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 effect of antifoam agents on mass transfer in bioreactors

The influence of antifoam agents on the liquid-phase mass transfer coefficient in stirred tank and bubble column bioreactors is studied. A physical model based on a surface-renewal concept and additional data in 40-dm³ bubble column bioreactor are presented. Comparisons between the physical model and the data indicate that the model predicts the maximum influence of antifoam agents on the liquid-phase mass transfer coefficient.List of Symbols a 1/m specific surface area - D m²/s diffusivity - D _c m bubble column diameter - d _vs m bubble diameter - g m/s² gravitational acceleration - k _L m/s liquid-phase mass transfer coefficient - k _LO m/s liquid-phase mass transfer coefficient for clean surface - N 1/s impeller speed - Sc Schmidt number (v/D) - U _sg m/s superficial gas velocity Greek Letters W/kg energy dissipation rate per unit mass - _g gas hold-up - Pa s viscosity - v m²/s kinematic viscosity - kg/m³ density - N/m surface tension     

Oxygen mass transfer into aerated CMC solutions in a bubble column

Differing findings on the volumetric mass transfer coefficients k(L)a in CMC solutions in bubble column bioreactors have been reported in the literature. Therefore, oxygen mass transfer was studied again in CMC solutions in a 14-cm-i.d. x 270-cm-height bubble column using different spargers. The k(L)a values were determined along with the dispersion coefficients by fitting the prediction of the axial dispersed plug model with the experimental oxygen concentration profiles in the liquid phase. Surprisingly, the obtained liquid phase dispersion coefficients for CMC solution are higher than one would expect from correlations. The k(L)a data depend largely on the flow regime. In general, they are lower than those reported in the literature. The data for developing slug and established slug flow are dependent on the gas velocity and the effective viscosity of the solution and can br correlated by a simple correlation. This correlation describes k(L)a values measured on fermentation broth of Penicillium chrysogenum with striking agreement.     

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.     

Characterization of the localized hydrodynamic shear forces and dissolved oxygen distribution in sparged bioreactors

Detailed, high-resolution numerical simulations of the bubbly flows, used for oxygen delivery and mixing in mammalian cell suspensions, have been performed. The hydrodynamics, shear and normal forces, mass transfer and mass transport from and around individual bubbles and bubble clusters were resolved for different operating conditions, that is, Weber, Morton, and Schmidt numbers. Suspended animal (e.g., mammalian, insect) cells are known to be susceptible to damage potentially leading to cell death, caused by hydrodynamic stresses and oxygen deprivation. Better knowledge of the magnitude of the shear forces and the extent of mixing of the dissolved oxygen in sparged bioreactors can have a significant impact on their future design and optimization. Therefore, the computed liquid-phase velocity fields were used to calculate and compare the local shear in different types of single bubble wakes and in bubble clusters. Oxygen mass transfer and dissolved oxygen transport were resolved to examine oxygen supply to the cells in the different types of flows.     

Oxygen transfer in slurry bioreactors

The oxygen transfer in bioreactors with slurries having a yield stress was investigated. The volumetric mass transfer coefficients in a 40-L bubble column with simulated fermentation broths, the Theological properties of which were represented by the Casson model, were measured. Experimental data were compared with a theoretical correlation developed on the basis of a combination of Higbie's penetration theory and Kolmogoroff's theory of isotropic turbulence. Comparisons between the proposed correlation and data for the simulated broths show good agreement. The mass transfer data for actual mycelial fermentation broths reported previously by the authors were re-examined. Their Theological data was correlated by the Bingham plastic model. The oxygen transfer rate data in the mycelial fermentation broths fit the predictions of the proposed theoretical correlation.     

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.     

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.     

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.     

Enhancing oxygen transfer in surface-aerated bioreactors by stable foams.

To enhance oxygen transfer in surface-aeration bioreactors, stabilized foams were generated to increase the gas-liquid interfacial area by slowly introducing coarse bubbles into media containing fetal bovine serum. The bubble sparging rates were so low (i.e., 20 and 50 mL/h) that the contribution to oxygen transfer from these bubbles was due to foaming instead of bubbling. Furthermore, no physical cell damage caused by bubble sparging was observed. Oxygen transfer coefficients, kLa, in the bioreactors were measured in cell-free media. Without the foam-stabilizing agent (i.e., serum), no appreciable change in kLa was observed due to the bubble sparging. On the other hand, with serum, kLa increased with increasing serum content and bubble sparging rate and corresponded well with the degree of foaming. With 10% fetal bovine serum and a bubble sparging rate of 50 mL/h, kLa increased approximately 90% compared with no foaming. The enhancing effect of foam on oxygen transfer in surface aeration bioreactors has been further demonstrated with hybridoma cultures simultaneously grown in three identical bioreactors with and without stabilized foams.     

An efficient system for carbonation of high-rate algae pond water to enhance CO2 mass transfer

High-rate algal ponds have the potential to produce 59 T of dry biomass ha(-1)year(-1) based on the specific productivity of 20 g m(-2) day(-1). Atmospheric air provides only 5% of the CO(2) to the pond surface required for photosynthesis. Hence, CO(2) is usually provided via bubbling of concentrated CO(2)-air mixture into the algae ponds. This process is, however, not significantly effective in terms of mass transfer. Use of bubble column to increase the interfacial area of contact available for gas exchange is proposed as an efficient alternative. A carbonation column (CC) was modeled and designed to measure CO(2) absorptivity in-pond water at various pH regimes. The CC performed at 83% CO(2) transfer efficiency. An air-to-pond mass transport coefficient of 0.0037 m min(-1) was derived. The proposed device can be used with any exhaust gas stream with higher concentrations of CO(2) in conjunction with raceways for optimizing algae production.     

CFD simulation coupled with population balance equations for aerated stirred bioreactors

Mammalian cells have been widely used to produce therapeutic proteins in stirred bioreactors in suspension culture. Local hydrodynamics can have a great impact on cell proliferation and protein synthesis, but there are few reports on spatial heterogeneity of nutrients, gas bubbles, and mass transfer coefficients. We have employed computational fluid dynamics (CFD) coupled with population balance equations to study local hydrodynamics in a 20 L stirred bioreactor. The flow patterns, energy dissipation rates, gas volume fraction, gas bubble size distribution and local mass transfer coefficient have been displayed throughout the whole bioreactor. Their implications for mammalian cell culture have been discussed. This study provides an insight into rational design and optimum operation conditions in a stirred bioreactor for mammalian cell cultivation.     

Evaluation of oxygen transfer rates in stirred-tank bioreactors for clinical manufacturing

Several methods are available for determining the volumetric oxygen transfer coefficient in bioreactors, though their application in industrial bioprocess has been limited. To be practically useful, mass transfer measurements made in nonfermenting systems must be consistent with observed microbial respiration rates. This report details a procedure for quantifying the relationship between agitation frequency and oxygen transfer rate that was applied in stirred-tank bioreactors used for clinical biologics manufacturing. The intrinsic delay in dissolved oxygen (DO) measurement was evaluated by shifting the bioreactor pressure and fitting a first-order mathematical model to the DO response. The dynamic method was coupled with the DO lag results to determine the oxygen transfer rate in Water for Injection (WFI) and a complete culture medium. A range of agitation frequencies was investigated at a fixed air sparge flow rate, replicating operating conditions used in Pichia pastoris fermentation. Oxygen transfer rates determined by this method were in excellent agreement with off-gas calculations from cultivation of the organism (P = 0.1). Fermentation of Escherichia coli at different operating parameters also produced respiration rates that agreed with the corresponding dynamic method results in WFI (P = 0.02). The consistency of the dynamic method results with the off-gas data suggests that compensation for the delay in DO measurement can be combined with dynamic gassing to provide a practical, viable model of bioreactor oxygen transfer under conditions of microbial fermentation.     

Investigation of the structure of two-phase flow fermentation model media in bubble-column bioreactors

Summary Electrical conductivity microprobes have been used to estimate the transverse variation of bubble size, local gas holdup and local specific gas/liquid interfacial area in bench scale bubble column bioreactors containing fermentation model media. Inserted O₂-electrodes and plane parallel windows alter the structure of the two phase flow. Even slight tilting of the column strongly influences the transverse profiles of the bubble size and local gas holdup. The larger bubbles are collected at the wall, where they can be redispersed. These observations open up new possibilities for the construction of bubble column bioreactors.     

Influence of pluronic F68 on oxygen mass transfer

Pluronic F68 is one of the most used shear protecting additives in cell culture cultivations. It is well known from literature that such surface‐active surfactants lower the surface tension at the gas‐liquid interface, which influences the mass transfer. In this study, the effect of Pluronic F68 on oxygen mass transfer in aqueous solutions was examined. Therefore, the gassing in/gassing out method and bubble size measurements were used. At low concentrations of 0.02 g/L, a 50% reduction on mass transfer was observed for all tested spargers and working conditions. An explanation of the observed effects by means of Higbie's penetration or Dankwerts surface renewal theory was applied. It could be demonstrated that the suppressed movement of the bubble surface layer is the main cause for the significant drop down of the k_La‐values. For Pluronic F68 concentrations above 0.1 g/L, it was observed that it comes to changes in bubble appearance and bubble size strongly dependent on the sparger type. By using the bubble size measurement data, it could be shown that only small changes in mass transfer coefficient (k_L) take place above the critical micelle concentration. Further changes on overall mass transfer at higher Pluronic F68 concentrations are mainly based on increasing of gas holdup and, more importantly, by increasing of the surface area available for mass transfer. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 29:1278–1288, 2013     

Oxygen transfer and axial dispersion in an aeration tower containing static mixers

Oxygen transfer from gas to liquid under steady-state cocurrent flow conditions was modeled using the dispersion model, and the oxygen transfer coefficients were estimated from available data for a column with Koch motionless mixers. The dispersion in the column was estimated for several different gas and liquid flow rates using steady-state tracer experiments. The estimated oxygen transfer coefficients were compared with those estimated using complete mixing and plug flow models. The results indicate that the dispersion model is the most appropriate model for estimating the mass transfer coefficient from the available data.     

Enhancement of oxygen transfer in highly viscous non-newtonian fermentation broths

The influence of short draft tubes covered by perforated plates on gas-liquid mass transfer was examined in external-loop airlift bioreactors. The volumetric mass transfer coefficients in a model external-loop airlift bioreactor were measured with water and non-Newtonian media. It was found that introduction of draft tubes covered with perforated plates in the riser significantly improved the mass transfer rate, particularly in higher viscous non-Newtonian fermentation media. The enhancement of mass transfer rate might be due mainly to an increase in bubble coalescence and redispersion. (c) 1994 John Wiley & Sons, Inc.