Page 361 - Adsorbents fundamentals and applications

346 SORBENTS FOR APPLICATIONS

and 4,6-dimethyldibenzothiophene are the refractory sulfur compounds that
make deep desulfurization by HDS extremely difficult. Accompanying deep
desulfurization is the saturation of olefinic compounds resulting in octane loss of
about 10 numbers (Avidan and Cullen, 2001).
Another need for deep desulfurization is for potential application in fuel cell.
Gasoline is the ideal fuel for fuel cell because of its high-energy density, ready
availability, safety, and ease in storage. However, to avoid poisoning of the
catalyst for the water–gas shift reaction and that in the electrode of the fuel cell,
the sulfur concentration should be preferably below 0.1 ppmw. To reduce the
sulfur content of diesel from 500 ppmw to this level, an estimate showed that
the HDS reactor size needed to be increased by a factor of 7 (Whitehurst et al.,
1998). Another estimate showed that in order to reduce the sulfur level in diesel
from 300 to less than 10 ppm, the HDS reactor volume needed to be increased
by a factor of about 15 at 600 psi, or by a factor of 5 at 1,000 psi (Parkinson,
2001; Avidan and Cullen, 2001).
Faced with the severely high costs of environmental compliance, a num-
ber of new technologies have been contemplated for post-treating of the FCC
naphtha (Avidan et al., 2001). One commercialized technology, named S Zorb,
has been announced by Phillips Petroleum Company, and it is claimed to remove
the refractory sulfur species such as 4,6-dimethyldibenzothiophene (brochure of
Phillips). No detailed information is available on the “sorbent.” The sorbent
is actually a highly sulfur-poisonable catalyst, used in a fluidized-bed reactor,
◦
for a hydrogenation reaction. The reaction is performed at 650–775 Fand
◦
100–300 psig in H 2 for gasoline, or 700–800 F and 275–500 psig in H 2 for
diesel. The reaction reported for benzothiophene is: benzothiophene + H 2 →
S(ads.) + ethyl benzene. The S atom is deposited on the catalyst/sorbent. The
catalyst is regenerated with air (forming SO 2 ), followed by reduction with H 2 .
The reduced catalyst is recycled to the reactor.
Another new technology using ultrasound was disclosed by Avidan and Cullen
(2001), Yen et al. (2002) and Mei et al. (2003). This is a two-step process, called
SulphCo process, named after the company that developed it. In the first step,
thiophenic compounds are oxidized in an ultrasonic reactor to form sulfoxides
(with 1 oxygen attached to the sulfur atom) and sulfones (with 2 oxygen atoms
attached to sulfur). The thiophenic compounds are oxidized in gas-bubble cavities
generated by ultrasound. The sulfoxides and sulfones are subsequently removed
by solvent extraction. Following Collins et al. (1997), H 2 O 2 (oxidant) and a
heteropolyanion (catalyst) were used for oxidation. Still another process was
disclosed by Research Triangle Institute (Chemical Engineering Progress, 2001).
A sorbent is reacted with the sulfur compounds in the naphtha from FCC in
a “transport reactor” designed to minimize the reactor volume. The sorbent is
regenerated with air to form SO 2 . Both reactions (sorption and regeneration) are
conducted at elevated temperatures.
The new challenge is to use adsorption to selectively remove these sulfur com-
pounds from transportation fuels (gasoline, diesel, and jet fuels). Since adsorption
would be accomplished at ambient temperature and pressure, success in this