Continuous sucrose crystallization

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Authors Ziebold, Steven Anthony, 1949-

Publisher The University of Arizona.

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CONTINUOUS SUCROSE CRYSTALLIZATION

bySteven Anthony Ziebold

A Thesis Submitted to the Faculty of the

DEPARTMENT OF CHEMICAL ENGINEERING

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 7 3

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of re­quirements for an advanced degree at The University of Arizona and is

under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg­ment the proposed use of the material is in the interests of scholar­ship. In all other instances, however, permission must be obtained from the author.

deposited in the University Library to be made available to borrowers

SIGNED:

APPROVAL BY THESIS DIRECTOR

This thesis has been approved- on the date shown below:

ALAN D. RANDOLPH.ALAN D. RANDOLPH/. Professor of Chemical Engineering

Date

ACKNOWLEDGMENTS

I am deeply indebted to my advisor. Dr. Alan D„ Randolph, with­

out whose assistance this thesis would not have been possible. I would

also like to extend my thanks to the faculty of the Chemical Engineering

Department for guidance in various areas related to the research work.

iii

TABLE OF CONTENTS

Page

LIST OF ILLUSTRATIONS „ . vi

LIST OF TABLES . . . . . . . . . . . .. . . . . . . . . . . . . . viii

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

CHAPTER

I. INTRODUCTION ........... 1

II. SUGAR CRYSTALLIZATION . . . . . . „ . . . . . . . ... . . . 3

Raw Sugar Production . . . . . . . . . . . . . . . . . 3Refining of Raw Sugar . . . . . . . . . . . . . . . . . 7

III. CURRENT CONTINUOUS CRYSTALLIZATION DEVELOPMENTS . . . . . . 9

Scope of Present Study . . . . . . . . . . . . . . . . 12Experimental Studies . . . . . . . . . . . . . . . 13Computer Simulations . . . . . . . . . . . . . . . 15

IV. THEORETICAL CONSIDERATIONS . . . . . . . . . . . .... . . . 17

MSMPR Technique . . . . . . . . . . . . . . . . . . . . 17Sucrose Growth and Nucleation Kinetics . . . . . . . . 20

Types of Nucleation . . . . . . . . . . . . . . . . 21Growth Rate Correlation . . . . . . . . . . . . . . 23

Salting-out Crystallization . . . . . . . . . . . . . . 23Crystallization Modifiers (Surfactants) . . . . . . . . 26

V. EXPERIMENTATION . . . . . . . . . . . . . . . . . . . . . . 28

Apparatus . . . . . . . . . , „ . . . . . . . . . „ . . 29Coulter Counter . . „ . . . . „ . . . . . „ . . . „ 34Start-up Procedure . . . . . . . . . . . . . . . . 35Sampling and Particle Counts . . . . . . . . . . . 39

Experimental Runs . . . . . . . . . . . . .............. 42Description of Individual Experiments . . . . . . . 43Calibration of the Coulter Counter . . . . . . . . 45

Experimental Data .................. 46

iv

VTABLE OF CONTENTS (Continued)

: :

VI. ANALYSIS OF EXPERIMENTAL RESULTS . . . . . . . ... . . . . . 56 '

Statistical Results . . o o o o . o . o o o o . o o o . 56 Results of D-Series Runs , , , , « . „ . „ , , , , „ . * 66Crys tal Hab3. t . © © © © © © © © © © © © © © © © © © © © 67Kinetics Correlation © © © © © © © © © © © © © © © » © 0 70

VII© PROCESS CALCULATIONS © © © © © © © © © © © © © © © © © © © © 75

Nucleator/Ripener/F©C© Crystallizer © © 0 © , © © © © © 75Solvent Recovery © © © © © © © © © © © . © © .© © © © © 78 Retention Time Effects © © © © « © © © © © © © © © © 78

VIII© SUMMARY © © © © © © © © © © © © © © © © © © © © © © © © © © 82

IX© CONCLUSIONS © © © © © © © © © © © © © © © © © © © © © © © © 83

X© RECOMMENDATIONS © © © © © © © © © © © © © © © © © © © © © © 85

APPENDIX : SOLUBILITY DATA © © © © © © © © © © © © © © © © © © © © 87

NOMENCLATURE . ©.. © © © © © © © © © © © © © © © © © © © © © © © © © © 88LIST OF REFERENCES © © © © © © © © © © © © © © © © © © © © © © © © 90

LIST OF ILLUSTRATIONS

Figure ' Page. 1 Sucrose Molecule . . ... ... ... . ... ............ 1

2 MSMPR Crysfcallizer o . . . . . 17

3 Schematic Flow Diagram of Crystallizer System . . . » . . 32

4 Main Crystallizer Unit (Front View) . . . . . „ . . ... . . 33

5 Crystallization System (Side View) . . . . . . . . . . . . 33

6 Coulter Counter and Sampling Stand . . . . . . . . . . . . 36

7 Population Density vs. Crystal Size, Run 1-A . . . .... . . 49

8 Population Density vs. Crystal Size, Run 2-B . . . . . . . 50

9 Population Density vs. Crystal Size, Run 4-B . . . . . . . 51

10 Population Density vs. Crystal Size, Run 1-D . . . . . „ . 52

11 Population Density vs. Crystal Size, Run 7-B . . . . . . . 53

12 Population Density vs. Crystal Size, Run 8-B . . . . . . . 54

13 Calculated Yield vs. Experimental Yield . . . . . . . . 55

14 Microphotograph of Run 6-B Product . . . . . . . . . . . . 6 8

15 Microphotograph of Run 8-B Product . . . . . . . . . . . . 68

16 Microphotograph of Run 3-D Ripened Product No. 1 . . . . 69

17 Microphotograph of Run 3-D Ripened Product No. 2 . . . . . 69

18 Microphotograph of Run 1-D Agglomerates . . . . . . . . . 71

19 Microphotograph of Run 1-D Agglomerate . . . . . . . . . . . 71

20 Correlation Plot of Predicted vs. Actual NucleationRates . . . . . . . . . . . . . . . . . . . . . . . . . . 73

vi

viiLIST OF ILLUSTRATIONS (Continued)

Figure . Page

21 Nucleator CSD and Ripener CSD for Two Production Rates . „ 76

22 Ripener CSD and F.C. Crystallizer CSD for Two .Production Rates . . . . . „ „ „ „ , „ . „ „ „ „ „ „ „ „ „ 79

23 Ripener CSD and F.C, Crystallizer CSD with Growth Rate Dependency on Size . . . . . . . . . . . . . . . . . . „ . . 80

LIST OF TABLES

Table Page1 Solubility of Sucrose in Ethanol/Water Mixtures . . „ . „ . 25

2 Summary of Experimental Conditions .0 o 44

3 Independent Variables Studied in B-Experiments 0 . » o 0 0 45

4 Summary of Experimental Results . . • . . . . • • • * • • • 4 7

5 Units for Experimental Results • 0 • • . . • • . . • . ■ • 48

6 Design Matrix for Series B Runs , o e •' o -• 58 ■

7 System Response = G T (Characteristic Length) 0 • « 60

8 System Response = ln(n ) . o « o . o o o . o 0 .o. . . • • • • 61

9 Test 2; System Response = Gt (Characteristic Length) , . . 64

10 Test 2; System Response = ln(n°) 64

viii

ABSTRACT

Research work at the University of Arizona on the sucrose

salting out technique centered on the optimization of process

conditionso The goals of the research were to produce a seed fondant

on a continuous basis with acceptable crystal habit, a nuclei density

of 5*0 x 10^ number/cc-micron, high yield, and population weighted

charactristic size of 10 microns0

As the research proceeded, it was concluded that the pertinent

variables affecting the system kinetics were the concentration of the

sucrose solution, the temperature of the crystallizer, and the holding

time0 The highest nuclei density (4,8 x 10^ number/cc-micron) was

achieved under conditions of high syrup concentration, low crystallizer

temperature, and long holding time. The corresponding characteristic

size was 10.9 microns.

The research work also included a study of two different alcohol/

solvent systems and two different types of surfactants.

Computer simulations of a three-stage continuous sugar process

demonstrated the feasibility of using the sucrose seed nucleator as a

grain source in a continuous sucrose crystallization pan.

CHAPTER I

INTRODUCTION

Sugar is the largest tonnage organic chemical produced in pure

form. It has been known to man for at least 3,000 years. Cane sugar

most likely originated in India, probably from the banks of the Ganges

River. We cannot be absolutely certain of this because today sugar cane

is not found anywhere in its wild state. A more complete history of

cane sugar is presented by Geerligs (1912).

Sucrose (Saccharose, Cane Sugar, Beet Sugar) is a disacharide

consisting of two monosaccharidic components: D-fructose and D-glucose.

It is a carbohydrate of the formula ^ 2^ 2 2 ^ 1 1 with the exact chemical

name given as Q'-D-glucopyranosyl-f3-D-fructofuranoside. The structural

formula is given as:

H C -----I

H C " " OHI

HO C ■'■■■■ HI

H C OHI

H 1 C------

0 CH20H

0 HO C " 1 HI

H ' C ■ ■ OH

H C------

CH20H

(glucose component)

c h 2o h

(fructose component)

Figure 1. Sucrose Molecule.

1

In plants, sugar is formed by the photosynthetic assimilation

process which is essentially the conversion of water and carbon dioxide

into carbohydrateSo Sucrose is the most widespread sugar present in

the sap of plants and the sap of sugar cane contains more than 17 per­

cent of sucroseo Hirschmuller (1953) presents a good description of

the sucrose molecule*

The increase of sugar consumption per capita since 1900 reflects

the growth and progress in methods of production and reduction in price0

It also reflects the increased appreciation of the benefits of sugar in

the diet by the consumer* Although there has been much research in the

past on the production of sugar there are still areas that if improved

would substantially benefit the economics of sugar production or would

improve the quality of the product itself*

Sugar represents one of the cheapest forms of all foods* It is

not only one of the most concentrated of all energy foods, but its

source plants are among the most productive of all agricultural plants*

Thus, in a world of exponentially increasing population it is desirable

to increase the consumption of sugar through technology*

CHAPTER II

SUGAR CRYSTALLIZATION

Much of the cane throughout the world is processed to final prod­

ucts in factories which use standard procedures. Chemical processing

of sugar by man is concerned with separating the sucrose from the various

materials with which it is associated in the plant. Sugar is purified

by the process of crystallization which could be considered one of the

oldest chemical engineering unit operations. The literature is extensive

on research efforts associated with refining sugar and the processes pro­

ducing raw sugar. Jenkins (1966) presents a fairly concise introduction

to cane sugar technology up to the production of raw sugar. Another

excellent source on the technology of crystallization of granulated

sugars is written by Alewijn and Honig (1959).

Grinding of the sugar cane is the first step in the production

of raw sugar from cane. The subsequent refining of raw sugar is gener­

ally a separate process. This dual nature in the industry leads to a

natural division between raw. and refined sugar.

Raw Sugar Production

The harvested cane is taken to the factory where the first step

consists of removing the sugar juices from the cells of the plant. The

efficiency of the standard milling process as gauged by the extraction

of sucrose and soluble impurities is usually about 95 percent. The non-

soluble material taken out with the extracted juice is a cblloidal

,

. 4

impurity and presents problems in subsequent processing. Raw sugar

mills are rated by their daily grinding capacity. An average size mill

processes from 2,000 to 3,000 tons of cane a day.

The juice coming from the grinding mills is strained to remove

dirt and is then clarified. Such purification is necessary in order to

reduce the content of nonsucrose impurities which could interfere with

the subsequent crystallization. The efficiency of the clarification is

tested quantitatively by the change in purity which is characterized by

boiling and freezing point data. Qualitatively, this is reflected in

the color, consistency, and crystal habit of the product obtained. Clar

ification is affected by heat and lime; however, other agents are added

to supplement their action. The coagulated impurities are then filtered

out of the system; what remains is raw sugar usually known as "thin

juice." This material contains about 85% water. In this mixture

there are approximately 630 pounds of water for every 100 pounds of

sucrose in addition to impurities.

It is next necessary to remove the large quantities of water in

the thin juice to extract the sugar. The first step is evaporation to

remove most of the excess water in which the thin juice is converted

to a heavy syrup. This process is controlled to stop short of satura­

tion. Multiple effect evaporators are used to achieve efficient evapora

tion, thus minimizing steam costs.

The next step is termed "sugar boiling." In this step, concen­

tration is carried further so that crystals form and are grown under

controlled conditions to the desired size. This "sugar boiling" process

is carried out in stages, the final products being raw sugar and molasses.

Generally, there are at least three stages,. After the boiling process,

the liquor from the first stage is separated from the slurry and is fed

to the next pan in line. In turn, the mother liquor from the second pan

forms the feed to the third stage. The small amount of syrup remaining

in the low-end "strikes," i.e., in the crystal slurries in the second

and third pans, is run into the first stage where further crystallization

takes place without additional removal of water. The massecuite (mixture

of crystals and syrup) is cooled at this station in order to accelerate

the final crystallization from the solution. Thus, "sugar boiling" in­

volves boiling a viscous liquid containing crystals in suspension.

Evaporators of a special design, known as vacuum pans, are used in single-

effect. Two designs of these single-effect evaporators are common-coil

and calandria, with the addition of forced circulation becoming more and

more common to either type. The work of the pan, which constitutes the

boiling of sugar manufacture, is one of the most important and strategic

steps in the entire sugar process. According to Van Hook (1949), the

sugar boiler is the true artisan of the sugarhouse, but scientific

knowledge of the boiling operation is reducing this step to one amenable

to exact control and duplication. Although concentration of the

massecuite by evaporation continues in the pan, the major pan function

is the induction of crystal formation and the continued growth thereof.

Thus the entire process of sugar manufacture culminates in the strike in

the pan. Sugar boiling could be considered strategic and critical in

the sense that the juice is at its maximum concentration or minimum

bulk and is most sensitive to degradation and spoiling by improper

'handling. The sugar boiling process commences with syrup from the

multiple effect evaporator at 65 -70 brix; this material is concentrated

to form crystals and grow them to the desired size. For maximum capacity,

it is desirable to carry the crystallization at each stage to the maximum

crystal content which can be handled. This means that at the conclusion

of each charge, the material or "massecuite" consists of a suspension of

crystals in a highly concentrated mother liquor or molasses,

A number of profound changes occur in the massecuite as soon as

crystallization starts. The immediate effect is a reduction in concen­

tration so that the driving force for crystallization drops immediately,

Tov maintain continued crystallization, there must be a simultaneous

evaporation of water from the solution. During all this time concentra­

tion of the noncrystallizing, nonsucrose solutes continues. There is

also the possibility of new nuclei being generated while the established

ones are growing. If uncontrolled, this situation, which is known in the

sugar industry as "false grain,u leads to a wide distribution in sizes

of the final product. Also conglomerates or intergrowth of crystals

already present must be avoided because part of the mother liquor is

then included within the crystal faces where it cannot be removed by

centrifugal force or washing.

The three-step operation of evaporation, sugar boiling and

crystallization, can be considered a semi-continuous process since the

raw juice is concentrated continuously by multiple-effeet evaporation

and the actual crystallization takes place batchwise in the pans.

Refining of Raw Sugar

Raw sugar is refined in a process similar to that used in the

raw sugar mill. Both the raw sugar and refining processes have distinct

similarities; however, refineries are generally characterized by their

large sizes in comparison to raw sugar mills. The refining process con­

sists of affination (washing), clarification, decolorizing, crystalliza­

tion, and drying and finishing. Crystallization of the sugar is again

accomplished in large vacuum pans. The pans, as in the raw factory, are

usually twelve to sixteen feet in diameter and the boiling procedures

in the two instances are practically identical. However, greater pre­

cautions are exercised in the refinery because of the higher quality of

the finished product.

Crystallization consists of three different phases, all requir­

ing separate specialized techniques and control. The first phase is the

graining or nucleation, the second is the growth of the crystals to the

required size and finally the last phase is the tying up or equilibra­

tion of the syrup to obtain the highest yield of crystallized sugar.

The major product of most refineries is the standard granulated

grade which is classified as a hard, white sugar, as distinguished from

soft sugars which may be anywhere from almost white to deep brown in

color. The objective in the majority of crystallization processes is to

produce individual crystals of regular form, purity, and size. For

example, twinning or agglomeration of the crystal can have adverse

effects upon packing, storing, and shipping characteristics due to

" - . - :: 8 variations in void volumes. Consumers often demand uniform crystal

size distributions due to precedents.

As with raw sugar production , sucrose refining can be considered

a semi-continous process; evaporation of weak syrup is done continuously

followed by batch crystallization. Considerable economy could be

achieved if the crystallization process could be made continous while

still producing high quality crystals of a narrow size distribution.

CHAPTER IIICURRENT CONTINUOUS CRYSTALLIZATION DEVELOPMENTS

The conventional processing of sugar today is characterized by

a semi-continuous operation. From a chemical engineering standpoint,

continuous processing of sugar appears, to have some distinct advantages,

primarily that of reduction of equipment size and energy requirements.

The literature describes some efforts to obtain a workable continuous

process and there are at least two Russian patents and one Dutch patent

associated with continuous processing.

Preliminary announcement of a new continuous sugar crystalliza­

tion system was made in 1970 by the Stark-Werkspoor Sugar Company in

the Netherlands. Their system calls for the use of a heater, evapora­

tor , and sugar-crystal generator. With the use of automatic controls,

the system is based on crystallization by flash evaporation following

the heating of the syrup to a closely controlled degree of supersatura­

tion. The sugar-crystal generator works under vacuum and is equipped

with an internal, positive displacement device for the removal of the

crystals, (Anonymous, 1970).

The Fives Lille Cail Company in France has developed a continu­

ous vacuum pan system which is essentially a horizontal cylinder having

an open area at the top and compartment cells in the lower portion

(Silver, 1970). Crystallization is carried out successively on a con­

tinuous basis progressively from one cell to the next. The benefits of

this system were determined to be fuel savings, power savings, labor

9 ' '

: iosavings, high quality product, elimination of a source of water pollu­

tion, and increased sugar recovery.

The concept of a continuous vacuum crystallizer has also been

described by G. Luce (1970), In this article the author's approach was

to consider discontinuous pan boiling and continuous pan boiling as two

separate processes. The continuous unit had the following character­

istics: (a) Continuous flow without the necessity of moving parts,

(b) identical and uniform flow patterns for all crystals, (c) elimina­

tion of hydrostatic head effect, (d) uniform rate of crystal growth

within the maximum safe working limits in all stages, (e) eliminations

of conglomerates and false grain resulting from "pulling" the strike

together, (f) improved distribution of feed syrup in all stages of

crystal growth, and (g) consistent flow of materials in the process

from feed to discharge,

Bennett and Petri (1966) have also made pioneer investigations

of continuous sucrose crystallization in a pilot pan. In the early

1960's, the Swenson Evaporator Company began developmental work in the

area of continuous sugar processing. This work led to the development

of a process centered around a forced circulation type crystallizer.

The Swenson unit was operated under various conditions to test the

nucleation and growth kinetics of sugar crystals. Bennett (1969) con­

cluded that the growth of sugar follows McCabe's A L law giving growth

rates of 0.007 to 0.090 millimeters per hour at typical pan super­

saturations. Bennett also reported expected savings of a continuous

process when compared to a conventional process with respect to

' ' • -: • ' : : : 11utilities and labor plus operational advantages, in. the area of control

and the simplification of equipment items. Pilot plant tests were

carried out by Refined Sugar and Syrups Corporation using the facilities

of the Whiting Corporation’s Swenson Division in Harvey, Illinois. The.

sugar produced contained less conglomerates compared with sugar boiled

in conventional batch type equipment.

Nucleation rates were low in the process; thus, attempts were

made to increase nucleation through mechanical stimulation by suspending

a circular saw blade in the masseciiite and whirling the blade at tip

speeds of five thousand feet per minute. Bennett.reported increased

nucleation using this method; however, the crystals produced from these

nuclei had a poor visual appearance due to the damage caused by the

device.

Sugar solutions support a high degree of supersaturation without

spontaneous nucleation. This phenomena seems to block the development

of a continuous sugar process, unless seeded externally from another

source. Such.external seed is indeed required in any continuous pro­

cesses attempted thus far. For example, if sugar seed were produced oh

a continuous basis and were added continuously to the Swenson forced

circulation crystallizer, then the inherent low nucleation levels en­

countered in the Swenson process could be overcome. The crystals would

serve as nuclei for further growth. Evidence suggests that seed crystal

with perfect habits produce product crystals with desirable habit.

Seeding is not a new idea in the sugar industry and many pro­

cedures use seeding to establish the proper grain centers for growth.

12The disadvantages of conventional seeding according to Van Hook (1959)

are in the availability of seeding stock, the difficulty in preparation

and cost, and in the amounts required0 Typically, seed is produced by

ballmilling off-grade product to chips and pieces in the 0 o 5 - 1 0 micron

size range. This material is usually ground in isopropyl alcohol

solution.

Thus, some research and development has been done in the area of

continuous sucrose crystallization. Progress has been hampered partly

by the lack of the development of theoretical methods for characterizing

and mathematically analyzing nucleation and growth rate phenomena as

well as due to the reluctance of sugar to nucleate spontaneously to any

significant degree. Minimal research has been carried out on the pre­

diction of crystal size distributions through simulation techniques as

a function of fluidity, supersaturation, and vaporization rates, al­

though the techniques necessary for such a study are at hand.

Scope of Present Study

The objective of this experimental work was to further develope

a method for producing high concentrations of well-formed sugar nuclei

on a continuous basis. Initial research on this process was carried out

by Crawford (1970); the results from his preliminary experimentation

seemed encouraging. The apparatus that Crawford used was a mixed

suspension mixed product removal (MSMPR) crystallizer of approximately

3 liters in volume. He confirmed earlier exploratory work at the

University of Florida by Randolph (1965) that it was possible to "salt-

out" sugar continuously from aqueous solution with low molecular weight

■ : ■ ' . ;. \ ^ ; 134 5alcohols and key tones with resulting nuclei densities of 10 -10 no<,/cc-

\ 4 : ■ /•-.micron. Nucleation rates of over 10 nuclei/cc-minute could also be

achieved. The product produced by "salting out" had a characteristic

single crystal form with exceptional crystal habit and color, provided

that the ethanol to water ratio (mass basis) was maintained around 1.4.

The population weighted mean crystal size of the product was 20-30 :

microns when the crystallizer was maintained at a holding time of approx­

imately 50 minutes at ambient temperature. Extending the holding time

of the crystallizer to periods over 1.0 hour had a resultant effect of

reducing the average crystal size without any crystal habit deteriora­

tion. Computer simulations carried out by Crawford indicated that seed­

ing by the salting out process should overcome the inherently low nuclea­

tion rates obtained in continuous forced circulation sugar crystallizefs,

Crawford recommended that further investigation of continuous

sucrose generation in an MSMPR crystallizer include careful control of

temperature so that the effects of temperature on the nucleation rate

could be studied. Another area to be further considered was that of the

effect of surfactants or additives since they often dramatically affect

nucleation rate as well as promote single-crystal growth. It was also

recommended that long periods of operation be studied to insure steady

state conditions and to study fouling.

Experimental Studies

With the above recommendations in mind, the continued research

work that was carried out over a period of nine months was executed as

follows:

141. A laboratory bench-scale salting-out MSMPR crystallizer was

modified and operated to produce sugar crystals from a sucrose/water/

solvent system. The solvent used was either a 50:50 MeOH/IPA mixture or

Vanzol A-l denatured ethanol (100 parts by volume of 190 proof alcohol,

1 part of methol isobutal keytone, 1 part of ethylacetate, 1 part of a

hydrocarbon solvent (Stoddard*s)),

20 Preliminary crystallizer runs were made to debug the system

and to make necessary modifications* A standard run was established

with set conditions to compare with Crawford’s data. Laboratory tech­

nique, which is very important, was also developed,

3* Optimization of chosen process conditions was carried out

with the objective of finding conditions which would produce a crystal7size distribution with a nuclei density approaching 10 no,/cc-micron

with a characteristic population-weighted size of 10 microns.

In other words, the primary objective of the research was to

produce a sucrose seed crystal fondant with the salting-out process

having superior single crystal form and with a nuclei population density

of at least 5,0 x 10^ no,/cc-micron, and a population-averaged size

(Gt ) of less than 10 microns. Exploratory runs were executed to map out

regions worthwhile to study in depth. This phase was guided by a

Plackett-Burman, one-level statistical analysis design to elucidate a

likely region in which to carry, out more detailed kinetic studies

(Isaacson, 1970). Independent variables of the salting-out nucleator

that were explored were:

r . ■ '■ . ; . : :''':- : 15(a) System temperature (50C-27°G).

(b) Feed concentration (680-750Brix)„

(c) Solvent type (50:50 MeOH/IPA vs. Vanzol A-l).

(d) Surfactants (Methyfamine HC1 and Fluorocarbon (FC-96),3M Co.).

(e) Holding time (r:1-3 hours).

In short, the objective was to find conditions under which the

growth rate was reduced and the nucleation rate increased, thus produc­

ing high population densities of small seed. This seed might then be

used directly in a full-scale process or as a seed source for a footing

crystallizer used to grow the seed to an intermediate size before seed­

ing a vacuum pan. Eight exploratory runs were made after the shakedown

runs to study the effects of high and low level values of the aboveovariables upon the dependent variables nuclei density n and the char­

acteristic size Gt according to the statistical design. An added con­

straint which was verified by microphotographs was that the crystal

habit be of a single-crystal form* The key variables were selected and

evaluated in a further kinetic study in an attempt to optimize the

number and quality of seed produced.

Computer Simulations

The kinetics developed were used in an extensive computer simu­

lation of continuous seeded sucrose processes* A computer simulation of

a three stage process comprised of a nucleator (with optimized condi­

tions) staged in series to a seed ripener and a forced circulation

continuous crystallizer was implemented to study the feasibility of this

system used as seed source in a continuous crystallizer. Auxiliary

computations were made to determine the feasibility of recovering

solvent from the nucleator by flash evaporation in the ripener.

CHAPTER IV

THEORETICAL CONSIDERATIONS

The theoretical considerations of continuous sucrose crystal­

lization can be separated into four categories: MSMPR crystallizer

techniques used to predict CSD, sucrose growth and nucleation kinetics,

salting-out crystallization, and crystallization modifiers (surfactants).

MSMPR Technique

The apparatus used in this research work could ideally be con­

sidered an MSMPR (mixed suspension mixed product removal) crystallizer.

This unit is a continuously fed mixed magma crystallizer of volume V

which contains crystals dispersed in mother liquor (see Figure 2).

Figure 2. MSMPR Crystallizer

17

;. • ; 18 It can be assumed that both growth and. nuoleatidh ate taking

place in the crystallizer because of supetsaturation» Supetsaturatinn

in MSMFR systems can be generated by chemical reaction, evaporation,

codling, and in our case, a precipitation or salting Out effedt which

will be described later*. It is required that the feed rate, composition,

and temperature as well as the crystallizer volume and temperature ros

main constant for steady-state operation of the MSMFR crystalliger*

in order to completely describe this crystallization gyatom,

mass, energy, and population balances must be sat up and solved along

acceptable paths provided by the process flows, the kinetics of growth,

and the kinetics of nucleation* Randolph and Larson (1071) present the

crystal population balance in an MSMPR crystallizer and derive the

following equation for the ideal expected number distributions of the

crystal products

n = n°exp(«L/gf) (1)

This expression was also derived by Saeman (19S6), in skis eqnaeion,

n is the populacion density whieh has dimensions of numher/length®0volume. Also, n is defined as the population density of the embryo*

siae crystals as these crystals approach zero sige. The characterizing

dimension of the crystal is given the symbol L and the rate at which

this dimension grows is termed the growth rate G. finally, f is the

drawdown time which is simply the crystallizer volume divided by the

suspension output, i.e., T = V/Q.

' 19

Some of the assumptions that must hold in order that equation

(1) be valid are:

1. McCabe’s AL law (McCabe and Smith, 1967), i.e., G 4 f (L) •

2. The feed to the crystallizer contains no seed (n_ = 0 ) .

3. The population density of the crystallizer product is the

same inside the crystallizer, i.e., no classification and perfect mixing.

4. The suspension volume remains constant in time.

The nucleation rate B° is related to the nuclei density and the

growth rate by the following equation:

(2)B° = n°G

Nuclei densities can be determined for various experimental runs by-

plotting ln(n) vSo L and determining the intercept at L = 0 o The

population-weighted characteristic size (Gt) can be found from the slopebof the line since r is known and then B is obtained from equation (2).

Randolph and Larson (1971) also relate the growth rate and the

particle solids concentration for the MSMPR case:

M t = 6kv pn°(GT) 4 (3)

Here, M̂ _ is the solids concentration, k^ is the crystal shape factor

which relates the cube of the characteristic size dimension of the

particle to its volume, and p is the crystal density.

Thus, equations (1) through (3) can be used to analyze the CSD

of the MSMPR crystallizer. It should be noted that each parameter of

equation (1) can be related to the kinetics of the system and the imposed

system constraints. The growth rate G is determined by the kinetics of

growth and T, the holding time9 which is known in any operating system*

These two quantities9 however, cannot be fixed independently. Generally

one fixes the mass and energy flows and the kinetics of growth and

nucleation inherent to the crystal system determine the growth rate

which exists. The adjustable controls in the sucrose system include the

syrup feed concentration, the rate of energy input or removal, and the

feed rate.

CSD analysis using the MSMPR technique has great utility in the

investigation and understanding of crystallizers. According to Randolph

(1970), the two goals of researchers who work on the CSD problem are to

be able to understand CSD so as to analyze and modify what a particular

operating crystallizer can and cannot do in the way.of producing accept­

able particle size, and to be able to conceive an a priori CSD design

in new units before they are operating.

Sucrose Growth and Nucleation Kinetics

Van.Hook (1959) in his chapter of Honig has written extensively

on the mechanisms of sucrose nucleation and growth. He summarizes that

the rate of adjustment of a metastable, homogeneous melt or solution is

prescribed by the consecutive acts of nuclei formation and growth. The

procreating nuclei: (a) may already exist latently in the melt or solu­

tion and merely require activation by cooling below the transition

point; (b) may be generated spontaneously as the equilibrium point is

exceeded; (c) may consist of fortuitous contaminating particle; or

(d) may be substances added as seeds which stimulate the change to the

stable configuration of the system.

21Types of Nucleation

Homogeneous nucleation is the formation of new crystals from the

liquid phase as a result of supersaturation alone. Van Hook (1959)

applies the theory of homogeneous nucleation to sugar solutions, and

Here, o' is the interfacial tension, M the molecular weight, N is

Avogadros ’ number, R the gas constant, T the temperature and p the

density of the separating phase. The degree of supersaturation S is

defined as the concentration in amount of solute per fixed amount of

solvent divided by the concentration of a saturated solution at the same

temperature. From S = 1,0 to 1.3 it has been observed that sugar

crystals do not form spontaneously under ordinary conditions, but that

adding of seed will cause the crystals to grow until the syrup is ex­

hausted. Essentially only growth occurs for supersaturations of 1.0 to

1.2 while from 1.2 to 1.3 the growing crystals form additional growth

centers or "false grain," The theory concurs with the general observa­

tion that homogeneous nucleation is a more difficult matter than sub­

sequent growth.

Heterogeneous nucleation refers to new particle formation

resulting from the presence of foreign insoluble material. The foreign

indicates that the well-known homogeneous nucleation rate equation

accounts fairly well for the behavior of sugar solutions. This equation

is given as:

33° =B> e x -16ttM 2ct3N .. (4)

material.provides sites where crystals can form because of reduced

surface energy requirements.

A third type of nucleation applicable to sucrose systems is that

pf secondary nucleation. This refers to nucleation induced by the

presence of suspended crystals of the solute. This mechanism is related

to the phenomena of false graining. Secondary nucleation is to some

degree similar to heterogeneous nucleation and is also called ancillary

nucleation. The actual mechanism of secondary nucleation is still as

yet poorly understood. Much recent effort has been aimed at determining

the mechanisms and kinetics of this phenomena. The. reason for this is

that in most cases of industrial application, the production of crystals

from batch or continuous crystallization is characterized primarily by

secondary nucleation.

In the sugar/water/solvent system, secondary nucleation is

probably predominately due to collision breeding. Nuclei result from

the collision of seed crystals with solid surfaces, which in the case of

a stirred crystallizer may be other crystals, impeller blades, or

crystallizer surfaces.

According to Youngquist and Randolph (1972), no fundamental •

theory for prediction or correlation of secondary nucleation exists;

most experimental data have been obtained using MSMPR crystallizers.

Data are frequently correlated using a power-law model of the form:

B° = k ^ s ^ J (5)

owhere B is the nucleation rate., s is the super saturation, and M is. t

the solids concentration of the slurry0 -

The mechanism of secondary nucleation (or self-nucleation) has

also been studied by Clontz and McCabe (1969) and by Randolph and Cise

(1972)o Their findings^ as pointed out before, seem to indicate that

contact secondary nucleation plays an important role in operating

crystallizerso This effect is very important in the understanding of

the nucleation phenomena* Part of the research in this thesis was

aimed at correlating the data according to equation (5 )*

Growth Rate Correlation

One useful correlation that was used during the course of the

computer simulations was the relationship between crystal growth rate

and supersaturation. The following linear equation was derived using

the data that Bennett (1969) presented:

S = (G +5*36)/(4*66) (6 )

where S is the coefficient of supersaturation and G is the growth rate

in microns per minute*

Salting-out Crystallization

In this study, advantage was taken of the low solubility of

sucrose in aqueous alcohol solutions to affect the crystallization of

sucrose* Sucrose precipitated in this manner is of extremely small

size and of high purity* Purest sucrose for exact scientific investiga­

tions is prepared by redissolving the best commercial sugar in water and

affecting precipitation with absolute ethyl alcohol*

' • ' 24Research in the area of preparing seed slurry, by the salting-out, ,

method for sugar boiling has been carried out by Gupta and Agarwal

(1968). The most recent research effort on the salting-out of sucrose

was conducted by Kelly and Keng (1972). In their work aqueous sucrose

(58.5% wt) solution was dispersed (2-10 volume %) in EtOH or Me

Cellosolve to prepare microcrystal nuclei for use as seed nuclei in

sugar crystallization. The high EtOH/HgO ratio used in this work would

appear to make this technique impractical on a commercial scale.

Crawford (1970) also studied the phenomena of salting-out

crystallization to precipitate sucrose from a sugar/water/alcohol system

in an MSMPR crystallizer as previously described.

Sucrose has the property of being very soluble in water as well

as slightly hygroscopic. In aqueous solution, sucrose molecules are

hydrated by not less than four molecules of water. When alcohol is

added to this system, these molecules displace one or more of the four

water molecules surrounding the sucrose, thereby forcing a decreased

solubility of sucrose in the mixture and thus affecting precipitation.

Although salting-out is employed industrially, relatively few

published data are available regarding its use in crystallization

operations. Kelly (1959) presents some data on the solubility of sucrose

at various temperatures; these data are shown in Table 1.

From the data, one can see that sucrose is sparingly soluble in

ethanol. The 2:1 weight alcohol/water mixtures used in this study were

approximately 70% (volume) alcohol.

25

Table 1. Solubility of Sucrose in Ethanol/Water Mixtures

Alcohol Sucrose in g/per 1 0 0 ml of SolutionVol.% . 25 °C 30°C 35 °C

2 0 78.5 89.26 93.0

35 62.8 77.05 81.38

50 52.8 60.25 64.50

65 31.5 35.79 41.17

80 9.3 - . 16.26 23.92

95 - 0.17 0.76

Although pure ethanol was used in only one experimental run in

this research, Vanzol A-l (denatured alcohol) was used for a number of

runs. The other solvent that was used was a 50:50 methanol/isopropyl

alcohol mixtureo ,

Previous exploratory research at The University of Florida

(conducted by Randolph 1965) dealt with the sucrose salting-out phenomena

using either methanol, acetone or mixtures of both. It was found that

sucrose did nucleate readily when salted out with acetone and methanol

mixtures.

Gunning (in Kelly, 1959) also found that at 15°C only 0,3 grams

of sucrose dissolves in 100 ml of pure methanol. At the same temperature

he found that 3.8 grams of sucrose dissolves in 100 ml of 80% methanol,

which fell to 0.60 and 0.45 for 90% and 95% methanol, respectively.

' : 26

Crystallization Modifiers (Surfactants)

Randolph and Larson (1971) have pointed out that trace chemical

additives are often used to suppress or promote nucleation and/or

promote better crystal growth and habit. Such additives are usually

considered proprietary information by industrial companies and therefore

few published data exist on this subject. It was also pointed out that

additives are usually quite specific in their effect on nucleation.

Surface active agents have been known to inhibit3 rather than accelerate?

nucleation in many systems.

Shor and Larson (in Randolph and Larson, 1971) present a

mechanism for the effects of surfactants upon the system kinetics in

an MSMPR crystallizer. Their work consisted in measuring the affects

of three surface-active agents: methylamine hydrochloride, dodecylamine

hydrochloride, and a fluorocarbon (FC-98, 3M Company) upon the nuclea­

tion and growth rates in a KNO^ system. Their data showed that

nucleation rate increases and the growth rate decreases with increasing

surfactant concentration. It was suggested that the surface active

agent was absorbed on the surface of the KNO^ crystal and that this was

the primary factor which caused the system kinetics to change. For this

reason, methylamine hydrochloride (CH^NH^HCl) and fluorocarbon were

selected for study with the sucrose system.

Other types of surfactants that affect crystal size distribution

of the sucrose system are discussed by Van Hook (1959). It was pointed

out that most impurities impede the crystallization velocity of sucrose

(growth rate). If an additive (e.g., an impurity or an electrolyte)

27can be found that will slow down the growth rate of the sucrose system,

and if this surfactant can be readily dissolved in aqueous-alcohol

mixtures, then a smaller crystal size in the sucrose seed nucleator may

be achieved* This is due to the fact that if growth rate is reduced,

the characteristic size of the CSD is also reduced. However, one must

also consider the toxic effects of additives if the crystalline product

is to be used for food purposes.

Nicol and Former (1972) have recently studied the effects of

lysine and glycine on the nucleation and crystallization of sucrose.

They found that the nucleation rate of sucrose was increased by lycine

because the impurity acted as a center for heterogeneous primary

nucleation.

Crawford (1970) tested the effect of the surfactant Aliquat 26

(which is Trimethyl Tallow Ammonium Chloride with some unsaturation

present in the alkyl chain) on the sucrose CSD. It was suggested that

the surfactant may have changed the system kinetics but no conclusive

evidence was given.

CHAPTER VEXPERIMENTATION

Exploratory sucrose research at The University of Florida by

Randolph(1965)was done with a continuous stirred tank crystallizer at room

temperature3 fed with near-saturated sucrose feed syrup» Mixtures of

methanol and acetone were used to affect salting-out conditions. It was

concluded from the work that a direct salting-out process for the contin­

uous crystallization of sucrose was not attractive due to extreme con­

glomerate habit3 occlusions of solvent in the crystals9 and the large

energy requirement necessary to recover solvent from mother liquor. It

was found3 however3 that if the process was carried out in a region of

low solvent addition, high nucleation with slow growth occurred to

produce a fine distribution of well-formed crystals. It was recognized

because of their good habit that such crystals would have potential as

a seed source in a conventional sugar boiler operated in a batch or

continuous mode.

Research to characterize the size and yield of these fine-sized

seeds to advance the idea of the nucleator as a grain source was carried

out by Crawford (1970) at The University of Arizona. His apparatus was

an MSMPR crystallizer with a capacity of around 3 liters. It was de­

termined that the nucleation rates produced by salting-out with a 2 / 1

volume ratio of methanol/acetone were high and yield was of the order of

0.4 grams of crystals per cc of mother liquor. Crawford later assembled

28

V ; ' 29data on the sugar/water/ethanol system9 obtaining yields between 0 « 2

4 5and 0,3 g/cc with nucleation rates in the order of 10 to 10 number/

cubic centimeter-minute. These crystals produced by the salting-out

method were of acceptable crystal habit provided that the ethanol to

water mass ratio was maintained around 1,4,

Apparatus

The MSMPR crystallizer used in this research work was constructed

from a 10 liter capacity polyethylene tank, A combination draft tube/

baffle/cooling system was constructed and mounted in the crystallizer,

This system was a hollow stainless steel coaxial cylinder with baffles

welded on the outside of the cylinder so as to conform to the shape of

the crystallizer. The baffle system extended about three-quarters of

the height of the overall crystallizer. Cooling or heating fluid

circulated through the hollow draft tube which then served to develop a

torroidal mixing pattern in the vessel.

The cooling unit that was used to obtain high and low tempera­

ture conditions (27°C vs, 5°C) was a model #2095 Refrigerated and Heated

Bath and Circulator manufactured by Forma Scientific, Inc, This unit

was connected directly to the baffle-cooling system through 3 / 8 inch

polyethylene tubing.

The impeller agitator used to mix the crystal magma was driven

by a type #60 stirrer built by the Heidolph Electro Co, A 3-bladed

impeller was used which was approximately 3 inches in diameter and was

seated through the baffle system into a mounting that was centered in

' / - ' - 30the bottom of the crystallizer0 The agitator was maintained at 1100

revolutions per minute during a crystallizer run.

An Eastern Industries U-17 (1/4-H.P., 1725 R,P.M,) circulation

pump was mounted directly on the side of the crystallizer about five

inches from the bottom, , Tygon plastic tubing (1/2 inch I,D,) was

connected to the discharge side of the pump and this was fed to a glass

tee connection through which one end circulated back to the bottom of

the crystallizer and the other end led to a Research Control Precision ..

air-to-close valve which led to the drain, A 3-way tee connection was

also placed on the downstream side of the valve so that a stream of

water was allowed to flow continuously through the discharge line to

prevent plugging of this 3/8-inch tygon tubing line.

The only modification to the circulation system, which is later

described in detail, was the outlet connection on the circulation pump

which plugged during the initial runs.

The level of the crystallizer was maintained at 8 liters by the

use of an automatic Matheson Lab Stat Controller. This unit operated by

sensing changes in the dielectric strength of a 1/4-inch sealed glass

probe inserted through the top of the crystallizer and immersed into the

liquid about 1/2-inch. As the liquid moved up the side of the probe,

the change in the dielectric strength of the probe caused the controller

to activate a solenoid which was connected to the air-valve. When the

solenoid was energized, the air pressure on the valve was released,

thus discharging about 150 milliliters of the crystal magma.

V ' 31The sucrose solution (6 8 ° or 75°brix) and the solvent (either

Vanzol A-1 or 50% MeOH/50% IPA) were fed to the crystallizer by a stain­

less steel Micro Bellows pump manufactured by the Research Appliance

Company. Both bellows were 1/2-inch I.D. and operated 180 degrees out

of phase with each other. The feedlines from the supply tanks were

3/8-inch polyethylene plastic tubing. Actual delivery rates through the

bellows pump ranged from 18 to 70 milliliters per minute.

Both polyethylene storage tanks had an 11 gallon capacity, and

were graduated in gallons to #id in feed preparation. A steam coil was

mounted inside the sucrose tank which regulated feed temperature via a2"Therm-o-watch" controller manufactured by the I R Company. Feed

sucrose was continuously mixed with a 1/10-HP, 2-R Lab Agitator, manu­

factured by the Bench Scale Equipment Company, to minimize formation of

crystals on the liquid surface. Feed syrup was maintained around 170°F;

there may have been some inversion occurring at this temperature.

A heating tape was wrapped around the stainless steel valve on

the sucrose bellows pump to prevent fouling inside the bellows.

Regulation of the sucrose flow rate presented operational

difficulties during the course of the research. A method for eliminat­

ing this problem is described in the next section.

A schematic flow diagram for the crystallizer system is shown

in Figure 3. Photographs of the crystallizer and the entire crystal­

lizer system are shown in Figures 4 and 5.

So lv en t S u c r o s e f e e d tonk

S t e a m line

I R Controller— -FtcI

R e se a r c h A p p l ia n c eDuplex Eciicws Pum p — 3

E a ste r n U - 17 c ircu la t in g pump

Crysta ll izer

Variable s p e e d a g i ta to r

CW fr o m r e f r ig e r a to r

B a f f l e / D r a f t - t u b e co o l in g s y s t e m

Circulating Waterline

4 ►To drain

P n e u m a t ic - o p e r a t e d ball v a lve

Figure 3. Schematic Flow Diagram of Crystallizer System

Figure 4. Main Crystallizer Unit (Front View)

Figure 5. Crystallization System (Side View)

Coulter Counter

Particle distributions were obtained with a sixteen channel

particle size distribution, analyzer, the Coulter Model "T" Particle

Counter. This counter is capable of analyzing a particulate sample

within twenty seconds. .The unit is manufactured by the Coulter

Electronics Industrial Division, a division of Coulter Electronics, Inc.

The unit represents the state-of-the-art in electronic particle counting

and sizing. •

The Coulter Counter works on the following principles.

Particles, suspended in electrolyte, can be sized and counted by passing

them through a specific path of current flow for some length of time. A

tube, containing an aperture of known dimensions is immersed in a con­

ductive electrolytic solution which contains the sample to be counted.

Electrodes on the inside and on the outside of the tube provide the path

of current flow. As particles pass through the aperture and displace

electrolyte, resistance in the path of current flow changes which in

turn changes the current. The magnitude of the change is directly pro­

portional to the volumetric size of the particle. The number of current

pulses within a specific sample is proportional to the number of

particles within the suspension.

Different apertures are used with the Coulter Counter. Currently

the range extends from a minimum size of 30 microns to a maximum of 560.

The work associated with this thesis was conducted with a 280 micron

aperture tube.

■ ■ ■ ' ■ : V , -The Coulter Counter model "T" consists of three separate inter­

connected assemblieso These are the sample stand, the main unit, and the

power supplyo A particulate sample suspended in electrolyte is intro­

duced to the sampling stand where counting occurs. The main unit

analyzes the information, reduces it to the desired data forms, and

provides readouts of the information. The power supply provides all

electrical power required by the circuitry.

The Model ,!Tn is a sophisticated electronic system which sepa­

rates particle size into fifteen ranges, or thresholds. The data forms

available are: (a) population differential,.(b) population cumulative,

(c) volume differential, (d) volume cumulative. Sample time is recorded

(channel 16) for any of the four modes, and weight percent is available

using the volume cumulative mode*

Aperture current setting was 3.2 microamps, the maximum current

setting. Calibration was carried out by using mono-sized ragweed pollen

with a mean diameter of 19.5 microns. The manometer mode setting was

used in this research work in preference to the preset time mode.

Aperture current polarity was automatically cycled by the main unit.

Digital paper printouts were obtained for all runs. The sample stand and

main unit were both grounded to minimize electrical noise. The Coulter

Counter and sample stand are shown in Figure 6 .

Start-up Procedure

The crystallizer was operated in the following fashion. First,

air pressure was turned on to close the air-valve which connected the

36

Figure 6. Coulter Counter and Sampling Stand

37crystallizer to the drain» A bleed stream of water was turned on which

ran through a tee connection on the downstream portion of the air valve0

The purpose of the bleed stream was to keep the discharge tubing from

plugging.

Next., initial charges of sucrose solution and solvent were

placed into the crystallizer in the proper mass ratios. Care was taken

in the manner of introducing these two quantities together in order to

avoid sudden changes in concentrations between the two fluid quantities9

i.e., it was noticed during one start-up procedure that when the sucrose

solution of either 6 8 ° or 75 brix was added after the solvent was al­

ready in the crystallizer (especially when the solvent was at a low

temperature) sudden crystallization occurred that produced many small

agglomerates. The problem was solved by putting the sucrose solution

in the crystallizer and then slowly adding the solvent (which was either

Vanzol A-l or 50% IPA 50% MeOH) with the agitator at a low speed until

the crystallizer was filled to 8000 ml. The agitator was then adjusted

to 1 1 0 0 revolutions per minute and the circulation pump was started.

Next,e the refrigeration unit was turned on if the operating conditions

required a low crystallization temperature. The level controller was

then activated to maintain the liquid in the crystallizer at the 8000 ml

level. Sucrose and solvent solutions were metered through the micro­

bellows pump. The solvent was easily maintained at a constant flow

rate; however, the sucrose feed flow rate usually varied during the

course of an experimental run due to temperature and viscosity changes

from the tank to the bellows pump (approximately 10 feet of 1/4 -inch

' 38tygon tubing). The sucrose flow rate had to be constantly checked

during the course of the run. This problem was later corrected by sub­

stituting a 1/4-inch bellows pump for the sucrose feed together with a

shortening of the sucrose line between the feed tank and the crystallizes

After these changes, the flow rate could be maintained at a more constant

level; however3 the stream still had to be checked and corrected

occasionally due to small fluctuations. A heating coil was also wrapped

around the top portion of the valve entrance to the sucrose bellows to

prevent temperature loss in the bellows. It was noticed from initial

runs that crystallization gradually occurred in the bellows. Therefore,

the bellows were flushed with hot water after each run to prevent

plugging.

The circulation line was found to plug during long retention

time runs due to a small diameter el-connection on the discharge side of

the circulation pump. This was corrected by replacing the circulation

line with a clear plastic 3/8 -inch tygon line stretched directly over

the discharge nozzle of the circulation pump.

The percent sucrose in the feed tank varied during runs due toO'evaporation at the high feed temperature (160-170 F) . Feed syrup con­

centrations were measured by an Abbe fefractometer and water was added

periodically during the run to maintain a constant feed concentration.

Regulation of the syrup concentration by this technique was accurate to

within l°brix.

Sugar was obtained in 100 pound bags. For a typical run, 50 to

60 pounds were dissolved with tap water to make up to the required feed

39concentration (68 or 75 brix) 0 The sugar in all experiments was

commercial grade granulated cane sugar manufactured by the C & H Company.

After dissolving, the syrup was slightly yellowish in color. No analyses

were made of the syrup composition but it was noticed that pH of the

solution was slightly acidic (pH of approximately 6 ). Sodium hydroxide

was added to one run to make the solution basic. It was noticed that

the basic sucrose solution became brighter yellow in color than the

regular acidic solution. After this basic solution was allowed to set

for three days either bacterial decomposition or chemical reaction took

place because the sucrose solution then became dark brown in color. The

experimental run using this solution was characterized by large

agglomerates in the crystal distribution. This run was aborted and the

sucrose feed solution for the remaining runs was left at the original

pH of 6.0. Sucrose feed solution should evidently be used immediately

after it is prepared to prevent chemical reaction (probably inversion)

or bacterial decomposition.

Sampling and Particle Counts

After the crystallizer was ̂ assumed to be at steady state

(usually 7-8 retention times), a 5 ml sample was pipetted directly from

the crystallizer using a 2 0 ml pyrex pipette that had the tip opening

widened to prevent plugging. The sample was filtered slowly to avoid

agglomeration. Filtering was done with a 3.0 cm diameter microfilter

funnel using No. 30 Whatman filter paper. This paper could not be

subjected to isopropyl alcohol for periods of over one minute since the

alcohol had a deteriorating effect on the paper. Speed of filtration

was important to prevent complete drying of the filtered sample* This

drying of the sample on the filter also caused agglomeration. Some

error occurred during sampling due to the difficulty in filtering

exactly 5 ml of solution*

Initial sampling was carried out by diluting the filtered sample

in an electrolyte solution by a factor of 1500 in order that the Coulter

counter could count the sample in a dilute medium* Another purpose of

this dilution was to decrease the number concentration of large particles

in the system so that plugging up of the Coulter aperture would be re- .

duced* It was found later that diluting the sample by a factor of 2000

achieved the best counting results* This was done by adding the 5.ml

filtered sample to 200 ml of IPA/electrolyte solution in a 250 ml pyrex

beaker* The beaker was placed in an ultrasonic bath to disperse the

particles* The bath used for this operation was a "Balsonic" Model 1009

manufactured by Crest Ultrasonics Corporation* The covered beaker was

subjected to ultrasonic vibrations for a period of approximately one

minute so that the agglomerates caused by the filtration process were

broken up. After this treatment the solution was thoroughly mixed and

4 ml of the sample was pipetted into 196 ml of electrolytic IPA solution

thus bringing the dilution factor to 2 0 0 0 *

The electrolytic solution used for counting was a 4% by weight

filtered solution of ammonium thiocyanate in absolute isopropyl alcohol*

The ammonium thiocyanate (NH^CNS) was analytical grade as manufactured

by the Mallinckrodt Chemical Company* The isopropyl alcohol was an

\ 41-

analytical reagent9 also manufactured by the same company0 Although

sugar is very insoluble in isopropyl alcohol the solution was saturated

with sugar, then filtered using a 0.45 micron filter to prevent the

dissolving of fine sugar particles during the sampling and counting

procedure.

While the above operations were being carried out, the Coulter

Counter was turned on so that the circuitry was allowed time to warm up.

The aperture was immersed in pure isopropyl alcohol and then a noise

count of the 4% ammonium thiocyanate/isopropyl alcohol solution was

taken. The Coulter Counter calibration setting was set prior to this;

the unit was used in the manometer mode to take the counts. Prior to

the counting, the vacuum pump was checked to insure adequate vacuum

within the system, A complete counting cycle usually took about 20

seconds; 6 for the actual count, several for the drawdown time of the

manometer, and about 10 seconds for the digital printout. At least 4

counts were taken of each sample in order to obtain a true representation

of the particle population. The sucrose sample was counted after the

noise count. These counts were displayed on the main unit as histograms

of the distribution which could be checked with the previous run for

reproducibility of counts.

An important factor in this measurement procedure was to insure

that the sample was thoroughly mixed while the count was being taken.

Best results were obtained by placing the propeller tip directly under

the aperture tube close to the bottom of the sample beaker. Large

variations in counts were observed to occur in the highest channel

(low sizes) due to noise as well as in the lowest channels due to poor

mixing, of the larger crystals. Intermediate counts were usually

constant.

It was necessary to watch the aperture during counting with a

small microscope mounted on the sample stand to observe if the aperture

became clogged (particularly when the crystal density was high)„ When

this occurred, the Coulter aperture current was turned off and the

crystals were dislodged from the aperture. The count was then repeated.

As mentioned, samples from the crystallizef were diluted by a

factor of 2000 in the electrolytic IPA solution prior to counting. This

dilution was necessary to get a counting frequency that was consistent

with the speed of the Coulter and also to minimize the effect of double

and triple particles being counted as singles. The counter has a con­

centration index meter calibrated in arbitrary units of 0-0.3. It was

found that concentrations of crystals resulting in a concentration index

of 0.2 to 0.3 gave inaccurate results. Indexes below 0.15 were found to

give reproducible results. The majority of runs were counted with the

concentration index scale below 0.15.

Experimental Runs

A total of fifteen runs were conducted in this research work.

Run 1-A, the first successful preliminary run, was used to compare with- ' - ■previous results (Crawford, 1970). Runs 1-B to 8-B were designed to

study the effects of five independent system variables upon the crystal

size distribution. These variables will be described later. The

results from the three C-series runs were used as an alternate method

of finding the standard error used in a statistical t-test to estimate

the main effects of the variables in the 33-series runs. Finally runs

1-D through 3-D were additional runs to check the optimized variables

from the B-series experiments.

Description of Individual Experiments. . • °

The experimental procedures utilized in this work were common to

all runs. However, as the course of research was carried out, the

counting techniques and the operational procedures were executed with

greater efficiency.

Table 2 gives a summary of experimental conditions associated

with the individual runs.

Run 1-A was conducted using an ethanol/sugar/water system.

However, the rest of the runs were executed using either Vanzol A-l or

50/50 (volume) isopropyl alcohol/methanol mixture as the salting-out

agent.

In runs 1-B through 8-B, five independent variables at high and

low level values were studied to estimate their effects on the continu­

ous nucleation and growth rate kinetics of the sucrose system. These

variables along with their high and low level values are shown in

Table 3.

Runs 1-C, 2-C, and 3-C are duplications of runs 3-B, 5-B and

8-B. These three sets of duplicate runs were used to calculate a

"pooled" standard error of the B-series runs in order to obtain.a more

Table 2„ Summary of Experimental Conditions

Run No * Solvent Solvent Holding Crystallizer Surfactants; Sucrose Feed pH of Date ofto Water Time/T" Temperature MH = Concentration Sucrose Run(Mass Ratio) (Hr,) 42 (°C) Me thy1amine HC1 41 (°Brix) Solution

FC= . . .

Fluorocarbon

1-A Ethanol 2/1 1.0 27. None 68, 6 5/18/721-B Vanzol A-l 2/1 . 3.0 27. FC - 100 PPM* 75. 6 7/19/722-B Vanzol A-l 2/1 . 3.0 27. MH - 100,PPM 68, 6 7/12/723-B 50/50 MeOH/IPA 2./1 1.0 27. None 75. 6 9/12/72

50/50 Me OH/‘I PA 2/1 3.0 5. ■ FC - 100 PPM . 68. 6 7/21/725-B 50/50 MeOH/IPA 2/1 1.0 27. FC -

MH100 PPM 100 PPM .

68. 6 9/14/72;

6 - B Vanzol A-l . 2/1 1.0 ' 13. . FC - MH -

100 PPM 100 PPM

. 74. . 6. . 9/5/72

7-B 50/50 MeOH/IPA 2/1 3.0 v.' MH - 100 PPM ■ 75 o’ . 8/19/728-B Vanzol A-l 2/1 1.0 8. : None : 68. 6 8/31/721-C 50/50 MeOH/IPA 2/1 1.0 27. . None 75. 6 7/7/722-C 50/50 MeOH/IPA 2/1 1.0 27. - FC -

MH -100 PPM 100 PPM

68. 6 5/27/72

3-C Vanzol A-l 2/1 1.0 10. None 68. : e 6/19/721-D Vanzol A-l 2.5/1 1.0 11. MH -

FC -100 PPM 100 PPM

74. • 10 6/26/72

2-D 50/50 MeOH/IPA 3/1 2.0 , 6. None 75. . 6 . 9/20/72 ,3-D Vanzol A-l 2/1 . ; 2.5: . 7. None ; 75. ■ 6 , 10/4/72* PPM t= Parts per million

f :

45

Table 3. Independent Variables Studied in B-Experiments

IndependentVariable Symbol High

Level (+1)LowLevel (-)

1. Crystallizer Temperature T 27°C, 5°C,

2. ResidenceTime T 3.0 hr. 1.0 hr.

3. Sucrose Feed Concentration V 75.07. 68.07.

4. Surfactants a) Methyl amineHC1 100 PPM None

b) Fluorocarbon (FC-96)-3M Co. 100 PPM None

5. Solvent Type -- 50/50 IPA/MeOH Vanzol A-l

accurate estimate of the main effects of the independent variables with

three degrees of freedom.

Run 1-D was carried out using a high solvent-to-water ratio

(R = 3.0) and a pH of 10.

In run 2-D, the conditions were similar to that of run 6-B;

however, the solvent-to-water ratio was 2.5.

Finally in run 3-D, optimized variables were chosen to study the

effects on the system kinetics.

Calibration of the Coulter Counter

The Coulter Counter was calibrated a total of four times during

the period of experimentation. The monosized particles used for this

operation were ragweed pollen with a spherical mean diameter of 19.5

: . - 46microns. All the calibrations gave a baseline for channel 5 of 40

microns. A standardized procedure was followed as described in the

Coulter Counter manual.

Experimental Data

After each run, nuclei densities were determined from the raw .

Coulter population counts. All the counts for a given sample and .

channel were averaged and the background noise was averaged and sub­

tracted. This quantity is denoted by N. The counts were then converted

to population densities by the following equation:

n = (N - DF) /2 A L (7)

The 2.0 multiplier in the denominator of this equation results

from the 2.0 ml sample volume used by the counter. DF represents the

dilution factor as previously defined. A L is the size width in microns

of the specific Coulter channel. Population densities were then plotted

on semilog paper versus the mean size L for each of the Coulter channels.

The intercept of a straight line through these data points gave the

nuclei density. The characteristic size (Gr) was then obtained from the

slope of the line. The method of least squares was used to plot these

lines (neglecting the nonlinear points at the small and large sizes of

the distribution). Table 4 is a summary of experimental results and

Table 5 lists the units for the respective columns of Table 4.

Some of the representative crystal size distribution plots are

shown in Figures 7 through 12. Figure 13 is.a correlation plot of

calculated yield via the Coulter Counter versus experimental yield by

filtration.

Table 4. Summary of Experimental Results

Run No. ResidenceTime

Solvent to Water Ratio

Nuclei " Density

Characteristic Size (GT )

CrystalGrowthRate

NucleationRate

Yield by Filtration

Yield byCoulterCounter

1-A 1.0 2.0 6.40xl05 14.3 0.238 1.52xl05 w- 0.1341-B 3.0 2.0 ,1.26xl06 13.8 .0767 9.56xl04 0.380 0.2262-B 3.0 2.0 5.85xl05 15.3 0.085 4.97xl04 . 0.242, 0.1613-B 1.0 2.0 8.76xl05 16.1 . 0.269 2.36xl0S 0.408 0.2934-B 3.0 2.0 1.25xl06 15.0 0.0835 1.04xl0S 0.304 0.3155-B 1.0 2.0 8.18xl05 14.7 0.246 2.02xl0S 0.251 0.1926-B 1.0 2.0 .. 5.45xl05 18.1 0.301 1.64xl0S ; 0.331 : 0.2917-B 3.0 2.0 4.83xl06

610.9 ■ . 0.0608 2.94xl05

5-0.423 .. 0. 347

8-B 1.0 2.0 1.22x10 13.6 .. 0.226 2.76x10 0.205 0.206l-C 1.0 2.0 7.40xl05 16.5 : 0.276 2.04xl0S 0.437 0.2702-C 1.0 2.0 3.30xl05 18.6 0.310 1.03xl0S 0.204 . ; 0.2663-C 1.0 2.0 7.70xl05 10.5 0.175 1.35xlOS 0.193 .047I'D 1.0 3.0 1.30xl06 0.4142-D 2.0 2.5 1.30x10* 13.9 0.231 3.0xiOS 0.326 0.2383-D 2,5 2.0 3.60x10* 11.9 0.0795 2.86xl0S 0.368 0.364

---n...,. ____ _

48

Table 5. Units for Experimental Results.

Column Title ' Units

2 Residence Time Hours

3 Solventsto-Water Ratio grams/grams

4 Nuclei Density number/cc-micron

5 Characteristic Size (Gt ) microns

6 Crystal Growth Rate microns/minute

7 Nucleation Rate number/cc-minute

8 Yield by Filtration grams/cc

9 Yield by Coulter Counter grams/cc

One can see that the majority of filtered yields were higher

than the calculated yields. This was probably due to error in the

sampling procedure or to some dissolving of the fine crystals in the

IPA suspending medium. •

RUN I-*A

S o l v e n t t o W a t e r R a t i o = 2 . 0 T = I.Ohr.S o l v e n t = E t h a n o lT e m p e r a t u r e o f C r y s t a l l i z e r * 2 7 e CNo S u r f a c t a n t sC o n c e n t r a t i o n o f S u c r o s e in f e e d

s t r e a m = 6 8 ° Br ix A g i t a t i o n = 1 1 0 0 R P M

2 0 4 0 6 0 8 0 1 0 0 120Crystal S i z e , microns

Figure 7. Population Density vs. Crystal Size, Run 1-A

Popu

lati

on

Den

sity

, n

o/cc

-mic

ron

50

to?

10*

105

10'

10'

t 0 ‘

__________ RUN 2 - B______________

S o l v e n t t o W a t e r R a t i o = 2 . 0 T = 3 . 0 h r s .S o l v e n t = V a n z o l - A - 1 T e m p e r a t u r e o f C r y s t a l l i z e r = 2 7 ° C Addi t ive * . M e t h y l a m i n e h y d r o c h l o r i d e ,

1 0 0 P P MC o n c e n t r a t i o n o f S u c r o s e in f e e d

s t r e a m = 6 8 ° Br ix A g i t a t i o n *= 1 1 0 0 R P M

JL2 0 4 0 6 0 8 0 1 0 0

Crysta l S i z e , microns120

Figure 8. Population Density vs. Crystal Size, Run 2-B

Popu

lati

on

Den

sity

, n

o/cc

-mic

ron

51

io6

105

104

103

2 0 4 0 6 0 6 0 1 0 0 1 2 0Crystal S i z e , microns

Figure 9. Population Density vs. Crystal Size, Run 4-B

R U N 4-B

X x d\ p

S o l v e n t t o W a t e r R a t i o = 2 . 0 ' T = 3 . 0 hr s .S o l v e n t = 5 0 % M e 0 H / 5 0 7 o I P A T e m p , o f C r y s t a l l i z e r e 5 e C A d d i t i v e : F l o u r o c a r b o n ( 1 0 0 P P M ) C o n c e n t r a t i o n o f S u c r o s e in f e e d

s t r e a m = 6 8 ° B r i x A g i t a t i o n » 1 1 0 0 R P M

\ o

\ o

Run D u r a t i o n O ^ 21 h o u r s

X - ^ s y s t e m p r o b a b l y not X k at s t e a d y s t a t e

O \

o \

-X . ©

i -1 _ L 1— __ J ___ j _

Popu

lati

on

Den

sity

, n

o/cc

-mic

ron

52

IQ?

106

I 0 5

104

I 0 3

10'

_________ RUN I - D____________

S o l v e n t t o W a t e r R a t i o = 3 . 0 2 . 0 hr.

S o l v e n t = M e O H 5 0 % / I P A 5 0 % T e m p e r a t u r e o f C r y s t a l l i z e r = 6 ° C N o S u r f a c t a n t sC o n c e n t r a t i o n o f S u c r o s e in f e e d

s t r e a m = 7 5 ° B r i x A g i t a t i o n = 1 1 0 0 R P M

2 0 4 0 6 0 8 0 1 0 0Crystal S i z e , microns

120

Figure 10. Population Density vs. Crystal Size, Run 1-D

Popu

lati

on

Den

sity

, n

o/cc

-mic

ron

53

RUN 7 - B____________

S o l v e n t t o W a t e r R a t i o = 2 . 0 T = 3 . 0 hrs .S o l v e n t = 5 0 % M e O H / 5 0 % I R A T e m p e r a t u r e = 5 ° C A d d i t i v e = M e t h y l o m i n e h y d r o c l o r i d e

i s . (CH3NH2HCn 1 0 0 P P M C o n c e n t r a t i o n o f S u c r o s e in f e e d

s t r e a m = 7 5 ° Br ix A g i t a t i o n = 1 1 0 0 R P M

100 120Crystal S i z e , microns

Figure 11. Population Density vs. Crystal Size, Run 7-B

Popu

lati

on

Den

sity

, n

o/cc

-mic

ron

54

RUN 8 - B

S o l v e n t t o W a t e r 1 .0 h r s

S o l v e n t = 5 0 7 o M e 0 H / 5 0 % I PA T e m p e r a t u r e o f C r y s t a l l i z e r 6 8 e C N o S u r f a c t a n t sC o n c e n t r a t i o n o f S u c r o s e in f e e d

s t r e a m = 6 8 ° B r i x A g i t a t i o n = 1 1 0 0 R P M

R a t i o

40 60 80 100Crystal S i z e , m icrons

Figure 12. Population Density vs. Crystal Size, Run 8-B

Cal

cula

ted

Yiel

d by

Cou

lter

C

ount

er,

9/cc

55

0.4

0.3

0.2

0.1

0.2 0.30.1 0.4E xper im en ta l Yield by F i l t r a t io n , 9/cc

Figure 13. Calculated Yield vs. Experimental Yield

CHAPTER VI

ANALYSIS OF EXPERIMENTAL RESULTS

Run 1-A was the only run that had experimental conditions

similar to Crawford* s (1970) <, The graphical representation of the CSD

along with the process conditions is shown in Figure 7« The system

kinetics of the run correspond to the growth rateenucleation rate curve

that Crawford plotted.

Statistical Results

The B-series experiments were run to find the effects of the

independent variables (from Table 3) on the system kinetics, i.e., the

nucleation rate and the population-weighted characteristic size.

Evaluation of these effects was made with the aid of the Placke11-Burman

analysis (Isaacson, 1970), which studies (n-1) independent variables in

n tests. This analysis consisted of a one-level statistical t-test,

i.e., no interactions were assumed between the real independent

variables. Thus, it was not possible to estimate the two-factor inter­

actions that could have been significant in the system and which may

have contributed to make some main effect appear highly significant.

This series of experiments consisted of studying six variables

in eight experiments. These variables are shown in Table 3 (the sur­

factants count as two separate variables), along with their chosen

high and low level values. One variable was assumed to be a dummy

variable for estimation of the standard error.

56 .

Each independent variable was studied at two levels; it was

important to choose as levels the values that were expected to produce

some significant change in the performance of the dependent variable.

This was considered the most important step in effectively applying

this experimental method.

The design matrix for the eight experimental runs is shown in.

Table 6. The + ' s and - ’s indicate the particular high and low settings

for the independent variables (designated by columns) for a given

experimental run (designated by rows).. The independent variables were

numbered as follows:

1. Crystallizer Temperature

2. Holding Time

3. Concentration of Sucrose Feed

4. Methyfamine Hydrochloride (surfactant)

5. Fluorocarbon (surfactant)

6. Solvent Type

7. Dummy Variable

After completion of the experiments, the effects of each inde­

pendent variable was calculated going from the low level to the high

level.

For each independent variable and dummy variable, the net effect

is calculated to be the difference between the average value of the re­

sponse obtained at the high levels and the average, value obtained at the

low levels. This may be described mathematically as:

58Table 60 Design Matrix for Series B Runs

Run Independent Variable1 2 3 4 • 5 6 7

1 + + '. + - + - -

2 + - '' + - ; — 4-

3 + ' - -f 4

4 - + - - + + 4

5 + - + + > -

6 — + + - 4

7

8

+ + + ~ +

-

Response at (+) Responses at (-) (8)E 1 = Number of Values (+) Number of Values (-)

where E 1 is the effect of variable 1„

The variance is equal to the average of the isquares of the dummy

effects and is calculated by : .

Djdummy (9)effects Number of Dummy Variables

Dummy variables are unassigned factors, and are used to obtain an

estimate of the variance0

The standard error of the effect is then obtained by taking the

square root of the variance of the effects:

59 •

. ■■ ■-■■■ ; ■: ■... .•Standard Error (S.E.) = (veffec;t:s) 2 (10)

The significance of each effect is determined by using the

familiar t-test in accordance with:

t-Ef£ect/S.B.e££ec< ’ V (11)

In other words, if the t-value is significant at an arbitrary level, it

means that in going from a low level to the high level of that variable

in the design, the change observed in the response or dependent variable

is truly due to changing the variable and not to random error„ Values

of t-significance at various probability levels are found in standard,

tables at n-degrees of freedom.where n is the number of dummy variables

(n = 1 in our case)„ Tables 7 and 8 list the results of the Plackett-

Burman analysis on the system variables of characteristic length Gr and

nuclei density, respectively.

The standard errors of both evaluations were high (see Tables 7

and 8). This was due to a high effect value of the dummy variable.

One dummy variable (one degree of freedom) cannot be considered a good

estimate of the standard error. The analysis would be improved if two

or three dummy variables (two or three degrees of freedom) were chosen

in order to lower the standard error. This would result in a more

accurate t-test.

Three possible explanations can be given for the large effect

of the dummy variable:

1. Interactions between the real variables that are confounded

with the dummy variable are significantly large.

60Table 7. System Response = G r (Characteristic Length)

VariableNumber Variable Name . Effect ' TT - Significance

- to + t-Value Percent

1 CrystallizerTemperature 0.585 0.204 12.8

2 Holding Time -1.860 0.645 36.2

3 Sucrose Feed Concentration 0.0075 .00202 « 0

4 Methylamine HC1 0.170 .0583 to 3

5 Fluorocarbon 1.410 0.492 29.0

6 Solvent Type -1.007 0.351 21.4

7 Dummy 2,864 1.00 --

Note: Standard Error = 2.864Degrees of Freedom = 1

2. Some important independent variables that have been ignored

or not held constant during the experimental program have varied to.a

significant level.

3. Considerable error exists in the measurement technique.

Therefore, another statistical t-test was carried out using an

alternative method of evaluating the standard error. As pointed out

before. Runs 1-C, 2-C and 3-C are equivalent to Runs 3-B, 5-B and 8-B,

respectively. The differences of the system responses in these

duplicate sets of runs were used to calculate a new standard error.

This method known as the combination of variances was presented by

Davies (1961).

. o 61Table 8. System Response = Infn0̂

VariableNumber Variable Name Effect

- to + t-Value SignificancePercent

. 1 CrystallizerTemperature -.5075 .712 39.3

2 . Holding Time .5625 00 hs .

42.0

3 Sucrose Feed Concentration -.3225 .453 27.9 \

4 Methylamine HCl -.0975 .137 « 9

5 Fluorocarbon -. 363 .510 30.0

6 Solvent Type .5425 .761 41.3

7 Dummy -.7125 1.0

Note: Standard Error = .7125Degrees of Freedom = 1

For each duplicate set of experiments the variance was calculated

to be:

" j

Sj2 = Y (yi " y)2/<Nj • V (12);; ;

where N. equals the number of duplicates in each set, y. is the_ "L

individual responses, and y the average response.

The three variances were then "pooled" to obtain an overall■ vvariance denoted by S :P.

62

„ 2 1)SX2 H- (N2 - PS,? + („3 .

P " (N, - 1) + (N - 1) + (N_ - 1) (13)

with (N^ - 1) + (Ng -.1) + (N^ - 1) degrees of freedom, ;

Since = 2, we have 3 degrees of freedom.

From the data in Table 4, the two pooled variances were

calculated to be: •

1, for In n°; S 2 = 0,1729P ■

22 0 for Gt ; = 4.08.

In order to relate the pooled variances to the effect values we start

from equation (8), namely:

g _ Response at (+) Responses at (-■)1 Number of Values (+) Number of Values (-)

which is equivalent to:

8e i - Z K i n / 4 < i 4 >

i=l

Here, is +1 if the response is at (+) and -1 if the response is at

(-). Thus

Var(E^) = (%)2Var(y1) + (%)2Var(y2> + ,„, + (-%)2Var(yg). (15)

where "Var11 stands for the variance*

Further, Var(y^) = Vav(y^) = VarCy^), etc.

Therefore, Var(E^) = 8/16 Var(y) = % Var(y).

Since,2Var(y) = Sp

One can thus generalize;

2Var(Ei) = % 8̂

The variance of the effect of the i-th independent variable is thus

equal to one-half of the pooled variance0

Therefore, the Standard Error of the effect becomes:

^ ;'

(SoEo) = (Var(Ei))2 (20)

with three degrees of freedom*

The alternate standard errors for ln(n°) and Gt were calculated

to be 0*294. and 2*04, respectively* The results of the t-test using

these calculated standards errors are shown in Tables 9 and 10*

Since there are two measures of standard error, one can apply

their ratio to an F-test to see if they are independent measures of

standard error* F was calculated to be 5*87 for the ln(n°) response*

It was then concluded after looking at an F-table (with three and one

degrees of freedom) that the two estimations of standard error are

indeed independent measures at about the 90 percent confidence level*

Since the alternate standard errors have three degrees of freedom, one

may assume that these are better estimates of the standard error*

63(16)

(17)

(18)

(19)

64Table 9. Test 2; System Response = Gr (Characteristic Length)

VariableNumber

VariableName

Effect t_value Significance to + Percent

1 CrystallizerTemperature 0.585 .409 28.3

2 Holding Time -1.860 1.300 71.33 Sucrose Feed

Concentration , 0.0075 .0053 « 04 Methylamine HC1 0.170 .119 < 10%5 Fluorocarbon 1.410 .985 60.36 Solvent Type -1.007 .703 46.6

Note: Standard Error = 1.43Degrees of Freedom = 3

Table 10. Test 2; System Response = In(n°)

VariableNumber

VariableName

Effect - to + t-Value Significance

Percent

1 CrystallizerTemperature -.5075 1.73 81.3

2 Holding Time .5625 1.91 83.83 Sucrose Feed

Concentration .3225 1.10 64.54 Methylamine HC1 -.0975 .332 23.75 Fluorocarbon -.3630 1.24 69.66 Solvent Type .5425 1.85 83.0

Note: Standard Error = .294 Degrees of Freedom = 3

Inspecting the data from Table 99 one can see that the highest

significance level was holding time at 71,3 percent0 It is possible

that a longer holding time will decrease the characteristic size; how­

ever, to verify this conclusion, more research is needed. The next

highest significance level was that of the fluorocarbon surfactant at

60.3 percento Again it is not possible to draw any definite conclusions

about the effect of this additive; however, the effect is in the direc­

tion of increasing Gt . It appears that adding this surfactant may be

defeating the objective of a low characteristic size. The rest of the

significance effects are below 50 percent so no main effects can be

assumed from the data.

Table 10 indicates that the highest significance level is holding

time (83.8%). It appears that longer holding times will increase In n .

The next highest percent is solvent type at 83.0 percent which means

that 83 out of 100 experiments will show that the solvent type has anoeffect on In n . If this is the case it may be advantageous to use

50/50 MeOH/IPA rather than Vanzol A-l solvent systems. A decrease in

crystallizer temperature causes an increase in the nuclei density with

a significance percent of 81.3 percent. Fluorocarbon was found to de­

crease the nuclei density (with a confidence level of 69.6%). This

effect is consistent with the effect of fluorocarbon increasing the

characteristic size Gr. Therefore, it is recommended that fluorocarbon

not be used. Finally, sucrose feed concentration increases In n°

(with a 64.5 confidence level). Certainly a high feed concentration

66increases the solids concentration in the crystallizer which would in­

crease the total number of seed. Therefore a high sucrose feed concen­

tration should be used.

In light of the statistical data that has been presented, it is

recommended that to increase nuclei density and decrease Gr, the

combination of long holding time, low temperature, 50:50 MeOH/IPA

solvent type, and high sucrose feed concentration should be used.

Results of D-Series Runs

Run 1-D was carried out with conditions which corresponded to

the optimized levels of the operating variables (see Figure 10),

These conditions were similar (if one assumes that methy1amine hydro­

chloride has no effects on the CSD) to run 7-B (Figure 11)9 except that

the holding time was decreased to 2,0 hours and the solvent mass ratio

was changed to 3,0, One other variable that was briefly considered is

pH, All of the runs shown in Table 2 except 1-D were conducted with a

pH of 6,0, Sodium hydroxide was added to the sucrose feed solution to

change the pH to approximately 10 in run 1-D, The resultant semi-log

population plot was extremely nonlinear due to multi-crystal growth

occurring above a 30 micron size range.

Run 2-D was similar to Run 6-B except that the solvent-to-water

ratio was changed to R = 2,5, In comparing the two runs, an increase

in R seems to have increased the nuclei density and decreased the

characteristic size. However, Run 2-D exhibited considerable multi­

crystal agglomerative-type growth.

Run 3-D was carried out with the optimum conditions for

nucleation which were determined from the B-series runs. The holding

time was decreased from 3o0 to 2,5 hours in an attempt to shorten the

required run duration in order to assure steady state operating con­

ditions, Run duration was 18 hours, however the system may not have

reached steady state.

Crystal Habit „

Microphotographs of crystal samples for runs 6 -B and 8 -B are

shown in Figures 14 and 15, These samples, were prepared by first

filtering the crystal slurry, treating them in the ultrasonic bath, and

then suspending them in the electrolytic solution used in the counting

procedure. One can see that the majority of these crystals are of

single crystal habit and that the largest crystals are approximately

100-150 microns in size,

A sample of the crystallizer product from run 3-D was suspended

in a 75 brix sucrose solution and allowed to ripen for approximately

15 minutes. Figure 16 is a color print (of run 3-D ripened crystals)

taken with a 35mm camera. The scale on this figure is 30 microns per

mm, A black and white photo taken with a Polaroid Land Camera is shown

in Figure 17, One can see that there are few agglomerates and also

that the ripening of the optimized seed produces many fine single

crystal particles.

Multi-crystal agglomeration seems to occur( at high solvent/water

ratios. Extreme agglomeration occurred in run 1-D having a solvent/water

68

Figure 14. Microphotograph of Run 6-B Product

V <vj P t

<30. ct

e

oW > m \

rq_5S6'Tv% ^Figure 15. Microphotograph of Run 8-B Product

69

% a

* • " % • • • v’

* - > ' •“ % V , e > »0 .

’ r f f *

» « < 5 o - . %Figure 16. Microphotograph of Run 3-D Ripened Product No. 1

,o o <?

Figure 17. Microphotograph of Run 3-D Ripened Product No. 2

70ratio of three0 These agglomerates are shown in Figure 180 The scale

for this photograph is 30 microns per mmo A close-up of an agglomerated

crystal is shown in Figure 19„ Here the scale is 143 microns per

centimetere In both pictures, the crystals have been suspended in the

electrolytic sampling solution^ This form of extensive agglomeration is

to be avoided at all costs if the nucleator is to be used as an effective

seed sourceo Further research is needed in order to precisely define

the limits of operating variables within which well-formed crystals, will

be producedo

It is likely that surfactants reduce agglomeration, but further

study is needed to establish their effectiveness. If a surfactant is

used and if it is absorbed into the crystalline structure while the

crystal is growing, then a necessary constraint is that it be harmless

for human consumption.

Thus high solvent/water ratios appear to produce high nuclei

densities at the cost of multi-crystal agglomerated habit. If a sur­

factant could be found which would suppress multi-crystal growth,

higher solvent/water ratios could be used with a concomitant increase

in nuclei production. Therefore, there is considerable incentive to

find such habit modifiers in future experimental studies.

Kinetics Correlation

An attempt was made to correlate the data according to the

following equation:

B° = V V

71

l

a o <sr= »

Figure 18. Microphotograph of Run 1-D Agglomerates

Figure 19. Microphotograph of Run 1-D Agglomerate

72Here, s is defined as (C - C ) where C is the concentration of theeqsucrose (on a clear liquor basis) inside the crystallizer and is

the equilibrium solubility of sucrose at the same crystallizer

temperature. An experimental plot of equilibrium sucrose solubility

data (for the two solvent systems that were studied) as a function of

temperature is given in the appendix. Sucrose mother liquor concentra­

tion was not measured experimentally and a material balance failed to

give reasonable values for C, Consequently, a correlation involving

growth rate, rather than supersaturation, was carried out. Data were

correlated from the eight experimental runs not involving the surfactant

fluorocarbon since this additive may have had an effect on the system

kinetics. The following equation was obtained with the aid of a

multiple linear regression computer analysis:

B° = 0,622 exp(4322/T) G0 o5 4 3M t 1 °2 1 2 (21)

Here, B° is the nucleation rate in no*/cc-minute, T is the crystallizer

temperature in degrees Kelvin, G is the crystal growth rate in microns

per minute, and is the solids concentration (gram/cc slurry) directly

measured from the crystallizer slurry0

The above equation yielded a good fit which is shown in the

correlation plot in Figure 20, All the coefficients in this equation

were calculated with a t-test to be at least 95% significant and an

F-test established the overall regression analysis to be statistically

significant above the 97,5% level. It should be mentioned that both

solvent types were involved in the above correlationo

B "(

pred

icte

d)

73

13.0

12.5 Zo

12.0O /

10.510.5 11.0 11.5 12.0 12.5B ° ( a c t u a l )

Figure 20. Correlation Plot of Predicted vs. Actual Nucleation Rates

>'■■■■ : , . 7 4

As was expected from the experimental results, one can see from

equation 2 1 that a decrease in temperature will increase the nucleation

rate over the range of the independent variables that were studied0

CHAPTER VIIPROCESS CALCULATIONS

Computer simulations of a three stage continuous sucrose

crystallization unit were carried out during the course of the research

worko Use was made of the steady state CSD Mark I-A program written by

Nuttal (1970)o In a typical simulation, the production and flow rates

along with seeding, classification and growth rate dependency (if any)

are read in as data and the program then calculates the level of the

driving forces that are required in order to produce the specified

production,

Nucleator/Ripener/F,C,Crystallizer

The first stage of the continuous sugar crystallizer was assumed

to be a 30 gallon MSMPR nucleator which operated at conditions similar to

run 7 -B but without any surfactants and with a pure ethanol solvent

system. The nucleator operated at a holding time of 3 hours and a

solids concentration of 0,42 grams/cc. The production in this stage was60.33 tons per day. Nuclei density was assumed to be 5.0 x 10 no./cc-

micron and the growth rate was determined to be 0i063 microns per minute.

The CSD for this stage is shown in Figure 21. The average size (popula­

tion weighted) for this distribution was calculated to be 11.4 microns.

On a weight basis, the average size was 45.54 microns and the coeffic­

ient of variation was 0.4999.

75

Wei

ght

Per

cent

pe

r M

icro

n2.0

Ripener CSD, ( P =0.977 ~ ; T s .3hr; s = 1.26)

Ripener CSD, (P = 0 .6 3 4 ^ ~ ;T=.3hr; s = 1.22)

1.0

0.5

Nucleator CSD

0.0 100 1208060 1404 020Size, microns

Figure 21. Nucleator CSD and Ripener CSD for Two Production Rates

ON

. ■ ■ ■ ■ ■ : ■ 77The second stage was assumed to be a crystal ripener of 20

gallon capacity operating at 38 C under a vacuum pressure of 0.24

atmosphereSo The feed streams to this stage were the product stream

from the nucleator (0o132 galIons/minute) and a 1»0 gallon/minute

(160 F, 75 brix) sucrose syrup stream* The purpose of the ripener was

to promote better mixing of the seed slurry produced by the nucleator*

This would insure a better chance of seed survival when injected into

the F o C » crystallizer (stage 3) 0 Another purpose of this stage was to

provide a means for solvent recovery* The ripener operated at a holding

time of 2 0 minutes and the nuclei density was assumed to be small

(io.e* 9 10 nuclei/cc-micron) * Figure 21 shows the second stage .OSD at

two different production rates* When the production was assumed to be

0*634 tons/day the crystals had a growth rate of 0*318 microns/minute

and the supersaturation was calculated from equation 6 to be 1*22* The

average size on a weight basis was 46*79 microns with a coefficient of

variation ( C * V * ) of 0*4631 (weight basis)* When production was in­

creased to 0*977 tons/day9 the supersaturation level was required to

increase to 1*26 with a corresponding increased growth rate of 0*511

microns/minute* On a weight basis, the average size was 51*82 microns

with a C o V * of 0*4525* One can see from Figure 21 that as the produc­

tion increases in the ripener stage, the required supersaturation level

increases (and hence the growth rate)* The C S D becomes slightly

narrower since the C*V. decreases and the average size of the distribu­

tion increases about 5 microns*

: - - ' . 78Solvent Recovery

Material and energy balances were carried out on the ripener .

stage assuming the operating conditions that were previously mentioned.

The sugar/water/ethanol system was assumed to behave similar to that of

a salt/water/methanol system in order to obtain an estimate of the

relative volatility of ethanolo It was.estimated that approximately

63% of the solvent could be recovered when the ripener operated at the

assumed conditions. This was equivalent to a 7% volume change in the

ripener as the ethanol and water were flash evaporated.

Retention Time Effects

Stage three was assumed to be a forced circulation crystallizer

with a 240 gallon capacity. This crystallizer was assumed to have a

solids concentration in the product slurry of 0,4 g/cc. Figure 22 shows

the CSD for the F„C, crystallizer at two different retention times and

thus at two different production rates. One can see that as the holding

time decreases, the required supersaturation and thus growth rate levels

increase. Production also increases from 3,2 to 9,6 tons/day (assuming

M t remains constant), The average weight size of the CSD increases

from 70,49 to 107,17 microns and the distribution becomes slightly wider

since the C,V, on a weight basis increases from 0,4176 to 0,4.186, In

both cases, the F,C, crystallizer is fed from the ripener which has a

production of 0,634 tons/day.

In Figure 23, the growth rate in the F,C. crystallizer is

assumed to be a function of crystal size, varying linearly between 0 , 5

to 1 , 0 on a dimensionless basis over the size range of 0 to 1 0 0 microns.

Wei

ght

Per

cent

pe

r M

icro

n2.0

Seed Ripener, (P= .634 Tons/day)

«r-F.C. Crystallizes (P= 3 . 2 ^ ; ^ = 2 . 4 hr; sM.I7)

1.0F.C. Crystallizes (P=9.6~~;T=0.8hr; s=l.27)

0.5

0.04 0 16080 200120 2 4 0

Size , microns

Figure 22. Ripener CSD and F.C. Crystallizer CSD for Two Production Rates

VO

Wei

ght

Per

cent

pe

r M

icro

n2.0

,<f—Seed Ripener, (P= 0 .634 Ton/day)

1.0F C. Crystallize^ ( P = 9 . 6 ^ ; ^ = 0 .8 ; s=l.3l)

0.5

0.0 200 240120 1604 0 80

Size , microns

Figure 23. Ripener CSD and F.C. Crystallizer CSD with Growth Rate Dependency on Size

ooo

When the crystals reach 100 microns the growth rate is assumed to be

constanto One can see by comparing Figures 22 and 23 that the growth-

rate dependency on size has the effect of widening the CSD (C.V.. varies

from 0.4186 to 0.4406) and increasing the average size of the distribu­

tion (from 107.11 to 121.12 microns). The larger size results due to the

"washout" of the slow-growing nuclei.

It should also be pointed out that the slight dip on the CSD

curve of the F.C. crystallizer in Figure 23 is due to the discontinuity

of the growth rate function at 1 0 0 microns.

These calculations confirm the feasibility of using the ethanol/

sucrose/water nucleator system as a seed source in a commercial continu­

ous sugar crystallizer. The required equipment, reproducibility and

costs of this process could be readily assessed from this research work.

A possible product quality improvement from the use of these well-formed

seed could only be determined from extensive pilot tests using industrial

syrups.

CHAPTER VIII

SUMMARY

1* A series of runs of the sucrose salting-out seed crystal

nucleator was executed with a rebuilt MSMPR crystallizer of the type

used by Crawford (1970)0

2o Crystallizer temperature, holding time, sucrose feed con­

centration, solvent type, and two surfactants were studied at high and

low level values in order to determine their effects on sucrose

nucleation. The magnitude of the effects of these variables was

estimated with a one-level statistical t-test.

3 0 Nucleation rate data were correlated according to the

kinetics power law expression B° = k^G^I^ ,

4 0 The feasibility of a continuous sucrose process was studied

via a computer simulation of a three stage nucleator/ripener/footing

crystallizer combination. z

5. Material and energy balances around the second stage ripener

were written to estimate the feasibility of solvent recovery from the

nucleation process. "

6 . Limited equilibrium solubility data were determined for the

two sugar/water/alcohol systems that were studied.

82

CHAPTER IX

. CONCLUSIONS

lo Larger holding times tend to increase the nuclei density and

decrease particle size of sucrose crystals generated by the salting-out

process.O2 0 Low crystallization temperatures of 0-5 C increase the nuclei

density. Average particle size was only slightly lowered at these low

operating temperatures.

3. 50:50 MeOH/IPA used as the salting-out agent results in in­

creased nuclei densities compared to Vanzol A-l (denatured alcohol); the

particle size was also slightly decreased.

4. Higher sucrose feed concentrations result in higher nuclei

densities in the crystallizer. Feed concentration has no observable

effect on the particle size.

5. Statistical evidence suggests that the surfactant fluoro-

carbon (FC-96, 3M Company) at 100 PPM has a deleterious effect on the

crystal distribution. Addition of FC-96 tends to decrease the nuclei

density and increase particle size. This surfactant is not recommended

for use in the sucrose seed nucleator.

6 . The surfactant methylamine hydrochloride at 100 PPM exhibits

no effect on the system kinetics.

7. High solvent/water ratios produce high nuclei densities at

the price of multi-crystal agglomerated habit. Ratios of less than two

(mass basis) should be used to produce seed of single-crystal form.

83

■ ■■ . ■ ■' ; r ' / : " ̂ ' 8480 The results of the computer simulation of the three stage

continuous crystallization system in which the ripened seed was added

to a forced circulation industrial crystallizer illustrated the feasi­

bility of the process used as a seed source in a large-scale process0

9. The technique of seeding a continuous crystallizer with these

continuously generated nuclei seems to be a viable alternative to current

ball mill seeding technique0

10o Solvent recovery in the second stage ripener appears

feasible.

CHAPTER X

RECOMMENDATIONS

1. Future experiments should quantitatively investigate the

effects of the key independent variables upon the system kinetics.

Wider ranges of these independent variables should be studied.

2. Extensive study of the effects of additives should be made.

It may be possible to optimize the system kinetics and/or reduce

agglomeration through the use of various non-toxic surfactants. If a

surfactant habit modifier could be found which would suppress multi-

crystal growth, higher solvent/water ratios could be used with a con-

committant increase in nuclei production. Thus, there is considerable

incentive to find such habit modifiers in future experimental studies.

3. Future research should investigate the effects of feed pH

and temperature on system kinetics. These variables are directly

related to the degree of inversion in the syrup. Work in the literature

indicates invert sugar affects crystal growth rate. The specific

effects of sucrose inversion on nucleation rate and crystal habit should

be quantitatively determined.\4. The differences of beet vs. cane sugar on nucleation kinetics

and crystal habit should also be studied. Different impurities exist in

these sugars which may have different effects on system kinetics.

5. The effect of agitation on crystal habit and system kinetics

shquId be quantitatively investigated.

85

866 e The apparatus should be rebuilt so that long-term unattended

runs can be made. This will enable study of possible habit improvement

to be gained with lower supersaturation driving forces and perhaps will

allow operation at higher solvent/water ratios without making crystals

of multi-crystal habit. . Long-term fouling tendencies can also be

studied with such unattended runs.

7o The seed crystal nucleation process of this study should be

implemented at the pilot plant level in a commercial sucrose refinery0

Product quality improvement using these seed should be quantitatively

determined.

Equi

libr

ium

C

once

ntra

tion

, 9

/cc

APPENDIXSOLUBILITY DATA

0 .3

0.2

O - S u g a r / w a t e r / V a n z o l A - 1

A - S u g a r / v / a t e r / 5 0 : 5 0 M e O H - I P A

0.0- 2 0 - 1 0 0 10 2 0 3 0

T e m p e r a t u r e , °C

Fig. 1-A Sucrose/Water/Alcohol Systems. Equilibrium solubility data for the two solvent systems studied in this research work were de­termined experimentally.

87

NOMENCLATURE

B° = nucleation rate*

C = outlet concentration of MSMPR crystallizer feed stream,,

C . = inlet concentration of MSMPR crystallizer feed stream,

E. = effect value of the i-th variable,

G = growth rate.

k = constant in kinetic nucleation model expression.n

k = crystal volumetric shape factor.v - ' ■ ■ ■ ■ ; : , ■

L = characteristic length. . ' ;

M = molecular weight.

M = total mass of crystals in suspension per volume,

n = population density.

n° = nuclei population density.

N = Avogadro s number„

N = averaged population counts minus the averaged noise counts.

P ,= production.

= input stream flow rate in general population balance.

Q = output stream flow rate in general population balance.° ' ' - -

R = universal gas constant,

s = supersaturation.

S = coefficient of supersaturation.

S. = the square root of the variance of the j-th duplicate set.

88

the square root of the pooled variance for the

standard error.

t-value.

temperature.

crystallizer volume.

ith response.

average response.

Coulter Counter channel width,

dilution factor,

crystal density,

interfacial tension,

residence time.

LIST.OF REFERENCES

Alewijn, W„ F. and Pieter Honig. "Technology of Sugar Crystallization," Principles of Sugar Technology, Vol. 2, Ch. 9. Edited, by Pieter Honig. New York: Elsevier Publishing Co., 1959.

Anonymous. "Continuous Sugar-Crystallization Process," Sugar Azucar., Vol. 65, No. 10, 1970.

Bennett, R. C. "Continuous Sugar Crystallization: A Chemical Engineer's Viewpoint," Chemical Engineering Progress Symposium Series,No. 95, Vol. 65, 1969.

Bennett, R. C„ and P. H. Petri. "Continuous Crystallization of Sucrosein a Pilot Pan," Unpublished research paper, Harvey,Illinois,1966

Clontz, N. A. and W. L. McCabe. Paper read at A.I.Ch.E. Annual Meeting No. 62, Washington, B.C. November 16, 1969.

Crawford, R. W. "The Continuous Nucleation of Sucrose," Master's Thesis. Tucson: The University of Arizona, Department of Chemical Engineering, 1970.

Davies, 0. L. Statistical Methods in Research and Production. New York: Hafner Publishing Company, 1961.

Geerligs, H. C. P. The World's Cane Sugar Industry. Manchester, England Norman Rodger, 1912.

Gupta, S. C. and S. K. D. Agarwal. "Preparation of Seed Slurry for Sugar Boiling," Int. Sugar J ., No. 836, Vol. 70, 1968.

Hirschmuller, H„ "Chemical Properties of Sucrose," Principles of Sugar Technology, Vol. 1, Ch. 1, edited by Pieter Honig. New York: Elsevier Publishing Company, 1953.

Isaacson, W. B. "Statistical Analyses for Multivariable Systems,"Chem. Engr., June 29, 1970.

Jenkins, G. H. Introduction to Cane Sugar Technology. New York:Elsevier Publishing Company, 1966.

Kelly, F. H. C. "The Solubility of Sucrose in Impure Solutions,"Principles of Sugar Technology, Vol. 2, Ch. 2, edited by Pieter Honig. New York: Elsevier Publishing Company, 1959.

91Kelly, F. H. C. and M. F. Keng. "Homogeneous Sucrose Nucleation

Studies," Int. Sugar J ., No. 881, Vol. 74, 1972.

Luce, G. W. "Continuous Vacuum Crystallizer," Int. Sugar J.. No. 857, Vol. 72, 1970.

McCabe, W. L. and J . C. Smith. Unit Operations of Chemical Engineering. New York: McGraw-Hill Book Co., 1967.

Nicol, W. M. and C. L, Former. "Effect of Impurity on the Nucleation and Crystallization of Sucrose," Suer. Beige Sugar Ind. Abstr., No. 91, Vol. 2, 1972.

Nuttal, H. Eric. "User Manual for CSD Simulation," Unpublished paper. Tucson: The University of Arizona, Department of Chemical Engineering, 1970.

Randolph, A. D. "Sucrose Nucleation Research Notebook," Tucson: The University of Arizona, Department of Chemical Engineering, unpublished paper, 1965.

Randolph, A. D. "How to Approach the Problems of Crystallization,"Chem. Engr., May 4, 1970.

Randolph, A. D. and M. D. Cise. "Nucleation Kinetics of the Potassium Sulfate-Water System," A.I.Ch.E. Journal, No. 4, Vol. 18, 1972.

Randolph, A. D. and M, A. Larson. Theory of Particulate Processes.New.York: Academic Press, 1971,

Saeman, W. C. "Crystal-size Distribution in Mixed Suspensions," A.I. Ch.E. Journal, No. 2, 1956.

Silver, B. S. "Continuous Vacuum Pan System," Sugar Azucar, No. 6,Vol. 65, 1970.

Van Hook, A. H. Sugar. New York: The Ronald Press Co., 1949.

Van Hook, A. H. "Nucleation in Supersaturated Sucrose Solutions,"Principles of Sugar Technology, Vol. 2, edited by Pieter Honig. New York: Elsevier Publishing Company, 1959.

Youngquist, G. R. and A. D. Randolph. "Secondary Nucleation in a Class II System: Ammonium Sulfate-Water," A.I.Ch.E. Journal, No. 2, Vol. 18, 1972.

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