Page 386 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

Membranes, Synthetic, Applications 321

FIGURE 33 An ultrahigh-purity water system for semiconductor manufacture. (Nitto Electric Industrial Co., Ltd.)

E. Membrane Extraction Processes periods of time. In extreme cases an emulsion may form
that is indefinitely stable. Whenever phase separation is
Membrane extraction encompasses a class of liquid-phase
incomplete, there is entrainment loss of one solution in
separations where the primary driving force for trans-
another. In addition to low overall separation efficiency,
port stems from the concentration difference between the
valuable products or extracting agents are lost.
feed and extractant liquids rather than a pressure gradi-
Using a solid, microporous membrane to define a sta-
ent, as is the case with most of the other processes dis-
tionary phase boundary during extraction may alleviate
cussed above. A microporous membrane placed between
this problem. The feed solution and the extractant flow
the feed and the extractant liquids functions primarily as
a phase separator. The degree of separation achievable is
determined by the relative partition coefficients among
individual solutes. This operationx is known as “mem-
brane solvent extraction.” If a nonporous, permselective
membrane is used instead, however, the selectivity of the
membrane would be superimposed on the partitioning se-
lectivity; in this case the process may be referred to as
“perstraction.” These “process” concepts are illustrated in
Fig. 34.

1. Membrane Solvent Extraction/Membrane
Contactors
Conventional liquid–liquid extraction is an established
unit operation for transferring one or more solutes from a
solution into a second, immiscible liquid. It is widely used
for separating ionic and nonionic species, for example,
on the basis of their preferential partitioning between an
aqueous phase and a nonaqueous phase, respectively. In-
dustrial liquid–liquid extraction equipment generally con-
sists of a mixer, where the feed solution and the extrac-
tant liquid are intimately mixed via agitation, and a settler
where the equilibrated phases are separated for further
processing.
Phase separation may or may not occur spontaneously
after mixing. If surface-active species are present, for ex- FIGURE 34 Membrane extraction processes: (a) membrane sol-
ample, the mixed phases may remain dispersed for long vent extraction and (b) perstraction.