Multiphase Bioreactor Design

Figure 6.8 Schematic diagram of a

dialysis membrane reactor (•—

enzyme, S—substrate, P—product,

—membrane)

Diffusion Membrane Reactors

In the class of diffusion membrane reactors, unlike the previous one, the substrate is not
added to the compartment that contains the enzyme. Instead, the contact between
enzyme/ substrate is established only after diffusion of the substrate molecules through
the membrane micropores to the other compartment where the enzyme is located (soluble
or insolubilised). After reaction, the products back diffuse to the unreacted substrate
stream that circulates by and leave the system (see Figure 6.9).
Hollow fibre modules are mostly used in this class of reactors, with the enzyme
usually placed in the shell side of the module and the substrate stream passing through
the fibre lumen. Closed tubings of dialysis membranes can also be used to contain the
enzyme solution (Lüthi & Luisi, 1984; Bednarski et al., 1987). The dialysis bag formed is
then submerged in the reaction mixture that contains the substrate, which diffuses
through the membrane. The product formed diffuses back to the bulk solution. Diffusion
membrane reactors act essentially as dialysers as solutes are transferred through the
membrane under a concentration driving force, rather than pressure. The majority of the
disadvantages presented by this type of reactors are related to the fact that diffusion is the
dominant transport mechanism. In many cases, the kinetic behaviour of enzymes in these
reactors is inferior when compared with a free enzyme because the permeation of the
substrates across the membrane is often the rate-limiting step. In addition, the control of
the environmental conditions in the vicinity of the entrapped enzyme is also limited by
the diffusion of chemical species in the media (Kitano & Ise, 1984; Kelsey et al., 1990).
The limited number of publications in the literature reporting the experimental operation
of such types of membrane reactors (Park et al., 1985; Bednarski et al., 1987; Tegtemeir
et al., 1988; Ishikawa et al., 1989a; Fujii et al., 1991; Miyawaki et al., 1990; Yonese et
al., 1990; Czermark & Bauer, 1991; Nakano et al., 1999), is probably a reflection of
transport limitations. Other arrangements can be implemented in this class of reactors
such as the one shown in Figure 6.10, where the substrate-containing stream is
recirculated from the module through an external vessel. This strategy provides one way
of increasing the residence time of the substrate molecules inside the reactor and
improves mixing. Furthermore, the presence of an external vessel is convenient in terms
of the operation of

Multiphase bioreactor design 156