Page 51 - Adsorbents - fundamentals and applications

36 SORBENT SELECTION: CRITERIA

Purge by Strong Adsorptive. As discussed above, under Co-Current Purge,
the feed gas stored in the void space of the saturated bed severely limits the
separation performance, especially for the product purity of the strong adsorp-
tive — hence the product recovery for the weak adsorptive. The CD step can
improve the separation. An illustrative example was given for the separation of a
50/50 hydrogen/methane mixture on activated carbon (Yang and Doong, 1985).
Using the Skarstrom cycle, the highest product purities were approximately 99%
for hydrogen and 80% for methane. With the addition of the CD step in the cycle,
the methane product purity was increased to approximately 90%. A more drastic
modification of the cycle must be made in order to further increase the methane
product purity.
A more effective method is to purge out the void spaces, after adsorption, by
using the strong adsorptive gas. The idea was first suggested in a patent to Tamura
(1974). In this cycle, a high-pressure purge step, in which the strong adsorptive
gas is used, follows the high-pressure adsorption step in the Skarstrom cycle.
The purge is conducted co-current to the feed direction.
There are two concentration wavefronts during co-current purge by the strong
adsorptive: (1) a zone filled with the pure strong adsorptive near the feed end;
(2) a displaced zone, which was the zone saturated with the feed mixture prior to
purge. Owing to the favored isotherm of the strong adsorptive, the two wavefronts
are both the self-sharpening type and result in good separation.
A disadvantage of the PSA cycle is that compression is required for the strong
adsorptive gas, which is available from the PSA process at a low pressure. There
is, however, room for optimization between the CD and strong adsorptive purge
steps. An optimal operation should exist in which the purge is performed after
some CD. During CD, the weak adsorptive is eluted from the saturated zone
(Yang and Doong, 1985), and the bed is enriched in the strong adsorptive. More
importantly, with the CD step, the bed pressure is reduced and hence lowers the
energy requirement for compressing the purge gas. A combination of these two
steps apparently has not been used.

Recent/Current Developments and Future Prospects for PSA
Simultaneous Purification and Sorbate Enrichment. The use of PSA as a tool
for enrichment gas been studied only recently (Ritter and Yang, 1991; Kikkinides
and Yang, 1991; Kikkinides and Yang, 1993; Kikkinides et al., 1993; Chue et al.,
1995; Chue et al., 2000). These were studies for air purification and concen-
tration in a variety of sorbates including CO 2 ,SO 2 , and gasoline vapors. The
basic PSA cycles were typically: (1) feed pressurization; (2) high-pressure feed;
(3) countercurrent evacuation; and (4) countercurrent vacuum purge. Evacuation
during desorption is critically important for enrichment, while the high pressure
is not important. An optimal purge/feed ratio is needed for the highest enrich-
ment. This type of process has been developed to the pilot plant stage at Korea
Institute of Energy Research by Cho and co-workers (Cho, 2001).
A recent study for enrichment was performed by Hirose et al. (Yoshida, 1998;
2000) for enriched Xe from air. A two-stage PSA employing two ideas was used