Department of Chemical EngineeringUniversity of San Carlos Technological CenterNasipit, Talamban, Cebu City

ChE 422LChemical Engineering Laboratory 1

Fluidization(Fluidization of a Packed Bed of Particles)

A laboratory report submitted toEngr. May TampusInstructor, ChE 422L

By

Date performed: January 15, 2015Date submitted: January 29, 2015

1. Introduction

In a packed bed of small particles, when a fluid enters enters at sufficient velocity from the bottom and passes up through the particles, the particles are pushed upward and the bed expands and becomes fluidized (Geankoplis, 2003). Fluidization is the operation by which the fine solids are transformed into a fluid-like state through contact with a gas or liquid (Kunii et al, 1991). Fluidized beds find use in a variety of industrial process such as drying, mixing, granulation, coating, heating, and cooling. Two general types of fluidization can occur: particulate fluidization and bubbling fluidization (Geankoplis, 2003). Particulate fluidization is characterized by the beds continuous expansion and homogeneity for a time as the fluid velocity continues to increase. This type of fluidization is very desirable in promoting intimate contact between gas and solids. Bubbling fluidization is characterized by the passage of gas, the fluidizing medium, through the bed as voids or bubbles which contain few particles and only a small percentage of the gas passes in the spaces between individual particles. The expansion of the bed is small as gas velocity is increased. Little contact occurs between the individual particles and the bubbles. Classification of particles according to their density and diameter ranges has been provided by Geldart (Geldart, 1973).

Figure 1. Geldart classification of particles

Fluidization proceeds as follows: a fluid, air or water, is passed upward through a bed of granular materials. If fluid is admitted at a very low rate, it merely percolates through the void space between the stationary materials, not causing the materials to move (static condition) or an increase in flow rate will cause the materials to move apart and vibrate and move about in restricted regions (expanded bed). Through the process of increasing the velocity, the pressure drop over the bed increases and will eventually equal the force of gravity on the particles and the grains begin to move. The bed expands slightly with the grains still in contact. The porosity increases, and the pressure rises more slowly than before. As the velocity is increased, the grains separate and true fluidization occurs. The bed is fluidized and is characterized by the particles moving about, traveling in random directions resembling a boiling liquid. The linear velocity of the fluid between the particles is much higher than the velocity in the space above the bed. Only the smallest grains are entrained in the fluid, even with rigorous fluidization. If the fluid velocity is reduced to and below the minimum velocity required for fluidization, the bed collapses once more; the pressure drop again follows the relationship for the fixed bed. The porosity may be higher than in the original bed. The pressure drop for a given velocity may be lower than what was before. As mentioned previously, when a fluid flows upward through a packed bed of particles at low velocities, the particles remain stationary. As the fluid velocity is increased, the pressure drop increases according to the Ergun equation (Geankoplis, 2003)

(1.1)

where p is the pressure drop across the bed, and is the viscosity and the density of the fluidizing medium, respectively, is the porosity of the bed at the given bed height L, Dp is the particle diameter and v is the superficial fluid velocity. This is characterized by line AB of Fig.2. Upon further increase in velocity, conditions finally occur where the force of the pressure drop times the cross-sectional area equals the gravitational force on the mass of particles, characterized by point B. Minimum fluidization occurs as the particles begin to move, characterized by point C. The fluid velocity at which fluidization occurs is called the minimum fluidization velocity and is based on the empty cross section of the tower. With any further increase in the velocity of the flow, the particles move in more rapid and more independent motion. Points D to E describes that large increments in velocity will only yield slight changes in pressure drop. Figure 2. The effect of v on the p for a fluid flowing upward through a bed of closely-packed particles

The porosity of the bed during fixed bed operation remains constant with increasing velocities until point B in Fig. 2 is reached. The porosity then increases continuously as a function of the Reynolds number up to the Reynolds number corresponding to the free-settling velocity of the individual particles as can be seen in Fig. 3 (Brown et al, 1973). The porosity of the bed when fluidization occurs is the minimum porosity for fluidization. The bed expands to this voidage or porosity before particle motion appears (Geankoplis, 2003).Figure 3. The effect of NRe on the of a bed of particles through which a fluid is flowing upward with a superficial velocity v.

2. Objectives of the Experiment2.1 Investigate the effect of column diameter on the power required to fluidize a bed of solid particles.2.2 Calculate the theoretical power required to fluidize a bed of solid particles and compare with experimentally determined values.2.3 Determine graphically and through visual observation the minimum fluidization velocity of solid particles packed in a cylindrical column2.4 Relate the power required for fluidization to the flow rate of the fluidizing medium and the porosity of the bed.

3. Methodology3.1 MaterialsWaterCeramic spherical packing3.2 Equipment and ApparatusVernier CaliperStopwatch ThermometerAnalytical Balance100-mL Graduated Cylinder Fluidized Bed EquipmentWeigh boat1-L Graduated Cylinder

Figure 1. The fluidized bed equipment

Figure 2. The sump tank

Figure 3. The small column (D =0.035m)

Figure 4. The big column (D =0.059m)

Figure 5. Ceramic packing

3.2 Procedure3.2.1 Preliminary Steps The average size, the average density and the percent void of the particles were determined. The average size was determined by taking 5 representative samples of the packing with the use of a Vernier caliper. The average density was determined by weighing a representative quantity of the material and dividing the amount by the corresponding volume. The percent void was determined by slowly introducing a measured amount of water into a graduated cylinder containing pellets up to a desired level.

3.2.2 Fluidization Experiment The sump tank was filled with water to 2/3 its capacity. Valves were labeled according to the specifications in the manual. Valves V1, V2, V4 and V7 were fully opened while valves V3, V5 and V6 were closed. The water pump was switched on. Water was allowed to fill the small column and this was done by slowly opening valve V3 while simultaneously closing valve V2. Valve V3 was further turned counterclockwise until the U-tube manometer gave a pressure drop reading. The flow rate was determined manually using a stopwatch and a graduated cylinder. The corresponding bed height was recorded and this was labeled as point A. V3 and V2 was regulated to adjust the flow rate. The flow rates at which the following conditions were observed, were noted: B-the point when the topmost particles of the bed begins to move in their places, C-the point when all of the bed particles are moving but not necessarily expanded to full column height, and D-when the bed has expanded to full column height. V1 was regulated to further increase the flow rate. At most 5 different flow rates and their corresponding pressure drops and bed heights in between the given conditions were measured and recorded. The water pump was then shut off. The steps were repeated using the bigger column.

4. Results and Discussion4.1 Operating Conditions

Table 1. Values of constants Diameter of particles [mm]5.65

Density of particles [g/cm3] 2.2048

Porosity0.539

Column height (both columns) [mm]935

Bed height (both columns) [cm]9

Inside diameter (small column) [mm]35

Inside diameter (big column) [mm]59

Cross-sectional area (small column) [m2]0.0009621

Cross-sectional area (big column) [m2]0.002734

T [C]32.5

Density (mercury) [kg/m3]13515.245

Density (water) [kg/m3]994.820

Viscosity (water) [Pas]0.000793

Surface Roughness (acrylic pipe) [mm]0.00152

Relative Roughness (acrylic pipe)4.3429E-05

Inner Diameter (water pipe) [mm]15.8

Cross-sectional area (water pipe) [m2]0.000196

4.2 Power requirement for fluidization (theoretical and experimental) versus column diameter

Table 2. Power requirement versus column diameterColumn Diameter (m)ExperimentalTheoreticalPercent Difference

P (kPa)P (W)P (kPa)P (W)

0.0359542.82800.48809158.19310.46834%

0.05916832.48822.42639157.86811.320184%

Table 2 shows how power requirement for fluidization varies with the diameter of the fluidization column. It can be observed that the power for the large column (D = 0.059m) is higher compared to that of the small column (D = 0.035m). This is because the volume of the bed in the large column is larger, thus offering more resistance to flow due to drag. The larger the bed volume, the higher the contact area and the more packing the fluidizing medium has to flow past through. This means that more driving force is required to make the fluidizing medium flow.

With regard to comparison between experimental and theoretical power requirement values, it is notable that the percent difference for the small column is very small compared to the large column. It is expected that the difference between calculated and observed power requirement values is large mainly due to: a.) the equipment and b.) the method of measuring of volumetric flow rate. The equipment has aged considerably thereby increasing the possibility of inaccurate parameter readings. Some scraped-off paint particles was observed in the bed, contributing to the resistance. The volumetric flow rate has to be obtained manually, yielding values that are not as accurate as those obtained if the equipment is appropriate and in good condition.

4.3 Determination of minimum fluidization velocity through graphical method and visual observation

When a fluid flows upward through a packed bed of particles at low velocities, the particles remain stationary. As the fluid velocity is increased further, conditions finally occur where the force of the pressure drop times the cross-sectional area equals the gravitational force on the mass of particles, the particles begin to move, and this is the onset of fluidization (Geankoplis, 2003). The fluid velocity at which fluidization begins is the minimum fluidization velocity based on the empty cross section of the tower (Geankoplis, 2003). At minimum fluidization velocity, all of the packing in the fluidized bed are in the loosest configuration and will expand when volumetric flow rate is increased. Two methods are employed to determine this: graphical method and visual observation.

One can determine the minimum fluidization velocity by manipulating the flow rate in the fluidization column and observing the packed bed. Below the minimum fluidization velocity none or only a few of the packing will be moving. Steadily increasing the flow rate causes more of the packing to move about. At a particular point where the height of the bed remains the same but all of the particles are already moving in their places, minimum fluidization velocity is reached. This corresponds to point C in the experiment procedure.

Figure 6. Logarithm of the pressure drop versus the logarithm of the velocity and determination of minimum fluidization velocity

Minimum fluidization velocity can also be determined graphically by either plotting pressure drop versus velocity or void fraction versus Reynolds number.

Figure 7. Logarithm of the void fraction versus the logarithm of the Reynolds number and determination of minimum fluidization velocity

Table 3. Visual observations at different pointsPointObservations

A-BNo significant change in bed height, manometer shows reading

B-CTopmost part of the bed shows movement, bed height is constant

C-DAll of the particles start moving, bed height increases

DParticles are all moving, bed height is at maximum

Both the visual observation and graphical method yielded approximately the same values of minimum fluidization velocity regardless of column diameter. This is because at minimum fluidization velocity, the terminal velocity of the particles equals the velocity of the fluidizing medium. The column diameter has no effect on the terminal velocity this is dictated by drag caused by the particles shape and surface.

Before fluidization is reached, the pressure drop increases with an increase in the fluid velocity according to the Ergun equation (Geankoplis, 2003). When fluidization starts, pressure drop only changes slightly while porosity increases with increasing velocity (and accordingly, Reynolds number). This trend is reflected on Figure 7 but not in Figure 6. The opposite is shown instead: with increasing velocity, pressure drop increases slightly before fluidization but increases greatly at higher velocities.

4.4 Power requirement versus flow rate or porosity

Figure 8. Power requirement versus flow rate

Figure 8 shows the relationship between power input and flow rate of the fluidizing medium. The general trend observed is that an increase in the flow rate corresponds to an increased power input.

Figure 9 however displays a relationship that changes at a certain point in the graph.

Figure 9. Power requirement versus porosity

For both the small and large columns, the power requirement is independent of porosity at some point the power requirement increases with the same value of porosity. However, at higher power inputs the relationship changes to that of an exponential increase of power input with respect to porosity. This change in relationship marks the start of fluidization where the bed height increases, resulting in increasing porosity.

5. Conclusion

The power requirement for fluidization for the bigger tower diameter is greater as compared to the power requirement for fluidization for the smaller tower diameter. A bigger tower diameter will allow for a greater drag. A bigger tower diameter will also allow for a greater volume of packing to resist flow. To overcome this resistance, a greater driving force should be applied.

There is a difference between the theoretical power requirement and the experimental power requirement for fluidization. This has been thought to have come from the inaccuracies caused by two factors: the degradation of equipment performance and the method of measuring the flow rate.

The minimum fluidization velocity determined graphically and through visual observation is approximately equal regardless of the column diameter. At minimum fluidization velocity, the terminal velocity of the particles equals the velocity of the fluidizing medium. The terminal velocity is dictated by drag caused by the particles shape and surface.

Before fluidization is achieved, the pressure drop increases with velocity by the Ergun equation. Once the minimum fluidization velocity is reached, the pressure drop and ultimately the power requirement, increases only slightly until such time that it becomes constant. As the fluid velocity is approaching the minimum fluidization velocity, the bed porosity is independent of fluid velocity. Once fluidization is achieved, an exponential increase in the porosity of the bed can be observed with an increase in the power requirement.

6. References

Brown, G. G. et al. (1973). Unit Operations. New York: John Wiley and Sons, Inc.Geankoplis. (2006). Transport Processes and Unit Operations, 4th edition. New Jersey: Prentice Hall.Geldart. (1973). Diagram of the Geldart Classification of particles. Kunii, & Levenspiel. (1991). Fluidization Engineering. Elsevier.Perry, R. H., & Green, D. W. (2008). Perry's Chemical Engineers' Handbook 8th Edition. McGraw-Hill Companies, Inc.

7. Appendix 7.1 TablesTable A-1. Diameter of PaticlesSampleDiameter [mm]

15.65

25.65

35.6

45.65

55.7

Average5.65

Table A-2. Density of ParticlesTrialMass [g]Volume of a particle [cm3]Number of particlesDensity of particles [g/cm3]

11.03990.094452.2023

21.03812.1985

31.06362.2525

41.04342.2097

51.02042.1610

Average2.2048

Table A-3. Percent VoidFixed water volume [mL]50.5

Water + Pellets volume [mL]60.5

Pellets added100

Bed + Void volume [mL]21.5

Bed volume10

Porosity0.5349

Table A-4. Column ParametersColumnHeight [mm]Inside Diameter [mm]Bed Height [cm]Cross-sectional area [m2]

Small9353590.0009621

Large9355990.0027340

Table A-5. Fluidization Data, Small Column DiameterSmall Column (D=35mm)

Volume [ml]time [s]Flowrate [m3/s]Bed Height, L [cm]h2 [cm]h1 [cm]h [cm]

A17.060.542.808E-079.022.3016.405.90

1100.05.981.672E-059.022.3016.455.85

2675.017.983.754E-059.022.5016.306.20

B725.020.003.625E-059.022.7016.256.45

1505.013.043.873E-059.022.7516.106.65

2825.019.764.175E-059.022.8515.956.90

3840.018.314.588E-059.022.9515.907.05

4868.018.544.682E-059.023.0015.857.15

C987.519.315.114E-059.023.0015.807.20

11000.015.096.627E-0511.423.2015.567.64

2862.58.281.042E-0418.123.6515.258.40

3875.06.511.344E-0425.324.2014.659.55

4864.03.922.204E-0470.526.1012.5513.55

5911.04.012.272E-0482.431.457.4024.05

D900.03.902.308E-0493.532.906.0526.85

Table A-6. Fluidization Data, Large Column DiameterLarge Column (D=59mm)

Volume [ml]time [s]Flowrate [m3/s]Bed Height, L [cm]h2 [cm]h1 [cm]h [cm]

A79.016.734.722E-069.023.2015.907.30

1911.012.287.419E-059.023.9015.558.35

2575.08.246.978E-059.023.7515.308.45

3875.07.441.176E-049.024.9014.3010.60

B862.56.941.243E-049.025.0013.9011.10

1843.06.471.303E-049.025.1013.7011.40

2893.06.781.317E-049.025.1513.5011.65

3862.56.441.339E-049.025.2013.4511.75

4907.06.291.442E-049.025.4013.3012.10

C800.05.551.441E-049.025.7013.0012.70

1998.04.682.132E-0411.427.6511.6016.05

2879.03.782.325E-0412.228.1511.2516.90

3929.03.882.394E-0412.828.4010.5517.85

4725.02.902.500E-0413.128.7010.1318.57

5875.03.252.692E-0413.429.359.8019.55

D812.53.132.596E-0413.629.509.6019.90

Table A-7. Fluid & Pipe PropertiesT [C]32.5

MercuryDensity [kg/m3]13515.245

WaterDensity [kg/m3]994.820

Viscosity [Pas]0.000793

Acrylic PipeSurface Roughness [mm]0.00152

Relative Roughness4.3429E-05

Water PipeInner Diameter [mm]15.8

Cross-sectional area [m2]0.000196

Table A-8. Experimental Power Determination, Small ColumnSmall Column (D=35mm)

v (m/s)h [cm]P (Pa)PT (W)log volog P

A0.00029195.907819.80.0022-3.53483.8932

10.01738095.857753.50.1297-1.75993.8895

20.03902016.208217.40.3085-1.40873.9147

B0.03767756.458548.80.3099-1.42393.9319

10.04025206.658813.90.3413-1.39523.9452

20.04339516.909145.20.3818-1.36263.9612

30.04768327.059344.00.4287-1.32163.9705

40.04866137.159476.60.4437-1.31283.9767

C0.05315317.209542.80.4880-1.27453.9797

10.06887877.6410126.00.6710-1.16194.0054

20.10826878.4011133.31.1597-0.96554.0466

30.13970159.5512657.51.7013-0.85484.1023

40.229087713.5517959.13.9583-0.64004.2543

50.236128324.0531875.77.2416-0.62694.5035

D0.239856726.8535586.88.2123-0.62004.5513

Table A-9. Experimental Power Determination, Large ColumnLarge Column (D=59mm)

v (m/s)h [cm]P (Pa)PT (W)log volog P

A0.00172727.39675.36720.0457-2.76273.9857

10.02713488.3511067.02970.8210-1.56654.0440

20.02552398.4511199.56890.7815-1.59314.0492

30.043017110.614049.16341.6523-1.36644.1477

B0.045457511.114711.85981.8284-1.34244.1677

10.047657311.415109.47761.9687-1.32194.1792

20.048175711.6515440.82582.0337-1.31724.1887

30.048986811.7515573.36512.0857-1.30994.1924

40.052742712.116037.25262.3125-1.27784.2051

C0.052723412.716832.48822.4263-1.27804.2261

10.077999316.0521272.55404.5363-1.10794.3278

20.085055716.922399.13795.2087-1.07034.3502

30.087577017.8523658.26105.6646-1.05764.3740

40.091442118.5724612.54386.1531-1.03894.3912

50.098476119.5525911.42876.4779-1.03894.4135

D0.094947819.926375.31627.3733-0.99034.4212

23

Table A-10. Theoretical Power Determination, Small ColumnSmall Column (D=35mm)

v (m/s)Efr,bed (J/kg)NRefFf (J/kg)hexhc

P (Pa)PT (W)log Plog log Nre,p

A0.00029190.53490.198813.36801.19695.447E-065.400E-073.730E-070.19889121.90.00263.9601-0.27171.1261

10.01738090.534911.8633796.08150.02013.244E-041.915E-031.323E-0311.86699133.60.15273.9606-0.27172.9010

20.03902010.534926.70461787.19850.00907.283E-049.652E-036.668E-0326.72169148.40.34343.9613-0.27173.2522

30.03767750.534925.78151725.70570.00937.032E-049.000E-036.217E-0325.79749147.50.33163.9613-0.27173.2370

B0.04025200.534927.55191843.62460.00877.513E-041.027E-027.095E-0327.57009149.30.35433.9614-0.27173.2657

10.04339510.534929.71491987.58510.00808.099E-041.194E-028.247E-0329.73599151.50.38213.9615-0.27173.2983

20.04768320.534932.66842183.98500.00738.900E-041.441E-029.957E-0332.69379154.40.42003.9616-0.27173.3392

30.04866130.534933.34262228.78780.00729.082E-041.501E-021.037E-0233.36899155.10.42863.9617-0.27173.3481

40.05315310.534936.44062434.52110.00669.921E-041.791E-021.237E-0236.47199158.20.46833.9618-0.27173.3864

C0.06887870.632822.60883154.78290.01122.837E-031.504E-031.039E-0322.61429144.30.60603.9612-0.19873.4990

10.10826870.768712.68624958.92440.00935.845E-033.716E-032.567E-0312.69839134.40.95153.9607-0.11423.6954

20.13970150.83459.34686398.61210.00879.037E-036.186E-034.273E-039.36639131.11.22733.9605-0.07863.8061

30.22908770.94064.480110492.67910.00762.138E-021.664E-021.149E-024.52979126.32.01153.9603-0.02664.0209

40.23612830.94923.988510815.15430.00762.271E-021.767E-021.221E-024.04119125.82.07323.9603-0.02264.0340

50.23985670.95523.616610985.92470.00762.344E-021.824E-021.260E-023.67099125.42.10593.9603-0.01994.0408

D0.00029190.53490.198813.36801.19695.447E-065.400E-073.730E-070.19889121.90.00263.9601-0.27171.1261

Table A-11. Theoretical Power Determination, Large ColumnLarge Column (D=59mm)

v (m/s)Efr,bed (J/kg)NRefFf (J/kg)hexhc

P (Pa)PT (W)log Plog log Nre,p

A0.00172720.53491.1766133.35400.12001.134E-052.571E-051.523E-051.176649122.90.04313.9601-0.27172.1250

10.02713480.534918.54322095.05180.00761.782E-046.345E-033.759E-0318.55359140.30.67813.9610-0.27173.3212

20.02552390.534917.43891970.67670.00811.676E-045.614E-033.326E-0317.4489139.20.63773.9609-0.27173.2946

30.04301710.534929.45473321.31350.01126.563E-047.973E-044.724E-0429.45669151.21.07623.9615-0.27173.5213

B0.04301710.534931.13513509.73550.01077.017E-048.903E-045.275E-0431.13729152.91.13753.9616-0.27173.5453

10.04545750.534932.65063679.57830.01077.713E-049.786E-045.798E-0432.6539154.41.19283.9616-0.27173.5658

20.04765730.534933.00793719.60240.01077.881E-041.000E-035.925E-0433.01039154.71.20583.9616-0.27173.5705

30.04817570.534933.56703782.23040.01027.787E-041.034E-036.126E-0433.56949155.31.22623.9617-0.27173.5777

40.04898680.534936.15754072.22150.00988.670E-041.199E-037.101E-0436.16029157.91.32053.9618-0.27173.6098

C0.05274270.534936.14414070.72490.00988.664E-041.198E-037.096E-0436.14699157.91.32013.9618-0.27173.6097

10.05272340.632825.63906022.25910.00901.735E-032.621E-031.553E-0325.64499147.41.95073.9613-0.19873.7798

20.07799930.656923.40366567.07270.00882.025E-033.117E-031.847E-0323.41069145.12.12663.9612-0.18253.8174

30.08505570.673021.38436761.74420.00882.147E-033.305E-031.958E-0321.39179143.12.18923.9611-0.17203.8301

40.08757700.680521.12647060.16350.00882.341E-033.603E-032.135E-0321.13459142.92.28573.9611-0.16723.8488

50.09144210.687620.02297060.16350.00852.253E-033.603E-032.135E-0320.03099141.82.28543.9610-0.16273.8488

D0.09847610.692221.67287894.75150.00852.817E-034.178E-032.669E-0321.68289143.42.55613.9611-0.15983.8973

7.2 Sample CalculationsVolume of a Particle

Density of Particles

Percent Void or Porosity,

Volumetric Flowrate

Pressure Drop, experimental

Efr,bed

Reynolds Number

Friction Losses along the Pipe

Expansion Head

Contraction Head

Total Friction Losses

Pressure Drop, theoretical

Power

Percent Difference