Page 359 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GLQ Final Pages
Encyclopedia of Physical Science and Technology EN009K-419 July 19, 2001 20:57

294 Membranes, Synthetic, Applications

penetrants near the permeating penetrant. Temperature penetrants (Singh and Koros, 1996). Moreover, the loss
is also an important factor which activates the diffusion in performance stability at high temperature, at high pres-
jumps and moderates the thermodynamic interactions be- sure, and in the presence of highly sorbing components
tween the sorbed penetrants and the matrix. limits the wider scale use of these otherwise versatile
The separation factor for component A vs B, α AB ,is membranes.
defined in terms of the downstream and upstream mole The values of permeability coefficients for He, O 2 ,N 2 ,
fractions (Y) of components A and B: CO 2 , and CH 4 in a variety of “dense” (isotropic) polymer
α AB = [Y A1 /Y B1 ]/[Y A2 /Y B2 ]. (13) membranes and the overall selectivities (ideal separation
factors) of these membranes to the gas pairs He/N 2 ,O 2 /N 2 ,
Under ideal conditions with a negligible downstream pres-
◦
and CO 2 /CH 4 at 35 C have been tabulated in numerous
sure of both components, the separation factor can be
reviews (Koros and Hellums, 1989; Koros, Fleming, and
equated to the ideal membrane selectivity factored into
Jordan et al., 1988; Koros, Coleman, and Walker, 1992).
its mobility and solubility controlled contributions, viz.,
Moreover, several useful predictive methods exist to allow

D A S A estimation of gas permeation through polymers, based on
∗
α AB = P A /P B = . (14)
D B S B their structural repeat units. The values of the permeability
mobility solubility coefficients for a given gas in different polymers can vary
controlled controlled by several orders of magnitude, depending on the nature of
factor factor
thegas.Thevaluesoftheoverallselectivitiesvarybymuch
For a defect-free ideal membrane, the selectivity is in- less. Particularly noteworthy is the fact that the selectivity
dependent of thickness, and either permeability ratios or decreases with increasing permeability. This is the well-
permeance ratios can be used for comparison of selectivi- known “inverse” selectivity/permeability relationship of
ties of different materials. Nonideal module flow patterns, polymer membranes, which complicates the development
defective separating layers, impurities in feeds, and other of effective membranes for gas separations.
factors can lower the actual selectivity of a membrane Typically, membranes with high gas permeabilities and
compared to tabulated values based on ideal conditions a low selectivities are comprised of “rubbery” polymers,
(Koros and Pinnau, 1994). i.e., T g < T , where T g is the glass-transition temperature
Currently, all commercial gas and vapor separa- of the polymer and T is the temperature at which the per-
tion membrane are either glassy or rubbery poly- meability is measured. Rubbery polymers are character-
mers (Spillman, 1989; Puri, 1996; Meindersma and izedbyhighintrasegmentalmobility,whereasglassypoly-
Kuczynskyi, 1996). Glassy materials generally derive per- mers exhibit the opposite characteristics. An interesting
mselectivity from their ability to separate gases based on exceptiontothisruleispoly[1-(trimethylsilyl)-1-propyne]
subtle differences in penetrant size with minor contribu- (PTMSP), which is a rigid glassy polymer but nevertheless
tions from the solubility controlled term. Rubbery mater- exhibits the highest intrinsic gas permeability of all known
ials, on the other hand, generally derive permselectivity synthetic polymers. The high permeability of PTMSP has
from favorable solubility selectivity with minor contri- been found to be due to an exceptionally large free volume
butions from the mobility term. In both cases, transport that appears to provide a system of interconnected microp-
˚
is postulated to occur upon the creation, next to the orous domains of about 5–15 A in size within the PTMSP
penetrant molecule, of a transient gap of sufficient size matrix (Stern and Koros, 2000). This material, therefore,
to accommodate the penetrant, thereby permitting a dif- appears to border on nanoporosity in its properties.
fusion step (Fig. 7A) (Koros and Hellums, 1989). These The above-mentioned “inverse” selectivity/permeabi-
transient gaps form and fade throughout the polymer due lity relationship of polymers has been summarized by
to thermally induced motions of the polymer chain seg- Robeson by means of log–log plots of the overall selectiv-
ments. Polymeric membranes tend to be more economical ity versus the permeability coefficient, where A is consid-
than other materials and thus dominate traditional gas ered to be the more rapidly permeating gas. These plots
separations. The low cost of polymeric membranes were made for a variety of binary gas mixtures from the list
results from their ability to be easily formed into hollow He, H 2 ,O 2 ,N 2 ,CO 2 , and CH 4 , and for a large number of
asymmetric fibers or spiral wound modules, due to rubbery and glassy polymer membranes. Such represen-
their segmental flexibility and solution processability. tations, shown in Fig. 8 and Fig. 9 are often referred to as
Extremely thin (less than 0.1 µ) separating layers (Fig. 2) “upper bound” plots (Robeson, 1991). The “upper bound”
are currently achievable with such materials (Zolandz and lines clearly show the “inverse” selectivity/permeability
Fleming, 1992). The segmental flexibility of polymeric relationship of polymer membranes. While these plots
membranes that makes them economical to prepare, in were prepared in 1991, only small advances have been
fact, limits their discriminating ability for similarly sized made to push the upper bound higher since that time.