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.

     

General relationship for volumetric oxygen transfer coefficient (k L a) prediction in tower bioreactors utilizing immobilized cells

A general relationship for prediction of the volumetric oxygen transfer coefficient (k_La) in a tower bioreactor utilizing immobilized Penicillium chrysogenum as function of air superficial velocity, suspension rheological parameters and liquid physical properties is proposed in this study. The relationship was applied to three different systems and a good agreement between the calculated values and the experimental data was obtained.     

Correlation of liquid dispersion and oxygen transfer in bubble column

Summary In steady state, attained by continuous aeration after oxygen saturation of water in a bubble column, vertical composition distribution of liquid and gas phases has been determined. It has been assumed that, as a result of absorption at the bottom of the column, desorption in the upper section and vertical dispersion of dissolved oxygen flux, a closed oxygen circulation is created. Determination of the axial dispersion coefficient from hydrodynamic and oxygen transfer data verifies the mathematical model proposed. The results allow conclusions to be drawn about supersaturation and desorption and other phenomena expected in biological systems.Abbreviations C[-] Dimensionless oxygen concentration Unit=0.21 bar oxygen partial pressure or dissolved oxygen level in equilibrium with latter - E[m²/s] Axial dispersion coefficient - F[m²] Horizontal cross-section area - k _L a[s^-1] Overall oxygen transfer coefficient - u; u ₂[m/s; cm/s] Superficial velocity: related to state of bubbles leaving the sparger - x; x _atm[-] Signal registered in the experiment; signal recorded in O₂ saturated water, or water vapor saturated air stream, at temperature identical to the experiment under atmospheric pressure - y[m] Water column height - [s^-1] Dimensionless oxygen flux Indices a asorption - d desorption - g gas - l liquid - k dispersion - m measured value/in the case of hydrodynamically measured E/ Dedicated to Professor Dr. H. J. Rehm on the occasion of his 60th birthday     

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.     

Promotion of oxygen transfer in three-phase fluidized-bed bioreactors by floating bubble breakers

The objective of the present study was to investigate a method to enhance the volumetric rate of oxygen transfer in three-phase fluidized-bed bioreactors. The rates of oxygen transfer from air bubbles to viscous liquid media were promoted by floating bubble breakers in three-phase fluidized beds operated in the bubble coalescing regime. The liquid-phase volumetric oxygen transfer coefficient has been recovered by fitting the axial dispersion model to the resultant data, and its dependence on the experimental variables, such as the gas and liquid flow rates, particle size, concentration of bubble breakers, and liquid viscosity, has been examined. The results indicate that the liquid-phase volumetric oxygen transfer coefficient can be enhanced up to 20-25%. The coefficient exhibits a maximum with respect to the volume ratio of the floating bubble breakers to the fluidized solid particles; it increases with increases in the gas and liquid flow rates and size of fluidized particles, while it decreases with an increase in the liquid viscosity. An expression has been developed to correlate the liquid-phase volumetric oxygen transfer coefficient with the experimental variables.     

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.     

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.     

Combined sulfite method for the measurement of the oxygen transfer coefficient k(L)a in bio-reactors

The combined sulfite method is proposed for the measurement of oxygen transfer coefficients, k_La, in bioreactors. The method consists of a steady-state and a dynamic measurement which are carried out under the same experimental conditions and thus yield data for both methods during one experiment. The applied experimental conditions are shown to avoid chemical enhancement during the steady-state measurement. Moreover, no parallel sulfite oxidation occurs during the oxygen saturation phase of the dynamic measurement. Under the applied experimental conditions, no information about the sulfite oxidation kinetics is required and possible metal ion impurities in sulfite salts do not influence the measurement. The characterization of a laboratory-scale bioreactor aerated with pure oxygen yields k_La values during the steady-state and the dynamic measurements that are in good agreement with the dynamic pressure method, the correctness of which is generally accepted. When air is used for absorption, the steady-state measurement yields k_La values that correlate to the correct variant of the standard dynamic method. The dynamic measurement with air absorption yields a k_La value which considers the influence of the non-uniform bubble size distribution present in bubble-aerated bioreactors.     

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 factors influencing the growth of Anabaena variabilis in a bubble column reactor

The combined effect of superficial gas velocity, pH, initial phosphate concentration, and light intensity on cell growth was investigated for the mass production of cyanobacterial cells. The light intensity was manipulated to maintain a specific irradiation rate (q(i)) at a constant level for high cell density culture. The optimum condition for the batch culture was achieved at a superficial gas velocity of 2.0 cm/s, pH 7.0, and an initial phosphate concentration of 55 mg/l when the specific irradiation rate was controlled above 11.5 micromol/s/g dry cell. In this condition, the specific growth rate and cell productivity were 1.47 day(-1) and 0.98 g dry cell/l/day, respectively.     

k(L)a as a predictor for successful probe-independent mammalian cell bioprocesses in orbitally shaken bioreactors

The aim of this study was to gain a better understanding of orbitally shaken bioreactors (OSRs) operated without controllers for pH and dissolved oxygen (DO) concentration. We used cylindrical OSRs with working volumes ranging from 250mL to 200L to determine that the volumetric mass transfer coefficient of oxygen (k(L)a) is a good predictor of the performance of OSRs at different scales. We showed that k(L)a values of 7-10hour(-1) were required to avoid DO limitations and to prevent conditions of low pH during the cultivation of CHO cells. Overall, cell cultures in probe-independent OSRs of different nominal volumes ranging from 250mL to 200L achieved similar cell densities, recombinant protein concentrations, and pH and DO profiles when having the same k(L)a. We conclude that k(L)a is a key parameter for probe-independent bioprocesses in OSRs and can be used as a scale-up factor for their operation.     

Influence of growth manner on nitrifying bacterial communities and nitrification kinetics in three lab-scale bioreactors

The effects of growth type, including attached growth, suspended growth, and combined growth, on the characteristics of communities of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) were studied in three lab-scale Anaerobic/Anoxic_m-Oxic_n (AmOn) systems. These systems amplified activated sludge, biofilms, and a mixture of activated sludge and biofilm (AS-BF). Identical inocula were adopted to analyze the selective effects of mixed growth patterns on nitrifying bacteria. Fluctuations in the concentration of nitrifying bacteria over the 120 days of system operation were analyzed, as was the composition of nitrifying bacterial community in the stabilized stage. Analysis was conducted using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and real-time PCR. According to the DGGE patterns, the primary AOB lineages were Nitrosomonas europaea (six sequences), Nitrosomonas oligotropha (two sequences), and Nitrosospira (one sequence). The primary subclass of NOB community was Nitrospira, in which all identified sequences belonged to Nitrospira moscoviensis (14 sequences). Nitrobacter consisted of two lineages, namely Nitrobacter vulgaris (three sequences) and Nitrobacter alkalicus (two sequences). Under identical operating conditions, the composition of nitrifying bacterial communities in the AS-BF system demonstrated significant differences from those in the activated sludge system and those in the biofilm system. Major varieties included several new, dominant bacterial sequences in the AS-BF system, such as N. europaea and Nitrosospira and a higher concentration of AOB relative to the activated sludge system. However, no similar differences were discovered for the concentration of the NOB population. A kinetic study of nitrification demonstrated a higher maximum specific growth rate of mixed sludge and a lower half-saturation constant of mixed biofilm, indicating that the AS-BF system maintained relatively good nitrifying ability.     

The effects of cells on oxygen transfer in bioreactors

Cells may affect oxygen transfer rates by three mechanisms: respiration of cells accumulated at the gas/liquid interface, physical presence of cells as solid particles, and modification of the medium by cells. These effects were studied experimentally in bubble-aerated bioreactors using baker's yeast at different cell concentrations, agitation speeds, aeration rates, and specific oxygen uptake rates. The overall effect of cells was to enhance oxygen transfer rates. The physical presence of cells as solid particles was found to retard oxygen transfer, presumably due to the lower oxygen permeability in the cell layer accumulated near the bubble surfaces. Cell respiration and medium modification, on the other hand, enhanced oxygen transfer rates. The retardation by nonrespiring cells and the enhancement due to cell respiration were found stronger at higher agitation speeds and lower aeration rates employed. This was attributed to the higher interfacial cell accumulation associated with the smaller bubbles produced under these conditions in the systems studied.     

The effects of non-Newtonian fermentation broth viscosity and small bubble segregation on oxygen mass transfer in gas-lift bioreactors: a critical review

This review paper deals with the effects of non-Newtonian fermentation broth viscosities on gas–liquid mixing and oxygen mass transfer characteristics to provide knowledge for the design and development of gas-lift bioreactors, which can operate satisfactorily with high viscosity fermentation broths. The effect of small bubble segregation is also examined.     

Production of a microbial lipase by Staphylococcus carnosus (pLipMut2) in a bubble column reactor and a centrifugal field bioreactor

Extracellular lipase production by the recombinant strain Staphylococcus carnosus (pLipMut2) has been studied. First substrate optimization was carried out in shaken cultures. As a result, the best substrate yield of 20 units/g (peptone + yeast extract) and maximum lipase activity in the culture supernatant of 1.7 units/cm³ could be obtained by a nutrient rich complex medium consisting of 75 kg/m³ yeast extract, 15 kg/m³ tryptone, 5 kg/m³ glucose and 0.5 kg/m³ K₂HPO₄. Higher initial substrate concentration caused inhibition of growth. Antifoam agent at higher levels than 1 cm³/ dm³ resulted in a negative influence on lipase yield. Comparative fermentation studies have been carried out in a bubble column reactor and in a centrifugal field bioreactor. Direct proportionality between growth, lipase production and oxygen consumption was observed. In the bubble column reactor usual superficial air velocities (4 cm/s) caused intensive foam generation, thus fermentation was only possible after installation of a broader column head to allow coalescence. In the centrifugal field bioreactor higher productivities were obtained without foam problems at superficial gas velocities which were one order of magnitude lower than in the bubble column. Fermentations have been performed batchwise and without holding pH constant. Neither pH control nor glucose feeding could improve the substrate yield further. Compared to former fermentation studies with the strain S. carnosus (pLipPS1) lipase yield (lipase activity/cell density) could be improved by 300% and substrate yield (lipase activity/substrate concentration) by 600%.     

Simulation of algae growth in a bench-scale bubble column reactor

The growth of the marine red microalga Porphyridium sp. in a bubble-column photobioreactor was simulated. The proposed model constitutes a dynamic integration of the kinetics of photosynthesis and photoinhibition with the fluid dynamics of the bubble column, including the effects of shear stress on the kinetics of growth. The kinetic data used in the model were obtained in independent experiments run in a thin-film photobioreactor with defined light/dark cycles. The maintenance term was modified to take into account the effects of liquid flow in the bioreactor on the growth rate. A hybrid method proposed for the approximate solution of the equations gave an appreciable reduction of the calculation time. Extrapolations of the model indicated the possibility of predicting the optimal diameter for an assembly of bubble column photobioreactors. Satisfactory fit was found with the experimental results of biomass growth in a 13-liter bubble column.     

Enhancing gas-liquid mass transfer rates in non-newtonian fermentations by confining mycelial growth to microbeads in a bubble column

The performance of a penicillin fermentation was assessed in a laboratory-scale bubble column fermentor, with mycelial growth confined to the pore matrix of celite beads. Final cell densities of 29 g/L and penicillin titres of 5.5 g/L were obtained in the confined cell cultures. In comparison, cultures of free mycelial cells grown in the absence of beads experienced dissolved oxygen limitations in the bubble column, giving only 17 g/L final cell concentrations with equally low penicillin titres of 2 g/L. The better performance of the confined cell cultures was attributed to enhanced gas liquid mass transfer rates, with mass transfer coefficients (k(L)a) two to three times higher than those determined in the free cell cultures. Furthermore, the confined cell cultures showed more efficient utilization of power input for mass transfer, providing up to 50% reduction in energy requirements for aeration.