Stochastic modelling of axial dispersion in external-loop airlift bioreactors

The paper presents a model of the motion of a particle subjected to several transport processes in connection with mixing in two phase flow. A residence time distribution technique coupled with a one-dimensional dispersion model was used to obtain the axial dispersion coefficient in the liquid phase, D_ax. The proposed model of D_ax for an external-loop airlift bioreactor is based on the stochastic analysis of the two-phase flow in a cocurrent bubble column and modified for the specific flow in the airlift reactor. The model takes into account the riser gas superficial velocity, the riser liquid superficial velocity, the Sauter bubble diameter, the riser gas hold-up, the downcomer-to-riser cross sectional area ratio. The proposed model can be applied with an average error of ᆨ.     

A novel split-cylinder airlift reactor for fed-batch cultures

Conventional airlift reactors are not adequate to carry out variable volume processes since it is not possible to achieve a proper liquid circulation in these reactors until the liquid height is higher than that of the downcomer. To carry out processes of variable volume, the use of a split-cylinder airlift reactor is proposed, in the interior of which two multi-perforated vertical baffles are installed in order to provide several points of communication between the reactor riser and downcomer. This improves the liquid circulation and mixing at any liquid volume. In fed-batch cultures, it is important to know how liquid height affects the hydrodynamic characteristics and the volumetric oxygen transfer coefficient since this impacts on the kinetic behavior of any fermentation. Thus, in the present work, the effect of the liquid height on the mixing time, the overall gas hold-up, and the volumetric oxygen transfer coefficient of the proposed airlift reactor were determined. The mixing time was increased and the volumetric oxygen transfer coefficient decreased due to the increase of the liquid height in the reactor in all the superficial gas velocities tested, which corresponded to a pseudohomogeneous flow regime. The experimental values of the mixing time and the mass-transfer coefficient were properly described through correlations in which the U_GR/H_L ratio was used as the independent variable. Thus, this variable might be used to describe the hydrodynamic behavior and the oxygen transfer coefficient of airlift reactors when such reactors are used in processes where the liquid volume changes with time. However, these correlations are useful for the particular device and for the particular operating conditions at which they were obtained. These empirical correlations are useful to understand some factors that influence the mixing time and volumetric oxygen transfer coefficient, but such correlations do not have a sufficient predictive potential for a satisfactory reactor design. The overall gas hold-up values were not significantly affected when the liquid height was lower than the downcomer height. However, the values decreased abruptly when the reactor was operated with liquid heights over the downcomer height, especially at high superficial gas velocities.     

Region-dependent oxygen transfer rate in the rectangular airlift reactor

Gas hold-up and the oxygen transfer in the zones of the internal loop airlift reactor with rectangular cross-section was studied. It was found, that the downcomer to the riser gas hold-up ratio depends on the gas flow rate, the physicochemical properties of the system and on the reactor height. The ratio of the downcomer mass transfer coefficient to the global mass transfer coefficient was less than 6%. The ratio of the downcomer to the global mass transfer coefficient slightly increased with increase of the gas flow rate and decreased with increase of the liquid viscosity. The proposed correlation for the global overall mass transfer coefficient predicts the experimental data well within 16.6% deviation. It was confirmed that the reactor height is the important parameter for a design and a scale-up of the airlift reactors.     

Hydrodynamic characteristics and gas-liquid mass transfer in a biofilm airlift suspension reactor

The hydrodynamics and mass transfer, specifically the effects of gas velocity and the presence and type of solids on the gas hold-up and volumetric mass transfer coefficient, were studied on a lab-scale airlift reactor with internal draft tube. Basalt particles and biofilm-coated particles were used as solid phase. Three distinct flow regimes were observed with increasing gas flow rate. The influence of the solid phase on the hydrodynamics was a peculiar characteristic of the regimes. The volumetric mass transfer coefficient was found to decrease with increasing solid loading and particle size. This could be predominantly related to the influence that the solid has on gas hold-up. The ratio between gas hold-up and volumetric mass transfer coefficient was found to be independent of solid loading, size, or density, and it was proven that the presence of solids in airlift reactors lowers the number of gas bubbles without changing their size. To evaluate scale effects, experimental results were compared with theoretical and empirical models proposed for similar systems.     

Hydrodynamics in bubble column bioreactors with fermentation broths having a yield stress

Summary The hydrodynamics in a bubble column bioreactor with fermentation broths having a yield stress are studied. Specifically, the liquid velocity at the reactor axis, the axial dispersion coefficient, and the gas hold-up are examined. The liquid velocity at the reactor axis and the gas hold-up are measured in a 40-1 bench-scale bubble column fermentor using carboxypolymethylene (Carbopol) aqueous solutions as simulated broths. Theoretical correlations for the liquid velocity at the reactor axis, the axial dispersion coefficient, and the gas hold-up are derived on the basis of an energy balance and the mixing length theory. The correlations are compared with the present data and a reasonable agreement is found. The theoretical predictions are also in satisfactory agreement with the re-examined data for actual fermentation broths which are Chaetomium cellulolyticum and Neurospora sitophila cultured in a 1000-1 pilot-plant scale airlift fermentor.     

Bubble characteristics and mass transfer in an airlift reactor with multiple net draft tubes

A pilot scale airlift reactor with multiple net draft tubes was developed. The reactor, 29?cm in diameter and 300?cm height, had four modules of double net draft tubes. Bubble size, bubble number, gas holdup, and volumetric mass transfer coefficient were measured under different superficial air velocities. The air velocity had little effect on bubble size but had significant influence on bubble number. A bubble column was also investigated for comparison. The airlift reactor had a higher gas holdup and volumetric mass transfer coefficient than those in the bubble column. The draft tubes in the airlift reactor substantially improved the reactor performance.     

Continuous production of citric acid with recirculation of the fermentation broth after product recovery

An airlift reactor with double net draft tubes was developed. A sparger was located between the two draft tubes. The draft tubes had a significant effect on breaking bubbles into smaller ones. The assessment of the reactor performance was based on gas holdup, mixing time, and volumetric mass transfer coefficient. The proposed reactor had higher gas holdup and volumetric mass transfer coefficient, and lower mixing time in comparison with those of the bubble column. Application of the proposed reactor to fermentation of Saccharomyces cerevisiae demonstrated that the cultivation time was significantly shortened.     

Local gas hydrodynamics in an external-loop airlift reactor: newtonian and non-newtonian fluids

Measurements of local gas phase characteristics are obtained in an external-loop airlift reactor filled with newtonian or viscous non-newtonian liquids. A double-optical fiber probe technique is used. It allows the determination of the axial and radial profiles of gas hold-up, bubbling frequency, bubble size and velocity. In the case of air-water system, the results show a strong effect of radial liquid velocity variation on the gas flow characteristics at the bottom of the riser. In the case of highly viscous non-newtonian solution, the gas flow is strongly affected by the gas distribution just above the gas sparger. This study also points out the bubble coalescence and the break-up phenomena in different liquids and levels in the reactor. Furthermore, the local measurements of bubble size and velocity allows to gain more detailed information on the dynamics of the bubble-flow and shows a tendency of large bubbles to circulate in the column center.     

A novel rectangular airlift reactor with mesh baffle-plates

An experimental rectangular airlift reactor having mesh baffle-plates has been fabricated out of Perspex and compared with conventional airlift and bubble column in terms of gas holdup, volumetric mass transfer coefficient and mixing time. Mesh baffle-plates improved mass transfer and mixing with the mass transfer coefficient of the proposed reactor being up to 12% higher than that in a conventional airlift reactor under the same operating condition. The mixing time of the proposed reactor can be 95% lower than that of the airlift reactor.     

Influence of very small bubbles on kLa measurement in viscous microbiological cultures

In gas-liquid dispersions based on viscous non-Newtonian fluids, numerous very small bubbles are formed due to their high residence times in contacting devices such as bubble columns. The influence of these small bubbles on the measurement of the volumetric mass transfer coefficient in the contactor by the dynamic CO(2) gas analysis method is discussed. It is found that their effect on this measuring technique is insignificant compared to that when using the conventional dissolved-oxygen technique.     

Oxygen transfer in liquids

In the laboratory-type airlift tower reactor oxygen transfer from air in tap water and/or polyacrylamide solutions (Neuperm WF) was studied. In order to characterize the system, volumetric coefficient of oxygen transfer was determined by the gassing-out method. Two arrangements of the airlift tower reactor were compared, namely the reactor with and without motionless mixer. In addition, mean relative gas holdup and gas power output were determined for both arrangements.     

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.     

Absorption of oxygen in highly viscous newtonian and non-Newtonian fermentation model media in bubble column bioreactors

Summary Mean relative gas holdup, slip velocity, bubble size distribution, mean specific interfacial area, and volumetric mass transfer coefficient of oxygen were estimated in sparged columns 14 cm in diameter and 380 and/or 390 cm high with two different aerator types (porous plate and injector nozzle) in highly viscous Newtonian (glycerol solutions) and non-Newtonian (CMC solutions) fluids.For the Newtonian liquids the above properties were estimated as function of the viscosity of the liquid. For the non-Newtonian liquids they were determined as function of the fluid consistency index and flow behavior index. Significant differences between Newtonian and non-Newtonian systems appear. In Newtonian medium k_L a drops with increasing viscosity and already approaches a constant value at =40 cP. In pseudoplastic medium k_L a varies with the fluid consistency and flow behavior indexes in the entire investigated range.In both of these systems the primary bubble population changes into two or three populations along the reactor: the medium bubbles gradually disappear and small and large bubbles are formed.     

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.     

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.     

Hydrodynamics and mass transfer in miniaturized bubble column bioreactors

Miniaturized bubble columns (MBCs) have different hydrodynamics in comparison with the larger ones, but there is a lack of scientific data on MBCs. Hence, in this study, the effect of gas hold-up, flow regimes, bubble size distribution on volumetric oxygen mass transfer coefficient at different pore size spargers and gas flow rates in MBCs in the presence and absence of microorganisms were investigated. It was found that flow regime transition occurred around low gas flow rates of 1.18 and 0.85 cm/s for small (16–40 µm) and large (40–100 µm) pore size spargers, respectively. Gas hold-up and K_La in MBC with small size sparger were higher than those with larger one, with an increasing effect in the presence of microorganisms. A comparison revealed that the wall effect on the flow regime and gas hold-up in MBCs was greater than bench-scale bubble columns. The K_La values significantly increased up to tenfold using small pore size sparger. In the MBC and stirred tank bioreactors, the maximum obtained cell concentrations were OD₆₀₀ of 41.5 and 43.0, respectively. Furthermore, it was shown that in MBCs, higher K_La and lower turbulency could be achieved at the end of bubbly flow regime.

     

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.     

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.     

Effect of organic solvents on oxygen mass transfer in multiphase systems: Application to bioreactors in environmental protection

The absorption of oxygen in aqueous–organic solvent emulsions was studied in a laboratory-scale bubble reactor at a constant gas flow rate. The organic and the gas phases were dispersed in the continuous aqueous phase. Volumetric mass transfer coefficients (k_La) of oxygen between air and water were measured experimentally using a dynamic method. It was assumed that the gas phase contacts preferentially the water phase. It was found that addition of silicone oils hinders oxygen mass transfer compared to air–water systems whereas the addition of decane, hexadecane and perfluorocarbon PFC40 has no significant influence. By and large, the results show that, for experimental conditions (organic liquid hold-up ≤10% and solubility ratio ≤10), the k_La values of oxygen determined in binary air–water systems can be used for multiphase (gas–liquid–liquid) reactor design with applications in environmental protection (water and air treatment processes).     

Effect of scale-up on average shear rates for aerated non-Newtonian liquids in external loop airlift reactors

Average shear rates have been estimated experimentally in a 700-dm3 external loop airlift reactor. Aqueous pseudoplastic carboxymethylcellulose and xanthan gum solutions were used to simulate non-Newtonian behavior of biological media. Average shear rates of non-Newtonian solutions were found by analogy with Newtonian glycerol solutions using downcomer liquid velocity as the measurable parameter. Due to the complexity of local shear rate measurement, an average shear rate was assumed to exist and is proportional to superficial gas velocity. The data from this work and those in the literature were used in producing a new correlation for estimating average shear rates as a function of superficial gas velocity, geometry, and dispersion height. Wall shear rates were found to be significant. The ratio of wall shear rates to bulk shear rates were varied from 5% to 40%. Furthermore, it has been found that shear rates generated in airlift loop reactors are lower than those generated in bubble columns. Copyright 1999 John Wiley & Sons, Inc.