Gas-Liquid Mass Transfer In Cellular Systems

Mass transfer takes place by two basic processes; convection and diffusion. A full treatment of mass transfer therefore in principle requires a fully known flow field. However, a simplified treatment in which the overall mass transfer is schematically divided into different transfer steps is normally used with good results. An overview of important mass transfer steps in a fermentation process which shows the individual steps involved in oxygen transport from a gas bubble to the reaction site inside the individual cells (Bailey and Ollis, 1986).

These arise from different combinations of resistances are mentioned below

• Diffusion from bulk gas to the gas-liquid interface
• Movement through the gas-liquid interface
• Diffusion of solute via the relatively pure or unmixed liquid area beside the bubble into the finely mixed mass liquid
• Transport of the solute via the bulk liquid to a second relatively unmixed liquid region surrounding the cells
• Transport through the second unmixed liquid region associated with the cells
• Diffusive transport into the cellular floe, mycelia, or soil particle
• Transport across cell envelope and to intracellular reaction site

When the organisms take the form of individual cells, the sixth resistance disappears. Microbial cells themselves have some tendency to adsorb at interfaces. Thus, cells may preferentially gather at the vicinity of the gas-bubble-liquid interface. Then, the diffusing solute oxygen passes through only one unmixed liquid region and no bulk liquid before reach­ing the cell. In this situation, the bulk dissolved O₂ concentration does not repre­sent the oxygen supply for the respiring microbes.

For most processes one or more of these steps are in a pseudo-steady state. The transport through the well-mixed liquid is normally very rapid in laboratory-scale bioreactors because of the reasonable assumption of homogeneity in the medium. Furthermore Steps 5, 6, and 7 are relevant only for processes in which pellets or cell aggregates appear. Intracellular transport resistance is normally also neglected because of the small size of most cells.

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