Multiphase Bioreactor Design

withdrawn from a quiescent zone of the crystalliser, and dissolved by the supply of either
solvent or heat; the resulting solution is recycled.
Control schemes for effective control of continuous crystallisers require on-line
measurements of crystal properties like crystal-size distribution, supersaturation, and
crystal purity. As with the existing sensors, measurements are either not feasible or
extremely difficult, significant progress can be made by the development of new robust
on-line measurement techniques. Besides, measurement of secondary variables such as
turbidity and the density of the liquor solution, which are related to the fines suspension
density and supersaturation, offer potential for the control of crystallisation processes
(Rohani, 1995).

Downstream Processing

As a rule in solids production processes, downstream processing consists of solid-liquid
separation by centrifuges or filters, followed by drying of the wet crystals. With respect
to centrifugation, Mersmann (1995a) reported that for obtaining a low mass ratio of
residual liquid to crystals (kg solution/kg crystals) the median crystal size is very
important; the mass ratio decreases with increasing median crystal size and decreasing
coefficient of variation of the crystal size. After centrifugation or filtration, the crystals
with adherent liquor are dried by flowing a preheated agent (hot air) through the solid
material. In this process, the need for a large median crystal size becomes even more
pronounced, as the specific energy consumption per mass unit of crystals increases with
increasing mass ratio of residual liquid to separated crystals. However, at a median
crystal size larger than 500 μm, and thus at a low mass ratio, the energy consumption
remains almost constant (Mersmann, 1995a).

BATCH OR CONTINUOUS?

Assuming that the proposed continuous systems for solid-to-solid bioconversions
(Figures 8.4 and 8.5) can be applied, the question arises which mode of operation to
choose for a specific solid-to-solid bioconversion. Now that the first batch systems have
been reported in which very high amounts of solid product per reactor volume can be
attained at high rates (Erbeldinger et al., 1998a), a shift to complex continuous operation
should be carefully thought over.
Process optimisation is generally focussed on minimisation of the costs per kg of
product, In a bioreactor the cost factors are: 1) the substrate, 2) the biocatalyst, 3)
investments and operation, and 4) downstream processing. For commercial applications,
the choice between batch or continuous operation is often based on a comparison of the
costs per kg of product for the two systems. For each of the systems, therefore, the
process conditions yielding minimum overall costs per kg of product should be
determined first. Selection of these conditions is highly case specific, as they depend on
the substrate and equipment used, the product produced, and the desired final purity of
the product. In this section, a general method is presented that can be used to calculate the
overall costs in both batch and continuous systems. If this method is coupled to an

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