Multiphase Bioreactor Design

Batch Reactors

Batch reactors are the most commonly used type of reactor when soluble enzymes are
used as catalysts. The soluble enzymes are not generally separated from the products and
consequently are not recovered for reuse.
Since one of the main goals of immobilising an enzyme is to permit its reuse, the
application of immobilised enzymes in batch reactors requires a separation (or an
additional separation) to recover the enzyme preparation. During this recovery process,
appreciable loss of immobilised enzyme material may occur as well as loss of enzyme
activity, therefore the use of immobilised enzymes in a batch operation is generally
limited to the production of rather small amounts of fine chemicals. Traditionally, the
stirred tank reactor has been used for batchwise work. Composed of a reactor and a
stirrer, it is the simplest type of reactor that allows good mixing and relative ease of
temperature and pH control. However, some matrices, such as inorganic supports, are
broken up by attrition in such vessels, and alternative designs have therefore been
attempted. A possible laboratory alternative is the basket reactor, in which the catalyst is
retained within a “basket” either forming the impeller “blades” or the baffles of the tank
reactor.
Another alternative is to change the flow pattern, using a plug flow type of reactor: the
total recycle reactor or batch recirculation reactor, which may be a packed bed or
fluidised bed reactor, or even a coated tubular reactor.
This type of reactor may be useful where a single pass gives inadequate conversions.
However, it has found greatest application in the laboratory for the acquisition of kinetic
data, when the recycle rate is adjusted so that the conversion in the reactor is low and it
can be considered as a differential reactor. One advantage of this type of reactor is that
the external mass transfer effects can be reduced by the operational high fluid velocities.

Continuous Reactors

The continuous operation of immobilised enzymes has some advantages when compared
with batch processes, such as ease of automatic control, ease of operation, and quality
control of products.
Continuous reactors can be divided into two basic types: the continuous feed stirred
tank reactor (CSTR) and the plug flow reactor (PFR).
In the ideal CSTR the conversion degree is independent of the position in the vessel,
as a complete mixing is obtained with stirring and the conditions within the CSTR are the
same as the outlet stream, that is, low substrate and high product concentrations. With the
ideal PFR the conversion degree is dependent on the length of the reactor as no mixing
device at all exists and the condition, within the reactor are never uniform.
While a nearly ideal CSTR is readily obtained, since it is only necessary to have good
stirring to obtain complete mixing, an ideal PFR is very difficult to obtain. Several
adverse factors to obtaining an ideal PFR often occur, such as temperature and velocity
gradients normal to the flow direction and axial dispersion of substrate.
Several considerations influence the type of continuous reactor to be chosen for a
particular application. One of the most important criteria is based on kinetic

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