Page 354 - 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 289

TABLE IV Membrane Classification
Separation Methods of Typical
material Structure Morphology Geometry fabrication applications
Polymers Homogeneous Microporous Flat-sheet, tubular, Phase-inversion casting Microfiltration, membrane
hollow fiber or spinning, sintering, distillation, affinity
track-etching, biaxial separation
stretching, anodizing
Nonporous Flat-sheet, Extrusion, casting Diaysis, electrodialysis,
hollow fiber, controlled release
Asymmetric Microporous Flat-sheet, tubular, Phase-inversion casting Microfiltration, ultrafiltration,
hollow fiber or spinning membrane reactors
Nonporous, Flat-sheet, tubular, Phase-inversion casting Reverse osmosis, gas
skinned on hollow fiber or spinning separation, pervaporation,
microporous perstraction, membrane
substrate reactors
Composite Nonporous barrier Flat-sheet, Direct coating, interfacial Reverse osmosis, gas
on microporous hollow fiber polymerization, plasma separation, perstraction
substrate polymerization
Inorganic Isotropic or Microporous Tubular, Sol-gel inversion, sintering, Microfiltration, ultrafiltration,
(ceramic, asymmetric multichannel calcining, anodizing, membrane reactors
metal, carbon) monolithic carbonizing of polymeric
precursors
Liquid Continuous Liquid immobilized Flat-sheet, Impregnation Membrane extraction, gas
in microporous hollow fiber separation, coupled
substrate transport
Emulsion Micellar Microdroplets Single- or multistage Emulsified liquid membrane
emulsification extractions
Colloidal Transient Colloidal barrier Tubular Formed in place during Ultrafiltration, reverse
(dynamic) gel-like layer on porous operation osmosis
coating substrate
Gas Continuous Gas trapped in Flat sheet, Formed-in-place Recovery of volatile
microporous by hollow fiber substances from liquids
external liquid

membrane device. Cost decreases as processing capacity stacks readily accept high flow rates; they are also more
per module increases. This consideration favors devices conveniently cleaned by mechanical means or by disas-
with high packing density, or large membrane area per sembly. However, these configurations provide relatively
unit module volume. Another consideration is to prolong low membrane area per unit module volume. Spiral-
the useful life of the module, hence reducing the frequency wound modules have a higher packing density. Capillary-
of membrane replacement. Membrane lifetime is affected like hollow fibers are prone to fouling, but they offer
mainly by the interaction between the membrane and the the highest packing density. Hollow fibers with internal
feed material, and by operating conditions that control the diameters from about 0.1 to 1 mm combine relatively
rate of reversible and permanent performance degrada- high packing density and the flexibility of lumen- or
tion. All membrane modules have finite lifetimes and ulti- shell-side feed at moderate flow rates.
mately require replacement. Some of them are designed to Special considerations apply to the design of products
be disposable devices intended for single use; their values intended for single use but to allow analysis to be con-
are less dependent on length of service than the need to ducted rapidly and on a relatively small scale. For exam-
maintain a consistent level of performance. ple, a membrane may be packaged in a small holder that
High flow rates across the membrane surface help re- attaches to the tip of a syringe to filter milliliter quanti-
duce the accumulation of solutes rejected by the mem- ties of solution. Centrifuge tubes may have a membrane
brane (referred to as concentration polarization) and impu- partition built in so that biological samples may be sepa-
rities lodged on the membrane surface (i.e. “fouling”). rated or rinsed with buffer solution as a part of centrifu-
(See Section 1bii.) Tubular membranes and flat-sheet gation (Fig. 6a). With the advent of biotechnology, large-
membranes installed in thin channel plate-and-frame scale screening procedures demanded dramatic increases