Multiphase Bioreactor Design

For a CSTR, because the component concentrations are constants, the effectiveness
factor does not change in the reactor. While for a BSTR or a PFR the component
concentrations change with time or the length of the reactor, respectively, which
influences the effectiveness factor, as this factor is dependent on the substrate
concentration.

The average effectiveness factor, can be obtained by the following equation:

(58)

Effect of back-mixing on the performance of immobilised biocatalyst

reactors

In real tubular (or column) reactors there is, usually, a back-mixing effect which
influences the performance of the ideal plug flow reactor. This axial dispersion is higher
for fluidised bed reactors than for packed-bed reactors, although comparatively lower
than for continuous stirred tank reactors, where the mixing is complete.
The modelling of real immobilised enzyme column reactors, mainly the fluidised bed
reactors, has been described by mathematical models based on the dispersion concept, by
incorporating an additional term to account for back-mixing in the ideal plug flow
reactor. This term describes the non-ideal effects in terms of a dispersion coefficient.
The design equation is:

(59)

where Du is the dispersion coefficient, u is the superficial fluid velocity, L the reactor
length or bed height and Z the normalised distance along the reactor length (=1/L). When
there are simultaneously dispersion and mass transfer effects, the equation (59) involves
the effectiveness factor η:

(60)

Effect of enzyme inactivation on the performance of immobilised

biocatalyst reactors

The performance of immobilised biocatalyst (enzyme) reactors is influenced by enzyme
inactivation during operation, mainly due to the thermal denaturation, desorption of the
biocatalyst from the solid support, disintegration or solubilisation of the support, and
microbial attack.
The overall effect of these factors can be determined experimentally, and it is
convenient to reduce them in order to increase the operational stability of the biocatalyst.
The prediction of the loss of the performance of an immobilised enzyme reactor due to
thermal denaturation can be quantified taking into account enzyme inactivation kinetics
models. The most used is the exponential decay model:

Multiphase bioreactor design 118