Ind. Eng. Chem. Res. 1988, 27, 1502-1505
1502
Lagow, R. J.; Margrave, J. L. In Process in Inorganic Chemistry; Lippard, S . J., Ed.; Wiley: New York, 1979; Vol. 26, p 162. Paciorek, K. J. L.; Kratzer, R. H.; Kaufman, J.; Nakahara, J. H. J. Appl. Polym. Sci. 1979, 24, 1397. Roelands, C. J. A. Correlational Aspects of the Viscosity-Temperature-Pressure Relationship of Lubricating Oils; Druk V. R. B.: Groningen, The Netherlands, 1966; p 52. Sianesi, D.; Zamboni, V.; Fontanelli, r.; Binaghi, M. Wear 1971,18, 85. Sianesi, D.; Pasetti, A.; Fontanelli, R.; Bernardi, G. C. Chem. Znd. (Milan) 1973, 55, 208. Snyder, C. E., Jr.; Dolle, R. E. ASLE Trans. 1976, 19, 171. Snyder, C. E., Jr.; Gschwender, L. J.; Tamborski, C. Lubr. Eng. 1981, 37, 344. Snyder, C. E., Jr.; Gschwender, L. J.; Campbell, W. B. Lubr. Eng. 1982, 38, 41. Snyder, C. E., Jr.; Tamborski, C.; Gopal, H.; Svisco, C. A. Lubr. Eng., 1979, 35, 451.
CF2))40CF&F,, 113488-74-7; CF3CF20CF2C(CF3)2CF,OCF2CF,, 114636-27-0; (CF30CF2)4C, 114636-28-1.
Literature Cited Brice, T. J.; Coon, R. I. J. Am. Chem. SOC.1953, 75, 2921. Gumprecht, W. H. ASLE Trans. 1966, 9, 24. Gumprecht, W. H. “The Preparation and Thermal Behavior of Hexafluoropropylene Epoxide Polymers”. Presented at the Fourth International Symposium on Fluorine Chemistry, Estes, Park, CO, July 1967. Jones, W. R., Jr.; Snyder, C. E., Jr. ASLE Trans. 1980, 23, 253. Jones, W. R., Jr.; Johnson, R. L.; Winer, W. 0.; Sanborn, D. M. ASLE Trans. 1975, 18, 249. Jones, W. R., Jr.; Paciorek, K. J.; Ito, T. I.; Kratzer, R. H. Ind. Eng. Chem. Prod. Res. Deu. 1983,22, 166. Jones, W. R., Jr.; Paciorek, K. J.; Harris, D. H.; Smythe, M. E.; Nakahara, J. H.; Kratzer, R. H. Ind. Eng. Chem. Prod. Res. Deu. 1985, 24, 417. Kawa, H. Ph.D. Thesis, Tokyo Institute of Technology, Tokyo, Japan, 1982.
Receiued for review December 5, 1986 Accepted September 25, 1987
Attrition in a Liquid Fluidized Bed Bioreactor Thomas B. Nelsont and J. M. Skaates* Department of Chemistry and Chemical Engineering, Michigan Technological University, Houghton, Michigan 49931
Attrition of porous alumina spheres in a liquid fluidized bed was measured a t three different liquid velocities. Attrition rate varied with superficial fluid velocity in a manner different from that reported for gas fluidized beds. An attempt was made to correlate the attrition rate with the number of particle collisions in the bed. The latter was estimated by a model based on an analogy with the kinetic theory of gases. The model failed to predict the measured increase of attrition rate with superficial liquid velocity. Reasons for the failure of the model are discussed. In liquid fluidized bed bioreactors, the incoming liquid containing the substrate fluidizes porous solid particles having enzymes chemically bonded to the pore walls. In principle this permits continuous operation without expensive steps to recover the enzyme from the substrate. However, attrition of the solid support due to collision of particles in the fluidized bed results in loss of enzyme and in contamination of the product stream with enzyme and solid fines. Attrition rates were measured in a laboratory bioreactor as part of a larger study of enzyme deactivation in fluidized beds. Attrition in gas fluidized beds has been extensively studied. Kunii and Levenspiel (1969) modeled slow attrition by simple zero-order and first-order rate equations: d(d,)/dt = -h d(d,)/dt = -k’d,
gas velocity. Vaux and Fellers (1981) identified 1 2 different sources (locations) of attrition in a typical industrial fluidized bed and developed a laboratory apparatus for testing solids for attrition tendency. It is apparent that most of the conclusions reached in the above studies are not applicable to the liquid fluidized bed bioreactor. Superficial velocities are much lower than in the gas fluidized bed (0.01-0.05 m/s versus 1-5 m/s). Densities of the solid and fluid phases are sufficiently close so that a smooth particulate fluidization occurs rather than the vigorous bubbling fluidization of commercial gas fluidized beds. Jets and cyclones are absent so that only one of the four attrition mechanisms identified by Vaux and Keairns (1980) is present (surface abrasion due to collision of slow-moving particles).
Vaux and Keairns (1980) identified four mechanisms for attrition of solids in a gas fluidized bed: thermal stress, caused by unequal temperatures in particles; chemical stress, caused by structural differences between solid reactants and solid products in a particle; static mechanical stress, caused by weight of overlaying layers on particles at bottom of bed; kinetic stress, slow-moving particles suffer surface abrasion on collision or fast-movingparticles (in cyclones or high-velocityjets) shatter completely. Kono (1981) studied the attrition rate of Mullite particles in three types of fluidized beds (fluidized, spouted, and spouted fluidized) and found that in all cases the attrition rate was proportional to the cube of the effective superficial
Theory
Present address: Department of Chemical Engineering, Michigan State University, East Lansing, MI 48824.
0888-5885/88/2627-1502$01.50/0
An attempt was made to correlate the amount of attrition measured experimentally in a liquid fluidized bed with the number of particle collisions, as predicted from fundamental principles of particulate fluidization. Many workers have drawn analogies between particle-particle encounters in a liquid fluidized bed and the molecular collisions in a gas. Furukawa and Ohmae (1958) proposed that the product of superficial liquid velocity and viscosity characterizes average particle kinetic energy in the same way that temperature determines the molecular kinetic energy of a fluid. Gelperin and Einstein (1971) also drew an analogy between the superficial velocity in a liquid fluidized bed and the temperature of a pure fluid. Carlos and Richardson (1968) photographed the movement of tracer particles in a liquid fluidized bed of identical 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 8, 1988 1503 Table I. Properties of Porous Alumina Spheres surface area 33 m2/g porosity 0.67 helium density 3.72 g/em3 total pare vol 0.550 cm3/g apparent 1.22 g/cm3 median pare diameter 800 A density
glass beads. The root-mean-square particle speed was proportional to the square root of superficial liquid velocity over the range 1.6 5 u/ud 5 3.1, with mean free path of the particles given by
For all three components (axial, radial, tangential) of particle velocity, the velocity distributions followed the Maxwellian distribution for molecular velocities. However, the fluidized bed was anisotropic in that the axial component of particle velocities greatly exceeded radial or tangential Velocities. This arose from the fact that the fluid medium was flowing in the axial direction and had no net radial or tangential motion. Latif and Richardson (1972) studied the same system at higher fluid velocities and found that in the range 2 5 u/umr5 8 average particle speed is proportional to superficial liquid velocity to the first power. Bernard et al. (1981) measured fluid velocities in the interior of a liquid fluidized bed with a laser-Doppler velocimeter and provided the first detailed maps of mean fluid velocities and turbulence levels inside a fluidized bed. The data showed that the fluid turbulence is nearly homogeneous and isotropic, suggesting that the turbulence is generated by the random motion of the solid particles. As the fluid moves up the fluidized bed, it imparts momentum to the solid particles. However, because of the frequent collisions among the particles, their motion becomes randomized, and their subsequent interaction with the surrounding fluid leads to isotropic turbulence. From the kinetic theory of gases (Adamson, 1979), the mean binary collision frequency, F,,, is given by
F,, = 0.5nF1 (3) where n is the total number of molecules and Fl is the frequency of collision of a specific molecule with other molecules. In a particulate fluidized bed, the frequency of collision of a particle with other particles is F, = up/X
(4)
where up is the average particle speed and Xis the particle mean free path, given by eq 2. In a photographic study of tracer particles in a liquid fluidized bed, Latif and Richardson (1972) found that the average particle speed increased linearly with liquid superficial velocity. Their data can be correlated by the equation up = 0.418(u/ud) + 3.556 cm/s (5) Although Latif and Richardson used larger particles (6.2-mm diameter versus 0.85-mm diameter in this study) and a more viscous liquid (dimethyl phthalate), their
Figure 1. Secondary electron photomicrograph of a single alumina pellet showing size, shape, and topography (65X).
particle Reynolds numbers (48-208) are not too far from those reported here (9-28). The range of u/u,r in both studies is identical (2-8). Combination of the above equations yields 4.242n(1 - c)(0.418(u/umf) + 3.556)
FII=
2,
binary collisions/(gs) (6)
where n is now the total number of particles per gram of fluidized solids. Experimental Section The fluidized particles used in this study were 14-28mesh F-100 controlled pore alumina spheres supplied by the Alcoa Technical Center of the Aluminum Company of America. Purified bovine liver catalase was immobilized on the pore walls via an azo linkage, using the method developed by Talbot (1982). Pertinent data supplied by Alcoa for the porous alumina spheres are given in Table I. A size classification of the alumina particles was performed by using an Allen-Bradley sonic sifter, and the results are given in Table 11. Also shown are basic parameters of the fluidized bed, as estimated from standard correlations (Kunii and Levenspiel, 1969). The morphology of the alumina particles was examined by using a JOELCO-U3 scanning electron microscope. Secondary and backscatter electron modes were used to determine surface morphology with respect to size, shape, and topography of macropores. Photomicrographs of representative pellets are shown in Figures 1 and 2. A batch recirculation reactor incorporating a fluidized bed of porous alumina spheres was used in this study (Figure 3). Peristaltic tubing pumps were used in the recirculation loop, and a flow integrator was installed upstream of the reactor to dampen oscillations which might impose a periodic motion on the fluidized particles. The fluidized bed reactor (Figure 4) was a Glenco Series 3500 jacketed liquid chromatography column 1.5 cm in
Table 11. Size Classification of Alumina Spheres diameter range, cm d, cm mass of particles, g no. of particles X 10" 0.059-0.071 0.065 9.09 51.56 0.071-0.084 0.078 14.17 47.83 0.092 12.62 25.28 0.084-0.100 0.409 11.28 13.45 0.100-0.119 surface area mean diameter terminal velocity in water at 25 "C minimum fluidization velocity in water at 25 "C
mass fraction 0.193 0.3CQ 0.268 0.239 dp = 0.084 em ut = 9.52 em/s umr= 0.397 cm/s
no. fraction 0.373 0.346 0.183 0.098
1504 Ind. Eng. Chem. Res., Vol. 27, No. 8,1988
Table 111. Effect of Superficial Liquid Velocity on Attrition Rate and Binary Collision Rate time of fines av attrition u,cm/s u/ud collection, b collected, g rate, g/(g.h) trial 1 1.0 2.52 0.529 48.0 0.0107 0.022 x 10-3 2.0 3.0 1.0 2.0 3.0
trial 2
5.04 7.56 2.52 5.04 7.56
0.647 0.721 0.529 0.647 0.727
16.0 12.6 47.0 18.3 20.0
0.0200 0.0213 0.0054 0.0243 0.0305
binary collision frequency, collisions/(g.h) 1.16 X 10' 1.07 X l@ 0.978 X 10' 1.16 x 109 1.01 x 109 0.978 X 10'
0.125 X 10.' 0.169 X 10.' 0.011 x 10-3 0.133 X 0.153 X
'Void fraction in fluidized bed, calculated from G = (u/ui)O1OO (Richardson and Zaki, 1954).
/ Glass Spheres
Figure 2. Secondary electron photomicrograph of alumina pellet showing surface morphology (6500x1. Ugulpmonl "lagram ,sr
lmmobllilrd
E"ZFW
Figure
4.
Reactor cross section showing details of the calming
section.
Figure 3. Schematic diagram of equipment used to study desctivation of immobilized enzymes in a liquid fluidized bed reactor. diameter and 60 cm in length. The reactor was preceded by an extensive calming section consisting of (1) a section filled with Teflon turnings, (2) a stainless steel disk perforated with 200-wm-diameter holes, (3) a layer of 3-mm glass spheres, and (4) another perforated disk. The reactor was surrounded by a water jacket through which water thermostated at 25 "C was circulated. The reactor feed temperature was maintained at 25 "C hy a thermastatically controlled immersion heater in the substrate reservoir. The average fines production rate in the fluidized bed was determined gravimetrically. To collect the catalyst fines, the reactor effluent was passed through a preweighed filter (Gelman A/E) that removed 99.98% of the particles larger than 0.3 pm. Fines collection continued until the pressure drop across the filter became excessive, as indicated by the torque required to turn the pump. At the end of the collection period, the filter was reweighed, and the average attrition rate was calculated as grams of fines collected attrition rate = howgrams of fluidized particles
:q
,
,.
