Biochemical Reactions - A First Look
51
16
Glucose
30
0
0
0
0,1
0,2
0,3
0,4
0,5
Dilution rate (h*1)
Figure 3.2
Schematic view of the concentration profiles of biomass, ethanol and glucose at different
dilution rates in a continuous, stirred tank bioreactor containing the yeast
S. cerevisiae.
The despair of experimenters who have waited many hours to obtain a steady state in continuous
stirred tank reactors when the pH control for no apparent reason fails or a rubber tube breaks is
all too well known. Likewise the loss of plasmids leading to gradual loss of productivity of the
culture is a well-known source of frustration. Still, the steady state continuous reactor is the ideal
equipment for physiological studies - and also to obtain trustworthy data for design of an
industrial production. The set of steady state data is the foundation of a quantitative treatment of
bioreactions, and deeper layers of metabolic response are revealed in the transients from one
steady state to the next. These transient experiments give the necessary input for the modeling of
non-ideal reactors in which the effects of spatial in-homogeneities of e.g. glucose or oxygen on
the performance of industrial bioreactors are investigated (see Section 11.3.6). Without at least a
semi quantitative knowledge of how rapidly a change in e.g. a vitamin concentration changes the
productivity of a desired metabolite the detailed calculations of flow patterns in the bioreactor
obtained by computational fluid dynamics are of little value.
Based on the measured feed and effluent concentrations in the steady state continuous bioreactor
the reaction rates are easily calculated from steady state mass balances for the bioreactor. Thus all
three equations (3.2) to (3.4) express that the mass of the compound produced by the reaction is
equal to the difference in mass between a liquid feed and the outlet from the reactor.
qs V
+ v(s; / - s .) = 0
(3.2)
(3.3)
qxV
+ v(xy -
x)
= 0
(3.4)
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