Page 160 - Chiral Separation Techniques

138 5 Membranes in Chiral Separations

Fig. 5-9. Total number of stages and total membrane surface area versus membrane selectivity for the
–2
–1
-2
separation of 1 kg s of a racemic mixture at a membrane permeability of 1.6 × 10 kg m .s, yielding
both enantiomers at 95 % purity [55].

5.3 Membrane-Assisted Chiral Separations

One of the major advantages of membrane processes for the separation of racemic
mixtures lies in the ease of scale-up. As scale-up procedures are well established,
this enables the implementation of membrane processes for separations on a multi-
kilogram scale. From the foregoing sections, it will be obvious that enantioselective
membranes are still in an early stage of development, and large-scale applications
are not expected in the short term [5]. Nevertheless, enantioselective membranes do
have considerable potential for large-scale separations because of the existing expe-
rience with techniques such as reverse osmosis and ultrafiltration. When looking at
membrane processes for chiral separations which have found full-scale implementa-
tion, or which are close to this, the range is clearly limited to membrane-assisted pro-
cesses.
Nonselective membranes can assist enantioselective processes, providing essen-
tial nonchiral separation characteristics and thus making a chiral separation based on
enantioselectivity outside the membrane technically and economically feasible. For
this purpose several configurations can be applied: (i) liquid–liquid extraction based
on hollow-fiber membrane fractionation; (ii) liquid– membrane fractionation; and
(iii) micellar-enhanced ultrafiltration (MEUF).