plate and frame, tubular, hollow fibres, spiral wound and ultrafiltration cells (Prazeres &
Cabral, 1994). While the permeate flow is usually perpendicular to the membrane
surface, the main flow in the concentrate side of these modules can be tangential or
perpendicular, as in dead-end designs. The use of tangential flow is usually preferable
since mass transfer and permeation flux are improved. Some module designs include
turbulence promoters, such as spacers or screens, as a way of increasing permeation flux.
Improved filtration performance can be further obtained by coupling a controlled
secondary flow to the main flow. This can be achieved for instance in devices where the
membrane tubes or fibres are helically wrapped (Figure 6.5a), originating secondary flow
structures named Dean vortices (Luque et al., 1999; Kluge et al., 1999). Another
approach is to use two rotating concentric cylinders, one containing the membrane, in
order to produce the secondary flow structures known as Taylor vortices (Figure 6.5b). In
both cases, the vortices formed enhance back migration through increased wall shear rate
and increased convective flow away from the membrane, increasing permeation flow
rates (Luque et al., 1999; Kluge et al., 1999).
Figure 6.5 Module designs that add
secondary flow structures to the main
flow: a) helically wound hollow fibre
design: secondary flow- Dean vortices
(adapted from Luque et al., 1999) and
b) rotating concentric cylinders:
secondary flow-Taylor vortices.
CLASSIFICATION OF MEMBRANE REACTORS
Enzymatic membrane reactors can be broadly classified in three categories, according to
the mechanism by which enzymes and substrates are brought into contact (Prazeres &
Cabral, 1994).
Enzymatic membrane reactors 153