Page 353 - 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

288 Membranes, Synthetic, Applications

In certain cases, the separation medium of a membrane actions (i.e. affinity separation) (q.v.). These applications
is a liquid that is immiscible with the feed stream. The call for a high internal surface area in the membrane, such
very high permeability of liquids relative to solid materials as that afforded by a finely porous, open-cell morphology.
offers a productivity advantage. Usually the selectivity of Membrane thickness does not affect productivity directly
liquids derives from differential partitioning of permeants. insuchcases.Indeed,thickermembranesmaybepreferred
Liquids may also be used as a solvent for specific com- because they permit longer residence times for more com-
plexing agents that do not form membranes themselves. plete reaction or capture of target species, so long as the
Finally, transient deposits of colloids can be used as selec- flow of reactants and products is not unduly hindered.
tive barriers in the so-called dynamic membranes, which Two common membrane geometries are flat-sheet and
offer very high productivities when moderate degrees of tubular (including hollow fibers). Flat-sheet membranes
separation are adequate. are made by casting, coating, or extrusion. A nonwo-
ven fabric backing is often used to provide mechanical
reinforcement. Tubular and hollow-fiber membranes are
B. Membrane Structure and Geometry
made by spinning or extrusion, depending on diameter.
For membrane separation processes, productivity is of- Inducing phase separation in a polymer solution—either
ten measured in terms of permeation flux. High fluxes thermally or by controlled mixing with a nonsolvent—
are achieved by using thin membranes. The invention and typically forms the microporous structure. Liquid mem-
widespread use of several types of membranes with sub- branes are either microdroplets in the form of emulsions
micron separation layers is largely responsible for the prepared and handled by liquid–liquid extraction equip-
phenomenal growth of applied membrane technology. In ment, or immobilized in a porous support to assume a
“asymmetric” membranes, the structural density changes stable physical form.
from one surface of the membrane to the other, with the
part of highest density being the functional separation C. Membrane Modularization and Packaging
layer. “Composite” membranes have a multilayered con-
struction: a thin separation barrier supported by a rela- Synthetic membranes are delicate and fragile by nature.
tively thick, nonselective substrate. Both types of mem- There are instances in which individual sheets of mem-
branes are used extensively for industrial separations of brane are used in holders or housings, particularly in a
low-molecular-weight substances. laboratory setting. Careful handling and controlled en-
Another means of classifying membranes is accord- vironments are essential to protect the membrane from
ing to their ability to retain substances of different sizes. damage or contamination.
Some membranes are capable of size discrimination at Independent of which type of membrane is being used,
the molecular level—for example, with gases or liquids— a large amount of membrane area must be accommodated
while others exhibit selectivity toward particles of mi- in an efficient system. Since compactness is important,
croscopic dimensions. As will be shown in the following clever designs have evolved to incorporate large amounts
sections, membrane processes in conjunction with appro- of membrane area in efficient modules. Virtually all mem-
priate membranes can achieve separation over a broad size branes used industrially are packaged as modules. Packag-
spectrum. ing also protects the membranes from damage, and facili-
“Homogeneous” membranes have a uniform structure tates changes in capacity by changing the number or size
(even if they are microporous or nanoporous) throughout of devices. Secondary factors such as the need to control
their thickness. Membranes used as depth filters generally external phase fluid dynamics are sometimes important
have this structure. They are also preferred when the appli- in practical module selection when phenomena known as
cation calls for membranes with a nondirectional charac- concentration polarization and fouling must be dealt with.
ter,asinelectrodialysis(q.v.),whenthematerialisdifficult (see Section B.1b). Flat-sheet membranes may be pack-
to fabricate into asymmetric or composite membranes, or aged as spiral-wound elements or pleated cartridges, or
when high fluxes are not important, as in controlled re- used in single sheets in plate-and-frame modules. Tubular
lease. Table IV lists separation membranes by materials and hollow-fiber membranes are usually formed into bun-
and structural features. dles secured by potted tube sheets at one or both ends and
Membranes used in nonseparation applications may housed in a cylindrical shell. Some common commercial
have special structural requirements. Examples are mem- module designs are shown in Fig. 5.
branes that serve as flow-through chemical reactors (q.v.), The choice of a preferred module design is determined
in which reactants are converted to products by contact by technical and economic factors specific to each appli-
with catalysts inside the pores of the membrane, or as a cation. Two key variables govern cost: the productivity
reversible adsorption matrix based on biospecific inter- per unit membrane cost, and the life expectancy of the