Page 345 - 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

280 Membranes, Synthetic, Applications

Membrane reactor Device for simultaneously carrying ence principles, such as diffusion or fluid flow. By under-
out a reaction and membrane-based separation in the standing the principles controlling function, the required
same physical enclosure. structure to enable that function becomes clear. This will
be illustrated for several examples, and the broader topic
of additional physical science phenomena that are poten-
1
SYNTHETIC MEMBRANES are thin barriers that al- tially useful in future or emerging membrane processes
low preferential passage of substances on a microscopic will also be noted, even if practical commercial examples
or molecular size level. Starting with this single attribute, may not yet exist.
a broad area of science and technology has evolved over In use, most synthetic membranes involve a transport of
the past century where membrane processes are used as one or more components from an “upstream” side of the
efficient and economical methods of separation and pu- membrane to a “downstream” side. Although microscopic
rification. Today, membrane processes contribute to many interpretations differ between the various applications, de-
sectors of scientific research and development, industry, scription of the transport process for a component, A, from
medicine, and management of natural and man-made re- the upstream to the downstream side of the membrane is
sources. Many membrane applications are so deceptively possible in terms of Eq. (1):
simple that the physical science governing their use is eas-
n A = [(Driving Force) A ]/(Resistance) A = [(DF) A ]/ A ,
ily overlooked. The field is best partitioned into smaller
topical areas to understand the diverse types and uses that (1)
membranes have in nature and industry. The present article
where n A is the flux of A, equal to the rate of transfer of
is organized according to this systematic approach.
component A per unit area per unit time. The net driving
A membrane, whether naturally occurring or synthetic,
force (DF A ) acting on component A between the upstream
is taken to be a structure with a large aspect ratio in which
one of its three dimensions is much thinner than the other and downstream membrane face and the net resistance re-
two dimensions. The simplest form of a membrane is thus tarding movement of A( A ), while simple to write, may
a flat diaphragm, but the above description also applies to have complex physical chemical origins that differ greatly
hollow fiber, or even a spherical or bag-like encapsulation between the various types of membrane applications. De-
domain surrounding living cells. spite these limitations, Eq. (1) is useful to unite the dis-
cussion, since it provides a framework to understand the
essential nature of most membranes.
I. GENERAL PRINCIPLES One can devise an almost unlimited number of net driv-
ing force terms, DF A , by imposing a difference in any
The discussion of synthetic membranes can be structured intensive thermodynamic variable between the upstream
in terms of the “function” or the “structure” of the mem- and downstream membrane faces. Coupling between the
braneusedinaparticularapplication.Forinstance,onecan effects can occur, but generally one driving force, e.g.,
consider whether a membrane is used to separate mixtures pressure, temperature, concentration, or voltage, is suffi-
of gas molecules vs particles from liquids (function) vs ciently dominant in a given application to allow focusing
whether the membrane structure is primarily microporous on it primarily.
or dense (structure). In fact, function and structure are The resistance term in Eq. (1), A , usually increases
linked, but to facilitate the consideration of physical sci- directly with the membrane thickness, so reducing
ence issues related to membranes appropriate for this ref- thickness by some percentage generally increases flux by
erence, emphasis on functional aspects are probably most the same percentage. This generalization has some ex-
appropriate. This approach reflects the fact that the use of ceptions. For instance, reaction or complexation kinetics
a membrane generally involves one or more physical sci- within the membrane or nonhomogeneous morphologies
within the membrane can cause such exceptions in some
1 The most obvious division of the membrane world occurs between cases (Crank, 1975).
synthetic(man-made)andbiological(naturallyoccurring)materials.The
present discussion will focus only on synthetic membranes, which alone
is an enormous area. Biological membranes have been the topics of books
A. Major Membrane Application Types
and reviews (Yeagle, 1992) at least as extensive as that of synthetic mem-
branes. Despite sharing interest in the large aspect ratio nature common
To facilitate the discussion, conventional terminology
to all membranes, the two fields have developed quite separately. In any
used to refer to the most common types of membrane-
case, the physical science related to synthetic membranes is fairly well
based processes is presented in Table I along with typical
understood and provides a useful basis for understanding many aspects
of the more complex biological membrane topical area. driving forces used in each application.