424
Chapter 10
3.
Diffusion of oxygen through a relatively stagnant liquid region adjacent to the gas bubble
i.e. from the gas-liquid interface to the well-mixed bulk liquid.
4.
Transport of oxygen through the well-mixed liquid to a relatively unmixed liquid region
surrounding the cells.
5.
Diffusion through the stagnant region surrounding the cells.
6.
Transport from the liquid to the pellet cell aggregate etc.
7.
Diffusive transport of oxygen into the pellet etc.
8.
Transport across the cell envelope (see Section 2.1.2).
9.
Transport from the cell envelope to the intracellular reaction site e.g. the mitochondria.
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 (see Chapter 11). 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. The most
important mass transfer phenomena are thus of two kinds: (1) Gas liquid mass transfer (Section
10.1) and (2) Molecular diffusion of medium components into pellets or cell aggregates (Section
10.2)
. Both of these mass transfer phenomena are discussed at great length in chemical engineering
textbooks (see e.g. Bird
et ai,
2001 or Cussler, 1997), which should be consulted for an in-depth
analysis of the subject. Here we will treat the subject only to enable the reader to combine the
kinetic models of the previous chapters with simple models for the mass transport phenomena
relevant to bioreactions.
Figure 10.1, Overview of steps in the overall mass transfer of oxygen from a gas bubble to the reaction site
inside the individual cells. (Reprinted by permission from J. E. Bailey and D. F. Ollis (1986), Biochemical
Engineering Fundamentals, McGraw-Hill),
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