Page 154 - Chiral Separation Techniques

132 5 Membranes in Chiral Separations

In general, high selectivities can be obtained in liquid membrane systems. However,
one disadvantage of this technique is that the enantiomer ratio in the permeate decreases
rapidly when the feed stream is depleted in one enantiomer. Racemization of the feed
would be an approach to tackle this problem or, alternatively, using a system contain-
ing the two opposite selectors, so that the feed stream remains virtually racemic [21].
Another potential drawback of supported enantioselective liquid membranes is the
application on an industrial scale. Often a complex multistage process is required in
order to achieve the desired purity of the product. This leads to a relatively complicated
flow scheme and expensive process equipment for large-scale separations.

5.2.2 Polymer Membranes

As the main disadvantage of liquid membrane systems is the instability over a longer
period of time, another approach would be to perform separation through a solid
membrane [22]. Enantioselective polymer membranes typically consist of a nonse-
lective porous support coated with a thin layer of an enantioselective polymer. This