Page 365 - Adsorbents fundamentals and applications

350 SORBENTS FOR APPLICATIONS

selectivities toward thiophene, the thiophene capacities were fairly low in all
cases. A summary of the thiophene capacities is given in Table 10.14. Even with
the low thiophene capacities, an adsorption process, named Irvad Process, has
already been commercialized by using activated alumina as the sorbent (Irvine,
1998; Ondrey, 1999).
The sorbent used in the Irvad process is Alcoa Selexsorb (CD grade), which is
tailored for polar compounds. The process is essentially a TSA process. Adsorp-
tion is performed in a countercurrent moving bed with a slurry of fine-sized
alumina. Desorption is performed with hydrogen at various temperatures up to
◦
520 F. Hydrogen is used for its high thermal conductivity and heat capacity, as
well as its ready availability in the refineries. Gasoline products with sulfur as
low as 0.5 ppmw were claimed (Irvine, 1998).
Ag-exchanged faujasite was claimed for thiophene removal (Michlmayr, 1980).
◦
Curiously, the preferred temperature for adsorption was 200–350 C, and the sor-
bent was not dehydrated by heat-treatment prior to adsorption. Consequently, the
sulfur capacities were very low, at 0.07–0.15 wt % for Ag-Y. The highest sul-
fur capacity (of 0.2 wt %) was obtained with the lowest Ag content, using USY
zeolite. This sorbent was apparently intended for bonding the sulfur atom with
Ag. It was clearly not intended for π-complexation (Michlmayr, 1980).
Vansant et al. (1988) investigated Cu(II)Y for thiophene removal. Curiously
also, the Cu 2+ exchanged Y zeolite was purposely heat-treated in air (to
◦
200–550 C) in order to maintain the Cu 2+ in the divalent state, rather than
treating in an inert or reducing atmosphere. Again, adsorption by π-complexation
was clearly not intended. As a result, low thiophene capacities (the highest was
1.6 wt %) were obtained.
Ma et al. (2001, 2002a) described a sorbent for removal of thiophenic com-
pounds from a jet fuel. Their sulfur breakthrough result is shown in Figure 10.52.
The jet fuel contained 490 ppm sulfur. The sulfur capacity was given as 0.015 g
sulfur per ml of sorbent. No details were given on the sorbent except that it was a
transition metal compound supported on silica gel at 5 wt % loading (Ma et al.,
2001) or simply given as a transition metal (Ma et al., 2002). It was obviously
intended for the transition metal to form a bond with the sulfur atom of the thio-
phenic compound. The sulfur capacity of this sorbent was not high. Moreover,
from Figure 10.52, it is seen that the breakthrough had already begun at the sec-
ond sampling data point (∼13 min), which indicates that the sorbent lacked selec-
tivity. Ma et al. (2002b) subsequently reported the breakthrough curve of a model
diesel on the same sorbent, showing a breakthrough capacity of only 1 cc/g.

10.7.3. π-Complexation Sorbents
A systematic approach has been taken by Yang and co-workers in the search for a
thiophene selective sorbent, leading to the π-complexation sorbents (Yang et al.,
2001; Yang et al., 2002; Takahashi et al., 2002; Hernandez-Maldonado and Yang,
2003). Effective π-complexation sorbents for sulfur removal (Yang et al., 2002)
include Cu(I)Y, AgY, CuCl/γ -Al 2 O 3 ,AgNO /SiO 2 , and others. The preparation
3
and characterization of these sorbents are described in Chapter 8.