Page 136 - Adsorbents fundamentals and applications

CARBON MOLECULAR SIEVES 121

A promising development in CMS membrane separation is the “selective
surface flow” carbon membranes, by Rao, Sircar, and co-workers (Rao et al.,
1992; Rao and Sircar, 1993a; Rao and Sircar, 1993b; Rao et al., 1994). Three
types of fluxes occur in pores: convective flux, Knudsen diffusion flux, and flux
by surface diffusion. Surface diffusion has been reviewed and discussed exten-
sively by Kapoor et al. (1989). When the surface concentration is high (e.g.,
for a strongly adsorbed component at a high concentration) or when the other
fluxes are low (e.g., in liquid phase), surface diffusion can become the dominant
flux for pore diffusion. Furthermore, when the pore dimension is about twice
that of the strongly adsorbed molecule, the (back-to-back) adsorbed molecules
effectively reduce the pore opening, thereby eliminating or hindering the fluxes
for the other non-adsorbed or weakly adsorbed molecules. The net result is
enhanced separation.
This concept of “selective surface flow” was first proposed in the 1950’s
when the effect one adsorbed species had on blocking the flux of another species
was lively discussed in the literature (e.g., Kammermeyer and Wyrick, 1958).
Kammermeyer and Wyrick (1958) studied the separation of propane and carbon
dioxide by a plug of porous glass, and enrichment factors as high as 100 for
propane were achieved. The separation was caused by differences in surface
fluxes because the gas-phase fluxes were about equal. However, the pores in
the porous glass were not small enough to achieve complete blockage by the
adsorbed molecules. This was achieved subsequently by Ash et al. (1963). Ash
et al. studied separations of four binary mixtures (H 2 /SO 2 ,N 2 /CO 2 ,Ne/CO 2 ,and
A/N 2 ) by a plug of carbon black powder that was highly compacted. They used
the case of H 2 /SO 2 for discussion, that the flux of H 2 would be enriched if the
carbon powder was not highly compacted. When it was highly compacted, the
enrichment was reversed, that is, SO 2 was enriched due to surface flow. Their
results are shown in Figure 5.28. The flux of H 2 was reduced to nearly 20% when
the relative saturated adsorption of SO 2 reached 20% and was completely blocked
at 60% SO 2 adsorption. Similar results were obtained for other mixtures. This
separation principle was subsequently demonstrated and studied by Barrer, Ash
and co-workers, as well as others-including Okazaki, Toei, and their co-workers
(e.g., Ash et al., 1973).
Successful application of this principle for separation depends on the fabrica-
tion of membranes with controlled and uniform pore sizes. This was done by Rao,
Sircar, and co-workers by controlled pyrolysis of PVDC supported on macrop-
orous alumina tubes. A schematic of their CMS membrane and the principle of
separation by the membrane is shown in Figure 5.29. The membrane has pores
of uniform sizes in the range between 5–10 ˚ A. The pore sizes can be tailored
for different separations.
Golden et al. (1998) described a demonstration unit with seven CMS/alumina
tubes (3.5-ft long) mounted in a shell-and-tube type housing. The feed mixture
was passed through the bore of the membrane, and the permeate was with-
drawn counter-current to the feed. Recovery of H 2 from H 2 /hydrocarbon mixtures
was first demonstrated. The H 2 product is produced at a high pressure, which