A Simple but Effective Fluidized-Bed Experiment

(••3 ..blllliii3L/a=b..:o:..:.r..=a:.:.t=o.:..:ry~ ______) A SIMPLE BUT EFFECTIVE FLUIDIZED-BED EXPERIMENT

CONAN J. FEE University of Waikato'" EXPERIMENTAL PREPARATION Hamilton, New Zealand Students are given a brief introduction to the concept of luidization is an aspect of chemical engineering not fluidization and it's use as a processing tool , including rea­ usually covered in depth at the undergraduate level, sons why one would want to contact a fluid with a solid in but engineers are as likely to meet with fluidization the first place. This section includes a qualitative description F of the desirability on the one hand of using small particles during their careers as they are some other, more extensively taught, unit operations. It has applications ranging from chro­ that have a high surface-area-to-volume ratio, and, on the matography and fermentation through filtration, drying, and other hand, the need to keep the pressure drop through such a catalysis and is likely to be encountered by non-engineering bed of particles low. professionals such as chemists and biologists. A comparison with a packed bed is made to highlight some advantages of the fluidized bed in terms of lower The laboratory experiment described in this paper was pressure drops, uniformity of temperature, a greater toler­ designed as part of a course in process technology for chem­ ance for solids in the feedstream, etc. The concepts of fric­ istry and biology majors. Often, such courses (intended to tional drag around a sphere and terminal settling velocities introduce "engineering principles" to science students) are are also qualitatively outlined, together with the notion given toward the end of the student's schooling, and it is that bed washout can be avoided. (The students have, by difficult to pitch material at an appropriate level to students this stage, had several lectures on fluid dynamics, so who are senior but who have little background in engineer­ the concepts of boundary layers and viscous drag are ing. For obvious reasons, the mathematical demands of this not new to them.) practical session are low, but it sti ll demonstrates basic engi­ neering principles well and provides an effective, interactive The students are given only the following four hypotheses learning experience for the student. The session is loosely to test, with the task of describing and explaining observed structured, giving students an opportunity to test out their bed behavior, especially noting any differences between the investigative ski ll s-but not so loose as to allow them to two beds provided. wander off-track. This approach avoids boring senior stu­ Hypothesis I: That at flowrates equal to and above the dents without demanding an unreasonably high level of en­ point of minimum (incipient) fluidization the bed behaves as gineering knowledge. a "liquid" with a density between the fluid and the solid The laboratory would suit first- or second-year chemical densities. engineering students and may be easily modified to include more sophisticated concepts for advanced engineering students. Hypothesis 2: That the pressure drop in the bed is a function of the superficial velocity, u, (m s· 1) , of the bed, where -.· . --~ Conan J. Fee received his BE in 1984 and his PhD in 1989 from the University of Canterbury u =_g_ (I) (New Zealand) and was a postdoctoral fellow s A c at the University of Waterloo (Canada) during . . . ·.•·~ ,. ~-- -~ 1989-90. He currently holds a joint appoint­ where Q is the volumetric flowrate of the fluidizing medium ' ment as a lecturer at the University of Waikato 3 1 (m s· ) and Ac is the cross-sectional area of the column and as a biochemical engineer at the Meat 2 ·~i/ Industry Research Institute of New Zealand. (m ). . ·. .Water exits the water column via a drain tube located on the side of the column, above the bed. Air is where vented directly from the top of the air column. 6P pressure drop in the bed (Pa) Adequate flowrate ranges are Oto 2.5 x 10·4m3s· 1 for the W weight of paiticles (N) air-fluidized bed, and Oto 5 x 10·5m3s· 1 for the water-fluid­ Lmr height of the bed at the point of minimum fluidization (m) ized bed. The cost of the apparatus was less than NZ$2000 Emr voidage at the point of minimum fluidization (-) 3 (US$l000). p, solid density (kg m· ) 3 Pr the fluid density (kg m· ) 2 RESULTS g gravity (m s· ) The students were enthusiastic, and they all remarked positively on the fluidization laboratory when they were Hypothesis 4: That above the point of minimum fluidiza­ asked in course appraisals to recall their most enjoyable tion the bed will expand (i.e., it's height will increase) with practical session. increasing fluid velocity. The apparatus lends itself well to demonstration. It is simple to operate, repeatable results are easy to obtain, and The amount of assistance given to the students regarding there is no setting-up time required before each session. methodology can easily be adjusted. Some will need little, if There is no deadtime or time spent waiting for steady state, any, help, while others will require a little more prompting. and measurements can be repeated literaJly in seconds. Also, For instance, some students can figure out on their own even the most hapless students would find it difficult to harm how to measure Em6 others might only need the hint to use either themselves or the equipment. a displacement method usi ng a measuring cylinder, while The magical moment when the block of wood floats in some may need the methodology spelled out for them in a bed of solids never fails to fascinate the students. Even greater detail. Engineering students ought to cope with the "coolest" student in the class cannot repress a childish developing the force balance expression given under Hy­ amusement and the urge to repeatedly push the block of pothesis 3 by themselves. wood into the bed and watch it bob back to the surface and float around. If the columns are mounted so they can be APPARATUS tilted, the liquid-like flow properties of the fluidized par­ Students are provided with two plexiglass columns (see ticles can be easily shown. Figure 1). The columns can be any size, but I wanted them to be large enough for students to be able to see what was going (Water Drain Tube in Water-Fluidized Bed) on internally. The columns should be large enough to allow the students to float or sink a few objects (such as a block of wood) in the (fluidized) bed. The columns used in my class Air Out are 0.45 m in height, with an inside diameter of 0.074 m. The first column, fluidized by air, contains 0.163-mm diameter glass beads to a height of 0.38 m (at rest), while the E second column, fluidized by water, contains 1-mm glass E beads to a height of 0.26 m. Each column has a plexiglass ..,.f6 support plate with 44 holes (3-mm diameter) and a fluid distributor plate, used also to retain the bed particles, made from 85-µm and 750-µm aperture sieve material for the air­ Air-Water and water-fluidized beds respectively. Each column has a Manometer valve and rotameter for setting and measuring volumetric flowrates and a manometer that allows measurement of the Figure 1. The air-fluidized bed apparatus. A similar pressure drop between two pressure probes (Pitot tubes) column is used for water-fluidization Summer 1994 215 The following experimental results for the air-fluidized system are typical. Those for the where E is the bed voidage and water-fluidized column are qualitatively similar. is the sphericity of the particles. For Hypothesis 1, the more mathematically literate students can derive an expression for The increase in bed voidage due the effective density of the bed at minjmum fluidization with little difficulty. Less quantita­ to the expansion of the bed with tively inclined students are satisfied with demonstrating that a block of wood with a density increasing superficial velocity between that of air and glass floats in the fluidized bed. above minimum fluidization can be calculated from measurements A plot (Figure 2) of total bed-pressure drop versus superficial velocity is sufficient to of bed height. If this is taken show the existence of a relationship, as predicted by Hypothesis 2. The pressure drop in an into account, Eq. (3) predicts equivalent packed bed can be calculated using the well-known Ergun equation[ 1l pressure-drop values which ini­ t.P tially are close to the experimen­ (3) L tal values, but which become higher than the experimental val­ ues as superficial velocity is in­ creased, as might be expected for

Bed Pressure Drop vs Superficial Velocity unrestrained particles. The slope of the pressure drop versus su­ 9000 perficial velocity predicted by Eq. Mlnlmum Fluidlzadon (3) changes at the point of mini­ 8000 --- mum fluidization. 7000

~ 6000 Figure 2 also illustrates that

~ 0 0 0 once the particles are fluidjzed, c 5000 0 0 0 e the frictional losses do not in­ i 4000 crease significantly with in­ ..,ii: i 3000 creased superficial velocities, in o Exparimef10il Values contrast to the behaviour of 2000 - - - - - Equation (3) packed beds. This is explained 1000 partly by the increased bed 0 voidage and also by the fact that 0 0.005 o.o, 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 the excess air flow (above that Suporflclal Voloclty {m/sJ required for fluidization) moves through the bed largely as Figure 2. Total bed pressure drop versus superficial velocity in the air-fluidized bed. bubbles, as can be seen through the column walls. The bubbles bypass the solids and therefore Bed Height vs Superficial Velocity do not contribute significantly to

0.45 the frictional losses, similar to the case of air bubbling through wa­ ter in which the pressure required 0.43 for air injection depends mainly on the static pressure head and

0 not on the flowrate. o. 1 Minimum Fluldlzatlon I • 0 l: Cl .; 0 To test Hypothesis 3, Emf must J:: 0 0 be known. The void fraction for i 0.39 0 non-fluidized beads, measured by 0 0 0 0 0 0 water displacement, is about 0.35 0.37 for both columns. At mjnimum fluidization, bed expansion over Emf 0.35 the rest state is negligible, so

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 is also 0.35. The density of the Superficial Velocity (mis] glass is given by the supplier as 3 2.09 g cm- . The pressure drop at Figure 3. Bed height versus superficial velocity in the air-fluidized bed. minimum fluidization is 5120 Pa, 216 Chemical Engineering Education giving a value for the left-hand side of Eq. (2) of qualitati ve fashion. Students have 2 found the session interesting and (5120 Pa)(0.0043 m ) = 22.0 enjoyable, and they relate well to compared to a right-hand side value of the engineering principles in­ 2 3 3 2 (0.0043 m )(0.380 m)( 1 - 0.36)(2090 kg m· - 1.2 kg m" )(9.8 1 m s· ) = 2 1.4 N volved, such as fluid-solid con­ tacting, pressure drop and fric­ The resultant force balance of within 3% is pretty good! tional losses, etc. In particular, the Hypothesis 4 is easily tested, as shown in Figure 3-but it requires some conceptual session is designed to stimulate thought by the students to explain the observed behavior, as follows. The frictional and hold the interest of relatively drag on a particle is a function of the relative velocity between the particle and the senior undergraduate students fluid moving through the void space surrounding it. At minimum fluidization the drag from outside of engineering, is just sufficient to support the weight of the particles, as proposed in Hypothesis 3. As whose limited quantitative back­ the flowrate is increased above mjnimum fluidization, the particles in the bed begin to grounds often constrain their en­ lift due to the extra drag force generated by the greater fluid velocity in the available gineering practical sessions to void space. But the void space correspondingly increases until the fluid velocity, more mundane topics. relative to the particle, reduces to the level at which the drag force again just balances the particle's weight. Hence the bed expands, but the particles are not washed out with NOMENCLATURE the fluid, and the explanation for this is consistent with the observation that the Ac cross-sectional area of bed, m2 pressure drop (frictional loss) in the bed is relatively constant above minimum fluidi­ dP particle diameter, m zation (at least within the limits of thi s experiment). g acceleration due to gravity, m -2 The superficial velocity at the point of minimum fluidization, umr, can be determjned s from either Figure 2 or 3, and in thi s case it is about 0.02 m s·'. The experimen­ L bed height, m tally determined value of Umr can then be compared to that calculated from packed Lmr bed height at minimum bed considerations by combirnng Eqs. (2) and (3), e1uating u5 with umr, and using fluidization, m 21 the voidage and bed height at mjnimum fluidization. The following quadratic in t.P axial pressure drop in bed, Pa Um f is obtained: Q volumetric fl owrate, m3 s·1 u, superfi cial velocity, m s" 1 umr superficial velocity at minimum fluidization, m s·' W weight of bed material, N Solving Eq. (4) for u r, using the data from the air-fluidized bed, yields a value for Umf 111 Greek Symbols of 0.02 m s· '. Given that Eq. (3) is expected to represent data only to within ±25%, this £ bed voidage result is in surprisingly good agreement with the values found from Figures 2 and 3. Emr bed voidage at minimum There are numerous points for discussion and they can be stimulated by rierusing fluidization 1 any standard fluid ization text, such as the one by Kunii and Levenspiet.l A few particle sphericity possibilities are: µ fluid viscosity, kg m·1 s·1 • Comparisons between the gas- and liquid-fluidized beds, such as bubbling and p, density of solid particles, kg slugging in the former, can be highlighted and the consequences for scale-up m -3 pointed out. Pr density of fluid, kg m·3 • How does bubbling affect fluid-solid contact? • Why are particles not washed out of the bed at superficial velocities immediately ACKNOWLEDGMENTS above m.inimwnfluidization, and does this have anything to do with bed expansion? I would like to thank Ms. Jijian • Why not place the pitot tubes at the very bottom or top of the bed? Lu for her technical assistance in • Would the fluid distributor at the base affect fludization ? developing the fluidized bed appa­ • What would heat transfer be like in the bed? ratus.

CONCLUSIONS REFERENCES We have developed a low-cost fluidized bed laboratory experiment, using apparatus 1. Ergun, Chem. Eng. Prog., 48, 89 that is safe and simple to operate and which requires no running costs. The level of (1952) experimental or theoretical complexity of the laboratory session can easil y be adjusted 2. Kunii, D. , and 0. Levenspiel,Flu­ to suit the backgrounds and capabilities of the students involved. idization Engineering, 2nd ed., Butterworth-Heinemann, Bos­ The experimental results can be presented and analyzed in both quantitative and ton, MA (1991) 0

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