materials have good mechanical properties, which is especially important in draft-tube
baffled stirred crystallisers (see below, Figure 8.4). However, the immobilisation
procedures are more harsh and difficult than for natural supports, resulting in much
biocatalyst inactivation; in PCS the diffusivity is lower. These characteristics indicate that
optimisation is still necessary (Leenen et al., 1996).
Equipment DesignThe design of the equipment is determined mainly by the desired product characteristics.
It can affect the product characteristics by its influence on the process variables. For
control and downstream-processing purposes, a large product crystal is generally aimed
for. In order to achieve this, Mersmann and Rennie (1995) recommend the mean and
especially the maximum (local) supersaturation to be limited by: a) excellent mixing of
the entire vessel contents, so that gradients in the supersaturation are absent; this can be
achieved by using a large ratio of stirrer to tank diameter, as such stirrers distribute power
more evenly; b) low concentrations of the reactant(s), which are here the substrate(s), and
of the biocatalyst; c) vigorous seeding especially at the feed point(s), and d) high
circulation rates of slurry with a high suspension density. Note that by the latter two (c
and d), the crystal hold-up, and thus the total crystal surface area that is available for
crystal growth, can be regulated.
Objectives a) to d) can best be met in a continuous-flow, stirred-tank reactor (CSTR).
Since both biocatalysts and crystals are sensitive to shear, axial flow impellers are
frequently used, as they produce more flow and less shear rate than radial flow impellers
at comparable power levels (Oldshue, 1993). A schematic view of a draft-tube baffled
continuous crystalliser for solid-to-solid bioconversions is shown in Figure 8.4. By
pumping the fluid down in the draft tube, an upward flow of (supersaturated) solution is
created in the annulus. This flow fluidises a bed of substrate and product crystals and of
solid support particles with immobilised biocatalyst; it will be desupersaturated during
passage through the annular fluidised bed. When the product crystals have grown large
enough, they settle from the bed to the bottom of the crystalliser. The solid substrate
particles (and product seeds) can be fed to the draft-tube baffled crystalliser either by
dropping them from a belt conveyer (Bennett, 1993), or by a suspension flow.
Another configuration that is often applied in continuous reaction crystallisation is a
fluidised-bed crystalliser (or Oslo growth-type crystalliser). A schematic view of an
fluidised-bed continuous crystalliser for solid-to-solid bioconversions is shown in Figure
8.5. This crystalliser works according to the same principles as described for the draft-
tube baffled crystalliser of Figure 8.4, and mainly differs from it by its fluid
Solid-to-solid bioconversions 247