when compared to traditional column reactors with enzymes immobilised on beads
(Nakajima et al., 1989, 1993).
Important advantages can also arise due to changes in the reaction rates caused by the
presence of a membrane, as described earlier: retention of substrate molecules to a certain
extent leads to higher reaction rates (Prazeres, 1995) and possibly to higher yields in
equilibrium limited reactions (van der Padt et al., 1991; Prazeres, 1996).
In the case of multi-product systems, the presence of a membrane may also be
beneficial. In such a case, if the membrane exhibits some selectivity towards different
products, an enrichment of the less rejected product is obtained in the outlet process
stream (Matson & Quinn, 1986). On the other hand, the product that is more rejected can
be concentrated inside the system. This may be disadvantageous if product inhibition is
present.
One of the early-recognised advantages of membrane reactors was detected in the
hydrolysis of macromolecules. In these cases, a membrane with the adequate cut-off
usually enables some control of the molecular weight of the hydrolysates. This leads to
an increase in lower molecular weight components in the permeate stream and to a
concentration of the heavier products behind the membrane (Cheryan & Mehaia, 1986;
Silva, 1990; Bouhallab et al., 1992, 1993, 1995).
As already mentioned above, membrane reactors also offer the possibility to conduct
two-phase reactions, without the emulsification problems; namely the inactivation of the
enzyme caused by the intensive agitation necessary to make and maintain emulsions and
the high power requirements (hence, energy costs).
DisadvantagesThe performance of membrane reactors during operation can be severely affected by
losses in the catalytic and mass transfer efficiencies, which are inherent to the geometry
and design of this particular type of reactor.
The stability of the enzyme during operation can be caused by several factors other
than temperature related deactivation (Deeslie & Cheryan, 1981). For instance, a gradual
decrease in activity may occur as a consequence of leakage of enzyme molecules through
the membrane pores (Silva, 1990; Jones et al., 1984). This may even occur in those cases
where the molecular weight of the enzyme is higher than the membrane cut-off, since the
shape of the enzyme molecules and the distribution of membrane pore sizes should be
also taken into account. Cheryan & Mehaia (1986) suggested that in order to prevent this
from occurring, a membrane with a cut-off at least 5–10 times lower than the enzyme
molecular weight should be selected. Small enzyme activators such as metal ions or
cofactors (Drioli et al., 1993; Hayakawa et al., 1985) may also cross the membrane,
decreasing the enzyme activity (Deeslie & Cheryan, 1981). In such cases,
supplementation of the leaking component is fundamental to ensure a successful
operation.
When the enzyme is used in its soluble form, unfavourable adsorption to the
membrane may occur. This contact with the membrane can lead to structural changes in
the enzyme molecule and poisoning which can contribute to a decrease in activity. This
means that the type of membrane material may influence the stability of the enzyme
(Nakajima et al., 1992; Alfani et al., 1990).
Enzymatic membrane reactors 161