tions. In these cases membrane separations will be more complicated, but the separa-
tion factor will be larger than using single separation mechanisms. In a membrane
diffusion cell, a concentration gradient was used as the driving force for the separa-
tion of medium-chain fatty acids. In an ultrafiltration reactor (X. Xu et al., unpub-
lished results), pressure was used as the driving force.
Membranes are the key factor for a promising membrane reactor, especially in
lipid reaction systems. Commercial membranes are produced mainly for water sys-
tems. The selection of membranes for lipophilic systems is the important step in
obtaining a potentially successful application. Membrane stability is, of course,
the important factor for lipophilic systems, both for practical and economical con-
siderations. It is often difficult to find a membrane that possesses high selectivity,
acceptable flux, and long stability in the lipid separation system. This may be one of
the major reasons for the slow progress of membrane applications in lipid processing,
as reviewed elsewhere (Snape and Nakajima, 1996; Parmentier and Fanni, 1999).
The kinetics of membrane reactors for lipase-catalyzed acidolysis depends on the
difference of reaction rate and transport rate. Normally, the transport is a limiting step
of the system, whereas the reaction rate depends on the transport rate. The highest
productivity can be obtained at the steady state where the reaction and transport rates
are identical:
VmaxS
KmþS¼kcCþkPP ð 22 ÞwhereVmaxis the maximum reaction rate,Sis the substrate concentration,Kmis the
Michaelis constant,kcandkpare the constants,DCis the concentration gradient, and
DPis the pressure gradient. The kinetics of membrane reactors depends much on the
reactor configurations and operations (Mulder, 1996). When the enzyme is immo-
bilized on the membrane wall, as for hollow-fiber membrane reactors, the modeling
of the membrane reactor is much more complicated.
208 11 Modification of Oils and Fats by Lipase-Catalyzed Interesterification
Figure 9. Effect of water content on the interesterification conducted in a microaqueous system for the
modification of oils and fats. (From Yamane, 1987.)