Liang-tseng Fan Kansas State University Manhattan
Fluidization as an Undergraduate Unit Operation Experiment
M a n y unit operations laboratory experiments are oftcn comulex and more or less fixed in their scope by the nature df the equipment used. It has been suggested that experiments be developed which would be inexpensive, flexible, and of course, uniquely characteristic of ohemical engineering alone (4-7). One such experiment would be on the subject of fluidization which is one of the newer fields in chemical engineering (1, 8, 8). An experiment of this nature is described below. TWERYDYETER
PRESSURE
FLOWMETER
Figure 1.
GAGE-
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flow rate until the frictional drag on the particles hecomes equal to their effective weight (actual weight less buoyancy). This point is called onset point of fluidization and further increase of the air flow rate beyond this point causes the individual particles to separate from one another and become freely supported in the fluid. This implies that the "fluidization" commences by increase of fluid flow rate beyond the onset point. The use of glass pipe enables the students to make visual observation of this onset point. 4. After the onset point is reached, the flow rate of the air is increased further until t,he point where particles begin to escape from the test section. The flow rate of air corresponding to this point is called maximum fluidization velocity. The pressure and temperature in the test section are maintained constant during the experimentation, and the air flow rate and corresponding pressure drop are recorded. The students are also asked to record any visual observation t.hey may make during the experimental runs. Channeling, slugging, fluidization onset, bed expansion, and elutriation of the particles from the column are the typical phenomena they may observe. Figure 2 illustrates a typical set of data. The line drawn represents the calculated pressure drop. For
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Assembly of flvidirotion oppardur.
A sketch and flow diagram of the apparatus is shown in Figure 1. The test section, one inch in diameter, may be constructed of iron pipe or preferably of glass pipe for visual observation. The Experiment
1. A measured quantity of solid particles is placed in the column. 2. Completely fluidize the particles and then allow to resettle in order to obtain reproducible porosity. 3. After the packed bed is formed following the preliminary fluidization, the npward flow of air is resumed with gradual increase of its rate. The,pressure drop across the bed increases with the increases in the air
I 0.5
.,,:
I 1.0
i 1.5
I
2.0
I 2.5
? L O W RATE OF blR. LB./ HR.
Figure 2. Cornpariron of experimental data i i f h calcvlottd pressure drop for 0 . 0 7 ~ .spheres. (True density of spheres-2.5 ~ / s m S Bulk density of spheres-1.5 g/cm3)
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the pressure drop through the packed bed before the fluidization, Ergun's equation (9)
was employed. Other equations are available in the unit operation textbooks (1,Z, 5 , s ) . Since the pressure drop across the fluidized beds is equal to their effective weight, it can be estimated as:
The air flow rate which corresponds to the visually observed onset point should be compared with the onset velocity estimated from the pressure drop versus fluid flow rate curve as indicated in Figure 2. Modifications
The basic scheme of this experiment outlined above may be modified or enlarged to cover other aspects of fluidization and other related phenomena of the solid particle-fluid contact operations. The various materials, other than spherical glass beads used in this experiment, are available as solid particles. The use of various shapes and sizes of the particles enables students to demonstrate and observe their effectson the smoothness or quality of the fluidization. The different weights of the same particles may be charged in the test section to demonstrate the fact that the onset velocity is independent of the total weight of the bed. The air used as a fluidizing medium can easily be replaced with water or other liquids to create an entirely different mode of fluidization. With a liquid as a fluidizing medium, the solid particle beds continue to expand as the velocity of liquid is increased, and these solid particle beds will maint,ain their uniformity. With a gas, however, except a t relatively low velocities, two separate phases are formed within the beds and the pressure drop across t.he bed will fluctuate considerably. The former mode of fluidization is called "particulate fluidization" and the latter "aggregative fluidization."
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Journal of C:~emicalEducation
The use of two differentsizes of the particles will allow the sludents t o measure the rate of elutriation and to correlate it in the form of the first order rate equation as suggested by Leva (8). In addition to the estimations of pressure drop and the onset velocity, the comparison of the falling rate of the particle with the maximum fluidization velocity and the correlation of bed expansion with respect to the operating conditions can also be carried out. If budget allows, the columns of other sizes may be used to show, at least qualitatively, the efiect of column size on the behavior of the fluidized beds. As can be seen from the above description, this experiment can illustrate not only the principles of fluidization, but also the fluid dynamics of fluid flowing through a packed bed and other related principles of solid particles-flnid contact operations. I n other words, we may count the flexibility and extensiveness of its scope as well as the relatively low cost and simplicity of the apparatus employed as advantages of this experiment. The author wishes to thank Dr. Henry T. Ward, Head, Department of Chemical Engineering a t Kansas State University, for his encouragement and advice in setting up this project. Literature Cited (1) BROWN,G. G., ET AL., "Unit Operations," John Wiley & Sons, Inc., New York, 1950, pp. 269-73. J. F., "Chemical Engi(2) C o u ~ s o J. ~ , M., AND RICHARDSON, neering Vol. 11," McGraw-Hill Book Co., I n , New York, 1955, pp. 393400, pp. 522-28. (3) LARIAN,M. G., "Fundementals of Chemical Engineering Operations," Prentiee-Hell, Inc., Englewood Cliffs, N. J., 1958, pp. 21&23. (4) LEMLICH, R., J. CAEM.EDUC.,31, 431 (1954). R., Ibid. 34,489 (1954). (5) LEMLICH, (6) LEMLICH, R., J. Eng. Edue., 48, 385 (1958). (7) LEWIS, H. C., "Chemical Engineering Laboratory Prohlems," Georgia Institute of Technology, Atlanta, Georgia,
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r.7.,, nc*
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( 8.) MCCABE. W. L.. AND SMITH. J. C.. "Unit O~erationsof Cbem-
icd ~ngineer&," ~ e ~ r i w - ~Book i i l c;., Ine., New York, 1956, pp. 261"70. S., Chem. Eng. Prog., 48, 89 (1952). (9) ERGUN,
