Page 387 - 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

322 Membranes, Synthetic, Applications

on opposite sides of the membrane and contact through this principle: emulsified liquid membranes, where dis-
its micropores, where mass transfer takes place. Disper- crete encapsulated droplets serve as selective reservoirs
sion/emulsification problems are avoided since the bulk for certain species in the surrounding solution, and im-
solutions do not mix. By using membranes with high pack- mobilized liquid membranes, where a microporous solid
ing densities, e.g., hollow fibers, a large phase contact support holds the liquid as a continuous barrier between
area can be obtained per unit volume. The most com- the feed and permeate streams. Both are intimately re-
monly prescribed membranes for this purpose are poly- lated to conventional solvent extraction in the selection of
olefin hollow fibers. There are many liquid–liquid extrac- extractants and the physical chemistry of the process. As
tion applications for which membrane solvent extraction further refinements of this configuration, selective carriers
is a viable alternative or an enabling technology. Thus far, may be incorporated into the immobilized liquid to en-
however, there are few known examples of commercial- hance extraction selectivity. Processes variously referred
scale membrane solvent extraction, due mostly to the to as “facilitated transport” and “coupled transport” are
relatively high cost of membrane systems compared to examples of this approach.
mixer/settler equipment. A second reason is the lack of
suitable membranes that are solvent-resistant and have
1. Emulsified Liquid Membranes
pores small enough not to allow breakthrough of one phase
into the other under modest pressure imbalances, or slow A liquid membrane can be prepared by emulsifying an
but nonnegligible emulsification. A viable alternative is aqueous solution in an organic liquid, then adding the
polyacrylonitrile hollow-fiber membranes with pore sizes emulsion to another aqueous solution. In this way, the
normally associated with ultrafiltration membranes. With organic liquid segregates the solutions but allows selec-
their good solvent resistance and a reduced tendency for tive diffusion of solutes across it. Similarly, oil/water/oil
phase breakthrough, these membranes hold the promise type emulsions can be formed in which the liquid mem-
as a generic membrane solvent extraction tool. brane is the water encapsulation layer. Very high rates of
mass transfer can be achieved because of the large effec-
2. Perstraction
tive membrane surface area represented by the emulsion
Perstraction is a process analogous to pervaporation, ex- droplets.
cept that a liquid extractant is used instead of a partial Separation in liquid membranes can take place in sev-
vacuum or sweep gas to carry the permeate away from eral ways, as shown in Fig. 35. The simplest mecha-
the permselective membrane. The liquid extractant is re- nism (a) is selective partition of solutes from the first
generated by passage through a stripping device. In prin- aqueous phase into the encapsulating organic liquid, fol-
ciple, perstraction offers the potential of higher selectiv- lowed by selective desorption into the second aqueous
ity than those achievable by liquid–liquid extraction or phase. Dissolved hydrocarbons have been separated using
membrane solvent extraction. To maximize the effective- this approach. However, the extraction capacity of each
ness of this approach, the membrane should be chosen membrane-encapsulated droplet is limited by its size be-
such that its permselectivity is complementary or additive cause the thermodynamic activity inside the droplet can-
with the equilibrium partitioning properties of the feed not exceed that in the feed. Backdiffusion can be prevented
solution/extractant pair. In practice, with the exception of by chemically converting the extracted solute (b) so as to
ethanol–water separation, the promise of additive selec- maintain the driving force for diffusion of unconverted
tivity is not well exploited to date because of the con- solute. To extract phenol from wastewater, for example, a
siderable development effort required to optimize a given liquid membrane prepared by encapsulating sodium hy-
separation. Successful applications will likely be limited droxide solution in a hydrocarbon liquid is used. Phenol
to separations of high-value products for which the devel- reaching the sodium hydroxide is converted into pheno-
opment of a unique permselective membrane for a single late ions, which is virtually insoluble in hydrocarbons and
purpose can be justified. cannot backdiffuse into the feed solution. A similar ap-
proach can be used in general to recover organic acids that
partition readily into hydrocarbons as neutral molecules
F. Liquid Membranes
and accumulate in dissociated form in the encapsulated
Permeation through liquids is orders of magnitude faster liquid. Even more complex reaction strategies may be im-
than that through solid polymers of comparable thickness. plemented as shown in mechanism (c). At this time, how-
This rate advantage is exploited for some separations by ever, there are relatively few liquid membrane extraction
using an immiscible liquid film as the membrane to me- systems in commercial use.
diate the transport of selected substances. Two somewhat The equipment used for emulsified liquid membrane
different separation technologies have evolved based on extraction, shown in Fig. 36 for a wastewater treatment