Page 361 - 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

296 Membranes, Synthetic, Applications

free-volume distribution, and the temperature (Koros and The above discussion raises a question regarding the
Hellums, 1989; Prasad, Notaro, and Thompson, 1994; degree to which the selectivity of polymer membranes to
Kesting and Fritzche, 1993; Gas Processors, 3132-84). specific gas pairs can be enhanced by structural modifica-
The diffusion coefficients of the components of a gas tions without significant loss in permeability. The ques-
mixture may also depend on composition. The solubil- tion posed is whether the lines in selectivity/permeability
ity coefficients depend primarily on unrelaxed volume plots, such as the dashed lines in Figs. 8 and 9, have an
in the polymer, the penetrant condensabiltiy, and to a upper limit. The consensus of these analyses (Singh and
lesser degree upon penetrant–polymer interactions (Spill- Koros, 1996; Park and Paul, 1997; Robeson, Smith, and
man, 1989; Zolandz and Fleming, 1992; Koros and Hel- Langsam, 1997; Alentiev, Loza, and Yampol’skii, 2000;
lums, 1989). Therefore, the permeability and selectivity Freeman, 1999) generally support the preceding quali-
coefficients depend on all of the above factors in view tative conclusions noted above that such an upper bound
of Eq. (14), but the overall selectivity of glassy polymer does exist for each gas pair using polymers that can be pro-
membranes depends mainly on the diffusivity selectivity. cessed by conventional solution casting methods. Specifi-
This diffusivity selectivity can vary by an order of mag- cally, it appears that the segmental flexibility of polymeric
nitude or more depending on the nature of the membrane membranes that makes them economical to prepare, in
and of the gas pair under consideration. The diffusivity fact, limits their size and shape discriminating ability for
selectivity, and hence the “sieving” ability, of glassy poly- similarly sized penetrants.
mer membranes is significant even when the difference in Molecular sieving materials are an alternative to poly-
the sizes of penetrant molecules is very small. For exam- mers. Like glassy polymers, such media rely primarily on
ple, the “kinetic” diameters of O 2 /N 2 pair differ by only differences in molecular size to achieve separation, but
˚
˚
0.18 A (3.46 vs 3.64 A; Koros and Hellums, 1989) while the detailed diffusion step is rather different in the two
˚
the He/CH 4 pair shows a “large” difference of 1.2 A in cases. Molecular sieve membranes are ultramicroporous,
kinetic diameters. with sufficiently small pores to exclude some penetrants
Fractional free volume, comprised of the average un- while allowing others to pass through (Fig. 7B). These
occupied space within the polymer matrix, are the most rigid membranes show extremely attractive permeation
commonly used parameters for correlating permeabilities, performance (Morooka and Kusakabe, 1999; Tsapatis and
and as noted earlier, group contribution methods exist to Gavalas, 1999) and maintain stability when exposed to ad-
assist in such estimations (Park and Paul, 1997; Robeson, verse conditions (high temperature, pressure, highly sorb-
Smith, and Langsam, 1997). Unlike rigid glassy polymers, ing components) that can cause polymeric membranes
rubbery polymers have a low ability to discriminate be- to plasticize. Under ideal conditions, minimum effective
tween penetrant molecules of different sizes and shapes, thickness layers similar to those achievable with poly-
due to the high segmental mobility of such polymers. As a meric membranes (∼0.05–0.2 µm) can be obtained with
result, the overall selectivity of such membranes to differ- some molecular sieving materials. Unfortunately, such
ent gases is controlled mainly by the solubility selectivity. membranes are difficult to process, are fragile, and ex-
The solubility of gases in polymers commonly increases pensive to fabricate into modules; thus, they are not com-
with increasing critical temperature, T c , of the penetrant mercially significant today except in niche applications.
gases—hence, the solubility selectivity of rubbery poly- As noted above, glassy polymers and molecular siev-
mer membranes to a gas pair will be larger the greater the ing materials preferentially permeate the smallest compo-
difference in the T c of the two gases. Therefore, rubbery nent in a mixture compared to larger sized components
polymer membranes are well suited for the separation of in the mixture. In certain separations, it may be advan-
easily condensable organic vapors with high T c ’s from tageous to permeate the larger sized penetrant and retain
light gases with low T c ’s, such as the components of air. the smaller component. These separations can be poten-
The solubility selectivity of a membrane for a specific tially achieved using “surface selective flow” membranes
gas pair could be increased (in principle) by inducing spe- (Rao and Sirkar, 1993, 1997). While rubbery polymers
cific interactions between the polymer and the more solu- show this property, the selectivity achievable is generally
ble component of the gas pair. For example, the substitu- not impressive except when comparing a highly conden-
tion of certain polar groups in some rubbery polymers has sible component and a supercritical gas like air. On the
other hand, uniformly nanoporous membranes have been
been found to increase their solubility selectivity for CO 2
relative to CH 4 (Story and Koros, 1991; Koros, 1985). reported that show a high degree of such “reverse selec-
Unfortunately, the increase in the polarity of a polymer tivity.” These nanoporous materials work by the selective
also tends to increase its chain packing density, and as a adsorption of the more strongly adsorbing components
result, decreases the gas diffusivity in membranes made on to the pore surface followed by surface diffusion of
from that polymer. the adsorbed molecules across the pore (Fig. 7C). The