Page 385 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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320 Membranes, Synthetic, Applications

be reclaimed by smelting, or decomplexed as a concen- rapidly and irreversibly. Pore plugging is reduced with
trated solution, and regenerating the polymer for reuse. asymmetric microfilters where penetration of particulates
The pulp-and-paper industry is a larger consumer of wa- below the membrane surface is reduced. Plugging can be
ter: about 70 tons of effluent water is generated for each further decreased by operating in the crossflow mode.
ton of paper produced from wood pulp. An ultrafiltration Depending on the application, microfiltration systems
system can potentially remove organic materials and re- may be designed for crossflow or dead-end operation.
duce biological oxygen demand in the effluent stream, Fluid management is more flexible in crossflow operation,
thereby helping compliance with increasingly stringent where high shear conditions can reduce concentration
effluent discharge regulations. A specific opportunity ex- polarization and pore plugging. On the other hand, a
its in the concentration of black liquor, an alkaline solution higher recovery of the feed fluid is possible with dead-end
laden with lignin and other organics from the kraft pulping microfiltration. Dead-end operation is also preferred
process. Black liquor is concentrated at present by flash for processing shear-sensitive feed materials such as
evaporation and incinerated for its fuel value, but the heat certain biomaterials. As with ultrafiltration, the transport
generated only marginally exceeds that required for evap- properties of the membrane can be strongly affected
oration. While the ultrafiltration system may improve the by concentration polarization, fouling, and interactions
energy balance, the membrane materials must be capable between the feed stream and the membrane.
of stable operation in the hot and corrosive environment. Microfiltration membranes are treated as single-use,
Better membrane materials have gradually appeared disposable items in many clinical, analytical, and labo-
over the past several years. Ceramic, carbon, and metallic ratory-scale applications where the high value of the prod-
membranes first introduced as microfilters are now com- uct or procedure justifies frequent membrane replacement,
mercially available in the ultrafiltration pore size range and/or the risks associated with reusing contaminated
˚
(ca. 40–1000 A). They are dominating small but signifi- membranes are unacceptable. Membranes used in large-
cant markets where their thermal and chemical resistance scale industrial MF systems are more often rejuvenated at
capabilities are enabling features. For many applications, regular intervals to maximize service life.
though, the high cost of inorganic membranes still deters The largest microfiltration application is for sterile fil-
their deployment. Investment in special module housings tration, or removal of microorganisms, in the pharmaceuti-
and membrane geometries discourages replacement even cal and biotechnology industries. Owing to the high value
as performance ultimately becomes marginal, as in the of the materials being processed, MF is deployed exhaus-
case of irreversible fouling. tively and prophylactically, leading to a substantial market
size and correspondingly large revenue base. Similarly,
MF is used extensively for clarifying fermentation broths
D. Microfiltration
as a component of an overall product recovery and purifi-
MF membranes are finely porous, with nominal pore cation scheme (see Sections VI and VII).
sizes ranging between 0.01 and 5 µm. Some of these Food and beverage processing represents an expanding
membranes are isotropic, i.e., uniformly porous through- area for process-scale microfiltration. Already in place are
out their thicknesses; others have an asymmetric, graded clarification systems for wine and beer, sugar, and gelatin,
porosity structure. Yet others have more unique morpholo- replacing existing practices such as diatomaceous earth
gies. For example, track-etched membranes are character- filtration. Less attractive economically are miscellaneous
izedbystraightcylindricalporesofuniformdiameter;they waste treatment applications, for which microfiltration is
are made by irradiating thin substrates, then etching away often a sophisticated but expensive alternative.
the irradiated paths where the local chemical resistance In semiconductor manufacturing, very-large-scale-
has been reduced. Biaxial orientation of polymer films or integration(VLSI)technologyandhigh-densityintegrated
fibers produces microporous membranes with connecting circuits are made by repeated deposition of extremely
fibrils within each pore. Anodized aluminum membranes fine patterns on silicon wafers. Between process steps,
with a high density of straight, closely packed uniform the wafers are cleaned using ultrapure water. The demand
pores have also been fabricated successfully. for increasing circuit density corresponds directly to
Separation takes place in microfiltration primarily increasingly sophisticated water treatment system designs
between solids and liquids, and many established that involve multiple stages of reverse osmosis, ultrafil-
applications are simply extensions of conventional filtra- tration, microfiltration, as well as other nonmembrane
tion into a lower particle size range. (See Section I.A.) A technologies. A typical integrated water supply system
homogeneous porous membrane used as a conventional is illustrated in Fig. 33. Microfiltration of electronics
depth filter traps particles on its surface and inside the chemicals also represents a large application area within
tortuous pores. The membrane can become clogged the electronics industry.