As in all aerobic bioprocesses, the oxygen transfer rate is a critical parameter that needs to be met for the satisfactory cultivation of animal cells. Oxygen in solution has to be continuously provided because of its low solubility in aqueous solution which is continually being utilised by the growing cells (at the current time, reaching a cell density of ~107 cells mL−1 in bioreactors up to 25 m3). Such a process requires a certain specific power input (or mean specific energy dissipation rate) to be used, which also has to provide a satisfactory level of other mixing parameters. However, though for animal cells, the specific power required is relatively low (typically < ~0.05 W/kg), because of the lack of a cell wall, there is still a perception that ‘shear damage’ may occur. Another aspect of oxygen mass transfer is the need to provide a continual inflow of oxygen into the bioreactor, typically by sparging (rates < ~0.01 vvm). However, especially at large scale, sparging may lead to excessive foaming requiring the use of antifoam to control it; and bursting bubbles damage cells unless protective agents are used. Both these additives negatively affect the rate of mass transfer. Another critical aspect directly linked to oxygen transfer is the molar equivalent production of carbon dioxide by the cells. Stripping of CO2 is essential to prevent physiologically damaging levels of pCO2 being produced as well as issues associated with pH and osmolality. Essentially, oxygen transfer, carbon dioxide stripping and mixing parameters are all intimately connected and need to be considered in an integrated way. In this chapter, oxygen mass transfer and carbon dioxide stripping theory and practice are considered in detail including non-stirred and single use bioreactors. In addition, other mixing parameters such as blending, heat transfer and scale-up/scale-down in stirred bioreactors are briefly considered as these are used commercially from the 15 mL ambrTM to the 25 m3 scale. Because the perceived ‘shear-sensitivity’ of animal cells to stirring and bubbling has impacted on how aeration and stripping have been addressed in practice, these topics too are worthy of consideration in any integrated approach. Finally, some recommendations regarding sparger selection and impeller choice are also made.

Comment on : Gas recirculation rate and its influences on mass transfer in multiple impeller systems with various impeller combinations

Mass Transfer and Mixing Across the Scales in Animal Cell Culture

The supply of oxygen to a growing organism, aeration, in bioreactors is a critical requirement in biotechnology because of the limited solubility of oxygen in water. Aeration involves transfer of oxygen from the air into the fluid surrounding the organism, from where it is transferred to the organism itself. During the course of a batch bioreaction, oxygen demand often passes through a marked maximum when the species is most biologically active. The basic principles that underlie aeration are exactly the same as those that determine the rate of transfer of any sparingly soluble gas (oxygen) from the gas stream (air) to the unsaturated liquid (broth). In each case, energy is dissipated to increase the rate of transfer beyond simple diffusion, and the mode of energy input defines the two-phase fluid dynamics. Though there are many different types of bioreactor, there are basically four ways of imparting the energy required: rotating agitators (stirred tanks), gas compression (bubble columns/loop fermenters), liquid circulating (jet loop reactors), and shaking (shake flasks, microtiter plates), the last one being common at the screening stage of process development and in clone selection. In addition, the transfer rate depends on the area of contact between the phases, the driving force available (ie, the difference in concentration of oxygen in the two phases), the chemical composition of the liquid, and its temperature. Aeration is used in bacterial, aerobic yeast, polysaccharide, and mycelial fermentations, animal, stem, and plant cell culture, single-cell protein production, and biological aerobic wastewater treatment.


Keywords:

aeration;
bioreactors;
mass transfer;
oxygen demand;
water treatment

Comment on : Gas recirculation rate and its influences on mass transfer in multiple impeller systems with various impeller combinations

Citations

Mass Transfer and Mixing Across the Scales in Animal Cell Culture

Aeration in Biotechnology

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