Page 423 - Elements of Chemical Reaction Engineering Ebook

394 Nonelementary Reaction Kinetics Chap. 7

catalysts. A common example of specificity in bioconversion production of a
single desired isomer that when produced chemically yields a mixture of iso-
mers is the conversion of cis-proenylphosphonic acid to the antibiotic (-)
cis- 1,2-epoxypropyl-phosphonic acid.
In biosynthesis, the cells, also referred to as the biomass, consume nutri-
ents to grow and produce more cells and important products. Internally, a cell
uses its nutrients to produce energy and more cells. This transformation of
nutrients to energy and bioproducts is accomplished through a cell’s use of a
number of different enzymes (catalysts) in a series of reactions to produce
metabolic products. These products can either remain in the cell (intracellular)
or be secreted from the cells (extracellular). In the former case the cells must
be lysed (ruptured) and the product purified from the whole broth (reaction
mixture).
In general, the growth of an aerobic organism follows the equation

“‘1
source source source source

culture media
Cell multiplication [CO,] + [H20] + [products] + more conditions
cells 1 iH, temperature, etc.) (7-98)

A more abbreviated form generally used is

substrate more cells + product (7-99)
The products in Equation (7-99) include COz, water, proteins, and other spe-
cies specific to the particular reaction. An excellent discussion of the stoichi-
ometry (atom and mole balances) of Equation (7-98) can be found in Wang16
and in Bailey and Ollis.17 The substrate culture medium contains all the nutri-
ents (carbon, nitrogen, etc.) along with other chemicals necessary for growth.
Because, as we will soon see, the rate of this reaction is proportional to the cell
concentration, the reaction is autocatalytic. A rough schematic of a simple
batch biochemical reactor and the growth of two types of microorganisms,
cocci (i.e., spherical) bacteria and yeast, is shown in Figure 7-13.

7.5.1 Cell Growth

Stages of cell growth in a batch reactor are shown schematically in Fig-
ure 7-14. Here, the log of the number of living cells is shown as a function of
time. Initially, a small number of cells is inoculated into (ie., added to) the
batch reactor containing the nutrients and the growth process begins.
In phase I, called the lag phase, there is little increase in cell concentra-
Lag phase tion. During the lag phase the cells are adjusting to their new environment,

16D. C. Wang et al., Fermentation and Enzyme Technology, Wiley, New York, 1979.
17T. J. Bailey and D. Ollis, Biochemical Engineering, 2nd ed., McGraw-Hill, New York,
1987.