Page 356 - 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 291

nanofiltration membranes preserves their basic pore size
distributions. These dried membranes can be used with
gaseous streams. Indeed, ambient temperature steriliza-
tion of air is possible with a membrane that removes
particulates less than the size of a virus (∼0.1 µm). In
microelectronics and pharmaceuticals, where not only
microbes but also their fragments can cause problems, this
is obviously an advantage. In general, however, membrane
separation is applied to gas or vapor mixtures to achieve
a molecular separation between the stream components.
An even wider diversity of mechanisms can effect
molecular level separations of gases and vapors as com-
pared to liquid mixtures. The simplest approach involves
applying a transmembrane mixed gas pressure across a
membrane. Depending upon the structure of the mem-
brane, this process may or may not cause separation of the
copermeatingcomponents.Forporousmembanes,thesize
of the pores relative to the mean free path of the molecules
under the conditions of the feed and permeate will deter-
mine the outcome. If the gas molecules collide preferen-
tially with each other instead of the pore wall (i.e., the pore
diameter exceeds the bulk mean free path), viscous flow
applies, and no separation occurs. On the other hand, if
the mean free path between collisions in a normal bulk gas
phase of equal pressure exceeds the pore size of the mem-
brane, separation occurs. This process, termed “Knudsen
diffusion,” is promoted by operation at low pressures or by
using membranes with small pores at elevated pressures.
The more rapidly moving low molecular weight gas exe-
cutes more frequent diffusional steps, since it hits the wall
more frequently. The ratio of wall collisions in this limit
scales with the inverse square root of penetrant molecu-
lar weight. Therefore, the Knudsen selectivity equals the
inverse square root of the molecular weight ratio of the
largest to smallest gas (Koros and Pinnau, 1994). This
principle was used for isotope enrichment on the Manhat-
FIGURE 5 (continued )
tan Project, but it is uneconomical for commercial sepa-
ration applications.
in productivity. Membranes are packaged in the form of
multiwell plates designed for automated equipment and
methods of analysis (Fig. 6b). B. Practical “Contender” Membranes
Specialty membrane devices used as sensing elements for Gas and Vapor Separations
and electrode components are often built permanently
Besides Knudsen diffusion, permselective transport of
into instruments. Diagnostic or medical devices are often
gases can occur by various mechanisms involving molecu-
single-use disposable items.
lar scale interactions of the sorption-diffusion type. These
can be broadly classified into three groups as described
below and pictured in Fig. 7.
III. GAS SEPARATIONS
A. Overview of Separation Processes 1. “Simple” Sorption-Diffusion Mechanism
Involving Gases and Vapors
The sorption-diffusion mechanism considers that some
As one considers gas and vapor feeds instead of liquids, thermally agitated motions (either in the matrix or by the
new issues emerge. Carefully drying of micro-, ultra-, or penetrant) provide opportunities for sorbed penetrants to