Page 364 - Adsorbents fundamentals and applications

DESULFURIZATION OF TRANSPORTATION FUELS 349

Table 10.13. Example of heteroatom contents in the FCC
gasoline
Heteroatoms Content, ppmw

Nitrogen 16.0
Oxygen 14.0
Mercaptan sulfur 24.2
Sulfide sulfur 7.3
Thiophene sulfur 61.9
C 1 thiophene sulfur 115.0
C 2 thiophene sulfur 130.6
C 3 thiophene sulfur 90.9
C 4 thiophene sulfur 88.0
Benzothiophene and 238.1
dibenzothiophene sulfur
Total 786.0

Irvine, 1998.

From these analyses, the sorbent to be developed must have the highest affini-
ties for the thiophenic compounds, medium affinities for the aromatics, and lowest
possible affinities for the alkanes and branched alkanes.

10.7.2. Sorbents Studied or Used

The use of sorbents for sulfur removal dates back to the use of bauxite (i.e.,
the ore for aluminum smelting) to adsorb mercaptans from various petroleum
fractions (Purdy, 1958). Red mud (the iron-rich waste from the Bayer process
for alumina extraction from bauxite) has also been used as a sorbent for sulfur
removal from petroleum oils.
The recent search for sorbents for liquid fuel desulfurization has taken two
paths, both based on trial-and-error. One approach has been simply testing the
commercial sorbents. The other approach has an origin in HDS catalysis. This
approach is aimed at sorbents that are good catalysts for HDS, with the hope
that such sorbents would form a bond with the sulfur atom of the thiophenic
compounds. Forming such a bond may require above-ambient temperatures. The
commercialized process discussed above, SZorb, is based on an interesting
hybrid of catalyst and sorbent. The use of π-complexation is an entirely different
approach and will be discussed separately.
The commercial sorbents (activated carbon, activated alumina, and zeolites)
have all been studied for sulfur removal. The zeolites included 5A; 13X (Salem,
1994; Salem and Hamid, 1997); various ZSM’s, including ZSM-5 and silicalite
(Weitkamp et al., 1991); and ion-exchanged zeolites (Michlmayr, 1980; Vansant
et al., 1988). The ion exchange was intended for the exchanged cation to form a
bond with the sulfur atom in thiophene. Although some of these sorbents showed