Page 162 - Chiral Separation Techniques

140 5 Membranes in Chiral Separations

m ⋅ F
E = D e (3)
D
F r

in which m and m are the distribution coefficients of the L and D enantiomers over
L D
both phases, and F and F are the extract and raffinate flow, respectively [63]. In
e r
order to achieve complete resolution, E and E should be opposite from unity. Due
L D
to the fact that most racemic mixtures dissolve preferentially in either the aqueous
or the organic phase, this often results in extreme flow ratios. In the example of D,L-
leucine separation, the flow ratio is chosen as 4, yielding extraction factors of 1.32
and 0.64, respectively.

Fig. 5-11. Fractional extraction with gel-coated hollow fibers [57].

Fig. 5-12. Separation of d,l-leucine in hollow-fiber membrane extraction using a N-n-dodecyl-l-hydrox-
yproline solution in octanol as the enantioselective extraction liquid. The modules used were 32 cm long
and contained 96 Celgard X-20 polypropylene fibers [57].