Membranes and ModulesThe majority of enzymes (10,000–100,000 daltons), whether native or modified, can be
retained in a membrane reactor with ultrafiltration (UF) membranes. Most of the
commercial UF membranes are asymmetric, i.e., the pore size varies continuously in one
direction. These membranes are formed by an ultra-thin layer deposited upon a sublayer
of higher porosity. The ultra-thin layer is formed by a network of micropores with pore
size distributions in the range 1–100 nm, corresponding to a nominal molecular weight
cutoff of 500 to 100,000 daltons. During operation, fluids flow through the membrane
due to a difference in hydrostatic pressure, and solutes are more or less retained by the
thinner layer according to their size. Apart from this size discrimination, a steric
exclusion process may be present for those molecules that have sizes inferior, but close to
the dimensions of the pores. The selectivity is therefore most likely associated to the ratio
of the molecule radius to the pore radius (Meireles et al., 1992). The chemical nature of
the membrane can also interfere with solute permeation due to non-specific interactions-
electrostatic, hydrophobic, van der Waals, dipole-dipole, etc. (Prazeres et al., 1993a)—
which lead to the formation of a second layer (gel layer) that decreases the permeation
(concentration polarisation phenomena). This is especially true if a chemical reaction
occurs simultaneously with the membrane separation process.
The asymmetric structure of ultrafiltration membranes allows higher permeate flow
rates and makes these membranes less susceptible to clogging and easier to clean
(Hildebrandt, 1991a). The materials commonly used in the manufacturing of
ultrafiltration membranes are synthetic polymers and ceramic materials (Santos et al.,
1991). Ceramic membranes when compared with polymeric ones are generally more
resistant to high temperatures and chemicals and are mechanically stronger under
pressure. The membranes used in the enzymatic reactors described in the literature have
used materials such as nylon, polypropylene, polyamide, polyacrylonitrile, cellulose
acetate, polysulfone, cellulose, polytetrafluoroethylene and carbon. The selection of a
membrane for a particular enzymatic process should be carefully made since the
membrane material can significantly affect the stability of the enzyme (Nakajima et al.,
1992; Alfani et al., 1990). This choice is usually made tentatively and should consider
characteristics such as morphology, molecular weight cutoff, porosity, pore size
distribution, chemical resistance, temperature, pH and pressure tolerance and price
(Hildebrandt, 1991a; Leuchtenberger et al., 1984). In the particular case of multiphase
reactors, the membrane material (wetting characteristics) and structure (pore size and
asymmetry) should be carefully chosen in view of the need to maintain a stable interface
between the two phases at the membrane (Vaidya et al., 1992).
Nanofiltration membranes can also be used in a membrane reactor if the retention of
small solutes (<200 g mol−^1 ) such as cofactors, substrates, products, and buffer
compounds is desirable. The separation here is performed on the basis of size, and also
charge (Lin et al., 1997; Seelbach and Kragl, 1997).
The UF membrane material is usually in the form of a flat sheet, a tube or a hollow
fibre. These are then assembled in modules with a determined geometry offering distinct
flow zones and hydrodynamics for the feed and permeate streams (Santos et al., 1991;
Hildebrandt, 1991b). Different types of modules have been used in membrane reactors:
Multiphase bioreactor design 152