k), as a function of the substrate
rejection coefficient, σ. The reactor
was modelled as a CSTR (see
Prazeres, 1995 for further details). θ—
residence time.
is exemplified in Figure 6.4 which presents the theoretical, steady state conversion
degree, X, obtained in a continuous enzyme membrane reactor operating with first order
kinetics (rate constant k), as a function of the substrate rejection coefficient, σ. As can be
seen, the conversion degree is affected not only by the kinetics of the reaction, but also by
the degree to which substrate molecules are retained by the membrane.
Furthermore, in those cases where chemical equilibrium affects the reaction yield, this
substrate retention may contribute to a favourable shift of the equilibrium towards the
product side (van der Padt et al., 1991, 1996a; Prazeres, 1996). The extent of substrate
retention depends mainly on the dimensions and/or chemical compatibility of the
substrate molecules with the membrane material. Another way of promoting the retention
of substrate molecules that are smaller than the membrane pores is by using charged
membranes or by applying an electrical field across the membrane. The prerequisite
necessary for this strategy to function is that the substrate can be rendered electrically
charged. For instance, negatively charged membranes have been used to retain negatively
charged cofactors [NAD(H), NADP(H)] inside reactors, thus preventing their leakage
from the system (Ikemi et al., 1990a; Kulbe et al., 1990; Nidetzky et al., 1996).
Electrically charged membranes can also be used to attract substrate molecules in those
cases where the enzyme is immobilised on the membrane (Chen et al, 1994).
The products generated during the bioconversion should in principle permeate through
the membrane, either by diffusion (induced by a concentration gradient) or convection
(usually induced by a pressure gradient). An electric field may also be used to force
charged products to migrate from the reactor and across the membrane into the permeate
stream (Lee & Hong, 1988; Furusaki et al., 1990; Righetti et al., 1997; Nembri et al.,
1997; Bossi et al., 1999).
Although the permeation of products through a membrane has been always regarded
as an essential requirement for the successful operation of a membrane reactor, cases
exist where the complete rejection of products might be desirable. For instance, if the
target product has a low solubility in the media and precipitates or crystallises during
reaction, the solid particles formed can be retained behind the membrane. The operation
of a membrane reactor of this kind which enables the continuous synthesis of dipeptides
(AcPheLeuNH 2 ) by a-chymotrypsin in reversed micellar media has been described
(Serralheiro et al., 1994; Prazeres et al., 1995; Serralheiro et al., 1999). Solid products
formed upon reaction in a membrane reactor can also be collected during operation if a
filtration unit is incorporated in the system (Furui et al., 1996). This avoids the continued
contact of solid particles with the membrane therefore preventing clogging and the
consequent decrease in permeate flux.
Enzymatic membrane reactors 151