On liquid-liquid mass transfer in two-liquid-phase fermentations

Almost all two-liquid phase bioprocesses are characterized by the presence of surface active materials (biosurfactants), which significantly influence the interaction between the phases. In order to predict mass transfer rates during cultivations of Pseudomonas oleovorans biosurfactant was isolated from the biosuspension and added in defined amounts to n-octane/water model-dispersions. Effects of biosurfactant concentration on interfacial tension, mean Sauter-diameter, drop size distribution, dispersion stability and liquid-liquid mass transfer coefficients were studied. A comparison was made between calculated solvent transfer rates (STR) and measured solvent uptake rates (SUR) of P. oleovorans cultures. With increasing interfacial surfactant concentration interfacial tension and mean Sauter-diameter decreased until a minimum for both, interfacial tension and mean Sauter-diameter, were reached. Interfacial tension measurements indicate that these minima have to be attributed to a maximum monomolecular surfactant concentration and the formation of polymolecular adsorption layers. Drop size distributions showed that, coalescence and droplet break-up disappear because droplets are stabilized by the biosurfactant adsorption layers at the interface. Mass transfer regime shifted from forced convection and surface renewal to diffusion. Comparison of solvent uptake rates (SUR) and solvent transfer rates (STR) showed that n-octane transfer usually will not be limiting P. oleovorans cultures, however, can become dominant in cultures where solvents with very low miscibilities like n-decane are used.     

Intensification of mass transfer in aqueous two-phase systems

A novel technique which intensifies conventional aqueous two-phase extraction by conversion of dispersed phase into colloidal gas aphrons (CGAs) has been developed for extraction of an enzyme. In the present work, amyloglucosidase (1,4-alpha-D-glucan glucohydrolase) was extracted using a polyethylene glycol-sodium sulfate-water system. The lighter phase, i.e., polyethylene glycol (PEG) rich phase, was converted into CGAs which were then dispersed into a salt rich phase. The effect of type of surfactant and its concentration, dispersed phase velocity, phase composition, and type of sparger on the dispersed phase mass transfer coefficient was investigated. The results suggests 9-16 times higher values of mass transfer coefficient compared to spray column. The multiorifice sparger at concentrations of 0.33 g/L of cetyl trimethyl ammonium chloride yielded best results. (c) 1993 John Wiley & Sons, Inc.     

Evaluation of oxygen mass transfer in Aspergillus niger fermentation using data reconciliation

Fermentation experiments using Aspergillus niger result in a very viscous broth due to the growth of filamentous microorganism. For viscous fermentation processes, it is difficult to estimate with confidence the volumetric oxygen mass transfer coefficient (K(L)a), which can be used for scale-up or design of bioreactors. In the present study, four methods based on dynamic and stationary approaches were used to measure K(L)a throughout the fermentation. Data reconciliation was used to obtain a more reliable and consistent K(L)a. The K(L)a value obtained by a data reconciliation technique was found to be more reliable since it takes into consideration both the reliability of all measured variables and the accuracy of all mass balance equations.     

Effect of additives on mass transfer in turbine aeration

Mass transfer coefficients and interfacial areas were determined for the aeration of aqueous solutions in a turbine agitated vessel. The mass transfer coefficients measured for water without additive and for sodium chloride solutions matched very well to measurements in the literature for air bubbles of the same diameter in free rise. Thus the only effect of agitation was to determine the bubble size which then in turn set the coefficient. Two surface active agents were studied: sodium dodecyl sulfate and Dow Corning Antifoam C. The rate of mass transfer increased with the former additive but decreased with the latter; however, the mass transfer coefficient was the exact same function of bubble diameter in both cases and the different rates are attributed to the quite different effects on interfacial area.     

Effect of the interfacial surfactant layer on oxygen transfer through the oil/water phase boundary in perfluorocarbon emulsions

The effect of interfacial surfactant molecules on oxygen transfer through oil/water phase boundary has been studied in FlurO(2) (TM) emulsions, i.e., perfluorocarbon (PFC) emulsions developed as oxygen carriers in cell culture. Measurements of oxygen permeability were made with a polarographic oxygen electrode in pure PFCs and in emulsions with various PFC volume fractions. Comparison of the experimental results with the theoretically derived values of relative oxygen permeability clearly indicates that the mass transfer resistance caused by the interfacial surfactant layer in PFC emulsions is insignificant. Therefore, oxygen dissolved in the enclosed PFC phase is readily available to cells growing in the aqueous media and FlurO(2) emulsions with very fine emulsion particles (< 0.2 mum) can be used to effectively enhance gas/liquid interfacial oxygen transfer in bioreactors. The inadequacy in describing mass transfer in heterogeneous systems, such as the PFC emulsions, by conventional concentration-based oxygen diffusion coefficients has also been discussed.     

Modeling multiphase fluid flow,mass transfer,and chemical reactions in bioreactors using large-eddy simulation

We present a transient large eddy simulation (LES) modeling approach for simulating the interlinked physics describing free surface hydrodynamics, multiphase mixing, reaction kinetics, and mass transport in bioreactor systems. Presented case-studies include non-reacting and reacting bioreactor systems, modeled through the inclusion of uniform reaction rates and more complex biochemical reactions described using Contois type kinetics. It is shown that the presence of reactions can result in a non-uniform spatially varying species concentration field, the magnitude and extent of which is directly related to the reaction rates and the underlying variations in the local volumetric mass transfer coefficient.     

Local overall volumetric gas-liquid mass transfer coefficients in gas-liquid-solid reversed flow jet loop bioreactor with a non-Newtonian fluid

The local overall volumetric gas-liquid mass transfer coefficients at the specified point in a gas-liquid-solid three-phase reversed flow jet loop bioreactor (JLB) with a non-Newtonian fluid was experimentally investigated by a transient gassing-in method. The effects of liquid jet flow rate, gas jet flow rate, particle density, particle diameter, solids loading, nozzle diameter and CMC concentration on the local overall volumetric gas-liquid mass transfer coefficient (K(L)a) profiles were discussed. It was observed that local overall K(L)a profiles in the three-phase reversed flow JLB with non-Newtonian fluid increased with the increase of gas jet flow rate, liquid jet flow rate, particle density and particle diameter, but decreased with the increase of the nozzle diameter and CMC concentration. The presence of solids at a low concentration increased the local overall K(L)a profiles, and the optimum of solids loading for a maximum profile of the local overall K(L)a was found to be 0.18x10(-3)m(3) corresponding to a solids volume fraction, varepsilon(S)=2.8%.     

A simple method to estimate the contribution of biological floc and reactor-solution to mass transfer of oxygen in activated sludge processes

In this study, the mass transfer coefficient of biological floc (K(L)a(bf)) was estimated from the mass transfer coefficient of the mixed-liquor (K(L)a(f)) and the reactor-solution (K(L)a(e)). The biological floc resistance (BFR) and reactor-solution resistance (SR) were defined as the reciprocal of K(L)a(bf) and K(L)a(e), respectively, by applying the concept of serial-resistance originally presented in two-film theory (Lewis and Whitman (1924) Ind Eng Chem 16:1215-1220). The specific biological floc resistance (SBFR) was defined as biological floc resistance per unit biomass concentration. The data indicated that an activated sludge process yielding low BFR/MLR and BFR/SR tended to produce higher oxygen transfer efficiency. Surprisingly, the reactor-solution posed the same level of resistance as clean water in all experiments, except in a 5-day SRT, non-nitrifying, completely mixed activated sludge (CMAS) process run. Furthermore, SBFR successfully represented biological floc and showed a positive correlation to sludge volume index (SVI). In addition, SBFR/SR and oxygen transfer efficiency (OTE(f)) followed an exponential relationship for the complete data set. The method of separating the mixed-liquor into biological floc and reactor-solution improved the understanding of oxygen transfer under process conditions, without resorting to intrusive techniques or direct handling of fragile biological floc.     

Polysaccharide concentration and molecular weight effects on the oxygen mass transfer in a reciprocating plate bioreactor

In most polysaccharide fermentations, the nature of the fermentation broth changes drastically with time and, as a result, the overall oxygen mass transfer coefficient (K(L)a) can vary by orders of magnitude. To obtain a better understanding of this phenomenon, an experimental program was devised to study the respective influence of molecular weight and concentration of dextran solutions on K(L)a. Experiments were conducted in a reciprocating plate bioreactor. This bioreactor uses a stack of perforated plates that is reciprocated axially in the column and it is therefore well suited for mixing viscous liquid broths and providing uniform overall mass transfer coefficients. The variation of K(L)a with the power input per unit volume and the superficial gas velocity were obtained for three ranges of molecular weights and five concentrations of dextran. In every medium, two regimes of operation were observed as a function of the power input per unit volume: a first regime, at low power inputs per unit volume where K(L)a remains constant until a threshold of power input is attained; and a second regime, which is characterized by a steep increase of K(L)a as a function of the power input per unit volume. The presence of dissolved biological macromolecules, not only because of their effect on the rheology of the medium but also because their effect on the gas-liquid interface, has a significant impact on K(L)a. It was found that, generally, small concentrations of polysaccharide favor oxygen mass transfer despite the increase in medium viscosity. However, the respective influence of polysaccharide concentration and molecular weight was different for the two regimes of operation. (c) 1996 John Wiley & Sons, Inc.     

Effect of hydrotropes on solubility and mass transfer coefficient of amyl acetate

This paper presents a comprehensive study on the effect of citric acid, sodium benzoate, sodium salicylate and urea (hydrotropes) on the solubility and mass transfer coefficient for the extraction of amyl acetate in water. The influence of a wide range of hydrotrope concentration (0–3.0?mol/l) and different temperatures (303–333?K) on the solubility of amyl acetate has been studied. The influence of different hydrotrope concentrations on the mass transfer coefficients for amyl acetate–water system has been ascertained. Setschenow constant, K_s, a measure of the effectiveness of hydrotrope has been determined for each case. The solubility of amyl acetate increases with increase in hydrotrope concentration and also with system temperature. Consequent to the increase in the solubility of amyl acetate, the mass transfer coefficient was also found to increase with increase in hydrotrope concentration. A Minimum Hydrotropic Concentration (MHC) was found essential to show a significant increase in the solubility and mass transfer coefficient for amyl acetate–water system. The enhancement factor, which is the ratio of value in presence and absence of a hydrotrope is reported for both solubility and mass transfer coefficients.     

Characterization of gas-liquid mass transfer phenomena in microtiter plates

Gas-liquid mass transfer properties of shaken 96-well microtiter plates were characterized using a recently described method. The maximum oxygen transfer capacity (OTR(max)), the specific mass transfer area (a), and the mass transfer coefficient (k(L)) in a single well were determined at different shaking intensities (different shaking frequencies and shaking diameters at constant filling volume) and different filling volumes by means of sulfite oxidation as a chemical model system. The shape (round and square cross-sections) and the size (up to 2 mL maximum filling volume) of a microtiter plate well were also considered as influencing parameters. To get an indication of the hydrodynamic behavior of the liquid phase in a well, images were taken during shaking and the liquid height derived as a characteristic parameter. The investigations revealed that the OTR(max) is predominantly dependent on the specific mass transfer area (a) for the considered conditions in round-shaped wells. The mass transfer coefficient (k(L)) in round-shaped wells remains at a nearly constant value of about 0.2 m/h for all shaking intensities, thus within the range reported in the literature for surface-aerated bioreactors. The OTR(max) in round-shaped wells is strongly influenced by the interfacial tension, determined by the surface tension of the medium used and the surface properties of the well material. Up to a specific shaking intensity the liquid surface in the wells remains horizontal and no liquid movement can be observed. This critical shaking intensity must be exceeded to overcome the surface tension and, thus, to increase the liquid height and enlarge the specific mass transfer area. This behavior is solely specific to microtiter plates and has not yet been observed for larger shaking bioreactors such as shaking flasks. In square-shaped microtiter plate wells the corners act as baffles and cause a significant increase of OTR(max), a, and k(L). An OTR(max) of up to 0.15 mol/L/h can be reached in square-shaped wells.     

Modeling multiphase fluid flow,mass transfer,and chemical reactions in bioreactors using large‐eddy simulation

We present a transient large eddy simulation (LES) modeling approach for simulating the interlinked physics describing free surface hydrodynamics, multiphase mixing, reaction kinetics, and mass transport in bioreactor systems. Presented case‐studies include non‐reacting and reacting bioreactor systems, modeled through the inclusion of uniform reaction rates and more complex biochemical reactions described using Contois type kinetics. It is shown that the presence of reactions can result in a non‐uniform spatially varying species concentration field, the magnitude and extent of which is directly related to the reaction rates and the underlying variations in the local volumetric mass transfer coefficient.     

A study of mass transfer kinetics in an enantiomeric separation system using a polymeric imprinted stationary phase

Chromatographic data pertaining to the enantioseparation of L- and D-phenylalanine anilide (PA) on a polymeric stationary phase imprinted with L-PA were studied from the viewpoints of phase equilibrium, mass transfer kinetics, and the thermodynamic properties of this enantiomeric separation system. The concentration dependence of the lumped mass transfer rate coefficient (k(m,L)) previously published was analyzed to obtain new information concerning the mass transfer characteristics in this chiral separation system. It was shown that intraparticle diffusion contributed much more to k(m,L) than adsorption/desorption. The positive concentration dependence of k(m,L) seemed to be interpreted by considering that of the surface diffusion coefficient, itself explained by the heterogeneous surface model. The characteristic features of the phase equilibrium, the mass transfer kinetics, and the thermodynamics of the enantiomeric separation system probably result from the adsorption energy distribution on the surface of the imprinted phase having an exponential decay.     

Prediction of gas-liquid mass transfer coefficient in sparged stirred tank bioreactors

Oxygen mass transfer in sparged stirred tank bioreactors has been studied. The rate of oxygen mass transfer into a culture in a bioreactor is affected by operational conditions and geometrical parameters as well as the physicochemical properties of the medium (nutrients, substances excreted by the micro-organism, and surface active agents that are often added to the medium) and the presence of the micro-organism. Thus, oxygen mass transfer coefficient values in fermentation broths often differ substantially from values estimated for simple aqueous solutions. The influence of liquid phase physicochemical properties on kLa must be divided into the influence on k(L) and a, because they are affected in different ways. The presence of micro-organisms (cells, bacteria, or yeasts) can affect the mass transfer rate, and thus kLa values, due to the consumption of oxygen for both cell growth and metabolite production. In this work, theoretical equations for kLa prediction, developed for sparged and stirred tanks, taking into account the possible oxygen mass transfer enhancement due to the consumption by biochemical reactions, are proposed. The estimation of kLa is carried out taking into account a strong increase of viscosity broth, changes in surface tension and different oxygen uptake rates (OURs), and the biological enhancement factor, E, is also estimated. These different operational conditions and changes in several variables are performed using different systems and cultures (xanthan aqueous solutions, xanthan production cultures by Xanthomonas campestris, sophorolipids production by Candida bombicola, etc.). Experimental and theoretical results are presented and compared, with very good results.     

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%.     

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).     

Kinetic study of the concentration dependence of the mass transfer rate coefficient in anion-exchange chromatography of bovine serum albumin.

The experimental results of a previous study of the mass transfer kinetics of bovine serum albumin (BSA) in ion-exchange chromatography under nonlinear conditions are reevaluated. The analysis of the concentration dependence of the lumped mass-transfer rate coefficient (k(m,L)) provides information on the kinetics of axial dispersion, fluid-to-particle mass transfer, intraparticle mass transfer, and adsorption/desorption. The new analysis shows that the contribution of intraparticle mass transfer is the dominant one. Similar to k(m,L), the surface diffusivity (D(s)) of BSA increases with increasing concentration. The linear concentration dependence of k(m,L) seems to originate in a similar dependence of D(s). The use of an heterogeneous-surface model for the anion-exchange resin provides an explanation of the positive concentration dependence of D(s). This work illustrates how frontal analysis data can be used for a detailed investigation of the kinetics of mass transfer between the phases of a chromatographic column, in addition to its conventional use in the determination of the thermodynamic characteristics of the phase equilibrium.     

Optimization of oxygen mass transfer in a multiphase bioreactor with perfluorodecalin as a second liquid phase

Oxygenation is an important parameter involved in the design and operation of mixing-sparging bioreactors and it can be analyzed by means of the oxygen mass transfer coefficient (k(L)a). The operational conditions of a stirred, submerged aerated 2-L bioreactor have been optimized by studying the influence of a second liquid phase with higher oxygen affinity (perfluorodecalin or olive oil) in the k(L)a. Using k(L)a measurements, the influence of the following parameters on the oxygen transfer rate was evaluated: the volume of working medium, the type of impellers and their position, the organic phase concentration, the aqueous phase composition, and the concentration of inactive biomass. This study shows that the best experimental conditions were achieved with a perfluorodecalin volume fraction of 0.20, mixing using two Rushton turbines with six vertical blades and in the presence of YPD medium as the aqueous phase, with a k(L)a value of 64.6 h(-1). The addition of 20% of perfluorodecalin in these conditions provided a k(L)a enhancement of 25% when pure water was the aqueous phase and a 230% enhancement when YPD medium was used in comparison to their respective controls (no perfluorodecalin). Furthermore it is shown that the presence of olive oil as a second liquid phase is not beneficial to the oxygen transfer rate enhancement, leading to a decrease in the k(L)a values for all the concentrations studied. It was also observed that the magnitude of the enhancement of the k(L)a values by perfluorodecalin depends on the biomass concentration present.     

Kinetic characterization of mass transfer limited biodegradation of a low water soluble gas in batch experiments--necessity for multiresponse fitting

A method was developed to characterize the kinetics of biodegradation of low water soluble gaseous compounds in batch experiments. The degradation of ethene by resting Mycobacterium E3 cells was used as a model system. The batch degradation data were recorded as the progress curve (i.e., the time course of the ethene concentration in the headspace of the batch vessel). The recorded progress curves, however, suffered gas:liquid mass transfer limitation. A new multiresponse fitting method had to be developed to allow unequivocal identification of both the affinity coefficient, K(aff), and the gas:liquid mass transfer coefficient, K(l)a, in the batch vessel from the mass transfer limited data. Simulation showed that the K(aff) estimate obtained is influenced by the dimensionless (volumetric basis) ethene gas:liquid partitioning coefficient (H). In the fitting procedure, Monod, Teissier, and Blackman biokinetics were evaluated for characterization of the ethene biodegradation process. The fits obtained reflected the superiority of the Blackman biokinetic function. Overall, it appears that resting Mycobacterium E3 cells metabolizing ethene at 24 degrees C have, using Blackman biokinetics, a maximum specific degradation rate, v(max), of 10.2 nmol C(2)H(4) mg(-1) CDW min(-1), and an affinity coefficient, K(aff.g), expressed in equilibrium gas concentration units, of 61.9 ppm, when H is assumed equal to 8.309. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 55: 511-519, 1997.     

Effect of mass transfer in a recirculation batch reactor system for immobilized penicillin amidase

The effect of external mass transfer resistance on the overall reaction rate of the immobilized whole cell penicillin amidase of E. coli in a recirculation batch reactor was investigated. The internal diffusional resistance was found negligible as indicated by the value of effectiveness factor, 0.95. The local environmental change in a column due to the pH drop was successfully overcome by employing buffer solution. The reaction rate was measured by pH-stat method and was found to follow the simple Michaelis-Menten law at the initial stage of the reaction. The values of the net reaction rate experimentally determined were used to calculate the substrate concentration at the external surface of the catalyst pellet and then to calculate the mass transfer coefficient, k(L), at various flow rates and substrate concentrations. The correlation proposed by Chilton and Colburn represented adequately the experimental data. The linear change of log j(D) at low log N(Re) with negative slope was ascribed to the fact that the external mass transfer approached the state of pure diffusion in the limit of zero superficial velocity.