504
Chapter 11
I
I
Position of impeller axis
Position of
turbine impeller
I
Fig. 11.11. Representation of an axi-symmetric compartment model of a stirred tank reactor equipped
with one turbine impeller giving a radial flow pattern. Each compartment is assumed to be ideally mixed.
The picture shows the right half of the reactor cross section. The thick arrows represent the main
circulation pattern (flow rate = CF along the central axis, which is split into two flows at the reactor wall),
and the dotted arrows represent the exchange flows between the compartments. The exchange flows, EFj,
need to be modeled for each individual compartment.
Use of multiple impellers on the same shaft tends to give rise to horizontally divided flow
patterns. Each such area can be modeled as a compartment, or a set of compartments as shown in
Fig. 11.10. To describe the overall reactor behavior with respect to e.g. gas-liquid mass transfer
or mixing, the compartments need to be connected via exchange flows (see e.g. Vrabel et al.,
2000).
The necessary
information for making
these models
are
thus;
the number of
compartments, a flow model for each compartment, and a model for the exchange flows between
the compartments. The exchange flow can be of two different kinds; the exchange caused by
turbulence and exchange caused by aeration-induced flow. It is possible to vary the level of
detail by changing the number of compartments used. The major weakness of these models is
probably the difficulty in getting an estimate of the exchange flows between compartments that
would allow extrapolations to different scales.
Example.11.4. A two-compartment model for oxygen transfer in a large-scale bioreactor.
An illustrative example of the usefulness of a simple compartment model is given by Oosterhuis and
Kossen (1983). These authors were concerned with the validity of using Eq. (10.27) for calculation of
oxygen transfer rates for widely different scales using the same parameter values. A bioreactor equipped
with two Rushton turbines was used in the study, and the basic data for the reactor is given in Table 11.7.
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