Characterization of shear rates in airlift bioreactors for animal cell culture

Hydrodynamics and Mass Transfer Simulation in Airlift Bioreactor with Settler using Computational Fluid Dynamics

In this work, the effect of inlet-gas superficial velocity over the circulation liquid velocity, gas holdup and mass transfer, from an airlift bioreactor with settler were studied by Computational Fluid Dynamics (CFD) modeling and contrasted with experimental results. Multiphase mixture model and κ-ε turbulence model were used to describe the two phases gas-liquid flow pattern in airlift bioreactor. The hydrodynamic parameters such as liquid circulation velocity and gas holdup were computed by solving the governing equations of continuity, moment and turbulence transport using the finite volume method. Global mass transfer coefficient was evaluated through the Higbie’s penetration theory and the two-phase fluid dynamic theory. Comparison between our numerical data and experimental data previously reported in the literature was done. Numerical and experimental data were very close, and the differences found were discussed in terms of the limitations of this study.

Influence of aeration-homogenization system in stirred tank bioreactors, dissolved oxygen concentration and pH control mode on BHK-21 cell growth and metabolism

This work focused on determining the effect of dissolved oxygen concentration (DO) on growth and metabolism of BHK-21 cell line (host cell for recombinant proteins manufacturing and viral vaccines) cultured in two stirred tank bioreactors with different aeration-homogenization systems, as well as pH control mode. BHK-21 cell line adapted to single-cell suspension was cultured in Celligen without aeration cage (rotating gas-sparger) and Bioflo 110, at 10, 30 and 50 % air saturation (impeller for gas dispersion from sparger-ring). The pH was controlled at 7.2 as far as it was possible with gas mixtures. In other runs, at 30 and 50 % (DO) in Bioflo 110, the cells grew at pH controlled with CO2 and NaHCO3 solution. Glucose, lactate, glutamine, and ammonium were quantified by enzymatic methods. Cell concentration, size and specific oxygen consumption were also determined. When NaHCO3 solution was not used, the optimal DOs were 10 and 50 % air saturation for Celligen and Bioflo 110, respectively. In this condition maximum cell concentrations were higher than 4 × 10(6) cell/mL. An increase in maximum cell concentration of 36 % was observed in batch carried out at 30 % air saturation in a classical stirred tank bioreactor (Bioflo 110) with base solution addition. The optimal parameters defined in this work allow for bioprocess developing of viral vaccines, transient protein expression and viral vector for gene therapy based on BHK-21 cell line in two stirred tank bioreactors with different agitation-aeration systems.

Macromixing hydrodynamic study in draft-tube airlift reactors using electrical resistance tomography

The present study summarizes results of mixing characteristics in a draft tube airlift bioreactor using ERT. This technique offers the possibility for noninvasive and nonintrusive visualization of flow fields in the bioreactor and has rarely been utilized previously to analyze operating parameters and mixing characteristics in this type of bioreactors. Several operating parameters and geometric characteristics were examined. In general, results showed that the increase in superficial gas velocity corresponds to an increase in energy applied and thus, to a decrease in mixing time. This generally corresponded to an increase in liquid circulation velocity and shear rate values. Bottom clearances and draft tube diameters affected flow resistance and frictional losses. The influence of sparger configurations on mixing time and liquid circulation velocity was significant due to their effect on gas distribution. However, the effect of sparger configuration on shear rate was not significant, with 20% reduction in shear rates using the cross-shaped sparger. Fluid viscosity showed a marked influence on both mixing times and circulation velocity especially in the coalescing media of sugar and xanthan gum (XG) solutions. Results from this work will help to develop a clear pattern for operation and mixing that can help to improve several industrial processes, especially the ones related to emerging fields of technology such as the biotechnology industry.

Quantification of the shear stresses in a microbial granular sludge reactor

Since a certain level of hydrodynamic shear force is needed in the formation of microbial granules for wastewater treatment, a method for quantifying the shear stresses in a microbial granular sludge reactor is highly desirable. In this work a novel energy-dissipation-based model was established and validated to quantitatively describe the shear stresses in a granular sludge sequencing batch reactor (SBR). With this model, the shear stress at the solid-liquid interface in an SBR was estimated and the relative magnitudes of shear stresses induced by fluid, gas bubble and collision on granules were evaluated. The results demonstrate that the effect of reactor geometry on the global shear stress was significant. Both the shear stress at the microbial granule surface and the biomass-loss rate increased with an increase in biomass concentration in the SBR. The gas bubble and the collision were found to be the main source for the shear stress at the granule surface.

Special Engineering Aspects

The specific characteristics of mammalian cells discussed in Chap. 2 require adapted solutions for bioreactor design and operation. Especially, cell damage by shear stress and aeration has to be considered. Therefore this chapter starts with a detailed discussion of shear stress effects on mammalian cells (anchorage-dependent and suspendable cells) in model systems and bioreactors, respectively, and consequences for reactor design. Appropriate oxygen supply is another critical issue, as adapted oxygen supply systems are required. Techniques for immobilization of cells, either grown on microcarriers in suspension culture or within macroporous carriers in fixed bed or fluidized bed reactors, are discussed as well. With respect to the operation of bioreactors, the characteristics of different culture modes (batch, fed-batch, chemostat, perfusion) are introduced and practical examples are given. Finally, concepts for monitoring of bioreactors, including offline and online methods as well as control loops (e.g. O₂, pH), are considered.

Production of exopolysaccharides by submerged mycelial culture of a mushroom Tremella fuciformis

The optimization of submerged culture conditions for mycelial growth and exopolysaccharide (EPS) production in an edible mushroom Tremella fuciformis was studied in shake flasks and bioreactors. The temperature of 28 degrees C and pH 8 in the beginning of fermentation in agitated flasks was the most efficient condition to obtain maximum mycelial biomass and EPS. The optimal medium constituents were as follows (gL(-1)): glucose 20, tryptone 2, KH(2)PO(4) 0.46, K(2)HPO(4) 1 and MgSO(4).7H(2)O 0.5. The fungus was cultivated under various agitation and aeration conditions in a 5L stirred-tank bioreactor. The maximum cell mass and EPS production were obtained at a relatively high agitation speed of 200 rpm and at an aeration rate of 2 vvm. The flow behavior of the fermentation broth was Newtonian and the maximum apparent viscosity (35 cP) was observed at a highly aerated condition (2 vvm). The EPS productivity in an airlift reactor was higher than that in the stirred-tank reactor. The morphological study revealed that the fungus grows in mainly three different yeast-like forms: ovoid, elongated, and double yeast forms. The high population of the elongated yeast has a very close relationship to high EPS production. The EPS were protein-bound polysaccharides consisted of mainly mannose, xylose, and fucose. The molecular weights of EPS were determined to be (1.3-1.5)x10(6).

Antibody production

The clinical and commercial success of monoclonal antibodies has led to the need for very large-scale production in mammalian cell culture. This has resulted in rapid expansion of global manufacturing capacity [1], an increase in size of reactors (up to 20,000 L) and a greatly increased effort to improve process efficiency with concomitant manufacturing cost reduction. This has been particularly successful in the upstream part of the process where productivity of cell cultures has improved 100 fold in the last 15 years. This success has resulted from improvements in expression technology and from process optimisation, especially the development of fed-batch cultures. In addition to improving process/cost efficiencies, a second key area has been reducing the time taken to develop processes and produce the first material required for clinical testing and proof-of-principle. Cell line creation is often the slowest step in this stage of process development. This article will review the technologies currently used to make monoclonal antibodies with particular emphasis on mammalian cell culture. Likely future trends are also discussed.

Influence of power supply in the feasibility ofPhaeodactylum tricornutumcultures

The influence of fluid-dynamic conditions on the yield of Phaeodactylum tricornutum microalgal cultures was analyzed in two stages: first, the influence of air flow rate; second, the influence of using fluid-moving pumps for recirculating the culture. With respect to the air flow rate, the yield of the cultures increased with the aeration rate up to values of 2.0 v/v/min, then stress was observed and the yield of the cultures decreased. With respect to the influence of mechanical power supply for liquid impulsion, three different types of pumps--centrifugal, pulse, and peristaltic--were essayed at different power supplies. The cultures were stressed for the three types of pumps essayed. For each pump, the higher the power supply the lower was the Fv/Fm value and the higher was the stress at which cells were exposed. The highest measured stress was when the culture was moved with the centrifugal pump. Despite measured stress, for all the experiments stable steady states were reached, thus indicating that cells reduced their yield but did not die, as was verified by cell viability measurements. It was observed that the increase of the power supply improved the frequency of light exposition thus enhancing the yield of the cultures. However, the higher the power supply, the lower the microeddy length scale; therefore, stress could appear. Data demonstrated that the microeddy length scale was always much higher than cell size and therefore the turbulence was not responsible for stress. Also, the mass transfer was discarded as responsible for yield reduction. It was concluded that the shear rate was the factor determining the existence of stress phenomena. The evaluation of these shear rates demonstrated that values above 30-80 s(-1) damaged the cells strongly. These data were verified in an outdoor pilot-scale tubular photobioreactor that was implemented with the same type of pumps, thus demonstrating the necessity to take into account this factor in the design and scale-up of microalgal photobioreactors.

Hydrodynamic damage to animal cells

Animal cells are affected by hydrodynamic forces that occur in culture vessel, transfer piping, and recovery operations such as microfiltration. Depending on the type, intensity, and duration of the force, and the specifics of the cell, the force may induce various kinds of responses in the subject cells. Both biochemical and physiological responses are observed, including apoptosis and purely mechanical destruction of the cell. This review examines the kinds of hydrodynamic forces encountered in bioprocessing equipment and the impact of those forces on cells. Methods are given for quantifying the magnitude of the specific forces, and the response thresholds are noted for the common types of cells cultured in free suspension, supported on microcarriers, and anchored to stationary surfaces.

Substrate controlled development of anaerobic acidifying aggregates at different shear rates in a gas lift reactor

The influence of liquid shear rates on the development of acidifying mixed-culture aggregates was studied in a gas-lift reactor. The glucose concentration was kept at a constant and relatively high level by operating the reactor in pH-auxostat mode. Size, strength, and wet density of aggregates cultivated at different superficial gas velocities (Ug) were investigated. Image analysis showed that the Sauter mean diameter (Ds) decreased with increasing Ug. A stirred tank was used to characterize the surface detachment rate (Rd) under non-growth conditions. An exponential decrease was observed in Rd with the applied Ug during cultivation, i.e., aggregates became stronger. The increased strength coincided with an increase in aggregate wet density. Size classified aggregates showed an increase in Rd with the square of the aggregate diameter (Dp), however, this contribution was much smaller than the effect of adaptation. Experiments in a similar gas-lift reactor under dynamic conditions without adaptation, showed that Rd increased exponentially with increasing Ug. So, two important contributions to Rd can be distinguished: adaptation, which induces stronger aggregates, and aggregate size, which makes them less susceptible to hydrodynamic shear. A general expression for Rd was derived, which depends on Dp and Ug. Combining this equation with the surface biomass growth rate (Rg) allowed for the estimation of the maximal diameter (Dmax) aggregates can reach at any Ug, and it was found that the estimated and measured Dmax were in good agreement.

Animal-cell damage in sparged bioreactors

The gas sparging of culture broth causes damage to suspended animal cells. However, despite this, sparged bioreactors remain the preferred means of cell culture because sparging is a robust method of supplying oxygen, especially on a large scale. This article examines the underlying mechanisms involved in bubble-associated cell damage and the methods available for controlling such damage.

Effect of scale-up on average shear rates for aerated non-Newtonian liquids in external loop airlift reactors

Average shear rates have been estimated experimentally in a 700-dm3 external loop airlift reactor. Aqueous pseudoplastic carboxymethylcellulose and xanthan gum solutions were used to simulate non-Newtonian behavior of biological media. Average shear rates of non-Newtonian solutions were found by analogy with Newtonian glycerol solutions using downcomer liquid velocity as the measurable parameter. Due to the complexity of local shear rate measurement, an average shear rate was assumed to exist and is proportional to superficial gas velocity. The data from this work and those in the literature were used in producing a new correlation for estimating average shear rates as a function of superficial gas velocity, geometry, and dispersion height. Wall shear rates were found to be significant. The ratio of wall shear rates to bulk shear rates were varied from 5% to 40%. Furthermore, it has been found that shear rates generated in airlift loop reactors are lower than those generated in bubble columns. Copyright 1999 John Wiley & Sons, Inc.

Prediction of dissolved oxygen and carbon dioxide concentration profiles in tubular photobioreactors for microalgal culture

A model is developed for prediction of axial concentration profiles of dissolved oxygen and carbon dioxide in tubular photobioreactors used for culturing microalgae. Experimental data are used to verify the model for continuous outdoor culture of Porphyridium cruentum grown in a 200-L reactor with 100-m long tubular solar receiver. The culture was carried out at a dilution rate of 0.05 h-1 applied only during a 10-h daylight period. The quasi-steady state biomass concentration achieved was 3.0 g. L-1, corresponding to a biomass productivity of 1.5 g. L-1. d-1. The model could predict the dissolved oxygen level in both gas disengagement zone of the reactor and at the end of the loop, the exhaust gas composition, the amount of carbon dioxide injected, and the pH of the culture at each hour. In predicting the various parameters, the model took into account the length of the solar receiver tube, the rate of photosynthesis, the velocity of flow, the degree of mixing, and gas-liquid mass transfer. Because the model simulated the system behavior as a function of tube length and operational variables (superficial gas velocity in the riser, composition of carbon dioxide in the gas injected in the solar receiver and its injection rate), it could potentially be applied to rational design and scale-up of photobioreactors. Copyright 1999 John Wiley & Sons, Inc.

Interaction between CO2-mass transfer, light availability, and hydrodynamic stress in the growth of phaeodactylum tricornutum in a concentric tube airlift photobioreactor

The microalga Phaeodactylum tricornutum was grown in a concentric tube airlift photobioreactor. A maximum specific growth rate of 0. 023 h-1 was obtained using a superficial gas velocity around 0.055 m/s. Lower or higher gas flow rates limited the culture performance. To establish if the observed limitation was due to CO2 or to the photosynthetically active irradiance, characteristic times for mixing, mass transfer and CO2 consumption, and the photon flux absorbed by the culture were analyzed. The CO2-gradients in the culture were shown to be responsible for the limitation during the exponential growth phase, and both CO2 and light irradiance were limiting in the linear growth phase. The decrease in specific growth rate relative to the maximum was found to be related to the specific gas-liquid interfacial area, the length scale of the microeddies and the shear rate. Copyright 1998 John Wiley & Sons, Inc.