Page 231 - Adsorbents fundamentals and applications

216 π-COMPLEXATION SORBENTS AND APPLICATIONS

8.4. BULK SEPARATIONS BY π-COMPLEXATION
Bulk separation/recovery of CO from synthesis gas by π-complexation has already
been commercialized worldwide since 1989. π-Complexation is highly promising
for other bulk separations such as olefin/paraffin and aromatic/aliphatic separa-
tions, either in vapor phase by PSA or in liquid phase by simulated moving bed
processes. Before discussing these processes, problems of deactivation or stability
of the π-complexation sorbents will be first addressed.

8.4.1. Deactivation of π-Complexation Sorbents
The deactivation behaviors of various π-complexation sorbents have been stud-
+
ied. Cu salts such as CuCl and Cu(I) zeolites are oxidized quickly into Cu 2+
state upon exposure to ambient air. Moisture is known to accelerate the oxida-
+
+
tion process. Ag salts and Ag -zeolites, on the other hand, are quite stable in
ambient air with minimized light exposure (Hutson, 2000).
Because H 2 and H 2 S are present in synthesis gas, cracked gases, and other
gas streams encountered in industry, their effects on the π-complexation sorbents
have been studied. The effects of exposure to 0.5 atm H 2 at various temperatures
on AgNO 3 /SiO 2 and AgY zeolite were discussed in detail by Jayaraman et al.
(2001). Severe deactivation of both sorbents occurred at temperatures above
◦
120 C. X-ray photoemission spectroscopy (XPS) studies of the deactivated sam-
0
+
ples showed that the Ag was reduced to Ag . However, these sorbents could
◦
be rejuvenated by oxidation with oxygen at 350 C when the valence of Ag
+
was restored to Ag .The π-complexation ability of the sorbent was tested by
adsorption of ethylene, and the deactivation and reoxidation behaviors are shown
in Figure 8.7.
The behavior of AgY zeolite in H 2 S was studied by Takahashi et al. (2001b).
◦
At 25 to 120 C, H 2 S chemisorbed on AgY, while reaction with H 2 S with contin-
◦
ual weight gain was observed at 180 C. XPS analysis showed the formation of
Ag 2 S. However, the adsorption capacities for 1-butene and 1,3-butadiene were
only slightly lowered, indicating the π-complexation capability of Ag 2 S.
The effects of H 2 and H 2 S on Cu(I)Y were studied by Takahashi et al. (2001a).
◦
Unlike AgY, exposure to H 2 and H 2 S at 120 C showed no effect on CuY,
demonstrating its excellent poison resistance toward H 2 and H 2 S.
The deactivation behaviors of the Ag and Cu(I) sorbents are summarized in
Table 8.10.

8.4.2. CO Separation by π-Complexation
Since 1989, CO separation/recovery by PSA using supported CuCl has been
commercialized worldwide. PSA separation results are available in the literature
(Kansai Coke & Chemicals Co., 1989; Chen et al., 1997; Golden et al., 1998).
CO is typically produced along with H 2 as synthesis gas, by steam reform-
ing of methane or naphtha. Separation and recovery of CO has been accom-
plished by cryogenic processes. CO is used as a raw material for production of