Figure 8.7 The substrate, biocatalyst,
investment and operating,
downstream-processing, and overall
costs per kg of product ($s, $e, $io, $dp,
and $ov, respectively) as a function of
the ratio of the initial biocatalyst
concentration (Ce(0)) and the initial
substrate concentration (Cs(0)) in a
batch reaction crystallizer;
were formulated. In batch systems, conversions of 80–100% are reported for every type
of solid-to-solid bioconversion, even when hardly any liquid phase is present, and it
seems that these systems can easily be scaled-up. Kinetic studies of these solid-to-solid
bioconversions give rise to further development of two kinds of systems in the future: 1)
batch systems with very high concentrations of undissolved substrate, and 2) continuous
systems for solid-to-solid bioconversions. In continuous systems an optimum
supersaturation (∆Copt) exists. The main advantage of continuous systems over batch
systems is that they are able to maintain a constant (optimal) supersaturation, resulting in
more homogeneous product crystals, and thus lower downstream-processing costs. In this
work, two continuous systems for solid-to-solid bioconversions are proposed: a draft-tube
baffled continuous crystalliser and a fluidised-bed continuous crystalliser, both with
immobilised biocatalyst. Crystallisation in the pores of the support of the immobilised
Solid-to-solid bioconversions 257