer D.F., "Fluidization and Fluid Particle Systems," Reinhold, (1960), 85-91.
Zheng L., Zheng Y. and Pan X., "Analysis of the Pressure Drop for Horizontal Plug Pneumatic Conveying of Particles", Bulk Solids Handling, Vol. 7, (1987), No. 2, 859-864.
329
PUBLICATIONS
During the course of this research programme, the following co-authored
papers were written.
Desai M. K. and McLean A. G., "Convenient Methods to assess Pneumatic
Conveying Characteristics of Powders", Proc. Third Int. Conf. on Bulk
Materials Storage. Handling and Transportation. Newcastle, N S W , Australia,
27-29 June 1989, 98-103.
Desai M. K. and McLean A. G., "Convenient Methods to assess Cohesion for
Selecting powder's Optimal Pneumatic Conveying Mode", Proc. 19th Aust.
Chem. Eng. Conf. - C H E M E C A 91. Newcastle, N S W , Australia, 18-20
September, 1991, 817-824.
Desai M. K. and McLean A. G., "Deaeration of Powders", To be submitted
to Indian Journal of Engineering.
Desai M. K. and McLean A. G., "Effect of Cohesion on Pneumatic
Conveying Characteristics of Powders" To be submitted to Indian Journal of
Engineering.
330
APPENDIX 'A'
PRESSURE DROP
A model to calculate the pipeline pressure loss for a pneumatic conveying system
including the solids friction component as proposed by Weber (1982) and Chambers
(1986) for circular pipes is presented below for completeness.
A.1 SUPERFICIAL TRANSPORT VELOCITY:
where ms = solids mass flow rate (kgs-1),
m * = m s / mf = phase density of the solids / gas flow,
mf = air mass flow rate (kgs-1),
Pf = air density (kg nr3),
A = 7iD2/4(m2),and
D = internal diameter of pipeline (m).
A.2 FREE SETTLING VELOCITY OR TERMINAL VELOCITY OF POWDER: vb„
Since the velocity of a single particle flowing vertically upwards often is defined
as the difference between the gas superficial velocity and the particle's terminal velocity.
It is appropriate to discuss briefly this characteristic velocity. This velocity has
paramount significance in governing the behaviour of powders in fluidized beds,
standpipes and flow of powders from orifices and hopper outlets. For these situations
the particle terminal velocity indirectly indicates the Froude Number applicable to the
flow situation. As will be shown shortly, the Froude No. is an important dimensionless
parameter in pneumatic conveying analysis.
For spheres,
vb~ = 4gd(pbd-pf)
3 C D p f
(A.2)
The drag coefficient C o has been shown to correlate with the Reynolds number.
In flow situations involving gas solids flow it is usual to adopt the following definition
for the Reynolds number,
R e ^ - - - ^ (A.3)
331
where Pf = absolute viscosity of air Pa-s, and
d = mean particle diameter, m
In regard to the drag coefficient for spheres the following correlations apply,
C D = I R ^ : A 1 + " 1 O - J ' Reb~^*° CA.4)
26 A
C D = 0 . 4 + - - ^ , l < R e b o o < 1 04 (A.5)
Reb« C D = 0.4, 1 0 4 < R e b o o < 1 0
5 (A.6)
where pb l = loose poured bulk density of solid (kgm"3)
By use of equations (A.2) and (A.6) v ^ can be evaluated. Obviously, in this
evaluation iterations are necessary. Assuming air at atmospheric pressure (101 kPa abs.),
air temperature at 293° K and spherical particles, the relationship between v ^ and d can
be plotted as shown in Figures A.l and A.2, the same can be used to determine v^.
A.3 FRICTION COEFFICIENTS:
A.3.1 Darcv Friction Factor for Air : X{
For air flow alone knowledge of the relative roughness of a pipe allows
evaluation of Xf from the Moody diagram (Figure A.3). For laminar and turbulent flows
in smooth pipes, dimensional analysis indicates that the pipe friction is a function of the
flow Reynold's number Re.
For a laminar flow, Xf = 64 / Re (A.7)
and for turbulent flow in smooth pipes Blasius derived that,
Xf = 0.316 /ReO-2* (A.8)
The Darcy-Weisback equation predicts the pipeline pressure loss due to air
friction to be,
PfVf2LXf
A P f = 2 D (A'9)
where Xf = friction factor due to air only,
Vf = the gas velocity
or
A pf = 500[(1012 + 0.004567 m/"85 L D " 5 ) 0 5 - 101 ] Pa (A.10)
Voo (m/s)
t 5 6 7 10 *- IC4
d ( p m )
Figure A.2: Settling Velocity in still air of Spherical Particles with Diameter d
Voo (m/s)
d (pm)
Figure A.l: Settling Velocity in still air of Spherical Particles with Diameter d
334
A.3.2 Effective friction coefficient due to solids. Xs
Stegmaiker (1978) determined a mean value for this coefficient by correlating the
data from a number of fine powder flow experiments and applying the mechanics of
similitude. The correlation was found to be,
Xs = 2.1 (m*)-0'3 Fr-1 Fr,0"25 (^) , d < 0.0005 m (A.l 1)
Xs = 0.082 (m*)-0'3 Fr-0"86 Fif25 (^) ' , d > 0.0005 m (A.12)
where, Froude number = Fr = Vf2 / gD and (A. 13)
F r ^ v ^ / g o * (A.14)
A number of research workers have analyzed the friction factor and reported
correlations for the same in the literature, a small number of which are presented in Table
Figure A.3: Moody Diagram.
TABLE A.1: FRICTION FACTOR
Research worker
Stemerding (1962)
Reddy and Pei (1969)
Expression
2fspp(l-e)v pw W T P D
2fsPp(l-£)vg D
Capes and Nakamura (1973) sPp^ p
2fsPp(l-e)vg
D
2fsPp(l-e)vg
D
2fsPp(l-e)vp2
D
Konno and Saito (1969)
Yang(vertical)(1978)
Yang(horizontal) (1978)
Mathur and Klinzing (1981) - p p
Stegmaiker (dense) (1978)
Muley, Mathur and Klinzing
(1982)
D
2fsPp(l-e)v2
D
2fsPpa-e)vg D
Friction factor
0.003
0.046vp - 1
0.046vp-1.22
0.028(gD)°-5vD-l
-.-0.979
0.0315
0.0293
55.5Dt' l.i
.0.64 J4). 26. 0.91 g
pJ3.26 C
H Pp
-T,r0.25nV s UJ
-0.1 m. - ..-U.-T- "1,-. -
2.1p Fr Frs
0.0172(l-e) 1.05 V f - v p
-1-1.28
L(gD) 05
[After Klinzing et al. (1987) ]
A.4 ACCELERATION PRESSURE DROP DUE TO FLUID. An.f The acceleration pressure drop for the transport fluid alone is,
AP_f = PfVf
(A.15)
336
A.5 ACCELERATION PRESSURE DROP DUE TO SOLIDS. An,.
When solids are transported in a transport fluid an additional acceleration pressure
drop occurs. This pressure drop is expressed by, APas = m * p f V f v s (A. 16)
where vs and Vf are in ms-1, d is in mm. and ps is in kgm
-3.
Wherein the particle velocity vs can also be calculated from the voidage e and the
solids mass flow rate m s using the following equation,
mc
' s ~ v.= ,_ ,' . (A.17)
(1-e) ps A
where ps = the density of solid and %
A = cross sectional area of the pipe.
Moreover, the particle velocity can be determined from experimental techniques as
discussed in Section 3.4.
Obviously, the superficial velocity Vf can be calculated from the system supply
flowmeter reading and the knowledge of the diameter of pneumatic conveying test rig.
From the superficial velocity Vf and the voidage e, the actual air velocity can be
calculated, viz.
vf = V f / e (A-18)
A.6 FRICTIONAL PRESSURE DROP. Apf The friction generated pressure drop Apf can be evaluated using the expression,
pfV?L7if m * p f V2 L X s ( A 1 9 )
Apf~ 2D + 2D where L = L^ + Lv (m)
Lh = total effective length of horizontal pipes (m),
Lv = total effective length of vertical pipes (m).
A.7 GRAVITATION^ PRESSURE DROP. AP-.
For vertical upwards pneumatic conveying system legs the pressure loss can be
evaluated using the same equations for horizontal pipes by taking into account an
additional term due to the pressure loss of lifting the solids against gravity. This term is
given by, A p ^ d + m ^ P f g L , KA-ZV)
where Ly = vertical length of pipe
337
Obviously, when conveying vertically downwards this pressure loss provides assistive effort.
A-8 PRESSURE DROP DUE TO RENfng, Apb
The pressure drops associated with bends can be evaluated using the following correlation,
B N B ( l + m * ) p f V f2
A p b = 5 (A.21)
where B = Bend coefficient,
Rg = Centreline radius of bend, and
N B = Number of bends.
For 90° bends in circular pipes the following bend coefficients apply as a function
ofRg/D.
TABLE A.2: BEND LOSS COEFFICIENT FOR 90° BENDS
Bend radius / pipe diameter
2
4
6 or more
B
1.5
0.75
0.5
A.9 TOTAL PRESSURE DROP. Ap
As can be appreciated this initial simplified correlation lacks theoretical basis nor
does it account for actual bend configuration. With knowledge of the individual
component pressure drop, the overall total system pressure drop is evaluated using,
Ap = APaf + APas + Apf + Apg + Apb (A.22)
Frictional pressure drop due to solids in dense phase conveying can be given as
(Wen et al. 1959),
A ps = 4.27 L Pds vs0"45 (§)°' 2 5 (A.23)
where d = the average particle diameter,
Pd- = the dispersed solids density.
338
APPENDIX 'B' POWDER PROPERTIES
B.l INTRODUCTION:
The purpose of this appendix is to present important aspects of powder properties
which were made during the literature survey for this investigation to which reference
was made. Inclusion of this information is considered appropriate to ensure the reader is
familiar with the numerous powder properties which characterize powder and to highlight
the complexity of powder properties and behaviour. Obviously, for brevity reasons not
all powder properties are discussed in this Appendix. Likewise space restriction prevents
discussion and mention of all terms used to describe powders. For a more complete
discussion the reader should consult the nomograph by Svarovsky (1987).
A brief discussion of the characteristics and significance of powder properties in
regard to pneumatic conveying is now presented.
B.2 PARTICLE HARDNESS: The hardness of a particle depends upon the type of structure and bonding, radii
distance of the atoms, relative atom or molecular size, the valence value and the continuity
of the bonding. In general, the harder the particle surface, the lower the interparticle
contact possible suggesting good flowability.
A knowledge of particle hardness is important for the design of pneumatic
conveying systems. It gives an indication of the extent of abrasion and erosion to key
components. Individual particles are small and the commercial equipment to measure
hardness like Vickers, Rockwell and Brinell types are not suitable for the powders. Since
scratching is the major mechanism of abrasive wear, scratch hardness can be used to
indicate the resistance of a metal of known hardness to abrasive wear. Thus, scratch
hardness is a suitable method for the measurement of hardness of powders. Particle
hardness is paramount for selecting the pneumatic conveying system piping material,
bearing and bearing seal requirements for rotary feeders.
B.3 ABRASIVENESS: In the design of pneumatic conveying system equipment to protect against wear, it
is important to have knowledge of a material's abrasiveness. Coke and foundary sand
will wear hoppers and pneumatic conveying systems. In dry wear situations, hardened
steels, wear-resistant liners and high-density plastics should be considered for contact
materials.
339
B.4 E L A S T I C I T Y :
Particle elasticity has an important role in pneumatic conveying. Elasticity
influences the rate of loss of kinetic energy during the interparticle collisions occurring
during flow and has some control on the duration of any particular collision.
B.5 ELECTROSTATIC PROPERTIES:
If the powder exhibits a tendency toward static electricity, it may be necessary to
install static eliminators in the filter receivers, cyclones or elsewhere to keep the powder
from building up on the filter bags or walls of the filter-receiver. It may also be necessary
to add moisture to the conveying air to reduce the static properties.
Electrostatic effects have been reviewed by Boothroyd (1971) and Soo (1971).
The use of dense phase conveying reduce the problems created by static electricity due to
lower conveying velocities and greatly reduced tendency of particle collisions as
suggested from the work conducted by Lippert (1966). The significance of electrostatic
charging is further supported by observations made by Clark (1952) that the pressure
drop increased, in some cases was up to 10 times, whenever powders were conveyed for
long periods.
B.6 EXPLQSIQN^CJIARAmTiRISTlCSi
W h e n conveying combustible materials the combinations of high concentrations of
solids in air mixtures in bins and pneumatic conveying systems and ideal ignition sources
give rise to frequent dangerous dust fires and dust explosions, if proper precautions are
not taken. Pneumatic conveying systems generally operate with a solid to mass ratio
considerably above the upper explosible limits and if the conveying velocity is kept as
high as possible, then the time of exposure of the dust particles to an ignition spark is
minimal. There are a variety of preventive measures for dilute phase pneumatic
conveying.
- avoiding dangerous material concentration
- avoiding ignition sources
- avoiding oxygen concentrations that render ignition possible
- by using special construction and protective barriers.
B.7 PARTJCJJEJSIZE_ANA_Jl^I^
A simple technique to determine the particle size analysis is to sieve the sample
powder into different fractions and weight average the results according to the sieve
openings. The distribution of particles over varying size ranges can be presented on a
340
mass or number basis. In particular, a sieve analysis presents the particle size information
on a mass basis, whereas, optical sizing is on a number basis.
The diameter of a particle is usually based on an equivalent sphere of the same
volume or mass. The sieve diameter is the width of the minimum square aperture through
which the particle will pass. Microscopic techniques may characterize particle size based
on the use of the mean value of the distance between pairs of parallel tangents to the
projected outline of the particles (Feret diameter) or the mean chord length of the
projected outline of the particle (Martin's diameter). This characterization of particle size
is further complicated by the fact that various forms of averaging are possible. The Sauter
mean diameter is a measure of the ratio of the total volume of particles to the total surface
area. The mean diameter is derived from the volume distribution. In general, powders
exhibiting bimodal distributions tend to be more cohesive and less permeable than
monosized powders of the same mean particle size.
B.8 SUREACE^AREA^i
One further important characteristics of the fine powders is the specific surface
area. This is a measure of the fineness and porosity of the powder. The specific surface
area increases with decreasing particle size.
The specific surface can be converted into an equivalent mean spherical diameter
XgV using the equation,
XsV = 6/Sv, (B.l)
where Sv = volume specific surface.
Some methods of measuring surface area are: permeametry, gas adsorption,
dynamic gas adsorption, gas diffusion, hindered settling, adsorption from solution, flow
microcalorimetry, dye adsorption and porosimetry. The actual method used to measure
surface area is selected depending on the purpose of the measurement.
The size of a particle relates to its surface area. The interrelationship of particle
size, surface area and porosity will determine the flow characteristic of a powder. In
general, a powder with large particle size and small surface area will be free flowing,
whereas a powder with small size and large surface area will be cohesive. Fine particles
with high porosity will also tend to be more floodable. Furthermore, a powder with large
surface area will display increased hygroscopicity, electrostatic activity, agglomeration,
solubility, deformability and reactivity.
341
B.9 PARTICLE nFMSITY-
This is the true density of a single particle. It is the density of a particle including
the pores or voids within the individual solids. It is defined as the weight of the particle
divided by the volume occupied by the entire particle. In general, dense particles do not
cohere to other particles and tend to be more free flowing than less dense particles. In
the design of pneumatic conveying system, this powder property controls, to a
significant extent, the conveying air pressure and volumetric requirements. In the design
of low velocity dense phase systems, particle density is an important parameter.
B.10 BULK DFNSTTY-
A powder consists of many particles grouped to form a bulk. This powder will
have an apparent bulk density, i.e. the mass of the bulk divided by the volume of particles
and voids contained. It is the overall density of the loose powder including the
interparticle distance of separation. This bulk density is dependent on the particle size and
size distribution, the particle shape, the particle density, moisture content, particle
packing, method of filling and the interparticle separation. Obviously, bulk density is
related to particle density through the interparticle void fraction.
It is important to recognize that bulk density does not have a unique value for a
particular bulk material, but will vary with the condition of the material. For example a
material that has just been pneumatically conveyed m a y be aerated and have a
considerably lower bulk density than when allowed to de-aerate.
The bulk density used to size a specific item of equipment in a pneumatic
conveying system should be relative to the condition of the powder at that point in the
system. The latter is not always easy to determine as most methods of evaluating bulk
density which, in turn, determines the change in volume of the powder, requires
knowledge of the changing consolidation conditions. Here, tests where both the poured
or aerated volume and the deaerated or settled volume are measured for a known mass of
powder provide useful benchmarks bulk density values for pneumatically conveyed
powders.
Ravenet (1983) stated three different bulk density values should be determined
experimentally to characterize powders. These are apparent density (without compression
or compaction), density under compression and density under compaction. For granular
products, the variation between apparent density and density under compaction is
between 9 and 21 per cent, while for cohesive powders, the difference is greater,
varying from between 22 and 87 per cent.
342
Seville (1987) has indicated that the Hausner ratio is an indirect measure of
interparticle forces. Harnby et al. (1987) also reported the variation of the Hausner ratio
with relative humidity for Ballotini. In particular, an increase of the Hausner ratio was
evident with a decrease in particle size and an increase in relative humidity.
B.ll COMPRESSIBTI.TTY-
Compressibility is a simple way of measuring indirectly the following
characteristics of flow:
- bulk density,
- uniformity in size and shape, %
- hardness,
- surface area,
- relative moisture content, and
- cohesiveness.
A simple quantification of compressibility is to define compressibility as the
difference between the aerated bulk density and the packed bulk density, divided by the
packed bulk density, times 100. This numerical value is referred to as the powder's
compressibility. This value varies, in general, with a powder's flow characteristics as
follows:
5 to 15 percent for free flowing granules - excellent flow
12 to 16 percent for free flowing powdered granules - good flow
18 to 21 percent for fair flow
23 to 28 percent poor flow
28 to 35 percent poor flow, fluidizable cohesive powders
33 to 38 percent for very poor flow, fluidizable cohesive powders
> 40 percent for very very poor flow cohesive powders.
The compressibility of a powder results in two effects: increase of bulk density
which affects the amount of powder held and an increase of cohesive strenguVwhich
affects the flowability of the powder. In general, a powder of wide size range is less
compressible than a powder of narrow-size range because of the increased number of
interparticle contacts. However, for the former case the powder m a y exhibit high
compressibility if the particles are fragile or soft. Obviously, the higher the relative
porosity, the lower the compressibility.
343
B.12 PARTICLE SHAPF-
Particle shape has a significant effect on the energy required to convey the powder
efficiently through the pipeline. Measurement of particle shape is difficult because of the
three dimensionality of particles and nonuniformity of size. Obviously, a particle that
has many irregular protrusions on its surface will have different flow behaviour to that of
a spherical particle. In general, irregular particles will join together and behave as a single
particle more easily than spherical particles. This tendency is reinforced by the fact that
irregular shaped particles have many points of contact. Hence, they display significant
surface forces. This suggests that as the size of the particles decreases, the importance of
shape becomes less. In general, the particles of powders vary not only in size but also in
shape. The qualitative terms used to describe powder particle shape and general flow
characteristics are presented in Table 5.5.
In general, spherical particles flow in pneumatic conveying without degradation
compared to the extent of degradation experienced when conveying sharp angular
particles, especially if the latter have a tendency to interlock. Yuasa et al. (1983) reported
that rougher particles have a higher interparticle coefficient of friction and are less likely to
pack as densely as less rough particles. Rough surface particles also display an increased
friction between the particles and the pipewall which reduces their flowability.
B.13 SURFACE PROPERTIES:
A knowledge of the general shape and structure of the particles comprising the
powder can be obtained by micrographs from either optical or electron microscopy.
Information in this visual form can provide the designer with information on the product
to be conveyed which can be linked to experience in handling similar shaped powders.
For example, the shape and structure of the particles m a y appear to be fragile indicating
that severe particle degradation m a y occur during transportation. A fibrous, thread-like
shape will indicate that the particles m a y lock together precipitating flow problems. The
sharp crystalline edges of a hard material m a y indicate the possibility of erosion of the
pipeline and system components.
The classification of surfaces on the basis of roughness depends on the scale of
observation. Surfaces appearing smooth from macroscopic examination by eye m a y
appear very rough under a high resolution microscope.
The use of microscope techniques to observe particle surfaces and shape provides
a powerful tool in modern particle technology studies. Use of microscopic techniques has
344
been facilitated by the development of modern electron microscopes access to which is
now common place.
However, the observation of powder surfaces by optical microscope is limited
clue to short length of focus. Whereas, in the electron transmission microscope, it is
limited by the sample preparation technique. Furthermore, the particles may alter during
examination by evaporation or decomposition of the particles which are opaque to
electrons. These effects results in the recording of shadowgrams.
Electron Microscopes take one of two forms; scanning (SEM) and transmission
(TEM). In respect to application to powder technology, the advantages of the Electron
Scanning Microscope are:
- direct examination;
- a large depth of focus (about 300 times that of an optical microscope) at resolutions of
15-20 n m as compared to 0.3-0.5 n m for the Transmission Electron Microscope (TEM);
- a higher range of magnification, 20 x to 100000 x is possible;
- minimal or no sample preparation, except for coating of nonmetallic specimens;
- a low specimen current;
- easy to operate;
- faster and more three dimensional details than T E M ; and
- samples as large 25 m m x 25 m m can be observed.
In view of the above ad/antages, it is apparent that the Electron Scanning
Microscope provides an excellent technique to observe powder size, shape, surface
roughness and texture, porosity and pore shape, microstructure and agglomeration
tendencies.
The only drawback to the wider application of SEM techniques is the need to
vapor-deposit a thin layer of metallic coating to prevent charging of the specimen.
Usually this metallic coating is gold.
In the scanning electron microscope, an electron beam of medium energy (5-50
keV) produced with an electron gun, is focused on the specimen through condenser and
objective lenses. These electrons interact with the sample and produce more electrons
which are classified as low-energy secondary electrons whose energy is less than 50 eV
and back scattered electrons of energy between 50 eV and the energy of incident
electrons. These secondary electrons and back scattered electrons are detected, amplified
345
and displayed on the screen of a cathode ray tube. Generally, these examinations are
made on photographic records of the screen.
B.14.1 POROSITY-
Porosity or voidage is defined as the volume of the voids within the powder bed.
The void volume includes the pores if particles are porous. It can be measured indirectly from particle and bulk densities.
B.14.2 PACKING;
The packing of solids in a material determines its porosity, permeability and the
bulk density. Moreover, the extent of packing and porosity is partly dependent on the
shape, size and size distribution of the particles. If the surface of the particles is rough,
then the interparticle friction may be greater and so prevent a denser packing condition.
The critical values of void ratio are dependent on particle geometry and the
interparticle friction angle. A powder bed or aggregate formed from equal spheres
exhibiting higher interparticle friction angle will have a critical void ratio higher than
aggregates formed from the same mono size spheres exhibiting lower interparticle
friction angle. When attractive forces exist between the spheres closer packing, higher
number of contacts and a lower void ratio results. Whereas, when repulsive forces exist
more separated packing, reduced number of contacts and higher void ratio results. In
general, powders having a wide particle size distribution exhibit closer packing and lower
porosity [Wood, (1986)].
B.15 THE FLOW OF FLUIDS THROUGH PARTICLE RF.ns-B.15.1 INTRODUCTION:
The flow of fluids through powder beds is controlled by the permeability of the
packing which depends to a significant extent, on the external surface of the particles.
The external surface area of the particulate system may be calculated from the Kozeny-
Carman equation by measuring the linear rate of flow and pressure drop of a fluid
through a particulate packing at various porosities. The permeability is low when the
particles consisting the bed have rough, porous surfaces and the calculated surface area
high.
B.15.2 PERMEAMETRY:
Permeametry is generally suitable for powders of average particle size between
0.2 and 50 microns but it can be also used with coarse particles, say upto 1000 microns
in size using a suitably scaled up test equipment. At low gas flow rates, the rate of gas
346
flow through a packed bed increases linearly with applied pressure drop. Bed
permeability or the permeability factor is the gas flow rate (m3/s) per unit pressure drop
(N/m2), per unit cross sectional area of the bed (m2) times the bed depth (m), giving the
final units for this factor in m4/N.s. This factor also depends on the gas viscosity.
In general, all permeametry methods are based on the Carman-Kozeny equation
which relates the superficial approach velocity u to the porosity of the powder e and the
specific surface of the sample Sw.
The measurement of permeability of a packed bed of powder can be conducted for
laminar gas flow. It can be measured either using continuous, steady-state flow (constant
pressure drop) or using variable flow (constant volume) instruments.
(a) Constant flow instruments:
The Lea and Nurse apparatus and Fisher Sub-sieve sizer are examples of
commercially available constant flow instruments.
(b) Variable flow instruments:
The most common variable flow instruments are the Griffin and George (oil
suction) and the Reynolds and Branson (mercury suction) permeameters.
B.16 INTERPARTICLE FORCES:
Adhesion is the phenomenon observed when particles stick to solid surfaces.
This is undesirable and must be accounted for in pneumatic conveying systems.
(a) V A N DER WAAL'S FORCES:
The molecular forces of adhesion are determined by Van der Waal's forces of
interaction between particle molecules and the supporting surface. They depend on the
properties and geometry of the contacting bodies, the roughness of the supporting
surface and the area of contact.
(b) CAPILLARY FORCES:
Capillary forces of adhesion are due to the condensation of water vapor into the
pores of loose solids. A liquid meniscus develops between the particles and the pipe wall,
resulting in the formation of surface forces. These forces press the solids onto the
pipewall. Capillary forces may be reduced by increasing the hydrophobicity of the
surface (i.e. by making it moisture resistant) through the addition of a liquid surface-
active agent in the loose material.
(c) ELECTROSTATIC FORCES:
Electrical forces of adhesion occur only during direct contact between particles and
the pipewall. During flow, the particles rub against each other and with the walls. This
creates electrical charges, causing a potential difference with the result that the particles
347
agglomerate. The greater the contact potential difference, the greater are the electrical
adhesion forces. The contact potential difference depends on the amount of charge on the
particle's surfaces and the supporting surface.
Coulomb's forces of adhesion arise, when charged particles approach the
pipewall. W h e n this happens, on the opposite side of the pipewall, charges are generated
that are equal to the particle charges, but are of opposite sign. The higher the wall's
conductivity, the less pronounced are Coulomb's forces and consequently, the smaller the
forces of adhesion. Surface wetness also promotes this process.
(d) V A R I A T I O N W I T H P A R T T C I . E ST7E-
Each type of adhesion force has a different effect dependent on particle size. In
terms of particle radius, the relationship are as follows:
Coulomb's forces are proportional to 1/r 2;
electrical forces, to r °-7;
molecular forces to r ;
capillary forces, rp (1 - r px l ) , for exponent x > 1.
(e) VARIATION WITH POWDER PROPERTIES:
Because of the radically different natures of the adhesion component forces, a
universal method for minimizing them is not possible. For example, while the
hydrophobilization of the pipewall decreases capillary forces, it tends to increase electrical
and coulombic forces; whereas increasing wetness decreases electrical and Coulomb's
forces but increases capillary forces. In comparison, Van-der Waals forces are
strengthened by adsorption layers; electrostatics and magnetic forces; liquid bridges and
solid bridges due to partial melting or crystallization of dissolved substance.
B.17 POWDER FLOWABILITY:
B.17.1 INTRODUCTION:
Two factors friction and cohesion act during the flow of solid bodies. They
correspond to two types of strength, shear and tensile. These strengths are directly based
on the packing structure of the particle assemblage through the degree of mechanical
interlocking among particles and the coordination number. As one expects the handling of
powders is associated with the frictional and cohesive properties and are fundamental to
the interpretation of particle behaviour.
(a) FRICTION:
For cohesive powders three angles of friction are important.
- The effective angle of friction, 8
- The angle of internal friction, ty
- The angle of internal friction between the powder and the wall, <j>w
348
These angles are functions of the size distribution, shape, surface properties of the
powders, bulk density, normal compressive stress. Knowledge of these friction angles
are important in the design of powders storage, handling and transportation equipment.
The angle of internal friction determines the stress distribution within a bed of
powder undergoing strain. Whereas, the angle of wall friction describes the stress
condition acting between the material bed and the walls of the container.
Since, the temperature and humidity can change the surface and frictional
properties of a material, atmospheric conditions will also affect the flow properties of a
bulk solid.
The internal and wall coefficients of friction of a powder may be determined by
shear tester and a M o h r circle stress analysis (Arnold et al. 1980). In regard to
determination of the internal friction, the frictional force, F required to shear a
consolidated powder sample under various normal loads, W , is measured. B y repetition
of the shear procedure on different consolidated samples a powder yield locus is
obtained,
x = f(G) (B.2)
where x = the shear stress parallel to the plane and
a = the compressive stress normal to the plane.
The slope of the powder yield locus determines the coefficient of internal friction,
p = tan<j) (B.3)
where <J)-= the angle of internal friction.
To determine the coefficient of the wall friction, pw, the base of the cell is
effectively lined with the wall material to be tested. The resulting yield locus is known as
the wall yield locus. The slope of the wall yield locus determines the coefficient of wall
friction,
p w = tan <|>w, (B.4)
where <J>W -= the angle of wall friction.
The accuracy of the measurements depend largely on the creation and maintenance
of a well defined failure plane within the cell. Modifications of the conventional shear
testers are the biaxial and triaxial testers.
Evaluation of results from wall friction tests is simple. The shear force necessary
to move the loaded cell is plotted against the applied normal load (note that both the axes
349
should have the same scale) and, where appropriate, a straight line is drawn through the
plot. The angle subtended by this line with the x-axis is the angle of wall friction.
Sometimes, there is a strong cohesion between the powder and the wall and this leads to
the above plot showing an intercept.
(a.l) ANGLE QF REPOSE; The angle of repose is that formed, when a sample of the powder is poured onto a
flat surface. It is used sometimes to indicate the type of metering and feeding devices for
bins or hoppers. Furthermore, the internal angle of friction can be approximated by the
angle of repose. This is a reasonable assumption only when the cohesive strength of the
powder is low.
Two methods for measuring this angle exist, namely discharging from a flat
bottom container or forming a heap on a horizontal surface. In first method, the angle of
repose is the angle formed between the surface of the powder remaining in the container
and the bottom wall. In the second method, the half angle of a heap formed on a flat
surface, measured at the top is taken as the angle of repose. As the measurements are
done at rest, these angles provide an indication of the static value of the internal angle of
friction of the powder.
(a.2) A N G L E OF SLIDE:
The angle of slide on a plate provides a convenient assessment of the angle of wall
friction.This convenience results from the fact that the tangent of the angle of slide
approximates the coefficient of friction of the product on the plate.
(a.3) C A S C A D I N G ANGLE:
The cascading angle represents the flowing state of powder rather than the
assembly at rest. It is defined as the angle formed by the inclined surface of a flowing
powder inside a horizontal rotating cylinder, and the horizontal. This angle is affected by
moisture content, particle size, shape and roughness (Yamashiro et al., 1982) and also
dependent on wall friction (Briscoe et al., 1985).
(b) COHESION: Cohesion is a fundamental property and hence is evaluated directly as a powder
flow property. The presence of cohesion in a powder causes the powder to stick together
to form a powder mass. Obviously, the higher the apparent surface cohesion of a
powder, the less the flowability and vice versa.
A relative measure of cohesion can be determined by extrapolating the yield locus
by a straight line across the low-stress region to intercept the shear stress axis. This
350
intercept is a measure of the shear stress at zero normal stress. This cohesion results
from the interparticle forces discussed in Section B.6.
Cohesive powders behave as rigid-plastic Coulomb solid. A rigid-plastic solid
possesses a yield locus which defines the limits of a range of stresses that cause no
permanent deformation, whereas stresses equal to the limiting cause either failure or
plastic flow. For a Coulomb powders the yield locus can be represented by:
T = C + CT tan <|> (B.5)
giving a linear relation between the shear stress, x, and the normal stress, CT. Thus, the
cohesion, C, and tensile strength, x, can be correlated with the angle of friction, <{>.
For different consolidations of a powder, there will be different yield loci.
Usually, powders display non linear yield loci. However, over a limited range of
consolidation, the yield locus can be approximated by straight lines. Furthermore, the end
points of the individual yield loci approximately lie on a straight line which goes through
origin. With cohesive powders the individual yield loci are a strong function of the
interstitial voidage, e of the bulk.
In practical terms, the cohesiveness of a powder is evaluated by evaluating the
flow function of a solid. In this evaluation, CTi is major consolidating stress and CTC the
unconfined yield strength of the powder. The ratio
ffc= CTl/ CTC, (B.6)
is defined as the flow function and is a measure of powder's flowability.
The larger the function ffc and the smaller the strength CTC , the better is the flowability of
a powder. If,
ffc < 2 then the powder is very cohesive and non flowing
2 < ffc < 4 then the powder is cohesive
4 < ffc < 10 then the powder is easy flowing and
10 < ffc then the powder is free flowing
By using a flow - no flow concept, gravity flow channel outlets can be sized for
reliable flow. In this concept, the strength of the powder is compared with the stresses
causing the powder to flow. Flow will occur when (Ti > CTC . That is the cohesive arch
across the outlet will fail since the stress CTi imposed by the hopper exceeds the
unconfined yield stress of the powder CTC. The critical value of CTi occurs at the
intersection point of the flowfactor and the powder flow function. That is for the flow to
be continuous.
351
A cohesive powder can support a static shear stress with no normal load applied.
For arching to be prevented and flow to be continuous, the strength of any arch must be
less than the forces tending to break it. In most situations, these forces only represent the
gravitational forces. However in some situations, there may be an additional force due to
a differential gas pressure across the arch.
In a free arch, both the shear and normal stresses in the plane tangential to the
arch must be zero, so that the stresses at any point on the surface of an arch must be
represented by a Mohr circle passing through the origin; this Mohr circle which touches
the material yield locus represent the stresses at a point in a powder on the verge of
failure. -
This limiting circle also defines the unconfined yield strength, CTC, of the powder
for a particular consolidation. This property is the yield strength that can be measured by
causing a radially unconfined cylindrical plug of preconsolidated powder to fail in axial
compression. The relation between the unconfined yield strength, the cohesion and
internal angle of friction, for a linear yield locus is approximately linear is,
c _ o c ( l - g i n 4 > ) ( B ? )
2 cos(t)
In the limiting case, the maximum shear strength possible is, 1/2 o c (B.8)
In this initial simple model, Jenike and Walker assume the point G(=l/2 CTC,)
which gives the most difficult and conservative case for maintaining flow.
Jenike's theory in combination with powder properties measured in the shear cell
predicts the arching dimension larger than the arching dimension found experimentally
as reported by Walker (1967). However, it is advisable to have a overdesign to be on the
safe side. In particular, Walker's equation for arching is,
CTC sin 2 (a + (j)) (B 9 )
R = —————^—— Pb
which considers the walls to be steep and smooth.
However, engineers who apply the Jenike method in practice consider the method
to be reliable and claim it is advisable to have a certain overdesign to be on the safe
side.The overdesign of 1 0 0 % is probably necessary to account for initial filling
conditions and other unaccounted factors (e.g. increase in moisture content, etc.).
Obviously more accurate outlet sizing can be determined using full scale experimental
facilities. Such testing would provide a lower bound to the expected critical outlet opening
dimension and the Jenike method the upper bound. T o approximate full scale
352
experimental facilities a novel arch tester was developed. Details of this arch tester is
outlined in Chapter 6. 12.
Using this tester, the unconfined powder yield strength was then evaluated from
knowledge of the observed opening span to effect flow, by use of the well known Jenike
critical outlet dimension equation (Jenike, 1970), viz. H (a) CTi
B o r D = "^i- (BI0)
where H(a) = function of a
a = hopper half angle
CTi = major principal consolidating stress
Pb = bulk density
In this evaluation the channel boundaries were assumed to be rough.
B.18 TENSILE STRENGTH:
B.18.1 INTRODUCTION:
Powder tensile strength is a fundamental failure property. Since this powder
property bears a strong relationship to powder cohesiveness, the tensile strength of a
powder is important during handling and processing of cohesive powders. The tensile
strength has been estimated from the failure curve obtained from shear tests or three
dimensional compression tests. Alternatively, it may be measured by purpose built
powder testers.
Rumpf (1970) suggested a model for tensile strength of solids arising out of
forces between individual particles. Rumpf s equation for tensile strength CTZ is,
CTz=^k| (B.ll)
where e = porosity
k = average coordination number
H = cohesive force at a contact point
dp = particle diameter
B.18.2 YIELD LOCUS EQUATION:
Farely (1965) reported that for any powder there exists a series of yield loci with
a state of compaction measured as bulk density as a parameter. It has been found that the
yield loci can be represented by the following equation:
where x = shear stress,
353
T = Ultimate tensile strength,
C = cohesion,
CT = compressive stress,
and n = shear index.
It was shown experimentally that this relationship holds for a large number of
powders tested. The value of the constant n varies between 1 and 2, being just over 1 for
free flowing powders and approaching 2 for cohesive powders. For a free flowing
powder, n = 1, T = C = 0 and the eqn.. (B.29) reduces to,
T = K ° (B.13)
where K = a value similar to the internal friction of the powde*
Nedderman (1978) has shown that the value of the constant n can not be larger
than two if Mohr's circle is to be tangential to the material yield locus.
Jimbo et al. (1984) confirmed experimentally by testing many powders that the
relationship between the tensile strength CTZ and the porosity e can be expressed by the
following equation over a wide range of porosity, except for agglomeration
phenomenon.
az = kx exp ( ~ - J (B.14)
where ki and b are empirical constants.
The expression relating the tensile strength CTZ, and pre-compression stress p can
be written as:
CTz = k 2 pm (B.15)
where k2 and m are empirical constants.
They also found that the porosity e can be correlated to the pre-compressive stress
p by the following equation.
p = k3exp(-~) (B.16)
where k3 and c are constants.
Moreover, Jimbo et al. (1984) found that the behaviour of powders can be
approximated, in the porosity range of about 0.4 ~ 0.9 by the expression,
(-i-~-)=10exp(-4.5e) (B.17)
354
In this range of porosity, the semi-theoretical equation proposed by Tsubaki et al.
(1984) to relate a powder tensile strength to bed porosity, pre-compression stress and
mean particle size as,
•--(¥) -.dp;
m
(B.18)
where P is the pre-compression force.
To overcome the limitations of the W.S.L. tester, notably the narrow limit of the
range of porosity of the powder bed, Jimbo et al. (1984) developed modified testers in
which both sides of the split cell are mounted on ball bearings incorporating a pushing
mechanism to reduce friction in the bearings. *
The tensile tester provides values of the powder's uniaxial tensile strength, in
comparison to that predicted from the Jenike's method which gives powder behaviour
when subject to a biaxial stress state.
C.19 FLUIDIZATTON:
Fluidization is the transformation of a static bed of particles into a liquid like mass
system induced by the flow of a gas or liquid. In most fluidization systems, the
fluidizing agent is air or a gas. In essence, particulate masses may range in characteristics
from static beds, through fluidized beds, through, in turn, fast fluidized to dilute
suspended particulate streams as those occurring in dilute pneumatic conveying.
In regard to fluidization behaviour, powders can be classified into two groups,
namely : Free-flowing and cohesive powders. With free flowing powders, the forces of
attraction between the particles are negligible, so that the powder can flow easily under
the action of gravity, e.g.dry Silica, Sand, dry Wheat. These powders give few problems
in regard to design and operation of pneumatic conveying systems. For cohesive
powders, there is more frictional resistance to flow and in some cases this resistance is
strong enough to prevent movement. These powders exhibit significant interparticle
forces and hence these powders are troublesome, when pneumatically conveyed. For
this reason, cohesion directly relates to the flowabiUty of a powder.
Thus, a free flowing powder is fluidized without any difficulty, whilst the
fluidization of cohesive powders is normally difficult and can require the modification of
either the flow properties of the powder or of the fluidization equipment. Further
classification of powders are:
1 free-flowing 6 fluidizable, high air retention
355
2 less free-flowing 7 very cohesive
3 fluidizable 8 abrasive
4 fluidizable, low air retention 9 friable
5 fluidizable, moderate air retention
The inter-particle forces that cause poor fluidization characteristics are small
particle sizes, strong electrostatic charges or the presence of moisture in the bed. Hence,
in regard to fluidization, the most important powder properties and their effects are now
listed.
Particle size: the finer the particles the stronger the interaction between the fluidizing
gas and the particles.
Flowability: the more flowable a fine powder, the more fluid-like and hence the more
readily it can be fluidized.
Hardness: controls rate of system wear, minimum effect with respect to fluidization.
Hygroscopicity: moisture works against fluidization.
Dispersibility: the less cohesion, the more readily fluidized the powder
Compressibility: the less the compressibility of a fine powder, the more fluidizable
the powder.
Size distribution: the wider the distribution, the less definite the fluidization and
greater tendency to segregate in the bed.
Particle shape: the more spherical the particle, the easier the fluidization. Irregular
shaped particles tend to interlock and hence tend to be difficult to fluidize.
Cohesion and particle friction: the stronger these forces, the more difficult the
fluidization.
Geldart (1973) suggested that powder particle size and density be used as a
criteria to classify various powders into four groups having different gas-fluidization
behaviour.
Group A powders are slightly cohesive and exhibit large bed expansion after
minimum fluidization and before the commencement of bubbling. Group B powders
bubble at minimum fluidization velocity with small bed expansion. Group C powders
are cohesive and difficult to fluidize, whereas Group D powders can form stable spouted
beds if the gas is admitted through a centrally positioned hole.
Group A powders deaerate at a constant rate, Group B powders deaerate rapidly,
Group C powders deaerate faster than Group A powders but remain in an aerated state
356
for a considerable time, decaying pressure at the base of the bed very slowly. In view of
this correlation, the deaeration rate is important in characterizing powders.
B.20 SLUGGING:
Dixon (1979) proposed slugging diagram (Figure B.l) based on Geldart's
fluidization diagram to predict the behaviour of powders in dense phase systems.
•
—
w
' * * / •
* 5—-J •v ',
(?) NO SLl
/ "'. '.
V
GC
-;/
K-\
IN
>-,
\
3
•
s, \
> -
Fg
\ ^
>.
JI (3-4 INC
\
D WEAK \SYM JLUG5
HPIPI
v \ \
MET
r SL
V \
lie
\ JG<
\ \
!>IN
\
\ \ 1
G D>
<°> STROI
*> AXISY ^ SLUG!
» G H A W
IG MME1
s \ \
RIC
N 20 50 100 600 1000
MEAN PARTICLE SIZE d (pml
Figure B.l: Typical Slugging Diagram [ Dixon, (1979) ].
In Figure B.l, Group A powders are the best candidates for dense phase
conveying. They are not natural sluggers but can be made to slug by using slug forming
techniques. Group B powders give problems if high solid gas loadings are used.
Group C powders are less suitable for dense phase conveying, whereas Group D
powders are good candidates which have natural slugging behaviour.
Flatt (1980) reported a graph for selection of dense phase systems whether by
pass or pulse phase systems taking into account the particle size distribution, whereas
Klintworth et. al. (1985) revealed particle size variation against mean particle size
indicating four classification zones for selecting discontinuous phase systems.
_ -
B.21 DEAERATION:
A n important property of fine powders is their ability to retain air within the voids.
W h e n powders are filled into containers, a large volume of air is entrapped within the
voids. O n settling, this air slowly dissipates and the level of powder drops, until it comes
to a some steady state level. After which time consolidation at almost constant voidage
commences.
Deaeration can be defined by noting the change in bed height with time after
aeration ceases or by noting the change in pressure differential across a unit of height of
powder with time. In regard to the observation of bed height, this apparently simple task
often proves difficult for many powders, as some powders tend to coat the interior of the
test rig with fine particles. ,
All powders that have been fluidized will deaerate over a certain period of time
during which their bulk density increases. The rate of deaeration process is important in
regard suitability for dense phase conveying. In practical filling operations, deaeration
takes place during the filling process.
The deaeration process forms the initial stage in time consolidation. Hence, to
prevent time consolidation, it is necessary to prevent the initial de-aeration. This can be
effected by passing air through an air distributor at the bottom of the bin. At the same
time, the formation of consolidation zones within the bed can be prevented. The
existence of these zones causes channelling, ii the powder is allowed to consolidate and
the air is introduced before powder filling commences. The powder contains zones of
higher bulk density and when air is passed, these zones will tend to separate causing the
fluid flow to channel through the less dense zones in the bed. The rate of diffusion
depends on the interparticle forces, size, shape and packing properties of the powder.
The quantity of air dissipated from the bed during deaeration is equivalent to the
volumetric change of the void during densification. Under most circumstances, the
voidage distribution settles down rapidly to steady state level, since most of the air is
dissipated while the permeability is still large. Thereafter, the deaeration is relatively
slow. Here, the displaced fluid caused by the change in bulk density percolates slowly
through the compacted bed.
Sutton described empirically the variation depicted by,
H = Hoe-k't (B.19)
where k' is diffusion coefficient
358
From these tests a value of k' was derived. Powders with k' > 100 cm/s are
unsuitable for aerated discharge, while powders which are aeratable i.e. deaerate slowly
have values of k' < 30 cm/s. This factor provides a measure of the tendency of the
particles to form agglomerates in the bed by interparticle attraction and which have a
structure different from the rest of the bed.
By use of a pressure transducer and recorder device, the time variation of column
base pressure can be observed and recorded. The relationship between the pressure drop
per unit length and time is of the form
Af = t . Ap/L , (B.20)
where A p = pressure drop per unit length L of the bed
t = time in seconds
A f = a constant defined as the deaeration factor with units of mbar.s/m
Mainwaring et al. (1987) proposed the permeability and air retention are two
important parameters in regard to feasibility of a powder in dense phase pneumatic
conveying. They developed the plot of the permeability factor, pf, versus quasi-steady
pressure drop per unit length corresponding to the fluidized condition, (Ap /L) c The
results for a number of powders tested by them indicate that powders exhibiting high
values of permeability factor generally convey in a plug type mode of dense phase
conveying, while powders exhibiting low permeability were conveyed in either a dense
phase moving bed type flow or could not be conveyed in dense phase at all.
This approach of utilizing the permeability and deaeration factors of the powder to
be conveyed ensures that the influence of the size distribution rather than a mean particle
size is used to categorize a powder for its likely conveying characteristics.
359
APPENDIX 'C
Cl CALIBRATION OE TRANSDUCER CHANNELS:
When conducting experiments on both the Sturtevant rig and low velocity rig, the
pressure transducers were calibrated before commencing each experimental run. Typical
values from the calibration are as follows, refer Table C l .
TABLE Cl: TYPICAL CALIBRATION VALUES
(A)
Sturtevant Rig
Channel No.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Calibration Values
0.223
0.251
0.354
0.293
0.298
0.309
0.293
0.228
0.009
7.809
95.61
0.299
0.330
0.305
0.311
0.139
(B)
Low Velocity Rig
Channel No.
0
1
2
3
4
5
6
7
8
9
10
*
Calibration Values
0.510
0.287
0.204
0.117
0.48
-
-
0.486
0.009
182.0
356.1
C.2 CALIBRATION OF WALL FRICTION RTO LOAD CELL:
This was calibrated by loading the piston with different weights and observing the
corresponding chart recorder readings. A typical calibration plot is presented in Figure
C l .
360
8-j — .
-35 -/.
UJ J3
o-l . , • , , 0 100 200 300
NUMBER OF DIVISIONS - m m
Figure C. 1: Calibration Plot.
C.3 CALIBRATION OF PRESSURE GAUGE IN WALL FRICTION RIG:
To measure the air pressure in the wall friction rig, a pressure gauge No. P / A
2125 - 466 of capacity 0 - 700 kPa x 254 m m . diameter was used. Before commencing
the experiments, it was calibrated by a static pressurized tank system, the results of which
are stated below, refer Table C.2.
TABLE C.2: CALIBRATION RESULTS
Test Pressure,
kPa
60
110
160
210
260
310
360
410
510
610
660
Gauge Reading,
kPa
57.2
106.1
156.4
206.7
254.9
305.2
356.9
405.1
504.4
604.9
654.6
Calibrated Pressure,
kPa
59.9
109.9
159.9
209.9
259.8
309.8
359.7
409.8
509.6
609.6
659.5
361
C.4 CALIBRATION OF THE DEAERATION RIG PRESSURE
TRANSDUCERS:
The calibration of the pressure transducers used in the deaeration rig to record the
interstitial air pressure, was effected using the Sturtevant blow tank as a pressurization
tank. A typical calibration recording is presented in Figure C.2.
Figure C.2: Typical Calibration Graph.
C.5 POWDER CONCENTRATION: This can be measured using a T.300 Tealgate transducer or calculated using the
data observed during each experimental run. In particular, the powder concentration can
be calculated using the equation :
C- = ^ (CD
where Vs = Volume of solids and
362
Vt = Volume of solids + air = total volume = Vs + V f (C.2) Vf = Volume of fluid (air)
Since, volume is related to mass M and density p,
Ms Vs - — (C3)
KS
Mf Vf = -
1 (c.4) Pf Therefore, the powder concentration,
Ms
r Ps ( -.
s Ms Mf C C 5 J
P7 P7 Simplifying eqn. (C5) gives,
1
1 + c*= —wt: (C6)
M s p f
where pf = air density = Patal + p f/RT
where Patm and pf are equal to the atmospheric pressure and the operating air pressure at
the point closest to the measuring instruments, respectively. R = gas constant (for air, R=0.287 kJ/kg/K)
T = operating air temperature.
C.6 CALIBRATION OF THE T.300 SERIES TRANSDUCER:
To measure powder concentration, the T.300 series transducer was used in
Sturtevant pneumatic conveying test rig system. A typical observed calibration graph is
presented in Figure C.3. Values of the transducer output corresponding to 30%, 5 0 % and 8 0 % concentration peaks are evaluated from the calibration graph for particular
reference. A typical concentration variation obtained by use of a Tealgate T.300
transducer is presented in Figure C 4 .
C.7 POWDER VELOCITY:
With knowledge of the particle concentration, the velocity of the conveyed
powder can be calculated as follows.
363
Since, the powder concentration is as per eqn. (Cl),
Ms C s" V^ ~ Ps V, A
(C'7)
where V s = volume of powder
Vt = volume of powder and air
It follows that the powder velocity can be calculated from a rearrangement of
equ. (C.7), viz.
M s
V = — s P s A C s
(C.S)
Figure C.3: Typical Calibration Graph.
364
ut«M tthV^^9'*npmiM%f^%pp%mptmmm9W9'Pmumm»mpmt»»M»vt>mi*im99paimtPP»»imamnmmtt3nii
iinuiHitirMMiiMtMMMii iv 4tiiiMi---WrWi«niitii<ir'wii»iiW*<'IWfiM winiiiwi
«-+ 1 1 -T ttt *
tct IS 4-1 ___: * 33 S mix
I -£ r 3"-
t-t trip ill nil IS S ca n; -8 -3-t cr;
c-i 3? 1 wm at m
Figure C.4 : Concentration Graph.
