Multiphase Bioreactor Design

Direct Contact Membrane Reactors

In this type of membrane reactor, the substrates are added to the same compartment of the
reactor that contains the enzyme. The enzyme, either in the soluble form or insolubilised
at the membrane surface, can thus act directly on the substrate molecules as soon as they
enter the system. This class of reactors can be further divided into recycle, dead-end and
dialysis reactors.
Recycle membrane reactors are usually formed by a stirred vessel coupled to an
ultrafiltration module in a semi-loop configuration (Figure 6.6). The substrate is added to
the vessel and the reaction mixture is continuously recycled from and to the vessel
through the UF unit. The reaction occurs both in the vessel and module if the enzyme is
in the soluble form, or solely in the module if the enzyme is immobilised in the
membrane. Products and excess solvent permeate through the membrane, being
continuously removed from the system. The liquid volume is kept constant in the vessel
as long as the inlet flow rate is equilibrated with the outlet permeate flow rate. If the
enzyme is immobilised on the membrane surface, external mass transfer limitations can
be reduced by increasing the recirculation flow rate (Lozano et al., 1990). Tubular
membranes and hollow fiber modules are the more used in this type of configuration.
Studies on the residence time distribution in these recycle reactors indicate that the whole
system behaves like an ideal CSTR (Deeslie & Cheryan, 1981; Bressolier et al., 1988;
Sims & Cheryan, 1992). This type of configuration is probably the most used and
referred to in the literature (Prazeres & Cabral, 1994).
Dead-end membrane reactors are formed exclusively by an UF module. In this
configuration, the reaction mixture is pressurised against the membrane and forced to
flow

Figure 6.6 Schematic diagram of a

recycle membrane reactor (•—enzyme,

S—substrate, P—product, —

membrane)

Multiphase bioreactor design 154