Figure 6.7 Schematic diagram of a
dead-end membrane reactor (•—
enzyme, S—substrate, P—product,
—membrane)
across the micropores (Figure 6.7). This type of operation can be carried out in the wide
spread ultrafiltration cells or, otherwise, in common ultrafiltration modules with the
appropriate ends sealed (Harrington et al., 1992).
In a continuous operation, the substrate should be added at a flow rate equal to the
permeate flow. Ultrafiltration cells are essentially operated as CSTRs and the permeation
of solutes is achieved by conventional filtration through a flat membrane placed
perpendicularly to the reactor bottom. Despite a very low ratio of membrane area to
reactor volume and low permeation fluxes caused by concentration polarisation,
ultrafiltration cells are extensively used in laboratory-scale studies to test operation
concepts and investigate kinetic mechanisms, probably due to their simplicity of
operation (Prazeres & Cabral, 1994).
In dialysis membrane reactors, two process streams are circulated at approximately
the same flow rate in each side of the membrane, thus minimising convective flow. The
substrate is directly added to the stream that contains the enzyme. The products formed
diffuse through the membrane towards the other stream as a consequence of the
concentration gradient established (see Figure 6.8). The disadvantages of this type of
configuration are mainly a consequence of a less efficient mass transport due to an
operating mode based exclusively on diffusion. Few examples of this mode of operation
have appeared in the literature (Chiang & Tsai, 1992a, b).
Enzymatic membrane reactors 155