such technology could be labeled ‘natural’, because of the absence of organic solvents
traditionally used in classical liquid systems.
Microbial solid/gas reactorsSuch solid-gas bioreactors have been tested for transformations involving more than one
step. In this field, the studies are essentially devoted to the removal of toxic products.
Nevertheless, the epoxidation of alkenes by bacteria has received some attention, in view
of the possible production of certain epoxides using a biotechnological process. Some
methods using immobilised enzymes have been developed, but the use of immobilised
micro-organisms is more promising. The major problem in this transformation is the
toxicity of the reaction product (ethylene or propylene oxide depending on the substrate
used) towards the biocatalyst. To avoid epoxide accumulation in the microenvironment of
the biocatalyst and thus inactivation, rapid removal of this product is essential. Ethylene
oxide is very soluble in water and no suitable extractant, immiscible with water, is
available. A solid-gas bioreactor has been used to promote a rapid and continuous
removal of the toxic ethylene oxide from the environment of the immobilised cells.
Mycobacterium Py 1 immobilised in alginate beads and on sand has been used. The
conversion of propylene or ethylene to the corresponding oxide is done by a mono-
oxygenase system requiring molecular oxygen and NADH or NADPH. The cofactor
regeneration is possible through endogenous respiration.
The same group has further studied the influence of immobilisation and reduced water
activity from the fundamental point of view, on gaseous-alkene oxidation by the same
type of bacteria in the solid-gas bioreactor (Hamstra et al., 1986).
Continuous production of propylene oxide has been successfully demonstrated using a
simple solid-gas heterogeneous bioreactor with Methylosinus sp CRL 31 coated onto
porous glass beads (Hou, 1984).
Trichloroethylene, one of the contaminants frequently detected in groundwater, has
been shown to be degraded by immobilised resting cells of Methylocystis sp M in the
same kind of reactor (Uchyama et al., 1992).
The alcohol oxidase enzyme of extruded pellets of Pichia pastoris has been tested for
aqueous, organic and vapor phase oxidations showing the feasibility of all the reaction
systems used (Duff and Murray, 1990). Again, the authors noted that the degree of
conversion and the catalyst longevity was influenced by the moisture of the biocatalyst.
OPERATING SOLID GAS BIOREACTORS
As noted in the case of enzyme gas/solid reactor uses, different parameters such as water
activity, temperature, and nature of the substrate are crucial for optimising the
performance of microbial solid-gas reactors or fermenters.
When dealing with reactors which contain growing cells, the moisture in the system
has to be carefully controlled in order to obtain a correct growth of the cells without
having the appearance of a well defined liquid phase. This type of bioreactor is very close
to gas-liquid-solid systems, where the liquid phase constitutes an interface between the
solid phase and the gaseous one, thus minimising greatly the external mass transfers and
Multiphase bioreactor design 264