Figure 8.3 Conversion of solid Ca-
maleate to solid Ca-D-malate by
permeabilized P. pseudoalcaligenes at
30°C and 250 rpm in a batch stirred
bioreactor seeded with Ca-D-malate
crystals; dissolved maleate (Ca-
maleate and maleate
2 −; ∆), total
maleate (in liquid and solid phase; ),
dissolved D-malate (D-malate^2 and Ca-
D-malate; ) and total D-malate (in
liquid and solid phase; ) (adapted
from Michielsen et al., 1999a).
CONTINUOUS SOLID-TO-SOLID BIOCONVERSIONS
The design of a continuous reactor for solid-to-solid bioconversions is governed by the
following demands:1) a high stability and appropriate retention of the biocatalyst; 2)
reliable and stable operation for a long period of time, and 3) a large product crystal size
for efficient downstream recovery (see below). The first demand is generally satisfied by
immobilisation. The last two demands are related; for reliable and stable operation,
product crystallisation by nucleation is usually minimised, which implies the formation of
large crystals at certain process conditions. To illustrate this, the fundamentals of
crystallisation are outlined shortly.
By definition, crystallisation consists of two processes, the formation of new crystals,
called nucleation, and crystal growth. These two processes occur if the solubility (C*) is
exceeded, resulting in supersaturation ( ). If super-saturation is the result of
a (biocatalytic) reaction and the reaction substrate originates from another phase, the
process of crystal formation is called heterogeneous reaction crystallisation. New crystals
(nuclei) are mainly formed at high supersaturation (by so-called primary nucleation), or
originate from attrition of existing crystals e.g. by collision or shear stress (so-called
secondary nucleation). Secondary nucleation dominates over primary nucleation at
intermediate and low supersaturation. At such a (lower) supersaturation in the presence of
product crystals, crystal growth also takes place. This results in polydispers crystals,
which may be characterised by a crystal-size distribution. The crystal-size distribution is
often summarised by the median crystal size and its coefficient of variation.
For reliable and stable operation of a continuous reaction crystalliser over a long
period of time, nucleation phenomena are generally minimised, as a good prediction of
nucleation rates is still not possible (Rohani, 1995). Besides, by minimising (primary)
nucleation the median crystal size increases (Mersmann and Kind, 1989), which is
advantageous for downstream processing. This can be achieved by seeding the
crystalliser with an increasing number of product crystals, so that the total crystal area
and hold-up in the crystalliser increases, assuming that the seed crystals have a constant
(small) size; by increasing the hold-up, the growth rate (per unit of crystalliser volume)
Solid-to-solid bioconversions 245