Oxygen mass transfer and gas holdup in a bubble column bioreactor with biosynthesis liquids

The influence of the rheology of some antibiotic biosynthesis liquids produced by Streptomyces aureofaciens, Nocardia mediterranei and Penicillium chrysogenum on the volumetric liquid phase oxygen transfer coefficient, k_La, and gas holdup, ε_G, together with the influence of superficial gas velocity, were studied in a bubble column bioreactor, using samples of fermentation liquids taken from industrial stirred tank fermenters, at 30-hour intervals during fermentation batch. The results were compared to those of previous studies from literature on non-Newtonian homogeneous fluids, such as CMC-Na, xanthan and starch solutions, respectively. In the heterogeneous broths, ε_G and k_La decreased with increasing apparent viscosity of the broth and increased with increasing superficial velocity. The experimental data were correlated using non-linear regression with correlation coefficients above 0.85.     

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.     

Liquid-phase axial mixing in a bubble column bioreactor containing yeast suspensions

Summary Liquid-phase axial mixing coefficients were evaluated in a 0.15 m x 2.0 m batch bubble column containing water and yeast-in-water suspensions of different concentrations. Air superficial velocities ranged from 0 to 0.06 m/s. Axial mixing coefficients were calculated from the residence time distribution to an NaCl tracer pulse using the Ohki and Inuoe model. No specific variations in the calculated coefficients were observed to result from the presence of yeast cells. There was fair agreement between the data thus obtained and the only available data on mixing in non-Newtonian CMC solution.Nomenclature C _E equilibrium tracer concentration g/l - C tracer concentration at time t g/l - d_h sparger hole diameter m - D _t tube diameter m - D _z axial mixing coefficient m²/s - g acceleration of gravity m/s² - H _B bubbling layer heigh m - L longitudinal dustance between tracer injection and detection points m - n 1,2,6 Eq. (3) - t time s - U_g gas superficial velocity m/s - U_t liquid superficial velocity m/s - V _r bubble relative velocity = m/s - V _t Linear relative velocity m/s - z axial distance m Greek _c wet cell volume farction - _g gas holdup - _l liquid holdup - _l viscosity of the liquid phase Pa/s - _l density of liquid or continuous phase g/ml     

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     

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.     

Oxygen mass transfer in a bubble column bioreactor containing lysed yeast suspensions

Summary Liquid-phase volumetric oxygen transfer coefficients were evaluated in a bubble column containing yeast suspensions, using the instationary oxygen absorption method and a polarographic oxygen electrode. The electrode time lag was found to be independent of both the system studied and the operating conditions. The volumetric oxygen mass transfer coefficients k _L a could be reasonably predicted by calculating k _L from the equation derived by Bhavaraju et al. or the empirical equation of Calderbank and Moo-Young and a from the experimental gas hold-up values.Nomenclature a Exponent in Eq.6 or specific gas-liquid interfacial area based on reactor volume m - b Exponent in Eq. 6 - C Constant in Eq 6 or oxygen concentration in the liquid phase g/ml - C ^* Equilibrium oxygen concentration g/ml - C ₀ Oxygen concentration in the liquid phase at t=0 g/ml - C _E Oxygen concentration as determined by the polarographic electrode g/ml - D _B Bubble equivalent diameter mm - D _l Oxygen diffusivity in the liquid phase m²/s - g Acceleration of gravity m/s² - K Consistency index Pasⁿ - K _L Liquid-phase mass transfer coefficient m/s - n Power law exponent - Pe _sw Peclet number based on bubble swarm velocity - S _C Schmidt number - Sh Sherwood number - i Time s - U _B Bubble rise velocity in infinite medium m/s - U _g Superficial air velocity based on column cross-sectional area m/s - U _sw Bubble swarm velocity defined by Eq.15 m/s - Y _MSW Mass transfer coeficient correction factor for mobile interfaces in pseudo-plastic fluids Eq. 7 - Y _MSW Mass transfer coefficient correction factor for immobile interface in pseudo-plastic fluids Eq. 8 Greek letters _l Density of liquid g/ml - _sus Density of unaerated suspension g/ml - _{wet cell} Density of yeast wet cells g/ml - _l Viscosity of the liquid Pas - _app Apparent viscosity of power law fluid Pas - _E Electrode time lag s - _l Time lag due to resistance of the gas-liquid interface s - _g Gas hold-up, volume fraction occupied by the gas phase - _l Liquid hold-up - _c Wet cell volume fraction     

Performance of a cell recycle bubble column fermentor with a membrane filter module for continuous vinegar production

The steady-state performance of a bubble column combined with a membrane filter module for cell separation and recycle is investigated numerically in the case of vinegar fermentation. The one-dimensional dispersion model for describing the longitudinal mixing of the liquid phase is employed. Kinetic expressions and their parameter values are taken from the available literature. Several characteristics of this fermentor system namely the concentration profiles of cells, substrate and product, the viability of viable cells relative to total cells, the washout condition for cells and the productivity of acetic acid are discussed. The average cell viability in the whole column and the critical dilution rate for washout are presented as equations. Low levels of the axial mixing are found to enhance the vinegar productivity. The optimum dilution rate giving the maximum productivity is determined and both are shown as figures with the Peclet number, the recycle ratio and the bleed ratio as parameters.     

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.     

Mechanical control of foaming in a bubble column

This study was performed to evaluate the effects of the air sparge rate, working liquid volume, liquid feed rate onto the disk, and disk diameter on the foam-breaking performance of foam-breaking apparatus with a rotating disk (FARD) installed in the bubble column. Experimental results showed that the smaller the air sparge rate and working liquid volume were, and the larger the liquid feed rate and disk diameter, the lower the critical disk rotational speed required for reduced foam-breaking. The presence of the effective ranges of the disk diameter and liquid feed rate for foam breaking was also confirmed. Furthermore, the quantitative predictions of the upper limits of the liquid feed rate, foam-breaking regions, and the required foam-breaking power were carried out, based on the results obtained above. Comparison of the FARD with two conventional mechanical foam-breaking spray-type apparati also demonstrated the highest level of the FARD in respect not only to foam-breaking performance but also to power requirements.     

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.     

Mass transfer of terpenes through a silicone rubber membrane in a liquid-liquid contacting system

Terpenoids are important compounds for the fragrance industry, and recently, biocatalytic methods have been developed to produce them from cheap monoterpenes, such as alpha-pinene oxide. The biotransformation of alpha-pinene oxide using resting cells of Pseudomonas fluorescens NCIMB 11671 produces isonovalal (cis-2-methyl-5-isopropylhexa-2,5-dienal), which is a fragrance. However, this biotransformation has technical problems including the following: alpha-pinene oxide undergoes autoxidation in water and light; it is hydrophobic and relatively toxic to the biocatalyst; and it suffers from product inhibition. Therefore, removal of isonovalal as it is formed should reduce its toxicity and increase volumetric productivity and production yield. Aqueous-organic two-phase (AOTP) systems can be used in the biotransformation of hydrophobic substrates and can protect biocatalysts from toxic substrates and products. However, the formation of stable emulsions makes further downstream processing and continuous operation difficult. One solution to these problems is to use a solid-phase membrane between the aqueous and organic phases in a Membrane Bioreactor for Biotransformation (MBB). Since there are no data in the literature on the behavior of terpenes and terpenoids with solid-phase membranes, or their mass transfer behavior, the objective of this work was to measure these parameters for a wide range of compounds so as to be able to design an MBB, and to gain a greater understanding of their behavior in these types of systems. Organic/aqueous (P (org) (aq)) and membrane/aqueous (P (mem) (aq)) partition coefficients were measured first, and subsequently used to quantify the overall mass transfer coefficients (k(ov)). The overall mass transfer coefficient (k(ov)) of alpha-pinene oxide through the membrane was found to be 2.5 x 10(-5) m x s(-1) using thicknesses of both 250 and 1,000 microm. Extraction kinetics were successfully described using a resistance-in-series model and were controlled by the aqueous boundary layer and/or membrane resistance (k(mem)), while organic film resistance played an insignificant role. Aqueous film resistance (k(aq)) was found to be the limiting step for mono- and diterpenes, and the effect of the hydrodynamics on k(ov) was successfully predicted using a Wilson plot. However, the extraction kinetics of larger terpenes, such as steroids, were influenced solely by the k(mem), suggesting that membrane diffusivity also depended on the size of the permeating molecule. Finally, the influence of other terpene byproducts on the flux of alpha-pinene oxide was investigated and found to decrease the flux into the organic phase by up to 10%.     

Cometabolic degradation of trichloroethylene in a bubble column bioscrubber

A bubble column bioreactor was used as bioscrubber to carry out a feasibility study for the cometabolic degradation of trichloroethylene (TCE). Phenol was used as cosubstrate and inducer. The bioreactor was operated like a conventional chemostat with regard to the cosubstrate and low dilution rates were used to minimize the liquid outflow. TCE degradation measurements were carried out using superficial gas velocities between 0.47and 4.07 cm s(-1) and TCE gas phase loads between 0.07 and 0.40 mg L(-1) Depending on the superficial gas velocity used, degrees of conversion between 30% and 80% were obtained. A simplified reactor model using plug flow for the gas phase, mixed flow for the liquid phase, and pseudo first order reaction kinetics for the conversionof TCE was established. The model is able to give a reasonable approximation of the experimental data. TCE degradation at the used experimental conditions is mainly limited by reaction rate rather than by mass transfer rate. The model can be used to calculate the reactor volume and the biomass concentration for a required conversion. (c) 1995 John Wiley & Sons Inc.     

Influence of antifoam agents on gas hold-up and mass transfer in bubble columns with non-newtonian fluids

Summary Experimental studies on the effect of antifoam agents on the performance of bubble columns with non-Newtonian fluids have been conducted. It is found that the gas hold-up and volumetric mass transfer coefficient in the case of water were reduced due to the addition of antifoam agents. It was found that this decrease in volumetric mass trasfer coefficient is substantial but in the aqueous solutions of polymers the effect becomes weaker as the concentration of polymers becomes higher. When the concentration of polymers became higher than a certain value, the volumetric mass transfer coefficient in the aqueous solutions of polymers with antifoam agents was higher than that without antifoam agents.Nomenclature a Specific surface area, 1/m - D _c Column diameter, m - d _max Diameter of the largest bubble stable against breakup, m - d _min Diameter of the smallest bubble stable against coalescence, m - g Gravitational acceleration, m/s² - H _l Clear liquid height, m - h Rupture thickness of the liquid film, m - K Consistency index in a power-law model, Pa·s ⁿ - k _l Liquid-phase mass transfer coefficient, m/s - n Flow index in a power-law model - u _sg Superficial gas velocity, m/s Greek letters Shear rate, 1/s - Gas hold-up - Energy dissipation per unit mass, m²/s³ - Viscosity, Pa·s - p Density, kg/m³ - Surface tension, N/m - Shear stress-Pa     

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.     

Modelling Xanthomonas campestris batch fermentations in a bubble column

Rate and yield expressions relating to biomass and xanthan formation and to nitrogen, glucose, and oxygen consumption were established for Xanthomonas campestris batch fermentations in a bubble column. Microbial growth was described by the logistic rate equation, characterized by a maximum specific growth rate mu(M) = 0.5 h(-1) and a maximum attainable cell concentration provided by nitrogenous compounds. With regard to carbon metabolism, the decrease with time in experimental yields and in the experimental specific rates of xanthan production and glucose assimilation demonstrated the inadequacy of the Luedeking-Piret model. These decreases were connected to the simultaneous drop in dissolved-oxygen tension observed during xanthan synthesis. The knowledge of metabolic pathways and energetic balance were used to establish the relationships between substrate utilization, ATP generation, and xanthan production. The model was structured by assuming the oxygen limitation of both the respiration rate and the efficiency of the oxidative phosphorylation mechanism (P/O ratio). Consequently, the specific rates and yield expressions became dependent on the dissolved-oxygen tension, i.e., of the volumetric oxygen transfer in the fermentor.     

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.     

Oxygen transfer rates in a mammalian cell culture bioreactor equipped with a cell-lift impeller

Measurements of k(L)a were carried out in 1. 5- and 5-L New Brunswick Scientific CelliGen(R) bioreactors. The measured k(L)a in water were identical for both vessel sizes operated in similar condition. The mass transfer rate increased with temperature, mixing speed, and aeration rate, with this last parameter being the most significant. Surface aeration alone gave k(L)a values of 0. 4 to 1. 6 h(-1). A 25% decrease in k(L)a was observed above an aeration rate of 1. 6 vvm. This was caused by the particular foam breaker of the CelliGen bioreactor. Measurements of k(L)a using a mammalian cell culture medium supplemented with 5% fetal calf serum (FCS) have confirmed the negative effect of the foam breaker on k(L)a The measured value in this medium was 1. 2 h(-1) for all aeration rates, more than 60% of which was attributed to surface aeration.     

Xanthan production in bubble column and air-lift reactors

A bubble column (0.05 m(3)) and an air-lift fermentor (1.2 m(3)) were used for the production of the exocellular microbial polysaccharide xanthan with Xanthomonas campestris in a synthetic medium. Upon oxygen depletion in the liquid, the xanthan production rate dropped sharply and then became a linear function of the oxygen transfer rate. The volumetric mass transfer coefficients for oxygen conformed to the correlation of Suh et al. Using this correlation in combination with the model for xanthan batch fermentation suggested by Peters et al., the xanthan fermentations in the bubble column were well described. The model also correctly predicted the time course of the molecular weight of the polysaccharide even when a complex medium was used. In the air-lift fermentor, however, the xanthan production rate and the xanthan yields with respect to oxygen and glucose were lower than expected at the overall oxygen transfer rate. The poor performance of the air lift was traced back to the lack of any oxygen supply in the downcomer.