Multiphase Bioreactor Design

The retention capability of a membrane towards a solute (enzyme, substrate or
product) can be quantified by a parameter called the rejection coefficient, , which is
defined as:

(1)

where Cp is the solute concentration in the permeate side, and Cc the solute concentration
in the concentrate side. A solute which is completely retained by a given membrane, will
have a rejection coefficient of 1, while a solute which permeates freely across the
membrane will have a rejection coefficient of 0.
In recent years, the functions of the membrane have extended with the systematic use
of membrane reactors in two-phase bioconversions. In these situations, the membrane
acts as a support for the interface between two distinct phases (liquid/liquid or
liquid/gaseous). The membrane not only separates the phases, but also provides
interfacial contact area and, together with the enzyme acts as an interfacial catalyst
(Matson & Quinn, 1986; Lopez et al., 1990, 1991; Matson & Lopez, 1991).

Enzyme Retention

The complete retention of the enzyme in the system by the membrane is the first and
most important requirement for the establishment of a successful continuous operation in
a membrane reactor. This retention, which can be accomplished by different mechanisms,
confines the enzyme to a specific region of the membrane reactor where reaction with
substrate occurs. The enzyme is usually present in two forms; soluble or insolubilised at
the surface or pores of the membrane matrix. If the enzyme is in solution, retention is
achieved by confining the enzyme to one side of the membrane. As shown in Figure 6.2,
this can be done by:
Size exclusion: enzyme molecules with sizes larger or close to the dimensions of the
membrane pores are excluded (Ohlson et al., 1984; Park et al., 1985; Ishikawa et al.,
1989a; Fujii et al., 1991; Bryjak et al., 1996)
Size exclusion via enlargement: enzyme molecules enlarged through
chemical/physical immobilisation onto an intermediate molecule—an inert protein
(Alfani et al., 1982), gels (Cantarella et al., 1989) or liposomes (Chang et al., 1991) are
excluded.
Electrostatic repulsion: charged enzymes are repelled by membranes with charges of
the same sign (Kulbe et al., 1990; Rôthig et al., 1990)

Multiphase bioreactor design 148