Free enzyme molecules in membrane reactors are subjected to shear forces and
frictions generated near the membrane walls. There is evidence that these shear fields and
related secondary effects such as interfacial inactivation, adsorption, local heating and air
entrapment can cause enzyme inactivation (Narendranathan & Dunnill, 1982; Virkar et
al., 1981; Thomas & Dunnill, 1979). Experimentally this effect is usually observed in the
form of a correlation between the loss of activity with an increase in recirculating rates
(Lozano et al., 1990; Narendranathan and Dunnill, 1982). These effects may be
significant in recycle reactors that usually operate with high recirculating flow rates. In
the case of membrane reactors using stirrers, shearing-related effects associated with
rotation may also contribute to deactivation (Alfani et al., 1990). Whenever a decrease in
operational kinetic stability caused by one of the factors described above occurs, fresh
enzyme should be added to maintain a constant productivity in the reactor.
Decreases in the performance of membrane reactors can also be attributed to losses in
mass transfer efficiency during the permeation process. Two distinct phenomena are
usually responsible for the reduction in membrane permeation ability: concentration
polarisation and fouling.
Concentration polarisation is the reversible build-up of dissolved and suspended
solutes (including enzymes) at the boundary layer adjacent to the membrane, which leads
to the formation of a concentration gradient. The gel layers formed at the interface act as
a second membrane and originate a diffusion flux of solutes (products or substrates) from
the membrane towards the bulk of the medium that decreases the net flux across the
membrane. The solvent flow through the membrane is also restricted due to the presence
of this additional hydrodynamic resistance that causes a decrease in the solvent filtration
flux (Hildebrandt, 1991b). In many cases, concentration polarisation-related phenomena
(adsorption, deposition, solute/membrane interactions) also contribute to an increase in
the rejection capability of the membrane (Fane & Radovich, 1990). Concentration
polarisation can be reduced and controlled by manipulating operating conditions such as
temperature and pressure, and by increasing flow rates or by introducing cyclic
backflushing or pulsatile flow (Park et al., 1985). Another way of reducing this
polarisation effect is by applying an electric current across the membrane. If the correct
pH is chosen, some solutes and enzymes can be rendered charged and will migrate in the
presence of the electric field (Lee & Hong, 1988). This strategy can be used to keep
enzyme molecules from adsorbing to the membrane if the migration proceeds in a
direction opposite to the permeate flow, and to accelerate the removal of electrically
charged products from the reactor into the permeate stream (Furusaki et al., 1990).
Usually, a complete restoration of original permeation fluxes after operation is achieved
by using efficient cleaning procedures.
The second limiting phenomenon most likely to occur during a filtration operation is
fouling. This is associated with a modification in the filtration properties of a membrane
as a result of the irreversible deposition or adsorption of solutes and particles at the
surface or inside the pores. This process leads to a progressive reduction in the efficiency
of a membrane, essentially due to a reduction in the solvent and solute permeate fluxes
and to an increase in the rejection of solutes (Santos et al., 1991; Hildebrandt 1991b). An
adequate pre-treatment of the membrane and substrate feed can contribute to a
minimisation of this effect, thus increasing the lifetime of the membranes.
Multiphase bioreactor design 162