380
Chapter 9
9.3
Dynamic Analysis of Continuous Stirred Tank Bioreactors
In Section 9.1 and 9.2 the design of fermentation processes using a stirred tank or a plug flow
reactor has been discussed. The design was primarily for steady state operation, although of course
both the batch reactor and the fed batch reactor operate in a transient mode.
Although time is a variable in the batch and in the fed batch design problem the physiological
state of the culture hardly changes during the entire exponential growth phase of a batch
fermentation, and the lag-phase can be adequately analyzed using the simple compartment
models discussed in Chapter 7 (Example 7.6). In the fed batch cultivation one would as
explained in Section 9.1.5 choose to work with a constant p (and consequently at constant
physiological state) unless insufficient mass- or heat transfer makes it necessary to switch to a
constant
qx
operation, s decreases in the constant qx period, but the physiological state of the cells
is not likely to change-certainly not for an aerobic yeast fermentation, and not either in most
other cases.
A dynamic analysis of the plug flow reactor is beyond the scope of this book, and the results of
such an analysis would have much less value than the results of a similar analysis of a typical
plug flow reactor for catalytic gas phase reactions in the chemical industry. For a bioprocess the
dynamic analysis would be overlaid with many unpredictable phenomena due to the complexity
of the bioreaction.
Fermentation in a stirred tank, continuous reactor is, however, very sensitive to disturbances in the
environment. Continuous stirred tank reactors can safely be predicted to have an increasing appeal
in future bioprocessing of cheap raw materials to make low value products. It is consequently
important, especially for the process control of these reactors to analyze the dynamics of stirred
tank continuous reactors. Since continuous stirred tanks are also the preferred choice for high
quality scientific studies of cell physiology it is, as already indicated in Chapter 3, very desirable to
include transients in the experimental study. To extract physiological information from the
transients one must be able to model the reactor response to different transients.
This analysis is the subject of the present section. First the response of the reactor to sudden
changes in the environment will be studied. Some fundamental aspects of reactor stability will be
the final result. Thereafter the response of the culture in a continuous stirred tank to infection and
to changes in the morphology of the culture will be described. The dynamics associated with
changes in the culture composition are of great importance to the fermentation industry.
9.3.1 Dynamic Response of the Reactor for Simple, Unstructured Kinetic
Models
A dynamic response is typically observed after a step change of the dilution rate D, after a
change in the feed substrate concentration
sfi
or after a pulse of substrate has been added to the
reactor.
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