Page 388 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Membranes, Synthetic, Applications 323

the target substances. The emulsion is coalesced chemi-
cally or electrostatically to release the encapsulated liquid
and to recycle the liquid membrane constituents.

2. Immobilized Liquid Membranes
An immobilized liquid membrane is formed by impreg-
nating a microporous support with an extractant liquid.
The liquid is held in place by capillarity and assumes the
flat-sheet or hollow-fiber geometry of the host membrane.
Immobilized liquid membranes can be used for virtually
all the liquid-phase separations achievable with emulsified
liquid membranes, but offer several important benefits.
There should be no entrainment loss because no mixing
occurs. Also, extraction and stripping of target species oc-
cur simultaneously on the upstream and downstream sur-
faces of an immobilized liquid membrane, respectively.
The size of the receiving phase can thus be virtually un-
limitedbycontinuallyregeneratingandrecyclingthestrip-
ping solution. Hollow-fiber devices may be used to favor
a high packing density of contact area between the im-
miscible phases. Finally, because it is supported in a solid
matrix, an immobilized liquid membrane is applicable to
the separation of gases and vapors.

3. Facilitated Transport and Coupled Transport
It is possible to achieve very high selectivities by incorpo-
rating complexing agents or carriers in immobilized liquid
FIGURE 35 Emulsified liquid membrane separation mecha-
membranes. These agents may be liquid ion exchangers or
nisms: (A) selective permeation; (B) chemical reaction inside
emulsion droplet; and (C) chemical conversion in liquid membrane chelating polymers; they form reversible complexes with
and further conversion inside droplet. Both (B) and (C) provide the target species on the feed side of the membrane and
quasi-infinite sink conditions for extraction from the feed solution.
release those species by dissociation on the downstream
side. As the overall selectivity of this process depends
system, bears much resemblance to conventional liquid– on the specificity of chemical recognition—sometimes at
liquid extraction systems. The liquid membrane formed is low concentration and often in the presence of interfer-
mixedwiththefeedwatertoallowextractiontooccur,then ing species—much effort has been focused on develop-
decanted off after the liquid membrane is saturated with ing sophisticated complexing agents such as macrocyclic

FIGURE 36 An emulsified liquid membrane wastewater treatment process.