biocatalytic conversion, product separation and/or concentration and catalyst recovery
into a single continuous operation.
It is well known that most of the enzymatic processes currently in industrial use are
carried out in batch reactors (Cheryan & Mehaia, 1986; Siebel, 1992). However, this
class of reactors suffers from a number of well-documented limitations, such as batch-to-
batch oscillations, high labour costs, frequent start-up and shut-down procedures, and the
need to recover the enzyme, or enzyme preparation, after each batch (Cheryan & Mehaia,
1986). The immobilisation of the enzyme in the reactor, with retention of its catalytic
activity, has emerged as a practical solution to overcome some of those disadvantages
that are associated to the use of free biocatalysts in solution. The technique not only
enables the recovery and re-use of the enzyme but also offers the opportunity to carry out
the process in a continuous mode if necessary. Further advantages of immobilisation
include better enzyme stability and process control, better productivity, more uniform
products and the integration of a purification step in the process (Cheryan & Mehaia,
1986; Cheryan, 1986).
The immobilisation of enzymes has been accomplished by chemical and physical
attachment to porous or non-porous solid surfaces (Gekas, 1986; Gerhartz, 1990). A wide
variety of surfaces with different geometry/morphology (beads, pellets, fibers) and
chemical composition have been used in different types of reactors: fixed bed, fluidised
bed, CSTR, expanded bed, etc. Rony (1972) first pointed out the advantages of using a
membrane as this solid surface, leading to the development of the first membrane
reactors. The unique features of membranes in these reactors enable them to accomplish
additional tasks other than immobilisation which are not commonly carried out in
conventional reactors: product separation, phase separation, enzyme
compartmentalisation, etc. (Matson & Quinn, 1992). Membrane reactors can therefore
complement and compete with other reactor types in the field of biotransformations.
THE CONCEPT OF A MEMBRANE BIOREACTOR
The basic feature of a membrane reactor is the separation of enzyme, products and
substrates by a semi-permeable membrane that creates a selective physical/chemical
barrier. Permeable substrates and products can be selectively separated from the reaction
mixture by the action of a driving force (chemical potential, pressure, electric field)
present across the membrane that causes the movement (diffusion, convection,
electrophoretic migration) of solutes. On the other hand, the enzyme is retained within
the system by the membrane, allowing the establishment of a continuous operation with
substrate feed and product withdrawal (Figure 6.1). The whole system can be set up by
assembling and interconnecting vessels and membrane modules.
Membranes can be used in a bioreactor exclusively as a matrix for the immobilisation
of the biocatalyst (reactive membrane), without any separation purposes (Furusaki &
Asai,
Multiphase bioreactor design 146