considerations. For Michaelis-Menten kinetics, the PFR is preferable to the CSTR as the
CSTR requires more enzyme to obtain the same degree of conversion as a PFR. If
product inhibition occurs, this problem is accentuated, as in a CSTR high product
concentration is always in direct contact with all of the catalyst. There is only situation
where a CSTR is more favorable kinetically than a PFR, namely, when substrate
inhibition occurs.
The form and characteristics of the immobilised enzyme preparations also influence
the choice of reactor type, and operational requirements are still another factor to be
taken into account. Thus, when pH control is necessary, for instance with penicillin
acylase, the CSTR or batch stirred tank reactor is more suitable than PFR reactors. Due to
possible disintegration of support through mechanical shearing, only durable preparations
of immobilised enzyme should be used in a CSTR. With very small immobilised enzyme
particles, problems such as high pressure drop and plugging arise from the utilisation of
this catalyst in packed bed reactors (the most commonly used type of PFR). To overcome
these problems, a fluidised bed reactor, which provides a degree of mixing intermediate
to the CSTR and the ideal PFR, can be used with low pressure drop.
Reactant characteristics can also influence the choice of reactor. Insoluble substrates
and products and highly viscous fluids are preferably processed in fluidised bed reactors
or CSTR, where no plugging of the reactor is likely to occur, as would be the case in a
packed bed reactor.
As it can be deduced from this outline, there are no simple rules for choosing the
reactor type and the different factors mentioned must be analysed individually for a
specific case.
Modelling of Immobilised Biocatalyst ReactorsThe modelling of immobilised biocatalyst reactors should take into account the several
factors which influence their performance. These factors are:
(a) the immobilised enzyme (or biocatalyst) kinetics;
(b) the external and internal mass transfer effects;
(c) the dispersion (back mixing) effects;
(d) the heat transfer effects;
(e) the operational stability of the immobilised biocatalyst.
The design equations of immobilised enzyme (biocatalyst) reactors can be obtained from
the basic chemical reactor equations and the rate of the biochemical reactions catalysed
by immobilised biocatalysts.
The total mass balance is the basis for the design of any reactor. The component
balances account for the mass of the individual chemical species which are transformed
by biochemical reaction. The general form for a component i, is:
(42)This general balance can be written in terms of measurable quantities: concentrations Ci,
reaction rate ri, flow rate Q, and reactor volume V:
Design and modelling of immobilised biocatalytic reactors 113