slight positive pressure may have to be applied in order to keep the interface in the plane
of the membrane and prevent the phases from mixing. In some cases the progressive
Figure 6.11 Schematic diagram of a
single pass multiphasic membrane
reactor (•—enzyme, S—substrate, P—
product, —membrane)
adsorption/desorption of products/substrates (e.g. surface-active compounds) to the
membrane may change the wetting characteristics of the material and cause a movement
of the interface or even the breakthrough of one of the phases (Vaidya et al., 1994). This
will of course change the performance of the reactor.
The enzyme is usually placed in the aqueous side of the membrane in order to prevent
contact with the hydrophobic phase—commonly an organic solvent—and to protect the
enzyme from denaturation. In certain cases (e.g. lipases), immobilisation on the
hydrophobic side is used (van der Padt et al., 1990, 1992; Janssen et al., 1991; Pronk et
al., 1992). Multiphase membrane reactors have been mostly used and seem specially
indicated in those cases where the enzymes display interfacial activation, such as lipases
and phospholipases. They are also especially suited for situations where substrates and
products have distinct solubility characteristics.
Configurations with recycle of one or both phases through external vessels are also
possible (Figure 6.12), with the advantages already mentioned in the previous section.
Although multiphase membrane reactors have exclusively used two phases, the
construction of an appropriate module can enable a membrane reactor to operate with a
higher number of phases. For instance, the installation of two independent bundles of
fibres in a hollow fibre module would permit the operation of a three phase membrane
reactor (Figure 6.13a). Phase I could serve as a substrate reservoir, phase II as a product
collector and phase III, located in the shell side of the module could contain the enzyme.
This type of module is commercialised by Hoechst (Liqui-Cel™ Contained Liquid
Membrane—CLM module), and is used to perform two-step extraction processes. A
similar process could also be implemented by coupling the shell side of two simple
hollow fibre modules (Figure 6.13b).
Multiphase bioreactor design 158