Experimental Study on the Local Hydrodynamic Behavior of a Three

Bubble coalescence prevails near the distributor, while bubble breakup is more important in the fully developed flow. The liquid ... of dynamic charac...
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Ind. Eng. Chem. Res. 2004, 43, 5432-5437

Experimental Study on the Local Hydrodynamic Behavior of a Three-Phase External-Loop Airlift Reactor Jing Lin, Minghan Han, Tiefeng Wang, Tongwang Zhang, Jinfu Wang,* and Yong Jin Department of Chemical Engineering, Tsinghua University, Beijing 100084, China

Detailed local three-phase hydrodynamic parameters, namely, the liquid velocity, bubble rise velocity, and gas holdup, were obtained for an external-loop airlift reactor using a fiber-optic probe and laser Doppler anemometry techniques. The radial and axial evolutions of these parameters and the effect of the solid fraction on the flow hydrodynamics were studied. The experiments showed that the bubble size has a large influence on the evolutions of these parameters because of radial forces acting on the bubbles. Bubble coalescence prevails near the distributor, while bubble breakup is more important in the fully developed flow. The liquid velocity and gas holdup decrease and the bubble rise velocity increases as the solid fraction increases. 1. Introduction External-loop airlift reactors (EL-ALRs) can be used in bioprocesses, wastewater treatment, and the chemical industry and have become increasingly popular in recent decades. Their advantages can be summarized as follows: simple construction without internals or moving parts and good heat- and mass-transfer capacity and good mixing properties with low energy consumption as the gas phase in the reactor serves the dual functions of aeration and agitation. Although much work had been carried out on the study of EL-ALRs, the proper design and scale-up of an EL-ALR remain difficult because any variation in the gas or liquid physical properties and the reactor structural feathers may have considerable effect on the hydrodynamics of an EL-ALR.1 Furthermore, the complexity increases with the presence of solid particles as a third phase. Work had been done to investigate the influence of the cross-sectional ratio of downcomer to riser,2,3 reactor height,1,4 gas-liquid separator configuration,5 and distributor type and location,6 all of which affect the flow characteristics considerably. However, most of the previous work on EL-ALRs, including those focusing on hydrodynamics, had only studied the global parameters, such as liquid circulation7-10 and average gas holdup in the riser.11-13 The hydrodynamics of airlift reactors can be treated on the basis of simple one-dimensional models based on the energy or momentum balance.14-17 However, they rely on empirical inputs, e.g., friction factors, which may change a lot with the reactor structure and the physical properties of the system. More important is that the details of the flow at the top and bottom of the reactor cannot be completely resolved. As a result, the initial stages of contacting and the position of the gas sparger and the phase separation zone cannot be fully incorporated into the design. It is recognized that computational fluid dynamics (CFD) might offer a way out of these drawbacks because it has the potential to assess the local, instantaneous flow phenomena in much more * To whom correspondence should be addressed. Tel.: +8610-62785464. Fax: +86-10-62772051. E-mail: [email protected].

detail. Presently, however, CFD is not a technique that can be used routinely in bubbly flows. Many questions remain unanswered, in particular on the modeling of phase interaction and turbulence in gas-liquid bubbly flows. To solve these problems, detailed information from experiments is required, not only for gaining insights into the physics of the flow but also for validating the CFD simulations. Unfortunately, detailed local information on the hydrodynamics of the airlift reactor is rather scarce in the literature. Utiger et al., Vial et al., and Wang et al. are among the few who reported experimental descriptions of the radial evolution of local gas holdup.18-20 However, data on the local parameters in a three-phase EL-ALR system were not given because of the lack of an appropriate experimental method. Hot-film anemometry, which was employed by Utiger et al.18 and Young et al.,14 is an accurate method, but the probe is fragile and so it is only used in gas-liquid two-phase flows. Ultrasonic Doppler velocimetry, which was used by Wang and co-workers,20 can only be used in liquid-solid two-phase flows or a three-phase flow with a low gas holdup because of the difficulties of signal discrimination at high gas holdup.21 Another method commonly used in multiphase flows is particle image velocimetry, but this can only give information in the region close to the wall because of the limited transparency of the system. In this work, we used modified laser Doppler anemometry (LDA) to measure the liquid velocity in a three-phase EL-ALR. Besides the radial evolution of flow parameters, the axial evolution of flow parameters is important as well. However, few resarchers investigated the axial evolution of local hydrodynamic parameters. Among them is Young et al.,14 but they only gave results at two different axial positions and claimed that the axial dependence is modest and most of the axial dependence is attributable to hydrostatic pressure effects. Up to now, there are few models that can give threedimensional predictions for airlift reactors. To develop useful CFD models for three-phase EL-ALRs and design these types of reactors properly, the local parameters of the gas phase and the local liquid velocity are measured in this work by a fiber-optic probe and the

10.1021/ie0304614 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/22/2003

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Figure 2. Calibration of LDA by an ultrasonic Doppler velocimeter.

Figure 1. Schematic representation of the experimental airlift reactor.

LDA technique, respectively. The experiments give radial and axial evolution of the local hydrodynamic parameters, which are helpful for the CFD modeling and reactor design. 2. Experimental Section The schematic diagram of the experimental setup is shown in Figure 1. The riser and downcomer have inner diameters of 0.230 and 0.190 m, respectively, and 4.8 m height. The height and diameter of the top section are 0.960 and 0.480 m, respectively, with a contraction that connects to a bend of 0.190 m diameter. The bottom section has the same diameter as the downcomer. The downcomer flow joins the riser 0.2 m above the distributor. Air was used as the gas phase and was introduced into the system through a distributor with holes of diameter 1 mm and a holed ratio of 0.25%. The superficial gas velocities, based on the riser crosssectional area, varied from 0.008 to 0.032 m/s. Water and glass beads with a diameter of 100 µm and a density of 2400 kg/m3 were used as the liquid and solid phases, respectively. The fiber-optic probe was fixed in the wall of the riser and is movable in the radial direction to measure the radial profiles of the gas holdup, bubble size, and bubble rise velocity. LDA was used to measure the local liquid velocity in the riser with a probe that is movable in the radial direction. Experiments were carried out at four different axial positions in the riser at 0.8, 1.6, 3.2, and 4.0 m above the gas distributor. 3. Measuring Techniques 3.1. Fiber-Optic Probe. Gas holdup, liquid velocity, bubble size, and bubble rise velocity are important parameters in a three-phase airlift. In this study, the local gas holdup and bubble behaviors were measured by a reflective fiber-optic probe developed by Wang and co-workers.22,23 The probe consists of two parallel optical

fibers with a diameter of 62.5 µm. Light reflection occurs at the tip of the optical fibers. The intensity of the reflected light is different when the probe fiber is in the liquid and in a gas bubble. The detected signal is higher when the fiber is in the gas phase. Because of the distance between the two fibers, the output signal from the downstream fiber is a little delayed compared to that from the upstream one. The delayed time can be determined by means of the cross-correlation method, and the bubble rise velocity can then be calculated by dividing the distance between the two optical fibers by the time delay. The bubble chord length is calculated by multiplying the bubble signal duration time and the bubble rise velocity. The bubble chord length distribution is obtained through statistical processing, which is transformed to get the bubble size distribution.22 The bubble frequency, local gas holdup, distribution of the bubble size, and bubble rise velocity are obtained by processing the output signal through algorithms for signal identification, cross correlation, and distribution transformation. 3.2. LDA. A backward scattering LDA system (system 9100-8, model of TSI) was used to measure the local liquid velocity in the riser. LDA is a standard technique in single-phase flows. However, its application in bubbly flows is more complicated. The reason is the reduced transparency of the bubbly system due to the presence of dispersed bubbles that act as scatterers of the laser beams. As a result, for larger equipment and moderate bubble fractions, the forward scatter mode cannot be employed. Hence, one is restricted to the backscatter mode with its low data rate. Mudde et al. take the view that LDA does not see the bubbles and measures only the local liquid velocity in the backward mode when the bubble diameter is larger than 1 mm.24 However, they did not get a sufficiently high data rate in the center because of the decreased probability that the laser beams can reach the region of measurement: the deeper into the column, the more likely it is that the light path is blocked by bubbles. We have modified the LDA probe to make it applicable to a three-phase system and obtained good results. A detailed description of the LDA modification can be found elsewhere.25 We have also calibrated LDA in a liquid-solid reactor with an ultrasonic velocimeter Dop2000 (model 2030, Singnal Processing SA). The results are presented in Figure 2. Errors are within (3%. The slip velocity between liquid and glass beads in our experiment is less than 0.01 m/s.

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Figure 3. Liquid velocity profiles at a solid content of 4 wt % and axial position h ) 3.8 m.

Figure 4. Turbulence intensity profiles at a solid content of 4 wt % and axial position h ) 3.8 m.

Therefore, the measured particle velocity is considered to be the liquid velocity. 4. Results and Discussion 4.1. Radial Profiles of the Hydrodynamic Parameters. Some investigations have been done concerning the radial gas profiles in vertical pipe flow.25-29 In some cases, the wall peak distribution was observed; in other cases, a peak in the pipe center was found. The gas profiles are the result of the nondrag forces, acting perpendicularly to the flow direction, which are caused by the liquid shear flow in the pipe. Zun derived a lift force,30 which acts in the wall direction. Tomiyama has proposed an extended definition of the lift force,31 changing its sign depending on the bubble diameter. In the present study, we proved that the bubble size is the main factor that determines the profiles of some important hydrodynamic parameters, such as the gas holdup, liquid velocity, and bubble velocity. The radial profiles of the liquid velocity in the riser at different gas superficial velocities are presented in Figure 3 with the data from Young et al.14 and a singlephase system.32 The liquid velocity has a radial profile with a higher value in the central region. Because the gas holdup has a larger value in the central region than near the wall and the rising bubbles carry liquid behind them in the trailing wakes, the local liquid velocity in the center of the column is greater in two-phase flow than in single-phase flow. Hence, for identical superficial liquid velocities, the radial profiles of the liquid velocity in gas-liquid two-phase flow will be more parabolic than those in single-phase flow. With an increase in the gas velocity, the liquid velocity increases in the riser, and the increase at the center of the riser is slightly more than that in the near-wall region. However, Young et al. observed liquid velocity profiles that become more parabolic with increasing superficial gas velocity.14 Liquid turbulence levels are shown in Figure 4 with the data from Utiger et al.18 and a single-phase system,32 which expresses the turbulence intensity as a percent of the mean local velocity. The relative turbulence intensity is found to be parabolic and nearly independent of the gas flow rate. The range of turbulence in the two-phase environment is substantially larger than that in the single-phase environment because of bubble-induced turbulence; e.g., centerline values are 30-35%, while it is around 10% in a singlephase system. The turbulence intensity is a little less than that reported by Utiger et al.18

Figure 5. Bubble velocity profiles at a solid content of 4 wt % and axial position h ) 3.8 m.

The bubble rise velocities are shown in Figure 5 along with the data from Young et al. and Utiger et al.14,18 The profile shape in Figure 5 is similar to that of the liquid velocity. Utiger et al. undertook similar studies and reported similar results in a two-phase system,18 while Young et al.’s profiles of bubble velocity are more parabolic14 than ours. As can been seen in Figures 3-5, the data obtained by Utiger et al.18 is more similar to ours than those by Young et al.14 We attribute the difference to the different gas distributors. Young et al.’s distributor produced larger bubbles and therefore more parabolic profiles of bubble velocity and liquid velocity. System viscosity has a smaller influence on hydrodynamics than bubble size does, and second-order flow properties, such as turbulent intensity, are a better indicator of the influence of system properties on hydrodynamics than first-order flow properties, such as bubble and liquid velocities. Thus, we use turbulent intensity to show the difference between the data of Utiger et al. and us. The bubble slip velocities are determined from the measured liquid and bubble rise velocities, and the results are shown in Figure 6. It can be seen that the slip velocity profiles are much flatter than the liquid or bubble velocity distributions. The hydrodynamic behavior of an individual bubble in a gas-liquid system generally differs from that of a single isolated bubble because of interactions with its neighboring bubbles.33 In the case of bubbles rising in line, the wake of the leading bubble is found to be a primary factor in the interaction between the trailing and leading bubbles,

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Figure 6. Slip velocity at a solid content of 4 wt % and axial position h ) 3.8 m.

Figure 7. Gas holdup profiles at a solid content of 4 wt % and axial position h ) 3.8 m.

which can significantly change the bubble rise velocity. Because bubbles attract each other to form clusters the

slip velocity is a little larger than the velocity of a single bubble rising in a stagnant liquid (0.20 m/s), which is used in most CFD simulation of gas-liquid flows. From Figure 7, it can be seen that the gas holdup increases with the superficial gas velocity. As can be seen, the gas holdup profiles are relatively flat at the lower superficial gas velocities and become more parabolic when the gas superficial velocity increases, with a more obvious change in the central region. This trend is different from that obtained byWang et al.20 The main reason is due to different bubble sizes. Wang et al. reported that the radial profiles of the gas holdup have slight peaks at the wall with bubbles smaller than 3.2 mm and the gas holdup is peaked at the core with bubbles larger than 4 mm.20 The bubble size in our experiment is about 5 mm (the method to measure bubble size is the same as that used by Wang et al.), and the gas holdup has radial profiles that peak at the core. 4.2. Axial Evolution of the Hydrodynamic Parameters. The axial evolutions of the bubble rise and liquid velocities are shown in Figures 8 and 9. The radial profiles of the bubble rise and liquid velocities are quite uniform at low axial positions because of a more uniform gas holdup distribution from the perforated plate and become more parabolic when flow increases along the axial direction. According to the analysis of forces acting on bubbles by Tomiyama,31 large bubbles tend to migrate to the central region. As a consequence, the bubble rise velocity is larger in the centerline because of the larger bubble population there. The gas holdup profiles in Figure 10 show that gas concentrates in the central region as flow develops. It is interesting that the bubble velocity increases between the heights 1.0 and 1.7 m, while gas is still accelerating in the liquid, especially at higher gas flow

Figure 8. Liquid velocity evolution along the axial direction: (left) Ug ) 0.016 m/s and s ) 8%; (right) Ug ) 0.032 m/s and s ) 8%.

Figure 9. Bubble velocity evolution along the axial direction: (left) Ug ) 0.016 m/s and s ) 8%; (right) Ug ) 0.032 m/s and s ) 8%.

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Figure 10. Gas holdup evolution along the axial direction: (left) Ug ) 0.016 m/s and s ) 8%; (right) Ug ) 0.032 m/s and s ) 8%.

Figure 11. Influence of solid loading on the liquid velocity.

rates. We attribute this phenomenon to bubble coalescence; namely, although the small holes in the distributor produce small bubbles, these are not stable during flow in the pipe and will not exist for long. Small bubbles approach each other and coalesce. At the same time, bubbles migrate toward the centerline and increase the bubble density there. Therefore, the bubble velocity profiles become more and more nonuniform as the flow proceeds along the riser. However, the large bubbles formed in the first stage are still not stable. They break up gradually under the agitation of turbulence. That is why we see a decrease in the bubble velocity in the upper half of the riser. The liquid velocity increases mainly because of the acceleratingeffect of the bubbles in the developing flow regime, while it gets higher mainly because of the reduced size of the bubbles, which leads to an increase in gas holdup. The tendency at a lower gas flow rate is similar except that the breakup effect is less evident than that at the higher gas velocity because of weaker turbulence. 4.3. Influence of the Solid Fraction. The influence of the solid fraction on the radial profiles of the liquid and bubble rise velocities is shown in Figures 11 and 12. The increase in the solid fraction causes an increase in the apparent viscosity of the liquid-solid suspension, which, in turn, increases the flow friction and consequently decreases the liquid velocity because of less driving force for liquid circulation. Moreover, an increase in the viscosity is also caused by an increase in the bubble diameter, which, in turn, increases the bubble rise velocity, especially in the low superficial gas velocity regime. The effect of the solid fraction on gas holdup is shown in Figure 13. The introduction of solids into the system results in a decrease in the riser gas holdup. This is due

Figure 12. Influence of solid loading on the bubble velocity.

Figure 13. Influence of solid loading on the gas holdup.

to increasing reduction of the flow areas of the gas and liquid phases and the consequent increase of coalescence. Similar results were obtained by many researchers.34-36 At the same time, the driving force decreases as a result of reduced gas holdup in the riser, namely, the density difference between the riser and downcomer, which causes the liquid circulation velocity to become smaller. 5. Conclusions A detailed description of the local hydrodynamic parameters of both gas and liquid phases has been obtained in the riser of an EL-ALR. This will give a better understanding of the flow in the riser and can be used for CFD simulation and validation. The experimental results show that large bubbles tend to gather in the center region of the reactor, which leads to stronger interactions between the bubbles and results

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in a higher bubble velocity in the central region. Bubble coalescence prevails near the distributor, while bubble breakup is the more important phenomenon in the fully developed reactor part. Along the axial direction, the bubble rise velocity first increases because of the formation of large bubbles and then decreases because of bubble breakup under the agitation of liquid turbulence. The apparent viscosity of the solid-liquid suspension increases with an increase of the solid fraction, results in a decrease of the gas holdup and liquid circulation velocity, and also leads to an increase of the bubble size and bubble rise velocity. Acknowledgment The authors gratefully acknowledge the financial support by the Chinese National Science Foundation (No. 20276035). Nomenclature h ) distance between probe holes and the distributor, m r ) radius, m R ) radius of the pipe, m ub ) bubble rise velocity, m/s Ug ) superficial gas velocity, m/s ul ) local liquid velocity, m/s Greek Letters g ) gas holdup s ) solid fraction

Literature Cited (1) Bentifraouine, C.; Xuereb, C.; Riba, J.-P. An Experimental Study of the Hydrodynamic Characteristics of External Loop Airlift Contactors. J. Chem. Technol. Biotechnol. 1997, 69, 345. (2) Bello, R. A. A Characterization Study of Airlift Contactors for Applications to Fermentations. Ph.D. Dissertation, University of Waterloo, Ontario, Canada, 1981. (3) Choi, K. H.; Lee, W. K. Circulation Liquid Velocity, Gas Holdup, Volumetric Oxygen Transfer Coefficient in External-Loop Airlift Reactors. J. Chem. Technol. Biotechnol. 1993, 56, 51. (4) Russell, A. B.; Thomas, C. R.; Lilly, M. D. The Influence of Vessel Height and Top-section Size on the Hydrodynamic Characteristics of Airlift Fermentors. Biotechnol. Bioeng. 1994, 43, 69. (5) Siegel, M. H.; Merchuk, J. C. Hydrodynamics in Rectangular Airlift Reactors: Scale-up and the Influence of Gas-Liquid Separator Design. Can. J. Chem. Eng. 1991, 69, 465 (6) Becker, S.; Sokolichin, A.; Eigenberger, G. Gas-Liquid Flow in Bubble Column and Loop Reactors: Part II. Comparasion of Detailed Experiments and Flow Simulations. Chem. Eng. Sci. 1994, 49, 5747. (7) Chisti, M. Y.; Hallard, B.; Moo-Young, M. Liquid Circulation in Airlift Reactors. Chem. Eng. Sci. 1988, 43, 451. (8) Jones, A. G. Liquid Circulation in a Draft Tube Bubble Column. Chem. Eng. Sci. 1985, 40, 449. (9) Hsu, Y. C.; Dudukovic, M. P. Gas Holdup and Liquid Recirculation in Gas-Lift Reactors. Chem. Eng. Sci. 1980, 35, 134. (10) Gavrilescu, M.; Tudose, R. Z. Study of the Liquid Velocity in External-Loop Airlift Reactor. Bioprocess. Eng. 1995, 14, 183. (11) Hills, J. H. The Operation of a Bubble Column at High Throughputs. I. Gas Holdup Measurement. Chem. Eng. J. 1976, 12, 89. (12) Siegel, M. H.; Merchuk, J. C.; Schugerl, K. Airlift Reactor Analysis: Interrelationships between Riser, Downcomer and GasLiquid Behavior, Including Gas Recirculation Effects. AIChE J. 1986, 32, 1585. (13) Joshi, J. B.; Ranade, V. V.; Gharat, S. D.; Lele, S. S. Sparged Loop Reactors. Can. J. Chem. Eng. 1990, 68, 705. (14) Young, M. A.; Carbonell, R.; Ollis, D. F. Airlift Bioreactors: Analysis of Local Two-Phase Flow Hydrodynamics. AIChE J. 1991, 37, 403.

(15) Glennon, B.; Al-Masry, W.; MacLoughlin, P. F.; Malone, D. M. Hydrodynamic Modeling in an Air-Lift Reactor. Chem. Eng. Commun. 1993, 121, 181. (16) Sa´ez, A. E.; Marquez, M. A.; Roberts, G. W.; Carbonell, R. G. Hydrodynamic Model for Gas-Lift Reactors. AIChE J. 1998, 44, 1413. (17) See, K. H.; Roberts, G.; Sa´ez, A. E. Effect of Drag and Frictional Losses on the Hydrodynamics of Gas-Lift Reactors. AIChE J. 1999, 45, 2467. (18) Utiger, M.; Stuber, F.; Duquenne, A.-M.; Delmas, H.; Guy, C. Local Measurements for the Study of External Loop Airlift Hydrodynamics. Can. J. Chem. Eng. 1999, 77, 375. (19) Vial, C.; Poncin, S.; Wild, G.; Midoux, N. Experimental and Theoretical Analysis of the Hydrodynamics in the Riser of an External Loop Airlift Reactor. Chem. Eng. Sci. 2002, 57, 4745. (20) Wang, T. F.; Wang, J. F.; Zhao, B.; Ren, F.; Jin, Y. Local Hydrodynamics in External Loop Airlift Slurry Reactor with and without Resistance-Regulating Element. Chem. Eng. Commun. 2003, in press. (21) Wang, T. F.; Wang, J. F.; Ren, F.; Jin, Y. Application of Doppler Ultrasound Velocimetry in Multiphase Flow. Chem. Eng. J. 2002, 92, 111. (22) Wang, T. F.; Wang, J. F.; Yang, W. G.; Jin, Y. Bubble Behavior in Gas-Liquid-Solid Three-Phase Circulating Fluidized Beds. Chem. Eng. J. 2003, In press. (23) Wang, T. F.; Wang, J. F.; Yang, W. G.; Jin, Y. Experimental Study on Gas-Holdup and Gas-Liquid Interfacial Area in TPCFBs. Chem. Eng. Commun. 2001, 187, 251. (24) Mudde, R. F.; Groen, J. S.; van den Aker, H. E. A. Liquid Velocity Field in a Bubble Column: LDA Experiments. Chem. Eng. Sci. 1997, 52, 4217. (25) Wei, F.; Lin, H. F.; Cheng, Y.; Wang, Z. W.; Jin, Y. Profiles of Particle Velocity and Solids Fraction in a High-Density Riser. Powder Technol. 1998, 100, 183. (26) Hibiki, T.; Ishii, M. Experimental Study on Interfacial Area Transport in Bubbly Two-Phase Flows. Int. J. Heat Mass Transfer 1990, 42, 3019. (27) Kataoka, I.; Serizawa, A. Interfacial Area Concentration in Bubbly Flow. Nucl. Eng. Des. 1990, 120, 163. (28) Krepper, E.; Prasser, H.-M. Measurements and CFXSimulations of a Bubbly Flow in a Vertical Pipe. CFX International Users Conference, Friedrichshafen, Germany, 1999. (29) Bertodano, M. L.; Lahey, R. T., Jr.; Jones, O. C. Phase Distribution in Bubbly Two-Phase Flow in Vertical Ducts. Int. J. Multiphase Flow 1994, 20, 805. (30) Zun, I. The Transverse Migration of Bubbles Influeced by Walls in Vertical Bubbly Flow. Int. J. Multiphase Flow 1980, 6, 583. (31) Tomiyama, A. Struggle with Computational Bubble Dynamics. Third International Conference on Multiphase Flow, ICMF ’98, Lyon, France, Jun 1998. (32) Hibiki, T.; Ishii, M. Experimental Study on Interfacial Area Transport in Bubbly Two-Phase Flows. Int. J. Heat Mass Transfer 1998, 42, 3019. (33) Katz, J.; Meneveau, C. Wake-Induced Relative Motion of Bubbles Rising in Line. Int. J. Multiphase Flow 1996, 22, 239. (34) Lu, W. J.; Hwang, S. J.; Chang, C. M. Liquid Velocity and Gas Holdup in Three-Phase Internal-Loop Airlift Reactors with Low-Density Particles. Chem. Eng. Sci. 1995, 40, 1301. (35) Velaan, P.; Tramper, J. Hydrodynamics, Axial Dispersion and Gas-Liquid Oxygen Transfer in an Airlift-Loop Bioreactor with Three-Phase Flow. International Conference on Bioreactors and Biotransformations, 1987; Paper 14, p 363. (36) Freitas, C.; Teixeira, J. T. Hydrodynamic Studies in an Airlift Reactor with an Enlarged Degassing Zone. Bioprocess. Eng. 1997, 18, 267.

Received for review May 30, 2003 Revised manuscript received September 4, 2003 Accepted September 5, 2003 IE0304614