Mass Transfer in Bioreactors-Lesson Summary
Mass transfer describes the transport of mass from one point to another. It may take place in a single phase or over phase boundaries in multiphase systems.
Mass is transferred under the influence of a concentration gradient in the system.
Gas-liquid mass transfer is extremely important in bioprocessing.
In aerobic processes, oxygen must first be transferred from gas bulk through a series of step onto the surfaces of the cell
Enhancement of gas-liquid mass transfer during aerobic fermentation is a priority.
Molecular diffusion is the movement of molecules under the influence of a concentration difference in the system.
Diffusion occurs in the direction to destroy the concentration gradient.
Continuous diffusion refers to supplying material to the region of high concentration and removing it from the region of low concentration.
Continuous diffusion is exploited for mass-transfer operations in bio-reaction systems.
The two theories of interphase mass transfer include the following:
Two-film theory: the entire resistance to transfer is contained in two fictitious films on either side of the interface in which transfer occurs by molecular diffusion.
Penetration theory: It assumes turbulent eddies travel from the bulk of the phase to the interface where they remain for a constant exposure time “t”.
Oxygen uptake in cell culture
The rate of oxygen transfer from gas to liquid is of prime importance in aerobic fermentation.
At high cell densities, oxygen is quickly consumed in aerobic cultures and must be constantly replaced by sparging.
The low solubility of oxygen limits the concentration difference to a very small value at all times.
Important factors which influence oxygen demand include:
Culture growth phase
Nature of the carbon source in the medium.
Factors affecting oxygen supply rate are:
Rheology of fermentation fluid
Fluids can be classified depending upon their fluid behavior as Newtonian or Non-Newtonian fluid.
The viscosity of a fluid measures its resistance to flow under applied shear stress.
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