Page 134 - Adsorbents fundamentals and applications

CARBON MOLECULAR SIEVES 119

of heating rate and the final temperature and time, as well as the inert gas used
versus vacuum. The ideal membrane has pore sizes in the range between 3 to 7 ˚ A
for the separation of the permanent gas molecules (e.g., H 2 ,O 2 ,N 2 ,Ar, CH 4 ,
and CO 2 ). A narrow range in the pore size is needed for a specific separation.
The ideal membrane also needs to have a uniform thickness of about 10–30 µm
and is mechanically strong and crack-free.
Jones and Koros (1994a) prepared CMS membranes by temperature-program-
med pyrolysis of an asymmetric hollow-fiber polyimide membrane. The heating
program involved decreasing heating rates until the final temperature of 500
◦
or 550 C was reached. Pyrolysis under vacuum was found to be better than
pyrolysis with inert gas purge. The membrane thickness was 30–35 µm. They
measured binary mixture separation properties with a single-fiber permeation
◦
system by using a CMS membrane with a diameter of 170 µm. The 500 C
pyrolysis protocol produced membranes with O 2 /N 2 selectivities ranging from
◦
8.5 to 11.5, with O 2 fluxes of 20 to 50 GPU (gas permeation units). The 550 C
pyrolysis protocol produced membranes with O 2 /N 2 selectivities ranging from 11
to 14, with O 2 fluxes of 15 to 40 GPU. The separation factor is defined as:

α AB = (Y A1 /Y B1 )/(Y A2 /Y B2 ) (5.9)

where α AB is the separation factor for component A and B, Y is mole fraction, and
subscripts 1 and 2 indicate downstream (1) and upstream (2). The gas permeation
unit (GPU) is defined as:
−7
−6
2
2
GPU = 10 cc(STP)/[s − cm − cmHg] = 3.35 × 10 gmol/[s − m − kPa]
(5.10)
The separation properties of these CMS membranes were compared with a very
good asymmetric polysulfone membrane for air separation (Jones and Koros,
1994a). The polysulfone membrane has an O 2 /N 2 selectivity ranging from 5.5 to
6.2, with an O 2 flux between 20 to 30 GPU. Hence the CMS membrane has better
separation properties. Jones and Koros also measured the separation properties
of these CMS membranes for CO 2 /N 2 ,CO 2 /CH 4 ,and H 2 /CH 4 and showed that
the CMS membranes outperformed the traditional polymeric membranes.
The effects of the pyrolysis conditions on the separation properties of the
CMS membranes from polyimides were further studied by Geiszler and Koros
◦
(1996). It was shown that by raising the pyrolysis temperature from 500 to 800 C,
the effective pore size was reduced, thereby making the CMS membranes more
selective but less productive (i.e., lower permeation fluxes). Vacuum pyrolysis
versus that in inert gases had the same effect as raising the temperature.
Drawbacks of the CMS membranes from hollow-fiber polymeric membranes
have also been reported (Jones and Koros, 1994b; Jones and Koros, 1995).
These membranes were vulnerable to organic vapors as well as moisture. These
molecules seemed to adsorb strongly in the pores and reduced the fluxes of the
permanent gas molecules. Membrane deterioration was observed with organic
contaminations at concentrations as low as 0.1 ppm. However, a promising regen-
eration process was reported by the use of propylene; significant recovery of the