Characterization of oxygen transfer in miniature and lab-scale bubble column bioreactors and comparison of microbial growth performance based on constant k(L)a

This work describes the engineering characterization of miniature (2 mL) and laboratory-scale (100 mL) bubble column bioreactors useful for the cultivation of microbial cells. These bioreactors were constructed of glass and used a range of sintered glass gas diffusers with differently sized pores to disperse humidified air within the liquid biomedium. The effect of the pressure of this supplied air on the breakthrough point for gas diffusers with different pore sizes was examined and could be predicted using the Laplace-Young equation. The influence of the superficial gas velocity (u(g)) on the volumetric mass transfer coefficient (k(L)a) was determined, and values of up to 0.09 s(-1) were observed in this work. Two modeling approaches were considered in order to predict and provide comparison criteria. The first related the volumetric power consumption (P/V) to the k(L)a and a good correlation was obtained for differently sized reactors with a given pore size, but this correlation was not satisfactory for bubble columns with different gas diffusers. Values for P/V ranged from about 10 to 400 W.m(-3). Second, a model was developed predicting bubble size (d(b)), bubble rising velocity (u(b)), gas hold-up (phi), liquid side mass transfer coefficient (k(L)), and thus the k(L)a using established theory and empirical correlations. Good agreement was found with our experimental data at different scales and pore sizes. Values for d(b) varied from 0.1 to 0.6 mm, and k(L) values between 1.7 and 9.8 x 10(-4) m.s(-1) were determined. Several E. coli cultivations were performed in the miniature bubble column at low and high k(L)a values, and the results were compared to those from a conventional stirred tank operated under identical k(L)a values. Results from the two systems were similar in terms of biomass growth rate and carbon source utilization.     

The influence of polymeric membrane gas spargers on hydrodynamics and mass transfer in bubble column bioreactors

Gas sparging performances of a flat sheet and tubular polymeric membranes were investigated in 3.1 m bubble column bioreactor operated in a semi batch mode. Air–water and air–CMC (Carboxymethyl cellulose) solutions of 0.5, 0.75 and 1.0 % w/w were used as interacting gas–liquid mediums. CMC solutions were employed in the study to simulate rheological properties of bioreactor broth. Gas holdup, bubble size distribution, interfacial area and gas–liquid mass transfer were studied in the homogeneous bubbly flow hydrodynamic regime with superficial gas velocity (U _G) range of 0.0004–0.0025 m/s. The study indicated that the tubular membrane sparger produced the highest gas holdup and densely populated fine bubbles with narrow size distribution. An increase in liquid viscosity promoted a shift in bubble size distribution to large stable bubbles and smaller specific interfacial area. The tubular membrane sparger achieved greater interfacial area and an enhanced overall mass transfer coefficient (K _La) by a factor of 1.2–1.9 compared to the flat sheet membrane.     

Hydrodynamic stress and lethal events in sparged microalgae cultures

The effect of high superficial gas velocities in continuous and batch cultures of the strains Dunaliella tertiolecta, Chlamydomonas reinhardtii wild-type and cell wall-lacking mutant was studied in bubble columns. No cell damage was found for D. tertiolecta and C. reinhardtii (wild-type) up to superficial gas velocities of 0.076 and 0.085 m s(-1), respectively, suggesting that high superficial gas velocities alone cannot be responsible for cell death and, consequently, bubble bursting cannot be the sole cause for cell injury. A death rate of 0.46 +/- 0.08 h(-1) was found for C. reinhardtii (cell wall-lacking mutant) at a superficial gas velocity of 0.076 m s(-1), and increased to 1.01 +/- 0.29 h(-1) on increasing superficial gas velocity to 0.085 m s(-1). Shear sensitivity is thus strain-dependent and to some extent the cell wall plays a role in the protection against hydrodynamic shear. When studying the effect of bubble formation at the sparger in batch cultures of D. tertiolecta by varying the number of nozzles, a death rate of 0.047 +/- 0.016 h(-1) was obtained at high gas entrance velocities. D. tertiolecta was cultivated in a pilot-plant reactor under different superficial gas velocities of up to 0.026 m s(-1), with relatively low gas entrance velocities and no cell damage was observed. There is some indication that the main parameter causing cell death and damage was the gas entrance velocity at the sparger.     

Different bioreactor configurations for the decolourisation of the azo dye reactive black 5 by Geotrichum sp. CCMI 1019

Decolourisation of the azo dye Reactive Black 5 by Geotrichum sp. CCMI 1019 was studied using stirred tank reactors (STR) and two types of bubble columns (porous plate (PP) bubble column and aeration tube (AT) bubble column). For the bubble columns, the k_La increased with the gas fractional hold-up (ε_G) and the aeration rate. A linear relationship between ε_G and superficial gas velocity was obtained for all reactors. At same aeration rates, the PP bubble columns showed higher k_La and hold-up values than the AT bubble column. In the STRs, large and dense aggregates were formed which adhered to surfaces whereas bubble columns gave smaller and less compact pellets.

Manganese peroxidase and laccase were detected in the extracellular media in all reactors. However, laccase was only detected after the onset of decolourisation, suggesting that additional enzymes may be involved. Mn peroxidase activity was detected (about 46 U/ml) in both the STRs and AT bubble columns but higher values (110 U/ml) were obtained with the PP bubble columns.

Out of the three reactor systems studied, the AT bubble columns gave the most favourable results for Reactive Black 5 decolourisation. Rapid and complete colour removal was obtained throughout the visible spectrum. Bubble columns are simple in design as well as operation and may be useful for the bioremediation of textile wastewater.     

Flow characteristics of a pilot-scale airlift fermentor

The effects of aeration on the flow characteristics of water in a glass pilot-scale airlift fermentor have been examined. The 55-L capacity fermentor consisted of a 15.2-cm-i.d. riser column with a 5.1-cm-i.d. downcomer tube. It was found that the average bubble size diminished with increased aeration. Typically, average bubble sizes ranged from 4.32 mm at a superficial gas velocity of 0.64 cm/s to 1.92 mm at 10.3 cm/s. A gas holdup of 0.19 was attained with superficial gas velocities (v_s) on the order of 10 cm/s, indicating the highly gassed nature of the fluid in the riser section of the fermentor. Circulation velocities of markers placed in the fermentor decreased with increasing aeration rates due to increased turbulence and axial liquid back mixing within the riser section. Actual volumetric liquid circulation rates remained relatively constant (0.36–0.49 L/s) for values of (v_s) up to 10 cm/s. Based on theoretical calculations, the ascending velocity of bubbles in a swarm reached 54 cm/s in the range of (v_s) values studied.     

Process-engineering characterization of small-scale bubble columns for microbial process development

Volumetric oxygen transfer rates and power inputs were estimated by a model of the formation of primary gas bubbles at the static sparger (sinter plate) of small-scale bubble columns and a common mass-transfer correlation for bubbles rising in a non-coalescent Newtonian electrolyte solution of low viscosity. Estimations were used to assess the dimensioning and possibilities of small-scale bubble column application with an height/diameter ratio of about 1. Estimations of volumetric oxygen transfer rates (<0.16 s^-1) and power inputs (<100 W m^-3) with a mean pore diameter of the static sparger of 13 µm were confirmed as function of the superficial air velocity (<0.6 cm s^-1) by measurements using an Escherichia coli fermentation medium. Small-scale bubble columns are thus to be classified between shaking flasks and stirred-tank reactors with respect to the oxygen transfer rate, but the maximum volumetric power input is more than one magnitude below the power input in shaking flasks, which is of the same order of magnitude as in stirred-tank reactors. A small-scale bubble columns system was developed for microbial process development, which is characterized by handling in analogy to shaking flasks, high oxygen transfer rates and simultaneous operation of up to 16 small-scale reactors with individual gas supply in an incubation chamber.     

Hydrodynamics, mass transfer, and yeast culture performance of a column bioreactor with ejector

A bubble column fitted with an ejector has been tested for its physical and biological performance. The axial diffusion coefficient of the liquid phase in the presence of electrolytes and ethanol was measured by a stimulus-response technique with subsequent evaluation by means of a diffusion model. In contrast to ordinary bubble columns, the coefficient of axial mixing is inversely dependent on the superficial air velocity. The liquid velocity acts in an opposite direction to the backmixing flow in the column. The measurement of volumetric oxygen transfer coefficient in the presence of electrolytes and ethanol was performed using a dynamic gassing-in method adapted for a column. The data were correlated with the superficial air and liquid velocities, total power input, and power for aeration and mixing; the economy coefficient of oxygen transfer was used for finding an optimum ratio of power for aeration and pumping. Growth experiments with Candida utilis on ethanol confirmed some of the above results. Biomass productivity of 2.5 g L(-1) h(-1) testifies about a good transfer capability of the column. Columns fitted with pneumatic and/or hydraulic energy input may be promising for aerobic fermentations considering their mass transfer and mixing characteristics.     

Different bioreactor configurations for the decolourisation of the azo dye reactive black 5 by Geotrichum sp. CCMI 1019

Decolourisation of the azo dye Reactive Black 5 by Geotrichum sp. CCMI 1019 was studied using stirred tank reactors (STR) and two types of bubble columns (porous plate (PP) bubble column and aeration tube (AT) bubble column). For the bubble columns, the k_La increased with the gas fractional hold-up (ε_G) and the aeration rate. A linear relationship between ε_G and superficial gas velocity was obtained for all reactors. At same aeration rates, the PP bubble columns showed higher k_La and hold-up values than the AT bubble column. In the STRs, large and dense aggregates were formed which adhered to surfaces whereas bubble columns gave smaller and less compact pellets.

Manganese peroxidase and laccase were detected in the extracellular media in all reactors. However, laccase was only detected after the onset of decolourisation, suggesting that additional enzymes may be involved. Mn peroxidase activity was detected (about 46 U/ml) in both the STRs and AT bubble columns but higher values (110 U/ml) were obtained with the PP bubble columns.

Out of the three reactor systems studied, the AT bubble columns gave the most favourable results for Reactive Black 5 decolourisation. Rapid and complete colour removal was obtained throughout the visible spectrum. Bubble columns are simple in design as well as operation and may be useful for the bioremediation of textile wastewater.     

Characterization of gas transfer and mixing in a bubble column equipped with a rubber membrane diffuser

Gas transfer and mixing were characterized in a 32-L bubble column reactor equipped with a commercially available rubber membrane diffuser. The performance of the membrane diffuser indicates that the slits in the membrane are best described as holes with elastic lids, acting as valves cutting off bubbles from the gas stream. The membrane diffuser thus functions as a one-way valve preventing backflow of liquid. Our design of the bottom plate of the reactor enabled us to optimize the aeration by changing the tension of the membrane. We thereby achieved mass transfer coefficients higher than those previously reported in bubble columns. A strong dependence of mass transfer on gas holdup and bubble size was indicated by estimates based on these two variables. The microalga, Rhodomonas sp. , sensitive to chemical and physical stress, was maintained for 8 months in continuous culture with a productivity identical to cultures grown in stirred tank reactors. Copyright 1998 John Wiley & Sons, Inc.     

Characterization of a pilot plant airlift tower loop bioreactor. I: evaluation of the phase properties with model media

Investigations were carried out in a 9 m high, 4 m(3) volume, pilot plant airlift tower loop bioreactor with a draft tube. The reactor was characterized by measuring residence time distributions of the gas phase using pseudostochastic tracer signals and a mass spectrometer and by evaluating the mixing in the liquid phase with single-pulse tracer inputs. The local gas holdup and the bubble size (piercing length) were measured with two-channel electrical conductivity probes. The mean residence times and the intensities of the axial mixing in the riser and downcomer and the circulation times of the phases as well as the fraction of the recirculated gas phase were evaluated. The gas holdup in the riser is nearly uniform along the reactor. In the downcomer, it diminishes from top to bottom. The liquid phase dispersion coefficients, D(L), are smaller than those measured in the corresponding bubble columns. In the pilot plant with tap water the following relationship was found: D(Lr) = cw(SG) (n); with c = 203.4; n = 0.5;D(Lr)(cm(2) s(-1);) and W(SG)(cm s(-1)) where D(Lr) is the longitudinal dispersion coefficient in the riser and W(SG) is the superficial gas velocity. The gas phase dispersion coefficients in the riser of the pilot plant, D(Gr), are also enlarged with increasing superficial gas velocity, W(SG), however, no simple relationship exists. Parameter D(Gr) is the highest in the presence of antifoam agents, intermediate in tap water, and the smallest in ethanol solution.     

Performance of a continuous foam separation column as a function of process variables

Enrichment and recovery of bovine serum albumin has been examined in a continuous foam separation column. The effects of the operating factors, superficial air velocity, feed flow rate, feed concentration and pH on the above characteristics was investigated. The protein enrichment decreased with the increase in the value of each of these parameters. Protein recovery increased with increasing air velocity, decreased with increasing feed flow rate and did not change very much with increasing feed concentration. Maximum protein recovery was obtained at the isoelectric point (pH 4.8) of the protein. Maximum protein recovery was found to be a strong function of the air velocity in the range 0.05-0.15 cm/s. Further increase in air velocity did not have much effect on recovery because of very large bubbles formed as a result of coalescence. Bubble size was determined as a function of the above factors in the liquid and foam sections of the column. It was found to be dependent on protein concentration, feed flow rate and solution pH. The effect was more significant in the foam section of the column. The bubbles in the foam section were significantly larger (about 3-10 times) than those in the liquid, with a sharp change at the foam-liquid interface. The bubble size measurements were used to calculate the interfacial area and it was shown that the rate of protein removal increases with increasing interfacial area.     

Response of <Emphasis Type="Italic">Tisochrysis lutea</Emphasis> [Prymnesiophycidae] to aeration conditions in a bench-scale photobioreactor

The effects of superficial gas velocity (U_gr), gas entrance velocity (ν), and bubble size on the growth of Tisochrysis lutea was investigated in 600-mL photobioreactors operated with airlift pumps. Superficial gas velocities, calculated from measured air flow rates, ranging from 7 to 93 mm s^?1 were created using a 1.6-mm diameter syringe. We tested the effects of sparger velocity over a range of 2.48 to 73.4 m s^?1 and the effects of bubble size by using two styles of air stones and an open glass pipette, which created a bubble sizes in the range of 0.5 to 5 mm. We calculated oxygen mass transfer coefficient, k_La, values for all experimental conditions. Cell growth increased linearly with increased superficial gas velocity and decreased with increased sparger velocity. Results indicated that smaller bubble size leads to some initial cell damage, but after time, the increased gas transfer as reflected by the k_La value produced higher growth than larger bubbles. Two mechanisms were observed to correlate with cell damage in T. lutea: increasing velocity at the sparger tip and bubble bursting at the surface. These results demonstrate a method to test sensitivity of T. lutea to aeration, which is important for the design of airlift systems.     

Membrane sparger in bubble column,airlift, and combined membrane–ring sparger bioreactors

The bubble column and the two internal loop airlift reactors (riser/downcomer area ratios of 0.11 and 0.58) characterized in this study were equipped with a rubber membrane sparger, which produced small bubbles, giving high mass transfer coefficients. The low mixing intensity in the bubble column was increased by an order of magnitude in the airlift reactors. We designed a novel aeration and mixing system by adding a ring sparger to the membrane sparger in the bubble column and maintained the advantages of both airlift configuration (good mixing properties) and bubble column configuration (efficient aeration, without any internal constructions). The combined membrane–ring sparger system has unique features with respect to the efficiency of utilization of substrate gasses and energy. Model experiments showed that the small bubbles from the membrane sparger do not coalesce with the large bubbles from the ring sparger. If different gases were added through the two spargers it was possible to transfer a hazardous or expensive gas quantitatively to the liquid through the membrane sparger (dual sparging mode). In the combined membrane–ring sparger system the energy input for mixing and mass transfer is divided. Therefore, the energy consumption can be minimized if the flow distribution of air through the membrane and ring sparger is controlled by the oxygen demand and the inhomogeneity of the culture, respectively (split sparging mode). The dual sparging mode was used for mass production of the alga Rhodomonas sp. as the first step in aquatic food chains. Avoiding mechanical parts removes an important risk of malfunction, and a continuous culture could be maintained for more than 8 months. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 64: 452–458, 1999.     

Comments on the communication: on the calculation of shear rate and apparent viscosity in airlift and bubble column bioreactors

The determination of the shear rate in bubble column and airlift bioreactors is an important question from both the perspective of cell damage and the correlation of hydrodynamic parameters in non-Newtonian fluids in these contractors. In the context of correlating hydrodynamic parameters in non-Newtonian fluids, a common approach involves assuming that there exists an average shear rate in the column that is proportional to the superficial gas velocity. This average shear rate is then used to evaluate an effective viscosity of the non-Newtonian fluid that is subsequently used to quantify the fluid's rheological behavior in correlation. Contrary to a recent communication, this report illustrates that this approach, which has mainly been applied to bubble columns, can also be applied to external loop airlift contractors, replacing the superficial gas velocity by the superficial gas velocity by the superficial gas velocity supplied to the riser of the contractor. This extension is based upon consideration of the relevant characteristic velocity in the active zone (i.e., the riser section) of the reactor.     

Outdoor helical tubular photobioreactors for microalgal production: modeling of fluid-dynamics and mass transfer and assessment of biomass productivity

The production of the microalga Phaeodactylum tricornutum in an outdoor helical reactor was analyzed. First, fluid dynamics, mass-transfer capability, and mixing of the reactor was evaluated at different superficial gas velocities. Performance of the reactor was controlled by power input per culture volume. A maximum liquid velocity of 0.32 m s(-1) and mass transfer coefficient of 0.006 s(-1) were measured at 3200 W m(-3). A model of the influence of superficial gas velocity on the following reactor parameters was proposed: gas hold-up, induced liquid velocity, and mass transfer coefficient, with the accuracy of the model being demonstrated. Second, the influence of superficial gas velocity on the yield of the culture was evaluated in discontinuous and continuous cultures. Mean daily values of culture parameters, including dissolved oxygen, biomass concentration, chlorophyll fluorescence (F(v)/F(m) ratio), growth rate, biomass productivity, and photosynthetic efficiency, were determined. Different growth curves were measured when the superficial gas velocity was modified-the higher the superficial gas velocity, the higher the yield of the system. In continuous mode, biomass productivity increased by 35%, from 1.02 to 1.38 g L(-1) d(-1), when the superficial gas velocity increased from 0.27 to 0.41 m s(-1). Maximal growth rates of 0.068 h(-1), biomass productivities up to 1.4 g L(-1) d(-1), and photosynthetic efficiency of up to 15% were obtained at the higher superficial gas velocity of 0.41 m s(-1). The fluorescence parameter, F(v)/F(m), which reflects the maximal efficiency of PSII photochemistry, showed that the cultures were stressed at average irradiances within the culture higher than 280 microE m(-2) s(-1) at every superficial gas velocity. For nonstressed cultures, the yield of the system was a function of average irradiance inside the culture, with the superficial gas velocity determining this relationship. When superficial gas velocity was increased, higher growth rates, biomass productivities, and photosynthetic efficiencies were obtained for similar average irradiance values. The higher the superficial gas velocity, the higher the liquid velocity, with this increase enhancing the movement of the cells inside the culture. In this way the efficiency of the cells increased and higher biomass concentrations and productivities were reached for the same solar irradiance.     

Hydrodynamics and mass transfer in Aspergillus niger fermentations in bubble column and loop bioreactors

The influence of Aspergillus niger broth rheology, bioreactor geometry, and superficial gas velocity on the volumetric liquid phase oxygen transfer coefficient (k(L)a(L)), riser gas holdup (epsilon(GR)), and circulating liquid velocity (u(LR)) was studied in a bubble column (BC) and two external-circulation-loop airlift (ECLAL) bioreactors. The results are compared to those of previous studies on homogeneous fluids and in particular with a recent study on non-Newtonian carboxymethylcellulose (CMC) solutions conducted in the same contactors used for the A. niger fermentations. As expected from the CMC-based studies, in the heterogeneous broths of A. niger epsilon(GR), k(L)a(L), and u(LR) decreased with increasing broth apparent viscosity; epsilon(GR) and k(L)a(L) decreased with increasing downcomer-to-riser cross-sectional area ratio, A(d)/A(r), whereas u(LR) increased with increasing A(d)/A(r). Gas holdup data in the airlift fermentations of A. niger were well predicted by the CMC-based correlation. However, the CMC-based correlations produced conservative estimations of k(L)a(L) and overestimates of u(LR) compared to the observed values in the A. niger broths.     

Effect of surface contaminants on oxygen transfer in bubble column reactors

Gas hold-up (ɛ_g), sauter mean bubble diameter (d₃₂) and oxygen transfer coefficient (k_La) were evaluated at four different alkane concentrations (0.05, 0.1, 0.3 and 0.5 vol.%) in water over the range of superficial gas velocity (u_g) of (1.18–23.52) × 10⁻³ m/s at 25 °C in a laboratory-scale bubble column bioreactor. Immiscible hydrocarbons (n-decane, n-tridecane and n-hexadecane) were utilized in the experiments as impurity. A type of anionic surfactant was also employed in order to investigate the effect of addition of surfactant to organic-aqueous systems on sauter mean bubble diameter, gas hold-up and oxygen transfer coefficient. Influence of addition of alkanes on oxygen transfer coefficient and gas hold-up, was shown to be dependent on the superficial gas velocity. At superficial gas velocity below 0.5 × 10⁻³ m/s, addition of alkane in air–water medium has low influence on oxygen transfer coefficient and also gas hold-up, whereas; at higher gas velocities slight addition of alkane increases oxygen transfer coefficient and also gas hold-up. Increase in concentration of alkane resulted in increase in oxygen transfer coefficient and gas hold-up and roughly decrease in sauter mean bubble diameter, which was attributed to an increase in the coalescence-inhibiting tendency in the presence of surface contaminant molecules. Bubbles tend to become smaller with decreasing surface tension of hydrocarbon, thus, oxygen transfer coefficient increases due to increasing of specific gas–liquid interfacial area (a). Empirical correlations were proposed for evaluating gas hold-up as a function of sauter mean bubble diameter, superficial gas velocity and interfacial surface tension as well as evaluating Sherwood number as a function of Schmidt, Reynolds and Bond numbers.     

Comparative evaluation of compact photobioreactors for large-scale monoculture of microalgae

Engineering analyses combined with experimental observations in horizontal tubular photobioreactors and vertical bubble columns are used to demonstrate the potential of pneumatically mixed vertical devices for large-scale outdoor culture of photosynthetic microorganisms. Whereas the horizontal tubular systems have been extensively investigated, their scalability is limited. Horizontal tubular photobioreactors and vertical bubble column type units differ substantially in many ways, particularly with respect to the surface–to–volume ratio, the amount of gas in dispersion, the gas–liquid mass transfer characteristics, the nature of the fluid movement and the internal irradiance levels. As illustrated for eicosapentaenoic acid production from the microalga Phaeodactylum tricornutum, a realistic commercial process cannot rely on horizontal tubular photobioreactor technology. In bubble columns, presence of gas bubbles generally enhances internal irradiance when the Sun is low on the horizon. Near solar noon, the bubbles diminish the internal column irradiance relative to the ungassed state. The optimal dimensions of vertical column photobioreactors are about 0.2 m diameter and 4 m column height. Parallel east–west oriented rows of such columns located at 36.8°N latitude need an optimal inter-row spacing of about 3.5 m. In vertical columns the biomass productivity varies substantially during the year: the peak productivity during summer may be several times greater than in the winter. This seasonal variation occurs also in horizontal tubular units, but is much less pronounced. Under identical conditions, the volumetric biomass productivity in a bubble column is 60% of that in a 0.06 m diameter horizontal tubular loop, but there is substantial scope for raising this value.     

Hydrodynamics and mass transfer coefficient in activated sludge aerated stirred column reactor: experimental analysis and modeling

The aerated stirred reactor (ASR) has been widely used in biochemical and wastewater treatment processes. The information describing how the activated sludge properties and operation conditions affect the hydrodynamics and mass transfer coefficient is missing in the literature. The aim of this study was to investigate the influence of flow regime, superficial gas velocity (U(G)), power consumption unit (P/V(L)), sludge loading, and apparent viscosity (mu(ap)) of activated sludge fluid on the mixing time (t(m)), gas hold-up (epsilon), and volumetric mass transfer coefficient (k(L)a) in an activated sludge aerated stirred column reactor (ASCR). The activated sludge fluid performed a non-Newtonian rheological behavior. The sludge loading significantly affected the fluid hydrodynamics and mass transfer. With an increase in the U(G) and P/V(L), the epsilon and k(L)a increased, and the t(m), decreased. The epsilon, k(L)a, and t(m), were influenced dramatically as the flow regime changed from homogeneous to heterogeneous patterns. The proposed mathematical models predicted the experimental results well under experimental conditions, indicating that the U(G), P/V(L), and mu(ap) had significant impact on the t(m), epsilon, and k(L)a. These models were able to give the t(m), epsilon, and k(L)a values with an error around +/-8%, and always less than +/-10%.     

Gas holdup and overall volumetric oxygen transfer coefficient in airlift contactors

The two major types of airlift contactors, concentric-tube and external-loop, were investigated for their gas holdup (riser and downcomer) and overall mass transfer characteristics. Results obtained in batch charges of tap water and 0.15 kmol/m(3) NaCl solution are reported for external-loop airlift contactors having downcomer-to-riser cross-sectional area ratios, A(d)/A(r), ranging from 0.11 相似文献