Page 169 - Chiral Separation Techniques

5.4 Concluding remarks 147

Fig. 5-20. Enantiomer separation in a cascaded ultrafil-
tration (UF) system of 60 stages [77]

Although the basic principles underlying this MEUF process for chiral separa-
tions have been established, no optimization has yet been performed. Based on the
ability of UF to treat large streams relatively inexpensively, this process will poten-
tially become an extension to the rather limited range of large-scale commercial sep-
aration processes. In addition, to facilitate further purification (e.g. by diastere-
omeric salt crystallization) a single-stage MEUF process can potentially serve as an
effective low-grade separation method.

5.4 Concluding remarks

In this chapter, we have discussed the potential of membrane technology for chiral
separations. As the focus of this chapter has been on large-scale separations, the var-
ious developments have been discussed from an engineering point of view. A signif-
icant effort is being put into the development of chiral polymer membranes by a
number of research groups. Nevertheless, these membranes still are in an early stage
of development, and for industrial use significant improvements on flux and/or enan-
tioselectivity are required. Once these limitations have been overcome, the applica-
tion of chiral membranes will be similar to reverse osmosis and ultrafiltration, which
will allow a relatively rapid implementation. Similarly, a large number of chiral
selectors have been developed for application in liquid membranes. A major issue
for the application of liquid membranes on an industrial scale lies in the problem of
staging. As a result of the many recycles in the process, selective liquid membranes
will require a large number of storage vessels, which is undesirable from a process
economics standpoint.
In the short term, we do not expect chiral membranes to find large-scale applica-
tion. Therefore, membrane-assisted enantioselective processes are more likely to be
applied. The two processes described in more detail (liquid–membrane fractionation
and micellar-enhanced ultrafiltration) rely on established membrane processes and
make use of chiral interactions outside the membrane. The major advantages of these