340
Chapter
9
Since the focus is now on the reactor rather than on the bioreaction we shall preferably use
kinetic models that are quite simple. The Monod model and its progeny (Section 7.3.1) is used
most of the time both when steady state and non-steady state operation of the bioreactor is
discussed. In Section 7.3 it was emphasized that the Monod model is unsuited for description of
cell behaviour during rapid transients, but it will nevertheless be used here as part of a transient
bioreactor model. The reason is that we do not wish to mess up the analysis of phenomena
associated with transient operation of bioreactors, such as the analysis of the stability of a given
steady state by including at the same time the complexities embodied in the structured models of
Chapter 7. The general principles o f bioreactor performance are equally well explained when
simple kinetic expressions are used. In a research situation the complexities of structured kinetics
and of non-ideal bioreactors (Chapter 11) must of course eventually be considered.
Reactors used in the chemical industry are roughly divided into two groups: Stirred Tank
Reactors and Plug Flow (or Tubular) Reactors. Reactors used in the bioindustry for fermentation
of microorganisms can be divided into the same two groups, but the stirred tank reactor is far
more popular. In the chemical and petrochemical industry it is the other way round. The reason is
that the volumetric rate of a bioreaction is proportional to the biomass concentration and
therefore the apparent reaction order in the substrate concentration is negative down to very low
levels of the substrate. Conventional chemical reactions are typically of order greater than zero in
the “limiting” reactant. Also aerobic fermentations cannot really be carried out in plug flow
reactors due to the very large difference in time constants between the bioreaction and the
passage of the gas through the reactor. This is o f course similar to liquid phase chlorination of
hydrocarbons in the chemical industry where some kind of stirred tank must be better than a plug
flow reactor. A specific problem with the use of plug flow reactors in the bioindustry is that the
feed must contain biomass— otherwise no reaction will take place unless biomass has been
immobilized on particles that remain in the reactor. A continuous feed of non-sterile medium to
the reactor would create all sorts of problems with infection and would not be chosen. Therefore
plug flow reactors are only used if the substrate conversion needs to be almost complete as is the
case in bioremediation of very toxic wastewater. Here the plug flow reactor is placed after a
stirred tank reactor as discussed in Section 9.2 to do the final clean-up of the waste stream.
The term “stirred tank reactor” is applied to the liquid phase alone. In an aerobic process it is
advantageous to contact the well mixed liquid phase with a gas phase that as far as possible
passes the reactor in plug flow since this will give the highest rate of mass transfer of a valuable
substrate from the gas phase and contribute to a high utilization of the gas phase or to lower cost
of compression of the gas phase if air is used. Consequently, a bioreactor may operate as a stirred
tank with respect to the biomass and liquid phase substrates such as glucose while it is desirable
to have plug flow for gaseous substrates. The design of bioreactors with both liquid phase and
gas phase substrates is an important topic to be discussed in Section 9.1.4
9.1
The Stirred Tank Bioreactor
As discussed in the introduction almost all bioreactors are stirred tanks operating at or close to
atmospheric pressure. The reactors are manufactured out o f stainless steel, and they are equipped
with devices to obtain good mixing o f the liquid phase and an efficient cooling of the medium.
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