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Hydrodynamic Characteristics in an External Loop Airlift Bioreactor Containing a Spinning Sparger and a Packed Bed Annie X. Meng, Gordon A. Hill,* and Ajay K. Dalai Department of Chemical Engineering, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan, Canada S7N 5C9
An external loop airlift bioreactor (ELAB) and the packed bed bioreactor (PB) design strategies have been combined into one vessel. The hydrodynamic behavior of the combined system has been investigated. Woven nylon packing was placed in the riser section of the ELAB to represent the packed bed. A novel spinning sparger was employed to generate air bubbles. The controlled input variables were packing porosity, packing height, superficial gas velocity, and sparger rotational speed. The hydrodynamic output variables included gas holdup, liquid circulation velocity, axial dispersion, and bubble-size distribution. Gas holdup continuously increased with increases in both the packing height up to 0.8 m and the porosity up to 0.99, but at a porosity of 1.0 (no packing), there was a significant drop in the gas holdup. Increased amounts of packing in the ELAB, whether in the form of packing height or packing density, decreased the liquid circulation rate in the bioreactor because of increased frictional resistance and tortuosity. Packing also decreased the Bodenstein number, indicating greater axial dispersion and enhanced mixing. Bubble sizes were more uniform and had smaller diameters after passing through the packing material. Empirical models are presented which accurately predict gas holdup and liquid circulation velocities as functions of all four independent variables (packing height, packing porosity, gas flow rate, and sparger spinning speed). The optimum hydrodynamic conditions were observed with packing at the highest porosity (0.99) used in this study. Introduction In relatively recent times, because of innovations and the growth in biotechnology, there has been considerable published information regarding the two-phase behavior of airlift bioreactors. Historically, extensive studies have been carried out for two-phase gas-liquid flows in packed beds due to their abundant use in the chemical process industries, primarily as mass-transfer vessels. Also, packing is used as a support for catalysts in chemical reactors1 or as a support for microorganisms or enzymes in biological reactors.2,3 For the best operation of packed columns or airlift reactors, a good understanding of hydrodynamic characteristics is very important in order to optimize mass-transfer and/or reaction conversions. Hydrodynamic conditions and mass-transfer characteristics in airlift vessels have been extensively reported and reviewed.4-7 An external loop airlift bioreactor (ELAB) in combination with a fluidized bed has also been recently studied.8 However, no information exists on the hydrodynamic behavior when an airlift reactor is combined with a packed bed, especially in the same vessel. This paper presents some hydrodynamic characteristics of an ELAB containing a packed bed. Because the ELAB, as with all types of bioreactors, normally operates at low air and liquid throughputs, it is important to characterize the hydrodynamic conditions at these operating conditions. Experimental Apparatus and Procedures The same ELAB as that used earlier5 (see Figure 1) was employed in this investigation. The ELAB has a * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (306) 966-4765. Fax: (306) 9664777.
Figure 1. Schematic of ELAB with a packed bed located in the riser.
degassed liquid volume of 1.2 × 10-2 m3 and a liquid height of 1.44 m above the sparger. A spinning sparger is used to generate small air bubbles. Woven nylon (solid density measured to be 647 kg/ m3) mesh consisting of ribbon strands of 1 × 10-4 m thickness and 1 × 10-3 m width was used as the packing material. When laid flat, the mesh produced nearly circular openings with a diameter of 3.7 × 10-3 m, but these could be easily stretched or compressed to different sizes. The mesh was randomly folded and inserted in the riser section of the ELAB such that it was
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homogeneously dispersed between the ends of the packing holder. The top end plate of the packing holder could be moved up or down to vary the height of the packing and the packing porosity. Both end plates of the packing holder contained large rectangular openings so that they did not interfere with the flow of fluids. The filled packing holder was inserted into the riser section of the ELAB and held the packing starting at a distance of 0.45 m above the sparger and extending to the variable depth of the packing (i.e., packing height, hP). Tap water and compressed air were used as sources of the continuous and dispersed phases, respectively. Earlier work5 had shown that identical hydrodynamic conditions were observed with pure water, saltwater, and culture media. The gas flow rate was measured with a calibrated rotameter. A tracer response technique9 using salt and two conductivity probes was applied to measure the liquid circulation velocity and axial dispersion profile as described earlier.5 Fraser and Hill5 showed that, over the range of salt concentrations used in this study, no effects of salt on hydrodynamics could be observed. An inclined tube10 was used for gas holdup measurements. Finally, bubble diameters were measured using a CCD camera and a stroboscope to capture images which were analyzed using an image analysis software package (Matrox Inspector, Matrox Electronics, Dorval, Quebec, Canada). The effects of the packing height (from 0.05 to 0.8 m) on the hydrodynamics of the ELAB were studied while maintaining a constant packing porosity of 0.95. The effects of the packing porosity (from 0.90 to 1.0) were determined at a fixed packing height of 0.3 m. Other important input variable ranges included the rotational sparger speed (0-45 rad/s) and the superficial gas velocity (3.1 × 10-3-1.6 × 10-2 m/s). Results and Discussion A total of 1400 experimental measurements were collected, all at 22 °C and atmospheric pressure. Gas Holdup. (i) Effects of the Gas Velocity and Sparger Speed. Meng et al.10 have recently reported on gas holdup in the ELAB used in this study without packing. Their equation was
GR ) 1.06JGR0.701(1 + UT)0.379
(1)
The gas holdup (GR) was found to be most sensitive to the superficial gas velocity (JGR), increasing by over a factor of 3 between 0.003 and 0.016 m/s, while the range of orifice speeds (UT, 0-0.99 m/s) resulted in an increase by a factor of 1.5 in gas holdup. The 95% confidence limits for the three coefficients in eq 1 were10 1.06 ( 0.03, 0.701 ( 0.001, and 0.379 ( 0.011. (ii) Effects of the Packing Height and Porosity. Figure 2 demonstrates the effect of packing height on gas holdup at various superficial gas velocities at a packing porosity (φS) of 0.95. In most cases, at a fixed gas velocity, the increase of gas holdup ranged from 0.5 to 2%, as the packing height increased from 0 to 0.80 m. An interesting phenomenon was observed regarding the effect of packing porosity on gas holdup (Figure 3). The gas holdup is seen to increase significantly with packing porosity over the range of 0.90-0.99; however, at a porosity of 1.0 (i.e., no packing), the gas holdup dropped noticeably at all gas flow rates and sparger spinning speeds. Thus, a small amount of packing with
Figure 2. Packing height effects on gas holdup at a packing porosity of 0.95 and an orifice speed of 0.88 m/s (lines represent eq 2).
Figure 3. Packing porosity effects on gas holdup at a packing height of 0.30 m and an orifice speed of 0.64 m/s (lines represent eq 2).
high porosity, in this case 0.99, maximized the holdup of air bubbles in the ELAB. The range of porosities that were studied spanned the capabilities of the apparatus because it was difficult to compress the nylon mesh to a smaller porosity than 0.90. Nevertheless, this range of porosities led to the same increase in gas holdups (0.5-2%) that was achieved by increasing the packing height between 0 and 0.80 m. It is believed that packing causes changes in the gas holdup due to greater flow tortuosity and therefore longer path lengths for bubbles to travel. Impacts between bubbles and with the packing material can greatly alter flow hydrodynamics, including gas holdup. Over the ranges of packing height and packing porosity used in this investigation, these features seem to be directly proportional to the amount of packing because the gas holdup is predicted to increase linearly with packing height and with packing porosity up to a porosity of 0.99. This is demonstrated by the straight lines drawn in Figures 2 and 3. The gas holdup dropped sharply at a porosity of 1.0 (i.e., no packing), indicating that between a porosity of 0.99 and 1.0 there is a dramatic change in column hydrodynamics that were not further investigated in this work. The linear increase of gas holdup with packing height and packing porosity could be accurately modeled by
GR ) (-2.75 + 0.272hP + 4.03φS)JGR0.701(1 + UT)0.379 (2) Equation 2 can be used to predict the gas holdup when packing is present in the riser. It is valid over the range
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Figure 4. Parity plot comparing the predictions of gas holdup using eq 2 to the measured gas holdup data.
of packing heights (0.05-0.8 m), packing porosities (0.90-0.99), gas superficial velocities (0.003-0.016 m/s), and orifice speeds (0-0.99 m/s) used in this study. For no packing (i.e., φS ) 1.0), eq 1 must be used to predict the gas holdup. Figures 2 and 3 show that, at the highest gas velocity (0.016 m/s), eq 2 begins to deviate from the experimental gas holdup data perhaps because of the onset of a change in the flow pattern in the riser, but Figure 4 demonstrates that eq 2 predicts all of the gas holdup data generated in this study with a standard percentage error of (5.2%. The 95% confidence limits of the new parameters in eq 2 were found to be -2.75 ( 0.03, 0.272 ( 0.004, and 4.03 ( 0.03. Liquid Velocity. (i) Effects of the Gas Velocity and Sparger Speed. An accurate ability to predict the liquid velocity and the factors that influence velocity is an essential requirement for the design, modeling, and operation of ELABs. To predict the liquid velocity in the riser, an equation similar in form to that used by Fraser and Hill5 was used
ULR ) CFE where
E)
(
(1 - GR)
GR -2
+ (AR/AD)
Figure 5. Determination of the effect of gas holdup on the liquid riser velocity with no packing in the riser (data from all sparger speeds and superficial gas velocities).
Figure 6. Packing height effects on liquid riser velocity at a packing porosity of 0.95 and an orifice speed of 0.77 m/s.
(3)
)
0.92
2
(4)
CF (m/s) represents the effect of friction on the velocity, while E represents the gas holdup driving force for liquid circulation as developed by Chisti et al.11 Fraser and Hill5 used a power exponent of 0.5 on the gas holdup forcing function to obtain a linear plot, but in this study the best-fit gas holdup power function was found to be 0.92 with a standard error of (0.09. Normally, the fluid flow is proportional to the square root of the forcing function, but in this situation, energy losses due to wakes around the spinning sparger must have altered this relationship and prevented the hydrodynamic pressure force from contributing purely to the axial fluid flow. In this investigation (see Figure 1), the spinning sparger was located above the downcomer return. This resulted in greater resistance to fluid flow around the ELAB and lower liquid riser velocities than those observed by Fraser and Hill.5 The sparger was acting as a partially closed valve in the riser column, and the liquid swirled around the sparger after it left the downcomer outlet. A plot of the measured liquid riser velocity (ULR) versus gas holdup in the form of eq 3 is shown in Figure 5. The data include the full range of superficial gas velocities and sparger spinning speeds
Figure 7. Packing porosity effects on liquid riser velocity at a packing height of 0.30 m and an orifice speed of 0.77 m/s (symbols and the line sequence are the same as those in Figure 6).
used in this investigation. Similar to the earlier study,5 there is significant scatter in the experimental data. The slope of the best-fit line (CF) was equal to 19.1 m/s (standard error of (0.6), which represents a measure of frictional resistance for liquid flow; the larger the value, the less the resistance. (ii) Effects of the Packing Height and Porosity. Measured liquid velocities in the riser as a function of the packing height and superficial gas velocity are shown in Figure 6 (symbols) at an orifice speed of 0.77 m/s and a packing porosity of 0.95. The packing height has a dramatic effect on the velocity in the ELAB. At a superficial gas velocity of 0.009 m/s, as the height of the packed bed increases from 0 to 0.80 m, the liquid riser velocity drops from 0.19 to 0.10 m/s. Figure 7 demonstrates the change in the riser liquid velocity with a change in the packing porosity at a fixed packing height
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of 0.30 m and a sparger spinning speed of 35 rad/s (orifice speed of 0.77 m/s). The more packing in the ELAB (lower porosity), the lower the circulation velocity. At a superficial gas velocity of 0.013 m/s, the liquid velocity falls from 0.27 to 0.10 m/s as the packing porosity drops from 1.0 to 0.9. The increased surface area and tortuosity of the packing material, whether caused by a greater packing height or a lower packing porosity, increases the frictional flow resistance, thereby slowing down the circulation velocity. It was found that the liquid riser velocity could be predicted using eqs 3 and 4 if the frictional term is calculated by
CF ) 19.1
without packing
CF ) -54.3-7.53hP + 71.4φS
(5)
with packing (6)
The solid lines in Figures 6 and 7 demonstrate reasonably good agreement between the theory and data for liquid velocity, considering that the model involves both the initial prediction of gas holdup by eq 2 followed by the solutions of eqs 3-6 to predict liquid velocity. The standard percentage error in the prediction of the liquid riser velocity compared to the experimental data was found to be (10.0%. The 95% confidence limits of the coefficients in eq 6 were determined to be -54.3 ( 0.8, 7.53 ( 0.08, and 71.4 ( 0.9. Axial Dispersion. Axial dispersion is normally expressed by the dispersion coefficient (D) and, for circulating vessels, the dimensionless Bodenstein number (Bo), which is defined as
Bo ) ULRL/D
(7)
where L is the length of the circulation loop. Axial dispersion in the ELAB was studied by measuring salt concentration fluctuations in the ELAB as described earlier.5 When packing was placed in the ELAB, the dispersion increased, with the Bodenstein number falling from 47 with no packing (at all gas flow rates and sparger spinning speeds) to 35 with 0.3 m at a porosity of 0.90. At this packing height, a decrease of the packing porosity from 0.99 to 0.90 resulted in a 30% decrease of the Bodenstein number, significantly increasing mixing as the liquid flows through the increased tortuosity created by higher packing densities. This increase in the dispersion coefficient with increased solid material in a vertical flow field had been reported previously.12 It was also observed that the Bodenstein number is not strongly dependent on the superficial gas velocity as reported earlier.5,13 Bubble Size. The bubble size was analyzed using the Sauter mean diameter. Low packing porosity causes narrower bubble-size distributions with lower mean bubble diameters, 3.2 mm at a packing porosity of 0.90 compared to 5.5 mm at a porosity of 1.0 at zero orifice velocity. Low orifice speeds generate the largest bubbles when no packing is present, but high packing densities (i.e., porosity of 0.90) results in relatively small diameter bubbles even at low orifice speeds. The packing seemed to help break the larger bubbles into fine bubbles. Thus, increased amounts of packing should enhance the mass transfer because of the increased bubble-liquid surface area created by smaller bubbles. It is expected that, because packing reduces the mean bubble size, similar to the smaller bubbles generated by the spinning sparger,14 improved mass transfer will occur with packing located in the riser compared to the unpacked ELAB.
When no packing was present, high orifice speeds decreased the mean bubble diameter. At an orifice speed of 0 m/s, the mean bubble diameter was 5.5 mm but decreased to 2.5 mm at an orifice speed of 0.99 m/s. This is similar to the trend reported by Fraser and Hill.5 At low packing porosity (φS ) 0.90), however, the same increase in orifice speed resulted only in a decrease in the mean bubble diameter from 3.3 to 2.7 mm. Conclusions Hydrodynamic conditions in an ELAB with nylon mesh packing in the riser section has been successfully measured and modeled. The dependence of gas holdup on the packing height was small, increasing less than 2% with an increase in the packing height from 0 to 0.80 m. Maximum gas holdup was found at a packing porosity of 0.99, with the gas holdup dropping both below that porosity and at a porosity of 1.0 (i.e., no packing). The packing height and packing porosity both affected the liquid flow rate in the ELAB, with increased amounts of packing decreasing both the flow rate (due to friction) and the Bodenstein number (higher liquid mixing). Higher spinning sparger speeds and increased packing density resulted in smaller bubble diameters inside the riser of the ELAB. It is concluded that, under the experimental conditions studied, the optimal hydrodynamic conditions for a packed ELAB occur at high packing porosity, observed to be 0.99 in this study, with the full packing height between the top of the gas sparger and the downcomer inlet. These conditions will permit high, immobilized biomass holdup attached to the packing; highest gas holdup to improve mass transfer; and large void space to reduce plugging and liquid frictional losses. Acknowledgment The authors are grateful for the financial support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC). Nomenclature AD ) downcomer cross-sectional area (m2) AR ) riser cross-sectional area (m2) Bo ) Bodenstein number (eq 8) CF ) friction loss variable (eq 4, m/s) D ) axial dispersion coefficient (m2/s) dS ) Sauter mean bubble diameter (m) E ) gas holdup function (eq 5) hP ) packing height (m) JGR ) superficial gas velocity (m/s) L ) length of the circulation loop (m) ULR ) liquid riser velocity (m/s) UT ) orifice speed (m/s) Greek Letters GR ) gas holdup φS ) packing porosity
Literature Cited (1) Agar, D. W.; Ruppel, W. Multifunctional Reactor for the Heterogeneous Catalyst. Chem. Ing. Tech. 1988, 60, 731. (2) Lin, S. H. Optimal Feed Temperature for an Immobilized Enzyme Packed Bed Reactor. J. Chem. Technol. Biotechnol. 1991, 50, 17.
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(3) Bales, V.; Palakovic, M.; Stefuca, B. Study of Hydrolysis of Sucrose in a Packed Bed Immobilized Cell Bioreactor. Chem. Eng. J. 1991, 5, 53. (4) Chisti, M. Y. Airlift Bioreactors; Elsevier Applied Science: London, 1989. (5) Fraser, R.; Hill, G. A. Hydrodynamic Characteristics of a Spinning Sparger, External Loop Airlift Bioreactor. Can. J. Chem. Eng. 1993, 71, 419. (6) Sajc, L.; Obradovic, B.; Vukovic, D.; Bugarski, B. Hydrodynamics and Mass Transfer in a Four-Phase, External Loop Air Lift Bioreactor. Biotechnol. Prog. 1995, 11, 420. (7) Wang, B.; Nikov, I.; Delmas, H.; Bascoul, A. Gas-Liquid Mass Transfer in a New Three-Phase Stirred Airlift Reactor. J. Chem. Technol. Biotechnol. 1998, 72, 137. (8) Guo, Y. X.; Rather, M. N.; Ti, H. C. Hydrodynamics and Mass Transfer Studies in a Novel External-Loop Airlift Reactor. Chem. Eng. J. 1997, 67, 205. (9) Verlaan, P.; van Eijs, A. M.; Tramper, J.; van’t Riet, K.; Luyben, K. Estimation of Axial Dispersion in Individual Sections of an Airlift-loop Reactor. Chem. Eng. Sci. 1989, 44, 1139.
(10) Meng, A. X.; Hill, G. A.; Dalai, A. K. Modified Volume Expansion Method for Measuring Gas Holdup. Can. J. Chem. Eng. 2002, in press. (11) Chisti, M. Y.; Halard, B.; Moo-Young, M. Liquid Circulation in Airlift Bioreactors. Chem. Eng. Sci. 1988, 43, 451. (12) Olazar, M.; San Jose, M. J.; Arandes, J. M.; Bilbao, J. A Model for Gas Flow in Jet-Spouted Beds. In Mixed Flow Hydrodynamics; Cheremisinoff, N. P., Ed.; Gulf Publishing Co.: Houston, TX, 1996; p 759. (13) Verlaan, P.; Vos, J. C.; Van’t Riet, K. Axial Dispersion and Oxygen Transfer in the Transition from Bubble Column to AirliftLoop Reactor. J. Chem. Technol. Biotechnol. 1989, 45, 181. (14) Fraser, R. D.; Ritchie, B. J.; Hill, G. A. Dynamic Mixing and Oxygen Transfer in Small, Airlift Loop Bioreactors: Model and Experimental Verification. Biotechnol. Prog. 1994, 10, 543.
Received for review October 9, 2001 Revised manuscript received February 19, 2002 Accepted February 22, 2002 IE0108301
