Biotechnol. h g . 1992, 8, 465-468
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Mass Transfer in an Airlift Reactor with a Net Draft Tube Wen-Teng WU,*Jiumn-Yih Wu, and Jian-Zhong Jong Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China Gas-liquid mass transfer in an airlift reactor with net draft tube is investigated. The effects of both the ratio of draft tube to reactor diameter and the reactor pressure on oxygen transfer are considered. The value of the volumetric mass transfer coefficient, KLU, increases with a decreasing diameter ratio a t higher air flow rates. The correlation of volumetric mass transfer coefficient with respect to the true superficial air velocity under different reactor pressures is determined. The k ~ value a decreases with increasing reactor pressure.
Introduction For an aerobic fermentation process, oxygen transfer between air bubbles and the surrounding liquid is always the rate-determining step for microbial growth and/or product formation. Hence, the volumetric gas-liquid mass , an important role in transfer coefficient, k ~ a plays characterizationof the performance of bioreactors (Blenke, 1979; 1985; Moo-Young and Blench, 1981). Bubble columns have been widely used in gas-liquid contactingprocesses (Viesturs et al., 1980;Yoshida, 1988). The columns have advantages such as no moving parts and ease of construction and maintenance. However, the columns still have some limitations such as gas-liquid mass transfer and liquid circulation. Modified bubble columns have been investigated (Deckwer, 1985;Kang et al., 1990). The airlift reactor is one of the modified bubble columns (Yoshida, 1988). Wu and Wu (1990) proposed a modified airlift reactor with net draft tube and found that the k ~ a value of the reactor with mesh-24 draft tube was double that of the conventional airlift reactor at the superficial gas velocity of 5.02 cm/s. Applications of the reactor to cultivations of baker’s yeast (Wu and Wu, 1991) and Breuibacterium divaricatum for glutamic acid production (Wu and Wu, 1992) gave higher cell mass than that of a bubble column under the same operating conditions. For glutamic acid production, the productivity in the bubble column was very low in comparison with that in the modified airlift reactor. In a conventional airlift reactor, Weiland (1984) investigated the effects of the ratio of draft tube to reactor diameter on four process parameters: gas holdup, oxygen transfer, liquid circulation, and mixing time. He showed that no single diameter ratio would produce favorable conditions for all the four process parameters. In fermentation processes,the effects of reactor pressure on microorganisms have been investigated (Sato et al., 1984; Onken et al., 1984; Onken, 1989). With increasing reactor pressure, both dissolved oxygen and dissolved carbon dioxide increase. A high concentration of dissolved carbon dioxide may have an inhibitory effect on cell growth (Mori et al., 1979). In this work, experimental study of the oxygen transfer in the airlift reactor with net draft tube for different ratios of draft tube to reactor diameters and different reactor pressures is investigated. Both the diameter ratio and
* To whom correspondence should be addressed. 87567938/92/300&0465$03.00/0
the reactor pressure have significant effects on the volumetric oxygen transfer coefficient of the reactor.
Materials and Methods Equipment. The schematic diagram of the experimental setup is shown in Figure 1. The reactor, 13 cm in diameter and 200 cm high, was the same as that of the previous investigation (Wu and Wu, 1991). The reactor contained a concentric draft tube of 100 cm in height. Four draft tubes with different diameters were tested. They were 6.5, 8.0, 9.0, and 10.4 cm, respectively. The mesh number of the draft tube was 24. Without the draft tube, the reactor becomes the bubble column. The reactor which was surrounded by a jacket for temperature control was made of stainless steel (AIS1 304). The working volume of the liquid in the reactor was 15 dm3. The thermocouple and DO sensor were at a position 45 cm from the bottom. The sensors were interfaced to an IBM PC/AT with an HP-3478A digital voltmeter and an HP3497A data acquisition unit. Methods. The volumetric oxygen transfer coefficients, k ~ awere , determined by using the dynamicmethod (Kamp et al., 1987;Ruchti et al., 1981;Sobotka et al., 1982).A DO sensor was utilized to measure the dissolved oxygen. The experiment was carried out using the air-water system. Mass balance gave the equation as = k,a(C, - C )
where C is the bulk concentration of dissolved oxygen and Ce is the saturated concentration of dissolved oxygen. Integration of eq 1 from COto C and defining a dimensionless dissolved oxygen concentration
gave In (1- C) = +,at
(3) where COis the initial concentration of dissolved oxygen. After the curve of In (1- e) was plotted with respect to t, the mass transfer coefficient k ~ was a obtained from the slope of the linear regression of the curve. The saturated concentration of dissolved oxygen, C,, depended on temperature (Sobotka et al., 1982) and pressure. An
0 1992 American Chemlcal Society and Amerlcan InstRute of Chemlcal Engineers
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Biotechnol. hog., 1992, Vol. 8, No. 5
outlet gas c
! P Thermostat
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3497A
"
Figure 1. Schematic diagram of the experimental setup.
00
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10
2 C
33 40 Usg(cm/sec)
60
50
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Figure 2. Volumetric mass transfer coefficient with respect to superficial air velocity for different draft tube diameters. Symbols: +, draft tube of 6.5 cm diameter; A,draft tube of 8.0 cm diameter; 0 , draft tube of 9.0 cm diameter; fr, draft tube of 10.4 cm diameter; 0,bubble column.
where T is the operation temperature in Kelvin and P is the pressure in atmospheres. In this work, the temperature was held at 303 K and the pressure was in the range of 1-2.36 atm.
r 160
c
140
-
120
-
100
-
mesuits ana ~ i u c u u s i o n
Effect of the Ratio of Draft Tube to Reactor Diameter. Four draft tubes of different diameter were investigated. They were 6.5,8,9,and 10.4 cm in diameter. Figure 2 shows the experimental results. The kLa value is proportional to the superficial air velocity for both the bubble column and the proposed reactor. As the superficial air velocity exceeded about 2.4 cm/s, the kLa values of the proposed reactor with draft tubes of 6.5,8, and 9 cm surpassed those of the bubble column. The reason is that at high superficial air velocity the air in the draft tube could break through the net tube. A large number of small bubbles accumulated in the downcomer of the reactor. However, the kLa values of the proposed reactor with the draft tube of 10.4 cm were lower than others. The result is different from the conventional inner-loop airlift reactor. Weiland (1984) suggested that the ratio of draft tube to reactor diameter was between 0.8 and 0.9 for the conventional airlift reactor. However, the suggestion is not suitable for our proposed reactor. In our proposed reactor with the draft tube of 10.4 cm, the volume of the downcomer might become a stagnant part. Effect of the Reactor Pressure. A pressure regulator was installed to adjust the pressure in the reactor. Five pressures were investigated. They were 1 atm, 1.34 atm (19.7 psia), 1.68 atm (24.7 psia), 2.02 atm (29.7 psia), and 2.36 atm (34.7 psia). The diameter of the draft tube was 8 cm. The experimental results of volumetric mass transfer coefficient with respect to air velocity for both the bubble column and the proposed reactor are shown in Figures 3 and 4, respectively. The values of kLa decreased with an increasing pressure for both the bubble column and the proposed reactor. Since the bubble size decreases with an
L
L
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1
60
-
40
+,
20 -
0 ' 0.0
I
1.0
I
2.0
I
I
I
3.0 4.0 5.0 Usg(cm/sec)
I
6.0
I
7.0
8.0
Figure 3. Volumetric mass transfer coefficient with respect to superficial air velocity for different pressures (bubble column). Symbols: U,reactor pressure of 1.00 atm; +, reactor pressure of 1.34 atm; X, reactor pressure of 1.68 atm; A,reactor pressure of 2.02 atm; 0,reactor pressure of 2.36 atm.
increasing pressure, the gas-liquid interfacial area per unit liquid volume decreases. For the reactor at higher pressures, the air velocity should be modified (Bello et al., 1985; Chisti, 1989). A local true superficial air velocity ug,c is expressed as (5)
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Biotechnol. Prog., 1992, Vol. 8, No. 5 300 180
-
160
-
140
-
120 ?loo
-
1 v
2
80-
t 0.0
1
1
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I
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1
1
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Figure 4. Volumetric mass transfer coefficient with respect to superficial air velocity for different pressures (proposed reactor). The symbols are the same as in Figure 3.
Figure 5. Correlation of the volumetric mass transfer coefficient with respect to superficial air velocity (bubble column). The symbols are the same as in Figure 3.
where q is the molar air flow rate, R is the gas constant, Tis the absolute temperature, A is the cross-section area of the reactor, subscript C denotes the length, and P is the pressure at the C position from the top of the reactor. The pressure is expressed as
P = Po + pLgc
(6)
where POis the pressure at the top of the reactor, p~ is the density of the liquid, andg is the gravitational acceleration. Substitution of eq 6 into eq 5 and integration from zero to L gives the true superficial gas velocity vg as
where Vis the working volume and L is the aerated liquid height. Plotting the volumetric mass transfer coefficient with respect to the true superficial gas velocity in the loglog scale,we obtained the curves which are shown in Figures 5 and 6. Linear correlations for both the bubble column and the proposed reactor at different pressures are obtained. For bubble column, we have k,a = 4 4 . 7 ~ : ~ ~ ~
(8)
The proposed reactor gives k,a = 36.3~:’~’
(9)
Conclusion The mass transfer coefficient depends on the equipment and the operating conditions. The diameter ratio of draft tube to reactor has a significant effect on the volumetric mass transfer coefficient. For low superficial air velocity, the kLa values of the proposed reactor were lower than those of the bubble column. As the superficial air velocity
-
P
in 0.1
1
10
v,(cm/sec)
Figure 6. Correlation of the volumetric mass transfer coefficient with respect to superficial air velocity (proposed reactor). The symbols are the same as in Figure 3.
increased, the kLa values of the proposed reactor with draft tubes of 6.5,8, and 9 cm in diameter surpassed those of the bubble column. The reactor presssure also has a significant effect on the volumetric mass transfer coefficient. Although the value of the volumetric mass transfer coefficient decreases when the reactor pressure increases, the saturated dissolved oxygen increases when the pressure increases. It is possible to improve the mass transfer by increasing the reactor pressure.
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Notation cross-section area, m2 bulk concentration of dissolved oxygen, mol-dm-3 dimensionless dissolved oxygen concentration saturated concentration of dissolved oxygen, molmdm-3 initial dissolved oxygen concentration, mol-dm-3 mass transfer coefficient of gas-liquid phase, h-I aerated liquid height, m length, m pressure, atm (1 atm = 1.01 X 10-5N.m-2) pressure a t the top of the reactor, atm molar air flow rate, mobs-1 gas constant, Jnmol-l.K-l temperature, K superficial air velocity, cms-1 local true superficial air velocity, cms-l true superficial air velocity, cms-1 working volume, m3 density of liquid, kgm-3
Kang, Y.; Min, B. T.; Nah, J. B.; Kim, S. D. Mass Transfer in Continuous Bubble Columns with Floating Bubble Breakers. AIChE J. 1990,36 (8), 1255-1258. Moo-Young, M.; Blench, H. W. Design of BiochemicalReactors: Mass Transfer Criteria for Simple and Complex Systems. Adu. Biochem. Eng. 1981,19, 1-69. Mori, H.; Yano, T.; Kobayashi, T.; Shimizu, S. High Density Cultivation of Biomass in Fed-Batch System with DO-stat. Chem. Eng. J . 1979,12, 313-319. Onken, U. Batch and Continuous Cultivation of Pseudomonas fluorescens at Increased Pressure. Biotechnol. Bioeng. 1990, 35,983-989.
Onken, U.; Jostmann, Th. Influence on Pressure on Growth of Pseudomonas fluorescens. Biotechnol. Lett. 1984, 6, 413418.
Ruchti, G.; Dunn, I. J.; Bourne, J. R. Comparison of Dynamic Oxygen Electrode Methods for the Measurement of k ~ a . Biotechnol. Bioeng. 1981,23, 277-299. Sato, S.; Mukataka, S.; Kataoka, H.; Takahashi, J. Effects of Pressure and Dissolved Oxygen Concentration on Growth of Pseudomonas aeruginosa. J. Ferment. Technol. 1984,62 (l), 71-75.
Sobotka, M.; Prokop, A.; Dunn, 1. J.; Einsele, A. Review of Methods for the Measurement of Oxygen Transfer in Microbial Systems. Annu. Rep. Ferment. Proc. 1982,5, Chapter 5,127210.
Literature Cited Bello, R. A.; Robinson, C. W.; Moo-Young, M. Gas Holdup and Overall Volumetric Oxygen Transfer Coefficient in Airlift Contractors. Biotechnol. Bioeng. 1985, 27, 369-381. Blenke, H. Loop Reactors. Advances in Biochemical Engineering; Springer-Verlag: New York, 1979; Chapter 13, pp 121-213.
Blenke, H. Biochemical Loop Reactors. Biotechnology; 1985; Vol. 2, Chapter 21, pp 465-517. Chisti, M. Y. Airlift Bioreactors; Elsevier Science Publishers Ltd.: New York, 1987; pp 287-292. Deckwer, W.-D. Bubble Column Reactors. Biotechnology; 1985; Vol. 2, Chapter 20, pp 445-464. Kamp, F.; Wase, D. A. J.; McMannamey, W. J.; Mendoza, 0.; Thayanithy, K. A Comparison of Some Methods of Estimating Volumetric Mass Transfer Coefficients in an External Loop Airlift Fermentor. Biotechnol. Bioeng. 1987, 30, 179-186.
Viesturs, U. E.; Sturmanis, I. A.; Krikis, V. V.; Prokopenko, V. D. Investigation of Fermentors with Various Contacting Devices. Biotechnol. Bioeng. 1980, 22, 799-820. Yoshida, F. Bubble Column Research in Japan. Chem. Eng. Technol. 1988, 11, 205-211. Weiland, P. Influence of Draft Tube Diameter on Operation Behaviour of Airlift Loop Reactors. Ger. Chem. Eng. 1984,7, 374-385.
Wu, J. Y.; Wu, W. T. Fed-batch Culture of Saccharomyces cereuisiae in Airlift Reactor with Net Draft Tube. Biotechnol. Prog. 1991, 7, 230-233. Wu, J. Y.; Wu, W. T. Fed-batch Culture of Breuibacterium diuaricatun an Airlift Reactor with Net Draft Tube. Bioprocess Eng. 1992 (in press). Wu, W. T.; Wu, J. Y. Airlift Reactor with Net Draught Tube. J. Ferment. Bioeng. 1990, 70 (5),359-361. Accepted July 22, 1992.
