Page 168 - Chiral Separation Techniques

146 5 Membranes in Chiral Separations

However, when complexation experiments are performed with both D- and L-enan-
tiomers (Fig. 5-18), this leads to selectivity values between 1.4 and 1.9. It was shown
that complexation by enantioselective micelles can effectively be described using
straightforward multicomponent Langmuir isotherms [74].

Fig. 5-18. Selectivity for D,L-phenylalanine, fitted with a multicomponent Langmuir isotherm [73].

Fig. 5-19. Cascaded system for countercurrent 99 %+ separation of racemic mixtures [75].

From the foregoing it will be obvious that a single-stage MEUF system will not
be capable of producing single enantiomers at high purity. Therefore, to achieve high
optical purities, a multistage separation process is required (Fig. 5-19) [75]. This
system is operated in a countercurrent mode, analogous to conventional extraction
and distillation processes. Here, the enantioselective micellar phase flows in oppo-
site direction of the water phase. In each stage a UF membrane separates the micel-
lar phase from its surrounding aqueous phase. To regenerate the saturated micelles
leaving the cascade, a simple decrease in pH leads to electrostatic repulsion between
selector and bound enantiomer [76]. Using the experimental Langmuir constants, the
process has been modeled in a cascade of 60 stages [77]. When the racemate is fed
to stage 34, the flows leaving stages 1 and 60 have an enantiomeric excess of 99.1 %
and 99.8 %, respectively (Fig. 5-20). An industrially important feature of the process
comes from the fact that the feed is diluted by a factor of only 3.