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Periodical changes of input air flowrate – a possible way of improvement of oxygen transfer and liquid circulation in airlift bioreactors
Chemical Engineering Science 54 (1999) 4937}4943
Periodical changes of input air #owrate } a possible way of improvement of oxygen transfer and liquid circulation in airlift bioreactors S[ . GodoH *, J. Klein, M. Polakovic\ , V. BaH les\ Department of Chemical and Biochemical Engineering, Faculty of Chemical Technology, Slovak University of Technology, Radlinske& ho 9, 812 37 Bratislava, Slovakia, Slovak Republic
Abstract The in#uence of small bubbles on the behaviour of airlift reactors during citric acid fermentation was studied. The small bubbles formed up to 70% of the total gas hold-up, so their distribution strongly in#uenced the predicted liquid circulation velocities. A periodically changing air #owrate could be used to signi"cantly improve the oxygen transfer capacity of airlift reactors. This could be achieved by the e!ective utilisation of the oxygen content of small bubbles, which originated from the foam. A procedure based on the evaluation of transient pO and gas hold-up pro"les was suggested for the determination of the period of sparging. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Airlift; Mass transfer; Gas hold-up; Liquid circulation; Non-Newtonian broth; Bubble
1. Introduction Several new types of bioreactors have been investigated in the last two decades in order to improve the fermentation performance. Bubble columns and their modi"cations, namely airlift bioreactors (ALR), are considered to be very promising, particularly due to their simpler construction, better de"ned #ow patterns, lower shear "elds and lower power input in comparison with conventional stirred tank reactors. Their wider application is hindered by the lack of theoretical knowledge, which would allow a reliable modelling of complex twoor three-phase #ow phenomena. A considerable e!ort has been devoted to the improvement of oxygen transport in viscous fermentation broths (Goto & Gaspillo, 1992; Karamanev et al., 1996; Stejskal & Potucek, 1985). In most cases static mixers have been used to redisperse larger bubbles. These may however slow down the liquid circulation and growing microorganisms may stick to their surface causing the clogging of the #ow path. It has been shown several times that in viscous mould broths a bimodal distribution of bubble sizes exists (Philip et al., 1990; Allen & Robinson, 1989; Kawase * Corresponding author. Tel.: 421-7-59325-265; fax: 421-7-396-743; e-mail address: [email protected].
& Tsujimura, 1994; Muller & Davidson, 1992). The role of small bubbles has unfortunately not been analysed in detail. Most authors (Chisti, 1989; Li et al., 1995) concluded that a long presence of small bubbles in the reactor was not desirable because the gas inside them would be in equilibrium with the bulk liquid in a relatively short time. Erickson et al., 1983 analysed the role of the recirculation of small bubbles in airlift reactors. They concluded that small bubbles could signi"cantly improve the oxygen transfer rate, provided that they could be regenerated in a rate comparable with their depletion. The main goal of this study is to show that fresh small gas bubbles could be generated and the depleted ones removed by choosing a certain periodic gas #owrate pro"le. In this way the oxygen transfer rate in ALR could signi"cantly be improved. This could surprisingly be achieved by decreasing the power input into the reactor. The knowledge of small bubble hold-up and its distribution between the reactor sections was found to be essential for the prediction of liquid circulation velocity.
2. Experimental Three citric acid fermentations on synthetic media (with sucrose as the only carbon source) using the mould
0009-2509/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 2 1 5 - 8
S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
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strain Aspergillus niger were carried out in a 195 l concentric tube airlift reactor made of glass. The details of reactor geometry are given in Table 1 and Fig. 1 and the reactor set-up is shown in Fig. 2. Air was introduced through a stainless-steel perforated plate with holes of diameter of 1 mm placed under the bottom of the draught tube (the riser section of the reactor). Rheological properties of the broth were measured o!-line with a coaxial cylinder viscometer Rheotest II (MLW PruK fgeraK te-Werk, Medingen, Germany). The dependencies of shear stress on shear rate were evaluated according to the Ostwald de Waele model. The liquid circulation velocity was measured through a visual observation of the movement of coloured #ow-follower particles of the same density as the broth. The gas holdup was measured by noting the dispersion height. The hold-ups of small and large bubbles were measured by noting the transient dispersion height after aeration stop and restart. The oxygen probe (Jenway, England) was "tted in the downcomer, 60 cm above the bottom of the reactor.
Table 1 Constructional details of the airlift reactor used in this study, D * col! umn diameter, H * liquid height, H * draught tube height, * "2 D * riser tube diameter, A /A * downcomer/riser cross-sectional 0 " 0 area ratio, H * bottom clearance, H * distance between measuring pO electrode and the reactor bottom, N * number of holes in gas sparger Working D ! volume (¸) (m)
H * (m)
H "2 (m)
D 0 (m)
A /A H " 0 (-) (m)
H (m)
N (-)
195
2.96
2.70
0.200
1.01
0.60
90
0.295
0.061
Fig. 1. 195 l internal loop airlift reactor.
Fig. 2. Reactor set-up.
3. Results and discussion 3.1. Flow regimes The production of citric acid is an example of a process where the dissolved oxygen concentration is a key factor for the success of the whole fermentation (Rohr et al., 1983). We have carried out three citric acid fermentations in a 195 l internal loop ALR with an aspect ratio (H /D ) * ! of ten. The time course of the measured dissolved oxygen concentration displayed a typical minimum at approximately 16}24 h, which was caused by a rapidly rising oxygen demand of the microbial culture. At the same time, rapid changes in rheological properties of the broth were observed. The time courses of the power law parameters are shown in Fig. 3. The broth became markedly pseudoplastic, which was accompanied by a strong coalescence of gas bubbles just above the sparger. The coalescence led to a change of the #ow regime from homogeneous bubbly to slug #ow. This #ow regime change seemed to be unavoidable, even when a sintered plate sparger was used. This indicates that the broth viscosity governs the size of bubbles formed. This phenomenon is well known from experiments with model (such as CMC) (Popovic & Robinson, 1987a; Popovic & Robinson, 1988) as well as with real fermentation media (Allen & Robinson, 1989). From the start of slug formation it was extremely di$cult to keep the oxygen level on the desired value. The simple increase of gas
S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
4939
Fig. 3. Time courses of the rheological parameters and the biomass concentration.
Fig. 4. Dissolved oxygen and gas hold-up pro"les during paused and resumed sparging. 113th hour of fermentation; air super"cial velocity ; "0.0812 m s\. %0
#owrate did not help too much, after a few minutes of moderate increase the oxygen concentration began to decrease again at any #owrate. The big Taylor bubbles passing the riser have a very short residence time and a small speci"c interfacial area; thus, they contribute little to oxygen transfer. Approximately from the onset of the slug #ow a relatively strong foaming occured. According to our visual observations, the foam was mostly generated at the top of the reactor by bursting of large bubbles. In fermentations, foaming is generally considered to be undesirable, because it represents a dead space (Hoeks et al., 1997). The small (about 1 mm of diameter) bubbles of the foam were drawn by the downward liquid #ow into
the downcomer. We observed visually that many small bubbles were accumulated along the walls of the downcomer. Thus, a bimodal bubble size distribution existed in the reactor, consisting of large bubble slugs (with the same diameter as the riser) and small bubbles, approximately 1 mm of diameter. 3.2. Gas hold-up and circulation velocity The measured gas hold-up curves during paused and resumed sparging are shown in Fig. 4. In the "rst seconds, solely big slugs formed the gas hold-up. The slow increase of gas hold-up after about 1 min of sparging is
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S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
Fig. 5. Measured downcomer liquid velocities. 113th hour of fermentation.
attributable to the entrainment of small bubbles, which were drawn by the liquid #ow into the downcomer and thus reduced the circulation velocity. The results of the measurement of the downcomer liquid velocities after restarted sparging, which are shown in Fig. 5. (113th hour of the fermentation), con"rm the above-mentioned accumulation of small bubbles in the downcomer. After the start of sparging, the initial liquid velocity was high due to the big holdup di!erence between the riser and downcomer (only large bubbles were present in the riser). This fast liquid #ow drew the foam, which began to form at the liquid surface, into the downcomer, thus reducing the riser}downcomer density di!erence, and hence the circulation velocity. The entrainment of the small bubbles into the downcomer and the decrease of velocity were the fastest at the start of sparging. The steady-state values of circulation velocity and total gas hold-up were reached approximately 13 min after the aeration restart. The circulation velocity decreased to a value of about 0.04 m s\ and was almost independent of the aeration rate. 3.3. Prediction of liquid circulation velocity The liquid velocity can be predicted from the energy balance model of Chisti and Moo-Young (1988). One should however take into account the hold-up of small bubbles and its distribution between the riser and downcomer. The correlation used by Chisti and Moo-Young (1988) their Eq. (18)). to predict the riser gas hold-up taken from Metkin and Sokolov (1982) predicts fairly well only the hold-up of large bubbles. This is shown in Fig. 6. It can be seen that the simulated lines approximate closely the measured large bubble hold-up ("lled symbols). For simplicity only the experimentally determined large bubble hold-ups for
113 h of the experiment are shown, but this part of the hold-up was almost constant during the whole experiment. This observation is in line with expectations, as the slug velocity should be independent of viscosity (Clift et al., 1978) and also with the predictions of Eq. (18) of Chisti (simulated e lines lie very close to each other % despite of viscosity changes). By comparison of the large bubble hold-up with the measured total gas hold-up (open symbols) it can be seen that the small bubbles made up about 60}70% of the total gas hold-up. The small bubbles are distributed between the riser and the downcomer. This distribution can be represented by a factor g, representing the portion of small bubbles present in the riser. Thus, e *g and e *(1!g) are the hold-up of %QK@ %QK@ small bubbles present in the riser and downcomer, respectively. The total riser gas holdup will be the sum of the large bubble hold-up and e *g, whereas in the %QK@ downcomer only small bubbles are present (e *(1!g)). %QK@ The di!erence between riser and downcomer hold-up, representing the driving force of liquid circulation, is therefore dependent on the distribution factor g. Any chosen g value will in combination with the measured large and small bubble hold-ups de"ne the driving force. Inserting these values in the model yields a calculated liquid velocity < . By comparing the experimental *" values of liquid circulation velocity with these predicted by the model we could calculate the correct value of g, thus the distribution of small bubbles between the riser and downcomer. This distribution is presented in Fig. 7. The curves (< sim.) indicate that the more small *" bubbles are present in the reactor, the more their distribution in#uences the predicted liquid circulation velocity. In 15 h of the experiment, when there are relatively less small bubbles (as shown in Fig. 6) the distribution coe$cient g may have values between almost zero and 0.8 (solid line), whereas in later stages its possible values
S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
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Fig. 6. Measured and predicted gas hold-up values - simulation made using the Eq. (18) of Chisti and Moo-Young (1988).
Fig. 7. Downcomer liquid velocity as function of the distribution of small bubbles.
range from approximately 0.23 to 0.55. The lower bound simply shows that if g would be less, the downcomer hold-up would be greater than that of the riser, which would cease the circulation. The correct values of the distribution coe$cient g are taken from the intersection of the calculated < !g lines (< sim.) with the experi*" *" mental < values (open symbols * < exp.). As can be *" *" seen they are between 0.17 and 0.31. The lowest value was calculated for the fermentation time of 15.5 h, just on the onset of slug #ow, when the foaming was not still so strong (compare the holdup for 15.5 h in Fig. 6). The distribution coe$cient depends probably on the reactor geometry, liquid properties and gas #owrate but further studies are needed to con"rm this hypothesis. Neverthe-
less, these preliminary results show that neglecting the hold-up of small bubbles may lead to serious errors in the estimation of the circulation velocity. The most important conclusion of the modelling was that despite of their virtually negligible rise velocity, these small bubbles are not distributed evenly between the riser and downcomer } a feature, which could be utilised, as shown in the next section. 3.4. Proxle of sparging The measured pO and gas hold-up curves after stopped and restarted sparging are shown in Fig. 4. It can be seen that after the start of sparging, the oxygen
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S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
Fig. 8. Time course of the dissolved oxygen level pO during periodical change of air #owrate: Q "21 Ln/min (; "0.0110 m s\) !1.5 min, %0 Q "153 Ln/min (; "0.0812 m s\) !13 min; ¹"203C, p"1 atm. %0
concentration increases at the beginning, but decreases to almost zero after a certain time despite of the slowly increasing gas holdup. As mentioned above, this happens at any gas #owrate. At this time, the small bubbles, which are accumulated at the downcomer wall, are mostly depleted of oxygen and do not contribute to the oxygen transfer any longer. Moreover, their simple presence in the downcomer causes a low circulation velocity, thus lowering the entrainment of fresh bubbles into it. The only possibility to overcome the oxygen shortage is therefore to get rid of the small depleted bubbles, and get fresh ones into the liquid. Airlift reactors possess a unique feature among pneumatically agitated reactors * a well-de"ned #ow path. The small bubbles are collected by the velocity pro"le along the walls of the downcomer and after a certain time they even form small `channel-like cloudsa. After stopping the sparging, these bubble swarms at the walls escape very fast from the reactor in contrast to those in the bulk liquid, which need a much longer time to leave the broth. This feature of the airlift reactor allows us to get rid of the small depleted bubbles in a relatively short time and represents an important di!erence between ALRs and bubble columns (mechanically stirred reactors also), where the small bubbles are distributed more evenly due to the di!erent #ow pro"les. It follows from the previous considerations that shutting o! or considerably decreasing the air #owrate for short time periods may markedly increase the oxygen transfer capacity of airlift reactors. As mentioned above, the fast liquid movement after restarted sparging draws fresh small bubbles into the downcomer, which causes a dramatic increase in oxygen transfer. We tested this hypothesis by periodically changing the air #owrate. The time periods for the minimum and
standard air #owrates were determined from Fig. 4. The time period necessary to let most of the small bubbles to escape was determined from the holdup curve after an air interruption (approxiamately 1.5 min). The time period of the standard #owrate was determined as the time from the start of standard aeration to the point, when the oxygen level fell below a critical value (approximately 13 min). Thus, the reactor was sparged at the standard #owrate for 13 min, then the air #owrate was decreased to a minimum level for 1.5 min and the sparging cycle was repeated. We could, in principle, have shut o! the sparging completely, but we needed to maintain a certain albeit low liquid movement around the pO probe. The resulting dissolved oxygen pro"le is shown in Fig. 8. The dissolved oxygen level increased after each air interruption and restart to levels, which could not be achieved by increasing the air #owrate. To achieve a more even pO level, the sparging cycle should be further optimised. Further studies of the initial transient liquid circulation and gas hold-up distribution will also be necessary.
4. Conclusions We have shown that for the success of prediction of liquid circulation velocities in non-Newtonian fermentation media it is decisive to know the holdup of di!erent bubble sizes. As in these media a bimodal bubble size distribution exists, the predicted liquid circulation velocity is very sensitive to the distribution of the small bubbles between the riser and the downcomer. We have observed and con"rmed by calculations that most of the small bubbles were accumulated in the downcomer.
S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
It was found that periodic changes of gas #owrate can increase the dissolved oxygen concentration in airlift reactors. This is due to the e$cient utilisation of oxygen contents of small bubbles originating from the foam. The time periods of di!erent sparging #owrates were optimised on the basis of unsteady gas hold-up and dissolved oxygen curves. Further detailed studies on the unsteadystate generation of small bubbles and their in#uence on oxygen transfer have to be carried out. This study has shown that keeping the ALR in a hydrodynamically unsteady state the reactor can be operated e$ciently also with highly viscous fermentation broths. Notation A D g
area, m diameter, m distribution coe$cient of small bubbles, dimensionless g"e /e , %QK@0 %QK@ H height, clearance, m H height of stainless section above the reactor bot tom, m H height of stainless-steel section/probe housing/, m K consistency index, Pa sL n Flow behaviour index, dimensionless P pressure, Pa pO dissolved oxygen level relative to saturation, % Q gas #owrate, m s\ ; super"cial velocity, m s\ < linear velocity <";/e, m s\ Greek letters e
gas hold-up, dimensionless
Subscripts B C D D¹ G ¸ R smb
bottom column downcomer draught tube gas liquid riser small bubble
4943
References Allen, D. G., & Robinson, C. W. (1989). Hydrodynamics and mass transfer in Aspergillus niger fermentations in bubble column and loop reactors. Biotechnology and Bioengineering, 34, 731}740. Chisti, M. Y. (1989). Airlift bioreactors. Elsevier: London. Chisti, Y., & Moo-Young, M. (1988). Prediction of liquid circulation velocity in airlift reactors with biological media. Journal of Chemical Technology and Biotechnology, 42, 211}219. Clift, R., & Grace, J. R., et al. (1978). Bubbles, drops and particles. New York: Academic Press. Erickson, L. E., Patel, S.A. et al. (1983). E!ects of viscosity and small bubble segregation on mass transfer in airlift fermenters. Process Biochemistry, 16}37. Goto, S., & Gaspillo, P. D. (1992). The e!ect of static mixer on mass transfer in draft tube bubble column and in external loop column. Chemical Engineering Science, 47(13-14), 3533}3539. Hoeks, F. W. J. M. M., & Wees-Tangerman, C., et al. (1997). Stirring as foam disruption (SAFD) technique in fermentation processes. The Canadian Journal of Chemical Engineering, 75(December), 1018}1029. Karamanev, D. G., & Chavarie, C., et al. (1996). Hydrodynamics and mass-transfer in an airlift reactor with a semipermeable draft tube. Chemical Engineering Science, 51(7), 1173}1176. Kawase, Y., & Tsujimura, M. (1994). Enhancement of Oxygen-Transfer in Highly Viscous Non-Newtonian Fermentation Broths. Biotechnology and Bioengineering, 44(9), 1115}1121. Li, G. Q., & Qiu, H. W., et al. (1995). E!ect of #uid rheological properties on mass-transfer in a bioreactor. Journal of Chemical Technology and Biotechnology, 62(4), 385}391. Metkin, V. P., & Sokolov, B. N. (1982). Hydraulic resistance to #ow of gas-liquid mixtures having non-Newtonian properties. Zhurnal Prikladnoi Khimii, 3, 614}619. Muller, F. L., & Davidson, J. F. (1992). On the contribution of small bubbles to mass transfer in bubble columns containing highly viscous liquids. Chemical Engineering Science, 47(13-14), 3525}3532. Philip, J., & Proctor, J. M., et al. (1990). Gas hold-up and liquid circulation in internal loop reactors containing highly viscous Newtonian and non-Newtonian liquids. Chemical Engineering Science, 45(3), 651}664. Popovic, M., & Robinson, C. W. (1987a). The speci"c interfacial area in external-circulation-loop airlifts and a bubble column * II. Carboxymethyl cellulose/sulphite solution. Chemical Engineering Science, 42(12), 2825}2832. Popovic, M., & Robinson, C. W. (1988). External-circulation-loop airlift bioreactors: study of the liquid circulating velocity in highly viscous non-newtonian liquids. Biotechnology and Bioengineering, 32, 301}312. Rohr, M., & Kubicek, P. C., et al. (1983). Citric acid. Verlag Chemie weinheim. In R. G. Rehm, & H. J. Weinheim, Biotechnology, vol. 3 (pp. 419}454). Stejskal, J., & Potucek, F. (1985). Oxygen transfer in liquids. Biotechnology and Bioengineering, XXVII, 503}508.
Periodical changes of input air #owrate } a possible way of improvement of oxygen transfer and liquid circulation in airlift bioreactors S[ . GodoH *, J. Klein, M. Polakovic\ , V. BaH les\ Department of Chemical and Biochemical Engineering, Faculty of Chemical Technology, Slovak University of Technology, Radlinske& ho 9, 812 37 Bratislava, Slovakia, Slovak Republic
Abstract The in#uence of small bubbles on the behaviour of airlift reactors during citric acid fermentation was studied. The small bubbles formed up to 70% of the total gas hold-up, so their distribution strongly in#uenced the predicted liquid circulation velocities. A periodically changing air #owrate could be used to signi"cantly improve the oxygen transfer capacity of airlift reactors. This could be achieved by the e!ective utilisation of the oxygen content of small bubbles, which originated from the foam. A procedure based on the evaluation of transient pO and gas hold-up pro"les was suggested for the determination of the period of sparging. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Airlift; Mass transfer; Gas hold-up; Liquid circulation; Non-Newtonian broth; Bubble
1. Introduction Several new types of bioreactors have been investigated in the last two decades in order to improve the fermentation performance. Bubble columns and their modi"cations, namely airlift bioreactors (ALR), are considered to be very promising, particularly due to their simpler construction, better de"ned #ow patterns, lower shear "elds and lower power input in comparison with conventional stirred tank reactors. Their wider application is hindered by the lack of theoretical knowledge, which would allow a reliable modelling of complex twoor three-phase #ow phenomena. A considerable e!ort has been devoted to the improvement of oxygen transport in viscous fermentation broths (Goto & Gaspillo, 1992; Karamanev et al., 1996; Stejskal & Potucek, 1985). In most cases static mixers have been used to redisperse larger bubbles. These may however slow down the liquid circulation and growing microorganisms may stick to their surface causing the clogging of the #ow path. It has been shown several times that in viscous mould broths a bimodal distribution of bubble sizes exists (Philip et al., 1990; Allen & Robinson, 1989; Kawase * Corresponding author. Tel.: 421-7-59325-265; fax: 421-7-396-743; e-mail address: [email protected].
& Tsujimura, 1994; Muller & Davidson, 1992). The role of small bubbles has unfortunately not been analysed in detail. Most authors (Chisti, 1989; Li et al., 1995) concluded that a long presence of small bubbles in the reactor was not desirable because the gas inside them would be in equilibrium with the bulk liquid in a relatively short time. Erickson et al., 1983 analysed the role of the recirculation of small bubbles in airlift reactors. They concluded that small bubbles could signi"cantly improve the oxygen transfer rate, provided that they could be regenerated in a rate comparable with their depletion. The main goal of this study is to show that fresh small gas bubbles could be generated and the depleted ones removed by choosing a certain periodic gas #owrate pro"le. In this way the oxygen transfer rate in ALR could signi"cantly be improved. This could surprisingly be achieved by decreasing the power input into the reactor. The knowledge of small bubble hold-up and its distribution between the reactor sections was found to be essential for the prediction of liquid circulation velocity.
2. Experimental Three citric acid fermentations on synthetic media (with sucrose as the only carbon source) using the mould
0009-2509/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 9 ) 0 0 2 1 5 - 8
S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
4938
strain Aspergillus niger were carried out in a 195 l concentric tube airlift reactor made of glass. The details of reactor geometry are given in Table 1 and Fig. 1 and the reactor set-up is shown in Fig. 2. Air was introduced through a stainless-steel perforated plate with holes of diameter of 1 mm placed under the bottom of the draught tube (the riser section of the reactor). Rheological properties of the broth were measured o!-line with a coaxial cylinder viscometer Rheotest II (MLW PruK fgeraK te-Werk, Medingen, Germany). The dependencies of shear stress on shear rate were evaluated according to the Ostwald de Waele model. The liquid circulation velocity was measured through a visual observation of the movement of coloured #ow-follower particles of the same density as the broth. The gas holdup was measured by noting the dispersion height. The hold-ups of small and large bubbles were measured by noting the transient dispersion height after aeration stop and restart. The oxygen probe (Jenway, England) was "tted in the downcomer, 60 cm above the bottom of the reactor.
Table 1 Constructional details of the airlift reactor used in this study, D * col! umn diameter, H * liquid height, H * draught tube height, * "2 D * riser tube diameter, A /A * downcomer/riser cross-sectional 0 " 0 area ratio, H * bottom clearance, H * distance between measuring pO electrode and the reactor bottom, N * number of holes in gas sparger Working D ! volume (¸) (m)
H * (m)
H "2 (m)
D 0 (m)
A /A H " 0 (-) (m)
H (m)
N (-)
195
2.96
2.70
0.200
1.01
0.60
90
0.295
0.061
Fig. 1. 195 l internal loop airlift reactor.
Fig. 2. Reactor set-up.
3. Results and discussion 3.1. Flow regimes The production of citric acid is an example of a process where the dissolved oxygen concentration is a key factor for the success of the whole fermentation (Rohr et al., 1983). We have carried out three citric acid fermentations in a 195 l internal loop ALR with an aspect ratio (H /D ) * ! of ten. The time course of the measured dissolved oxygen concentration displayed a typical minimum at approximately 16}24 h, which was caused by a rapidly rising oxygen demand of the microbial culture. At the same time, rapid changes in rheological properties of the broth were observed. The time courses of the power law parameters are shown in Fig. 3. The broth became markedly pseudoplastic, which was accompanied by a strong coalescence of gas bubbles just above the sparger. The coalescence led to a change of the #ow regime from homogeneous bubbly to slug #ow. This #ow regime change seemed to be unavoidable, even when a sintered plate sparger was used. This indicates that the broth viscosity governs the size of bubbles formed. This phenomenon is well known from experiments with model (such as CMC) (Popovic & Robinson, 1987a; Popovic & Robinson, 1988) as well as with real fermentation media (Allen & Robinson, 1989). From the start of slug formation it was extremely di$cult to keep the oxygen level on the desired value. The simple increase of gas
S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
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Fig. 3. Time courses of the rheological parameters and the biomass concentration.
Fig. 4. Dissolved oxygen and gas hold-up pro"les during paused and resumed sparging. 113th hour of fermentation; air super"cial velocity ; "0.0812 m s\. %0
#owrate did not help too much, after a few minutes of moderate increase the oxygen concentration began to decrease again at any #owrate. The big Taylor bubbles passing the riser have a very short residence time and a small speci"c interfacial area; thus, they contribute little to oxygen transfer. Approximately from the onset of the slug #ow a relatively strong foaming occured. According to our visual observations, the foam was mostly generated at the top of the reactor by bursting of large bubbles. In fermentations, foaming is generally considered to be undesirable, because it represents a dead space (Hoeks et al., 1997). The small (about 1 mm of diameter) bubbles of the foam were drawn by the downward liquid #ow into
the downcomer. We observed visually that many small bubbles were accumulated along the walls of the downcomer. Thus, a bimodal bubble size distribution existed in the reactor, consisting of large bubble slugs (with the same diameter as the riser) and small bubbles, approximately 1 mm of diameter. 3.2. Gas hold-up and circulation velocity The measured gas hold-up curves during paused and resumed sparging are shown in Fig. 4. In the "rst seconds, solely big slugs formed the gas hold-up. The slow increase of gas hold-up after about 1 min of sparging is
4940
S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
Fig. 5. Measured downcomer liquid velocities. 113th hour of fermentation.
attributable to the entrainment of small bubbles, which were drawn by the liquid #ow into the downcomer and thus reduced the circulation velocity. The results of the measurement of the downcomer liquid velocities after restarted sparging, which are shown in Fig. 5. (113th hour of the fermentation), con"rm the above-mentioned accumulation of small bubbles in the downcomer. After the start of sparging, the initial liquid velocity was high due to the big holdup di!erence between the riser and downcomer (only large bubbles were present in the riser). This fast liquid #ow drew the foam, which began to form at the liquid surface, into the downcomer, thus reducing the riser}downcomer density di!erence, and hence the circulation velocity. The entrainment of the small bubbles into the downcomer and the decrease of velocity were the fastest at the start of sparging. The steady-state values of circulation velocity and total gas hold-up were reached approximately 13 min after the aeration restart. The circulation velocity decreased to a value of about 0.04 m s\ and was almost independent of the aeration rate. 3.3. Prediction of liquid circulation velocity The liquid velocity can be predicted from the energy balance model of Chisti and Moo-Young (1988). One should however take into account the hold-up of small bubbles and its distribution between the riser and downcomer. The correlation used by Chisti and Moo-Young (1988) their Eq. (18)). to predict the riser gas hold-up taken from Metkin and Sokolov (1982) predicts fairly well only the hold-up of large bubbles. This is shown in Fig. 6. It can be seen that the simulated lines approximate closely the measured large bubble hold-up ("lled symbols). For simplicity only the experimentally determined large bubble hold-ups for
113 h of the experiment are shown, but this part of the hold-up was almost constant during the whole experiment. This observation is in line with expectations, as the slug velocity should be independent of viscosity (Clift et al., 1978) and also with the predictions of Eq. (18) of Chisti (simulated e lines lie very close to each other % despite of viscosity changes). By comparison of the large bubble hold-up with the measured total gas hold-up (open symbols) it can be seen that the small bubbles made up about 60}70% of the total gas hold-up. The small bubbles are distributed between the riser and the downcomer. This distribution can be represented by a factor g, representing the portion of small bubbles present in the riser. Thus, e *g and e *(1!g) are the hold-up of %QK@ %QK@ small bubbles present in the riser and downcomer, respectively. The total riser gas holdup will be the sum of the large bubble hold-up and e *g, whereas in the %QK@ downcomer only small bubbles are present (e *(1!g)). %QK@ The di!erence between riser and downcomer hold-up, representing the driving force of liquid circulation, is therefore dependent on the distribution factor g. Any chosen g value will in combination with the measured large and small bubble hold-ups de"ne the driving force. Inserting these values in the model yields a calculated liquid velocity < . By comparing the experimental *" values of liquid circulation velocity with these predicted by the model we could calculate the correct value of g, thus the distribution of small bubbles between the riser and downcomer. This distribution is presented in Fig. 7. The curves (< sim.) indicate that the more small *" bubbles are present in the reactor, the more their distribution in#uences the predicted liquid circulation velocity. In 15 h of the experiment, when there are relatively less small bubbles (as shown in Fig. 6) the distribution coe$cient g may have values between almost zero and 0.8 (solid line), whereas in later stages its possible values
S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
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Fig. 6. Measured and predicted gas hold-up values - simulation made using the Eq. (18) of Chisti and Moo-Young (1988).
Fig. 7. Downcomer liquid velocity as function of the distribution of small bubbles.
range from approximately 0.23 to 0.55. The lower bound simply shows that if g would be less, the downcomer hold-up would be greater than that of the riser, which would cease the circulation. The correct values of the distribution coe$cient g are taken from the intersection of the calculated < !g lines (< sim.) with the experi*" *" mental < values (open symbols * < exp.). As can be *" *" seen they are between 0.17 and 0.31. The lowest value was calculated for the fermentation time of 15.5 h, just on the onset of slug #ow, when the foaming was not still so strong (compare the holdup for 15.5 h in Fig. 6). The distribution coe$cient depends probably on the reactor geometry, liquid properties and gas #owrate but further studies are needed to con"rm this hypothesis. Neverthe-
less, these preliminary results show that neglecting the hold-up of small bubbles may lead to serious errors in the estimation of the circulation velocity. The most important conclusion of the modelling was that despite of their virtually negligible rise velocity, these small bubbles are not distributed evenly between the riser and downcomer } a feature, which could be utilised, as shown in the next section. 3.4. Proxle of sparging The measured pO and gas hold-up curves after stopped and restarted sparging are shown in Fig. 4. It can be seen that after the start of sparging, the oxygen
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S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
Fig. 8. Time course of the dissolved oxygen level pO during periodical change of air #owrate: Q "21 Ln/min (; "0.0110 m s\) !1.5 min, %0 Q "153 Ln/min (; "0.0812 m s\) !13 min; ¹"203C, p"1 atm. %0
concentration increases at the beginning, but decreases to almost zero after a certain time despite of the slowly increasing gas holdup. As mentioned above, this happens at any gas #owrate. At this time, the small bubbles, which are accumulated at the downcomer wall, are mostly depleted of oxygen and do not contribute to the oxygen transfer any longer. Moreover, their simple presence in the downcomer causes a low circulation velocity, thus lowering the entrainment of fresh bubbles into it. The only possibility to overcome the oxygen shortage is therefore to get rid of the small depleted bubbles, and get fresh ones into the liquid. Airlift reactors possess a unique feature among pneumatically agitated reactors * a well-de"ned #ow path. The small bubbles are collected by the velocity pro"le along the walls of the downcomer and after a certain time they even form small `channel-like cloudsa. After stopping the sparging, these bubble swarms at the walls escape very fast from the reactor in contrast to those in the bulk liquid, which need a much longer time to leave the broth. This feature of the airlift reactor allows us to get rid of the small depleted bubbles in a relatively short time and represents an important di!erence between ALRs and bubble columns (mechanically stirred reactors also), where the small bubbles are distributed more evenly due to the di!erent #ow pro"les. It follows from the previous considerations that shutting o! or considerably decreasing the air #owrate for short time periods may markedly increase the oxygen transfer capacity of airlift reactors. As mentioned above, the fast liquid movement after restarted sparging draws fresh small bubbles into the downcomer, which causes a dramatic increase in oxygen transfer. We tested this hypothesis by periodically changing the air #owrate. The time periods for the minimum and
standard air #owrates were determined from Fig. 4. The time period necessary to let most of the small bubbles to escape was determined from the holdup curve after an air interruption (approxiamately 1.5 min). The time period of the standard #owrate was determined as the time from the start of standard aeration to the point, when the oxygen level fell below a critical value (approximately 13 min). Thus, the reactor was sparged at the standard #owrate for 13 min, then the air #owrate was decreased to a minimum level for 1.5 min and the sparging cycle was repeated. We could, in principle, have shut o! the sparging completely, but we needed to maintain a certain albeit low liquid movement around the pO probe. The resulting dissolved oxygen pro"le is shown in Fig. 8. The dissolved oxygen level increased after each air interruption and restart to levels, which could not be achieved by increasing the air #owrate. To achieve a more even pO level, the sparging cycle should be further optimised. Further studies of the initial transient liquid circulation and gas hold-up distribution will also be necessary.
4. Conclusions We have shown that for the success of prediction of liquid circulation velocities in non-Newtonian fermentation media it is decisive to know the holdup of di!erent bubble sizes. As in these media a bimodal bubble size distribution exists, the predicted liquid circulation velocity is very sensitive to the distribution of the small bubbles between the riser and the downcomer. We have observed and con"rmed by calculations that most of the small bubbles were accumulated in the downcomer.
S. Godo& et al. / Chemical Engineering Science 54 (1999) 4937}4943
It was found that periodic changes of gas #owrate can increase the dissolved oxygen concentration in airlift reactors. This is due to the e$cient utilisation of oxygen contents of small bubbles originating from the foam. The time periods of di!erent sparging #owrates were optimised on the basis of unsteady gas hold-up and dissolved oxygen curves. Further detailed studies on the unsteadystate generation of small bubbles and their in#uence on oxygen transfer have to be carried out. This study has shown that keeping the ALR in a hydrodynamically unsteady state the reactor can be operated e$ciently also with highly viscous fermentation broths. Notation A D g
area, m diameter, m distribution coe$cient of small bubbles, dimensionless g"e /e , %QK@0 %QK@ H height, clearance, m H height of stainless section above the reactor bot tom, m H height of stainless-steel section/probe housing/, m K consistency index, Pa sL n Flow behaviour index, dimensionless P pressure, Pa pO dissolved oxygen level relative to saturation, % Q gas #owrate, m s\ ; super"cial velocity, m s\ < linear velocity <";/e, m s\ Greek letters e
gas hold-up, dimensionless
Subscripts B C D D¹ G ¸ R smb
bottom column downcomer draught tube gas liquid riser small bubble
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