Gas hold-up measurements in bioreactors

The gas hold-up of a gas-liquid dispersion is an important parameter in the fermentation industry. If it is too low, or too high, productivity can be adversely affected. Gas hold-up in fermentors cannot be calculated from physico-chemical correlations and, therefore, must be measured accurately for each fermentation.This article surveys a number of methods for measuring the gas hold-up in gas-liquid dispersions, making particular note whether these methods can be applied aseptically.     

Gas hold-up in external-loop airlift bioreactors

A new model of gas hold-up is proposed for external-loop airlift bioreactors. It is based on the similarity between the liquid circulation due to the local variation of gas hold-up in airlift bioreactors and the natural convection due to temperature difference. The model is developed to include the case of non-Newtonian fermentation media which are involved in many industrially bioprocesses. The capability of the model is examined using a wide range of experimental results including the present data. Reasonable agreement is obtained between the proposed model and the experimental data both for Newtonian and non-Newtonian media.     

Average shear rate in three pneumatic bioreactors

A method proposed in recent literature was applied to evaluate the average shear rate ( [(g)\dot]_av ) \left( {\dot{\gamma }_{\rm av} } \right) in three pneumatic bioreactors of 5-dm³ working volume: bubble column, split airlift, and concentric-tube airlift. The volumetric oxygen transfer coefficient (k _L a) is the appropriate characteristic parameter to assess the average shear rate ( [(g)\dot]_av ) \left( {\dot{\gamma }_{\rm av} } \right) in this methodology. Correlations for [(g)\dot]_av \dot{\gamma }_{\rm av} as a function of superficial gas velocity in the riser region (U _GR) and rheological fluid properties (consistency index, K, and flow index, n) were obtained for each model of pneumatic bioreactor studied. The [(g)\dot]_av \dot{\gamma }_{\rm av} values estimated by the proposed methodology lay within the range of values calculated by classical correlations. The proposed correlations were utilized to predict the [(g)\dot]_av \dot{\gamma }_{\rm av} during the Streptomyces clavuligerus cultivations carried out at the same specific air flow rate (3.5 vvm) in the different types of pneumatic bioreactors. The lowest values of [(g)\dot]_av \dot{\gamma }_{\rm av} related to the highest values of consistency index (K) were found for the bubble column bioreactor, and the highest values of [(g)\dot]_av \dot{\gamma }_{\rm av} related to the lowest values of K were found for the concentric-tube airlift bioreactor. Intermediate values were found for the split airlift bioreactor. The results showed that high [(g)\dot]_av \dot{\gamma }_{\rm av} values affect the structural health of the mycelia by the rupture of the hipha.     

A novel method of simulating oxygen mass transfer in two-phase partitioning bioreactors

An empirical correlation, based on conventional forms, has been developed to represent the oxygen mass transfer coefficient as a function of operating conditions and organic fraction in two-phase, aqueous-organic dispersions. Such dispersions are characteristic of two-phase partitioning bioreactors, which have found increasing application for the biodegradation of toxic substrates. In this work, a critical distinction is made between the oxygen mass transfer coefficient, k(L)a, and the oxygen mass transfer rate. With an increasing organic fraction, the mass transfer coefficient decreases, whereas the oxygen transfer rate is predicted to increase to an optimal value. Use of the correlation assumes that the two-phase dispersion behaves as a single homogeneous phase with physical properties equivalent to the weighted volume-averaged values of the phases. The addition of a second, immiscible liquid phase with a high solubility of oxygen to an aqueous medium increases the oxygen solubility of the system. It is the increase in oxygen solubility that provides the potential for oxygen mass transfer rate enhancement. For the case studied in which n-hexadecane is selected as the second liquid phase, additions of up to 33% organic volume lead to significant increases in oxygen mass transfer rate, with an optimal increase of 58.5% predicted using a 27% organic phase volume. For this system, the predicted oxygen mass transfer enhancements due to organic-phase addition are found to be insensitive to the other operating variables, suggesting that organic-phase addition is always a viable option for oxygen mass transfer rate enhancement.     

Liquid-phase mass transfer coefficients in bioreactors

Liquid-phase mass transfer coefficient in bioreactors have been examined. A theoretical model based on the surface renewal concept has been devloped. The predicted liquid-phase mass transfer coefficients are compared with the experimental data for a mycelial fermentation broth (Chaetomium cellulolyticum) and model media (carboxymethyl cellulose) in a bench-scale bubble column reactor. The liquid-phase mass transfer coefficient is evaluated by dividing the volumetric mass transfer coefficient obtained experimentally by the specific surface area estimated using the available correlations. The available literature data in bubble column and stirred tank bioreactors is also used to test the validity of the proposed model. A reasonable agreement between the model and the experimental data is found.     

Hydrodynamics and oxygen transfer in pneumatic bioreactor devices

Gas holdup and oxygen transfer studies in non-Newtonian suspensions of cellulose fibres conducted in two large (0.098 m(3) each) reactors are described. Both reactors-a bubble column and a similar internal loop airlift-were unusual in that they had rectangular cross-sections. In all cases gas holdups and k(L)a(L) declined with increasing solid concentration and, under identical conditions, the bubble column performed better than the airlift. The fluid systems used were carefully selected to represent mould fermentation broths.The behavior of true mass transfer coeffcient k(L) with changes in bubble size is discussed for these systems.     

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.

     

Enhancement of oxygen mass transfer in stirred bioreactors using oxygen-vectors 2. Propionibacterium shermanii broths

The previous works on simulated broths are continued and developed for Propionibacterium shermanii broths. The obtained results indicated the considerable increase of k _L a in presence of n-dodecane as oxygen-vector and the existence of a certain value of hydrocarbon concentration that corresponds to the maximum mass transfer rate of oxygen. The magnitude of the positive effect of the oxygen-vector strongly depends on operational conditions of the bioreactor, on broth characteristics and on P. shermanii concentration.     

Pressure drop,gas hold-up,and oxygen transfer in tower systems

Pressure drop, gas hold-up, and oxygen transfer were investigated in a sieve tray column, a column with Koch motionless mixers, and a bubble column. The oxygen transfer experiments were conducted using cocurrent flow of gas and liquid under steady-state conditions with oxygen transfer from the gas to the liquid phase. The oxygen transfer rates and efficiencies of the sieve tray column and the column with Koch mixers were found to be superior to those of the conventional bubble column. Gas hold-up was also greater when sieve trays or Koch mixers were inserted in the tower. The pressure drop was found to be primarily due to the liquid head in all three columns.     

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.     

Enhancement of oxygen mass transfer in stirred bioreactors using oxygen-vectors. 1. Simulated fermentation broths

Oxygen mass transfer represents the most important parameter involved in the design and operation of mixing-sparging equipment for bioreactors. It can be described and analyzed by means of the mass transfer coefficient, k_La. The k_La values are affected by many factors such as geometrical and operational characteristics of the vessels, media composition, type, concentration and microorganism morphology, and biocatalysts properties. The efficiency of oxygen transfer could be enhanced by adding oxygen-vectors in broths, such as hydrocarbons or fluorocarbons, without increasing the energy consumption for mixing or aeration. The experimental results obtained for simulated broths indicated a considerable increase of k_La in the presence of n-dodecane, and the existence of a certain value of n-dodecane concentration that corresponds to a maximum mass transfer rate of oxygen. The magnitude of the positive effect of n-dodecane depends both on the broths characteristics and operational conditions of the bioreactor.Notation d stirrer diameter, mm - d oxygen electrode diameter, mm - D bioreactor diameter, mm - h distance from the inferior stirrer to the bioreactor bottom, mm - H bioreactor height, mm - k_La oxygen mass transfer coefficient, s^-1 - l impeller blade length, mm - I oxygen electrode immersed length, mm - P power consumption for mixing of non-aerated broths, W - P_a power consumption for mixing of aerated broths, W - (P_a/V) specific power input, W/m³ - s baffle width, mm - v_S superficial air velocity, m/s - V volume of medium, m³ - w impeller blade height, mm - volumetric fraction of oxygen-vector - _a apparent viscosity, Pa*s - density, kg/m³     

Oxygen mass transfer and scale-up studies in baffled roller bioreactors

Oxygen mass transfer was studied in conventional, bead mill and baffled roller bioreactors. Using central composite rotational design, impacts of size, rotation speed and working volume on the oxygen mass transfer were evaluated. Baffled roller bioreactor outperformed its conventional and bead mill counterparts, with the highest k _L a obtained in these configurations being 0.58, 0.19, 0.41 min^?1, respectively. Performances of the bead mill and baffled roller bioreactor were only comparable when a high bead loading (40 %) was applied. Regardless of configuration increase in rotation speed and decrease in working volume improved the oxygen mass transfer rate. Increase in size led to enhanced mass transfer and higher k _L a in baffled roller bioreactor (0.49 min^?1 for 2.2 L and 1.31 min^?1 for 55 L bioreactors). Finally, the experimentally determined k _L a in the baffled roller bioreactors of different sizes fit reasonably well to an empirical correlation describing the k _L a in terms of dimensionless numbers.     

Modeling of mass transfer and optimization of stirred bioreactors

Models of power input and mass transfer are synthesized in bioreactor. A new constructive parameter of bioreactor the eccentricity of stirrer towards its rotation axis is introduced and investigated. Obtained values of constructive and regime parameters allow the design of a bioreactor with improved mass transfer characteristics.     

Characterisation of the gas-liquid mass transfer in shaking bioreactors

The maximum gas-liquid mass transfer capacity of 250ml shaking flasks on orbital shaking machines has been experimentally investigated using the sulphite oxidation method under variation of the shaking frequency, shaking diameter, filling volume and viscosity of the medium. The distribution of the liquid within the flask has been modelled by the intersection between the rotational hyperboloid of the liquid and the inner wall of the shaking flask. This model allows for the calculation of the specific exchange area (a), the mass transfer coefficient (k(L)) and the maximum oxygen transfer capacity (OTR(max)) for given operating conditions and requires no fitting parameters. The model agrees well with the experimental results. It was furthermore shown that the liquid film on the flask wall contributes significantly to the specific mass transfer area (a) and to the oxygen transfer rate (OTR).     

Determination of power consumption and volumetric oxygen transfer coefficient in bioreactors

A torque meter has been developed for determining the power consumption in a bench stirred tank. The device has been bonded in the stirrer shaft inside a commercial bench fermentor, in order to avoid frictional losses in the mechanical seal. Power consumption measurements in ungassed and gassed systems were obtained at different agitation and aeration conditions, for Newtonian and non-Newtonian fluids. Also, a "simple modified sulfite method" for volumetric oxygen transfer coefficient (k_La) determination was developed and the experimental data were correlated with the gassed power (P_g) by using well-known correlations presented in the literature.     

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.     

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     

Model analysis of biological oxygen transfer enhancement in surface-aerated bioreactors

A model was developed to evaluate the effects of cells and surfactants on oxygen transfer in surface-aerated bioreactors. The model assumed the presence of serial layers of adsorbed surfactants and microorganisms directly adjacent to the gas-liquid interface due to their surface activities, followed by a stagnant liquid layer to account for the oxygen transfer resistance in the liquid phase. The interfacial surfactant film, although posing as an additional resistance, was found to have negligible effect on the oxygen transfer rate because of its extremely small thickness as compared to the cell monolayer and the stagnant liquid layer. On the other hand, cells affect oxygen transfer by two mechanisms: the biological enhancement due to the respiration of interfacial cells and the physical blocking resulting from the semipermeable nature of cell bodies. Due to the low specific oxygen uptake rates of the sludges, the two mechanisms were found to be of comparable importance in activated-sludge systems; the oxygen transfer enhancement factor, E, varied from about 0.97 to 1.10 depending on the operating conditions. The biological enhancement effect, however, predominated in fermentations of actively growing bacteria. At relatively low agitation speed (e. g., 300 rpm), the value of E could reach about 3 to 5 in fermentations with high cell concentrations. Effects of other operating variables, such as the agitation intensity, the oxygen content in the mixed liquor, and the bulk cell concentration, on biological oxygen transfer enhancement were also studied. (c) 1992 John Wiley & Sons, Inc.     

Experimental and computational characterization of mass transfer in high turndown bioreactors

Single-use bioreactors (SUBs, or disposable bioreactors) are extensively used for the clinical and commercial production of biologics. Despite widespread application, minimal results have been reported utilizing the turndown ratio; an operation mode where the working range of the bioreactor can be expanded to include low fluid volumes. In this work, a systematic investigation into free surface mass transfer and cell growth in high turndown single-use bioreactors is presented. This approach, which combines experimental mass transfer measurements with numerical simulation, deconvolutes the combined effects of headspace mixing and the free surface convective mass transfer on cell growth. Under optimized conditions, mass transfer across the interface alone may be sufficient to satisfy oxygen demands of the cell culture. Within the context of high turndown bioreactors, this finding provides a counterpoint to traditional sparge-based bioreactor operational philosophy. Multiple monoclonal antibody-producing cell lines grown using this high turndown approach showed similar viable cell densities to those cells expanded using a traditional cell bag rocker. Furthermore, cells taken directly from the turndown expansion and placed into production showed identical growth characteristics to traditionally expanded cultures. Taken together, these results suggest that the Xcellerex SUB can be run at a 5:1 working volume as a seed to itself, with no need for system modifications, potentially simplifying preculture operations.