Gas–liquid Mass Transfer in Sonicated Bubble Columns. Effect of

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Gas−liquid Mass Transfer in Sonicated Bubble Columns. Effect of Reactor Diameter and Liquid Height N. Sainz Herrán, J. L. Casas López,* and J.A. Sánchez Pérez Department of Chemical Engineering, University of Almería, 04120 Almería, Spain ABSTRACT: The enhancing effects of ultrasound on different aspects of bioprocesses have been repeatedly reported in the literature. These have usually been attributed to the mass transfer increase induced by sonication in fermentation processes. However, until now, this has not been analyzed in detail. In this work, the influence of ultrasonication on the overall gas−liquid oxygen transfer coefficient, KLa, has been studied as a function of reactor geometry and liquid height. Bubble columns ranging from 0.5 to 20 L were used. The effect of temperature on KLa was checked and no significant variation was observed between 25 and 45 °C. Therefore, the temperature changes caused by the application of ultrasound had no effect on the KLa measurement. The effects of ultrasonication were only significant when applied to small volumes resulting in power densities above 400 kW·m−3, which are two orders higher than the standard mechanical power density supplied for mixing in fermentation (1−2 kW·m−3). Additionally, the enhancement obtained from ultrasound depended on the superficial gas velocity and the height/diameter ratio (H/D). Sonication only enhanced KLa at H/D ratios lower than 1, which corresponds more to the geometry of an ultrasonic bath than to a bubble column. Regardless of the ultrasound supply, when the H/D ratio increased from 0.3 to 3, the KLa significantly decreased regardless of the Ug value. However, for H/D ratios higher than 3, no influence of the H/D ratio on KLa was observed.

1. INTRODUCTION The initial application of ultrasound in biotechnology was used mainly to break down cell walls in order to liberate their contents.1−4 The intensity level needed is related to the location of those compounds, therefore only superficial cell rupture by low ultrasound intensity is sufficient in some cases.5 However, in recent years, biotechnological processes that use cells or free enzymes activated by ultrasound have been reported in the literature.6−9 The effect of ultrasound can be either destructive or constructive in biotechnology depending on the ultrasonic wave intensity and the exposure time.10 The mechanisms involved in certain productivity improvements taking place under sonication have not been totally identified yet.7,11 However, it is assumed that mechanical effects can generate improvements in gas−liquid12,13 and solid−liquid transfer processes.14,15 Another important factor is the movement and mixing of fluid that liquid currents can generate.16 Examples of the beneficial influence of cavitation and microcurrents are the stimulation of some enzymatic and microbiological reactions.17,18 Laugier et al., 2008, proposed that the main effect of sonication at 20 kHz (the frequency at which some sonochemical reactions are accelerated) is mechanical, so this effect is related to interfacial transfer kinetics due to an increase of the interfacial area in the ultrasonic bubbles breakdown process.13 For this reason, it is relevant to further investigate how ultrasound can influence mass transfer phenomena. Most of the references describe sonication experiments carried out on small volumes, in which the specific ultrasound power supplied has been very high, especially for biotechnology processes. In contrast, the possibility of generating mass transfer improvements in fermentation by ultrasound application has not been analyzed in detail yet. © 2012 American Chemical Society

The power density supplied by the ultrasound generator (W·m−3) is directly related to the density of energy, ω (J·m−3), and this is directly proportional to the square of the amplitude of the pressure wave P (Pa).6 Therefore, the higher the ultrasound power supplied, the higher becomes the amplitude of the pressure wave and the influence of the ultrasounds on the mass transfer properties of the system because of an increase in the turbulence and the gas liquid contact area due to the breakdown of the bubbles. However, effective sonication requires the energy input to exceed the cavitation threshold throughout the working volume, which can vary widely between 15 and 65 kW·m−3, depending of the liquid phase properties. In a sonobioreactor, the cavitational threshold energy is not exceeded in most of the reactor volume.6 The supplied energy should be considered as a possible limiting factor due to its great influence on culture cells: for example, high sonication intensity levels markedly affect the morphology of the fungus Aspergillus terreus, from pellets to dispersed hyphae, causing a decrease in lovastatin productivity. This consequence is mainly attributed to the metabolite secretion mode, which usually takes place with pellet morphology. At the same time, the ultrasound application cycle was proven to be the most influential parameter.19,20 The present work tackles the necessity to know the effect of ultrasound application in mass transfer enhancement in sonobioreactors. The overall gas−liquid volumetric oxygen transfer coefficient, KLa, as a function of superficial gas velocity, Ug, has been measured under ultrasound application in bubble Received: Revised: Accepted: Published: 2769

July 18, 2011 January 9, 2012 January 16, 2012 January 16, 2012 dx.doi.org/10.1021/ie201559e | Ind. Eng.Chem. Res. 2012, 51, 2769−2774

Industrial & Engineering Chemistry Research

Article

The pore size might affect the distribution of the gas and, hence, the values of the gas holdup and the mass-transfer coefficient. However, the same sparger has been used for all cases in the present work so there is no dependency on the pore size. Although a number of techniques have been developed to measure the volumetric gas−liquid mass transfer coefficient, dynamic methods are used as a preference, as they are fast, experimentally straighforward and applicable to various systems.21 In the present work, a fast-responding polarographic oxygen electrode (Crison 6050) was employed to measure the variation of dissolved oxygen concentration in the liquid phase over time and to calculate the volumetric oxygen transfer coefficient, KLa, as described by Casas López et al.22 The KLa measurements in the different bubble columns were carried out at different superficial gas velocities, Ug (m·s−1), maintaining the liquid at room temperature. The Ug values assayed were in the range of 0.0014−0.0096 m·s−1 for the lab scale columns and 0.0047−0.0189 m·s−1 for the pilot plan scale columns. The influence of ultrasound seemed to be higher in the desorption process than in the absorption process, which can be influenced by the different solubility of the gases used (nitrogen and oxygen).13 For this reason, the results present only KLa values obtained from the data of the desorption process. The experimental conditions are shown in Table 1.

columns. Several reactor volumes, ranging from 0.5 to 20 L, were used to evaluate the influence of reactor geometry and liquid volume on KLa enhancement by ultrasounds. Vessel diameter, D, and liquid height, H, are the geometric variables that have been considered for the study. The main objective was to extend the discussion on possible mass transfer improvements at different height/diameter (H/D) ratios in order to evaluate the advantages of sonication in reactors scale-up.

2. MATERIALS AND METHODS Experiments have been performed in bubble columns using an ultrasonic horn (diameter 22 mm, length 300 mm, model H22L3D; Dr. Hielscher GmbH, Stuttgart, Germany) connected to a generator (Ultrasonic Processor model UP400S; Dr. Hielscher GmbH, Stuttgart, Germany) with variable amplitude and cycle. The ultrasonic equipment operates at a fixed frequency of 20 kHz and delivers a maximum power of 320 W, from which 120 W are transmitted through the lateral surface and 200 W from the tip. Experiments were carried out at the maximum power supply. The sonotrode was inserted in the bubble column at the headplate (Figure 1).

Table 1. Experimental Conditions for KLa Determinationa diameter (m)

liquid height (m)

volume (L)

H/D ratio

ultrasound power density (kW·m‑3)

0.088

0.08 0.16 0.34 0.06 0.12 0.24 0.04 0.08 0.16 0.30 0.92 1.02

0.5 1 2 0.5 1 2 0.5 1 2 4 12 20

0.91 1.82 3.86 0.57 1.10 2.24 0.30 0.60 1.20 2.26 7.4 6.8

460 260 160 440 240 150 420 230 130 80 26 16

0.105

0.133

0.125 0.150 a

Last column refers to sonicated experiments.

The influence of temperature on the response of the oxygen electrode was checked at three temperatures, 20, 35, and 45 °C. The parameters to be considered of the first order model which usually follow the probes, described by several authors,23,24 were the delay time, tp, and the time constant of the electrode, which is calculated as the inverse of the time needed to reach 63.2% of the final response (final dissolved oxygen concentration). Measurements to check temperature effect were conducted in two cylindrical glass vessels with a volume capacity of 3 L (0.13 m diameter and 0.26 m height), provided with an external jacket. The liquid used was 2 L of distilled water and the air flow rate was maintained at 2 L/min (1vvm). Following a procedure similar to that proposed by Vandu and Krishna, one of the vessels was bubbled with air and another with nitrogen.23 The oxygen electrode was previously calibrated at 100% saturation in water sparged with air. Then it was placed in the vessel sparged with nitrogen until the reading of oxygen concentration dropped to zero. After that, the probe was

Figure 1. Gas liquid contactor set up.

Five bubble columns were employed to check the possible influence that ultrasound might have at such volumes on mass transfer processes. The assayed diameters were 0.088 m, 0.105 and 0.133 m, for lab scale volumes, between 0.5 and 4 L, and 0.125 and 0.150 m, for pilot plant scale volumes (12 and 20 L). In lab scale cases, the sonotrode was situated 1 cm from the sparger (A, Figure 1) so that the power density supplied depended on the liquid volume treated, which also determines the percentage of submerged sonotrode. In the pilot plant scale, the sonotrode was completely submerged and the tip situated 62 cm from the sparger in the 12 L bubble column and 72 cm in the 20 L bubble column (A, Figure 1). In all cases, air was sparged using a perforated plate located at the bottom of the contactor with 1 mm holes separated 1 cm. 2770

dx.doi.org/10.1021/ie201559e | Ind. Eng.Chem. Res. 2012, 51, 2769−2774


Article

sonotrode vicinity, where the energy exceeds the cavitation threshold,6 and cannot be extended effectively to larger volumes due to the waves attenuation. The effect of ultrasound in the bubble columns between 0.5 and 4 L was also investigated. Figure 2 shows the oxygen

rapidly submerged in the first vessel, to reach again saturation conditions. To determine the effect of temperature on the KLa measurement, distilled water in a bubble column with an internal diameter of 0.133 m was saturated with oxygen by air bubbling and its temperature was controlled. When the liquid had the desired temperature (25, 35, and 45 °C), a nitrogen flow was bubbled into the column to get desorption data.

3. RESULTS AND DISCUSSION 3.1. Influence of Temperature on Mass Transfer Coefficient Determination. The experiments about temperature influence on the oxygen electrode showed a delay time between 4.4−6.4 s and a time constant that varied from 0.06 to 0.11 s−1 for the temperature variation of 20−45 °C. These values are significantly higher than the KLa measured under sonication that are shown later, confirming the suitability of the selected electrode given its quick response for the proposed measurements. As expected, KLa increased with the gas flow rate, but no significant variation of KLa values with temperature was found in the range tested, which amply covers the operating conditions expected for the culture of microorganisms. In previous works, the temperature was controlled at 28 °C in all sonicated fermentations.19,20 Therefore, it is ensured that the temperature changes caused by ultrasound application have no effect on the KLa measurement. 3.2. Effect of Ultrasound Application on KLa in Bubble Columns. The velocity of ultrasound in dispersions (as in bioreactors) is different from in pure liquid, where it is faster than in gas. Sound is vibrational energy passed from molecule to molecule. It is easier for sound waves to travel through liquid than through gas because the distance between liquid molecules is shorter. In this sense, the attenuation of ultrasound energy is a function of the gas holdup,25,26 which usually increases with high superficial gas velocities. Under these conditions, where the bubble concentration dispersed in the aerated liquid also increases, the sound wave is rapidly attenuated and more of the ultrasonic energy is lost. However, high hold-up values involve higher interfacial area and a higher mass transfer coefficient.27 Therefore, it can be expected that the mass transfer coefficients measured will be higher, although the enhancing effects of ultrasound will be weaker at high superficial gas velocities. The effect of ultrasound application on KLa in bubble columns at a scale of 12 and 20 L is shown in Table 2, where no

Figure 2. Mass transfer coefficient variation under different superficial gas velocities and volumes at a bubble column diameter of 0.088 m, with and without sonication.

transfer coefficient in a bubble column of 0.088 m diameter at several liquid heights. The effect of US in mass transfer can only be appreciated with the lowest sonicated liquid volume, which corresponds to a water height of 0.08 m, resulting in an average increase of 21.47 ± 11.71%. For higher volumes in this same configuration, heights 0.16 and 0.34 m, there was no significant enhancement on mass transfer coefficients compared to those obtained without ultrasound application at the same superficial gas velocity. In the 0.105 m diameter bubble column (Figure 3), the measured KLa presented similar behavior as in the previous

Table 2. Mass Transfer Coefficient in 12 and 20 L Reactors with and without Sonication

12 L reactor 20 L reactor

aeration flow (L·min‑1)

Ug (m·s‑1)

KLa (s−1)

US KLa (s−1)

5 10 5 20

0.0068 0.0136 0.0047 0.0189

0.0066 0.0105 0.0047 0.0147

0.0065 0.0103 0.0051 0.0151


enhancement in the mass transfer coefficient was observed at ultrasound (US) power densities of 26 and 16 kW·m−3 for these volumes, respectively. Nonetheless, these values of power density supplied caused pellet disruption and hyphae growth morphology in Aspergillus terreus cultures.19,20 A reason for this difference in response to ultrasound application is that the amplitude of the sonic waves is only high enough in the

case. Sonication only influenced the mass transfer coefficient at the lowest liquid height of 0.06 m, the average increase being 26.53 ± 5.42%. 2771



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turbulence. Consequently, the effects on KLa decrease, giving only a small level of enhancement. Therefore, gas bubble distribution in the cavitationally active zone is decisive in intensifying mass transfer due to the presence of ultrasound. The presence of a dispersed phase contributes to wave attenuation and, thus, the active sonicated part is restricted to a zone located in the vicinity of the emitter.14 However, at greater superficial gas velocities, the H/D ratio has a higher effect on the mass transfer coefficient, regardless of the ultrasound power density supplied (Figure 5). Gas holdup

At increasing bubble column diameters, sonication had even less influence on the mass transfer process. For a 0.133 m bubble column diameter (Figure 4), the liquid height at which


KLa increased with sonication was 0.04 m, giving an average increase of 28.77 ± 9.48%; the larger the diameter, the lower the liquid height for which KLa enhancements caused by ultrasound were found. In the three cases, the ultrasound power density supplied to the liquid was over 400 kW·m−3 markedly higher than the mechanical power density supplied for mixing in fermentation (1−2 kW·m−3) or gas expansion in bubble columns (∼ 0.2 kW·m−3). The mass transfer coefficient showed a proportional relationship to the bubble column diameter; see Figures 2 and 4 for a liquid height of 0.016 m. This fact can be attributed to a better gas distribution through the liquid at larger diameters. The presence of ultrasound at a given gas flow rate is supposed to cause an increase in the mass transfer coefficient, which can be attributed to the additional interfacial turbulence created by ultrasonic irradiation.28 The analysis of the results indicates that the KLa enhancement obtained by ultrasound application depends on the superficial gas velocity, Ug. An increase of the Ug values produced an increase of the ultrasound effects on the KLa, nevertheless this influence disappeared for the highest Ug values assayed. This might be related to a more intense attenuation of the ultrasound waves due to the increased gas holdup rather than to the liquid turbulence. At lower gas flow rates, the energy associated with the gas is much less, so the ultrasound contribution is comparatively more important and the enhancing effects of ultrasound are observed more easily. The ultrasonic horn can generate convective currents that can distribute more evenly the bubbles paths, helping to maintain them longer in the system. The longer residence times in the active cavitation zone also favors a reduction in the bubble size, factors which sum up to enhance mass transfer.12 The intensification magnitude also depends on the attenuation of the ultrasonic energy by the presence of gas. At higher flow rates, the attenuation will be more intense,29 leading to a less ultrasonic energy available for the dispersion and breakage of gas bubbles or the creation of interfacial

Figure 5. Effect of H/D ratio on the overall mass transfer coefficient as a function of the superficial gas velocity at several ultrasound power densities.

variation is closely related to flow pattern in the bubble column so the differences are more noticeable at high values of superficial gas velocities. This relationship is in accordance with Cachaza et al., 2008, who also experimented at several H/D ratios.30 However, other authors have obtained contradictory results.23,31,32 In this sense, Vandu and Krishna, 2004, assert that the gas holdup decrease found at increasing column diameters is due to an increase of the liquid recirculation in the scale-up process. These authors also found that, for high superficial gas velocities, the influence of the diameter on the mass transfer coefficient is not noticeable, which is contradictory with the results presented in this work. Any significant comparison among the results of different authors should take into account the operational conditions and the dimensions of the columns used in the experiments. It has been demonstrated that large diameters and liquid heights (large liquid volume) induce a decrease in the ultrasound influence on the mass transfer coefficient and that sonication only enhanced KLa with H/D ratios lower than 1. These dimensions are rather similar to those of an ultrasonic bath. 3.3. Effect of H/D Ratio on KLa. Figure 6 shows the KLa variation with superficial gas velocity, with the H/D ratio as a parameter. Data were obtained from different columns with several diameters, as given in Table 1. As the H/D ratio increases from 0.3 to 3, the KLa significantly decreases for a given Ug value. The improvement in mass transfer at a low H/D ratio can be attributed to the formation of additional currents in the fluid that retain more bubbles and increase their residence time. At low superficial gas velocities, the fluid movement is insufficient to retain bubbles so the increase in H/D does not 2772



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(7) Lin, L.; Wu, J. Enhancement of Shikonin production in singleand two-phase suspension cultures of Lithospermum erythrorhizon cells using low-energy ultrasound. Biotechnol. Bioeng. 2002, 78, 81−88. (8) Pitt, W. G.; Ross, S. A. Ultrasound increases the rate of bacterial cell growth. Biotechnol. Process. 2003, 19, 1038−1044. (9) Zhang, L.; Liu, Z. Optimization and comparison of ultrasound/ microwave assisted extraction (UMAE) and ultrasonic assisted extraction (UAE) of lycopene from tomatoes. Ultrason. Sonochem. 2008, 15, 731−737. (10) Sinisterra, J. V. Application of ultrasound to biotechnology: an overview. Ultrasonics 1992, 30, 180−185. (11) Zhang, G.; Zhang, P.; Gao, J.; Chen, Y. Using acoustic cavitation to improve the bio-activity of activated sludge. Bioresour. Technol. 2008, 99, 1497−1502. (12) Kumar, A.; Gogate, P. R.; Pandit, A. B.; Delmas, H.; Wilhelm, A. M. Gas-liquid mass transfer studies in sonochemical reactors. Ind. Eng. Chem. Res. 2004, 43, 1812−1819. (13) Laugier, F.; Andriantsiferana, C.; Wilhelm, A. M.; Delmas, H. Ultrasound in gas-liquid systems: Effects on solubility and mass transfer. Ultrason. Sonochem. 2008, 15, 965−972. (14) Romdhane, M.; Gourdon, C. Investigation in solid−liquid extraction: influence of ultrasound. Chem. Eng. J. 2002, 87, 11−19. (15) Vargas, L. H. M.; Piao, A. C. S.; Domingos, R. N.; Carmona, E. C. Ultrasound effects on invertase from Aspergillus niger. World J. Microbiol. Biotechnol. 2004, 20, 137−142. (16) Schläfer, O.; Sievers, M.; Klotzbucher, H.; Onyeche, T. I. Improvement of biological activity by low-energy ultrasound-assisted bioreactors. Ultrasonics 2000, 38, 711−716. (17) Barton, S.; Bullock, C.; Weir, D. The effects of ultrasound on the activities of some glycosidase enzymes of industrial importance. Enzyme Microb. Technol. 1996, 18, 190−194. (18) Wood, B. E.; Aldrich, H. C.; Ingram, L. O. Ultrasound stimulates ethanol production during the simultaneous saccharification and fermentation of mixed waste office paper. Biotechnol. Prog. 1997, 13, 232−237. (19) Sainz Herrán, N.; Casas López, J. L.; Sánchez Pérez, J. A.; Chisti, Y. Effect of ultrasound on culture of Aspergillus terreus. J. Chem. Technol. Biotechnol. 2008, 83, 593−600. (20) Sainz Herrán, N.; Casas López, J. L.; Sánchez Pérez, J. A.; Chisti, Y. Influence of ultrasound amplitude and duty cycle on fungal morphology and broth rheology of Aspergillus terreus. World J. Microbiol. Biotechnol. 2010, 26, 1409−1418. (21) Gogate, P. R.; Pandit, A. B. Survey of measurement techniques for gas liquid mass transfer coefficient in bioreactors. Biochem. Eng. J. 1999, 4, 7−15. (22) Casas López, J. L.; Rodríguez Porcel, E. M.; Oller Alberola, I.; Ballesteros Martín, M. M.; Sánchez Pérez, J. A.; Fernández Sevilla, J. M.; Chisti, Y. Simultaneous determination of oxygen consumption rate and volumetric oxygen transfer coefficient in pneumatically agitated bioreactors. Ind. Eng. Chem. Res. 2006, 45, 1167−1171. (23) Vandu, C. O.; Krishna, R. Influence of scale on the volumetric mass transfer coefficients in bubble columns. Chem. Eng. Process. 2004, 43, 575−579. (24) Vasconcelos, J. M. T.; Rodrigues, J. M. L.; Orvalho, S. C. P.; Alves, S. S.; Mendes, R. L.; Reis, A. Effect of contaminants on mass transfer coefficients in column and airlift contactors. Chem. Eng. Sci. 2003, 58, 1431−1440. (25) Soong, Y.; Harke, F. W.; Gamwo, I. K.; Schehl, R. R.; Zarochak, M. F. Hydrodynamic study in a slurry-bubble-column reactor. Catal. Today. 1997, 35, 427−434. (26) Stolojanu, V.; Prakash, A. Hydrodynamic measurements in a slurry bubble column using ultrasonic techniques. Chem. Eng. Sci. 1997, 52, 4225−4230. (27) Supardan, M. D.; Masuda, Y.; Maezawa, A.; Uchida, S. Local gas holdup and mass transfer in a bubble column using an ultrasonic technique and a neural network. J. Chem. Eng. Jpn. 2004, 37, 927−932. (28) Vichare, N. P.; Dindore, V. Y.; Gogate, P. R.; Pandit, A. B. Mixing time analysis of a sonochemical reactor. Ultrason. Sonochem. 2001, 8, 23−33.

Figure 6. Mass transfer coefficient vs superficial gas velocity at different H/D values.

cause any noticeable changes in gas holdup, or in KLa.30 These ́ et al., results are in accordance with those obtained by Diaz 2006,33 and Ruzicka et al., 2001.34 For H/D ratios higher than 3 (up to 7.4 in the presented results), H/D barely influences KLa and, consequently, there is no need to take bubble column diameter into account.

4. CONCLUSIONS In sonicated and nonsonicated experiments, KLa increases with lower liquid heights at all of the assayed diameter configurations. This fact is derived from the significant effect of H/D ratio on bubble columns. In sonicated bubble columns, the effect of ultrasound supply in enhancing gas−liquid mass transfer can only be appreciated at low liquid volumes with an H/D ratio below 1. KLa increased with ultrasound supply at a power density above 400 kW·m−3. This ultrasound energy dissipation is too high for a bioreactor. Therefore, beneficial effects of ultrasound in some bioprocesses with living cells are not attributable to gas−liquid mass transfer enhancement. In the H/D ratio interval between 0.3 and 3, KLa significantly decreases with H/D for a given superficial gas velocity. Finally, there is no H/D influence on KLa at H/D ratios above 3.

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REFERENCES

(1) Mason, T. J.; Paniwnyk, L.; Lorimer, J. P. The uses of ultrasound in food technology. Ultrason. Sonochem. 1996, 3, S253−S260. (2) Chisti, Y.; Young, M. Disruption of microbial cells for intracellular products. Enzyme Microb. Tech. 1986, 8, 194−204. (3) Save, S. S.; Pandit, A. B.; Joshi, J. B. Microbial cell disruption: Role of cavitation. Chem. Eng. J. 1994, 55, B67−B72. (4) Kuboi, R.; Umakoshi, N.; Takagi, H.; Komasawa, I. Optimal disruption methods for the selective recovery of β-galactosidase from Escherichia coli. J. Ferment. Bioeng. 1995, 79, 335−341. (5) Balasundaram, B.; Pandit, A. B. Significance of location of enzymes on their release during microbial cell disruption. Biotechnol. Bioeng. 2001, 75, 607−614. (6) Chisti, Y. Sonobioreactors: using ultrasound for enhanced microbial productivity. Trends Biotechnol. 2003, 21, 89−93. 2773



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(29) Dähnke, S.; Keil, F. J. Modelling of linear pressure fields in sonochemical reactors considering an inhomogeneous density distribution of cavitation bubbles. Chem. Eng. Sci. 1999, 54, 2865− 2872. (30) Cachaza, E. M.; Díaz, M. E.; Montes, F. J.; Galan, M. A. Analytical solution of the mass conservation equations in gas−liquid systems: Applicability to the evaluation of the volumetric mass transfer coefficient (kLa). Ind. Eng. Chem. Res. 2008, 47, 4510−4522. (31) Eickenbusch, H.; Brunn, P. O.; Schumpe, A. Mass transfer into viscous pseudo-plastic liquid in large-diameter bubble columns. Chem. Eng. Process. 1995, 34, 479−485. (32) Terasaka, K.; Shibata, H. Oxygen transfer in viscous nonNewtonian liquid having yield stress in bubble columns. Chem. Eng. Sci. 2003, 58, 5331−5337. (33) Díaz, M. E.; Montes, F. J.; Galán, M. A. Influence of aspect ratio and superficial gas velocity on the evolution of unsteady flow structures and flow transitions in a rectangular two-dimensional bubble column. Ind. Eng. Chem. Res. 2006, 45, 7301−7312. (34) Ruzicka, M. C.; Drahoš, J.; Fialova, M.; Thomas, N. H. Effect of bubble column dimensions on flow regime transition. Chem. Eng. Sci. 2001, 56, 6117−6124.

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Gas–liquid Mass Transfer in Sonicated Bubble Columns. Effect of

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