biotransformations carried out in a two-phase system and the advantages of using such a
system are given by Vermuë and Tramper (1995a). In general one can chose a liquid-
liquid two-phase system when:
− the substrate dissolves poorly in the medium; the organic solvent acts as a reservoir;
− there is product inhibition; by using an organic solvent the product concentration in the
medium is lowered;
− the reaction equilibrium is unfavourable; the equilibrium products dissolve in the
organic solvent and the degree of conversion can be enhanced.
Another reason for using in situ extraction, the combination of bioconversion with the
first unit operation in the downstream processing, might be a facilitated product recovery.
Obviously, using an organic solvent in bioconversions also leads to disadvantages. For
example, the toxicity of the solvent might reduce the biocatalytic activity and stability
(Vermuë et al., 1995b). Furthermore, in any bioconversion with micro-organisms
involved, surface active agents are present, whether excreted by the micro-organism, or
present due to cell lysis. This can result in non-desirable, stable emulsions that are
difficult to separate (Vermuë et al., 1995b).
Inhibition due to direct contact between micro-organisms and the organic solvent can
be reduced by immobilising the micro-organism. Immobilisation may also reduce the
amount of surface-active agents, and consequently prevent a stable emulsion (Vermuë et
al., 1995b). Immobilisation of micro-organisms has been studied extensively (Wijffels et
al., 1996), and a particularly elegant way of immobilising micro-organisms is a method
that uses a natural gel solution (e.g. κ-carrageenan, alginate). Broth is mixed with the gel
solution and gel beads are made as described by, for instance, Hunik and Tramper (1993).
Obviously, immobilisation will only work when there is hardly any outgrowth of the
micro-organisms, and little excretion of proteins.
In operating an extractive bioconversion one should decide whether to use a bioreactor
with bioconversion and extraction integrated, or use a plant set-up with both processes
separated. In the latter case the medium has to be circulated between both apparatus at a
high velocity (see the section on loop reactors for more detail). Using, in one apparatus,
immobilised cells for a bioconversion and an organic solvent for extraction, a three-phase
system is automatically formed. As apparatus, stirred tank reactors, although favourable
for good mixing and a high interfacial area between medium and organic solvent, are not
very useful; the harsh conditions in the tank will destroy the gel beads. A stronger
immobilisation matrix would be required than the natural gels mentioned above. Another
option is using column reactors. For this purpose a liquid analogue of the air-lift loop
reactor, the liquid-impelled loop reactor, was designed (Tramper et al., 1987) and
hydrodynamically characterised by van Sonsbeek (1992a). Different biotransformations
have been executed in this type of reactor (Vermuë et al., 1995; Buitelaar et al., 1991;
van den Tweel et al., 1987; Mateus et al., 1996). The application of this type of reactor is
discussed in the section on loop reactors. Another simple bioreactor is the liquid-liquid-
solid three-phase fluidised bed. The reactor is almost identical to a liquid-impelled loop
reactor, but the water flow is controlled with a pump and not induced by a density
difference. The hydrodynamics of this reactor have been studied: the hold-up of the
different phases as a function of the fluxes of both phases (van Zessen et al., 2000a), as
well as the mixing of the medium phase (van Zessen et al., 2000b). This type of reactor
Multiphase bioreactor design 354