Page 376 - Adsorbents fundamentals and applications

REMOVAL OF AROMATICS FROM FUELS 361

detection limit (∼0.28 ppmw) for prolonged periods of time, and clear break-
throughs occurred only after these periods of time. The thiophene capacity for
AgY was 7.5 wt % for a feed containing 2000 ppmw. This was higher than all
other sorbents reported in the literature (Table 10.14). From the results shown in
Figure 10.61, the thiophene capacity of Cu(I)Y was 21.42 wt % for a feed con-
taining 2000 ppmw. The breakthrough curve for Cu(I)Y with a feed containing
500 ppmw thiophene is not shown. A “sulfur-free” effluent was also produced
and the total thiophene capacity was 10.75 wt % (Hern´ andez-Maldonado and
Yang, 2003). Thus, Cu(I)Y is by far the best sorbent, in both sulfur selectivity
and capacity, for sulfur removal from gasoline and diesel fuels (Yang et al., 2002
U.S. and foreign Patent application).
Hern´ andez-Maldonado and Yang (2003b) also reported breakthrough curves of
commercial gasoline and diesel on Cu(I)Y as well as Cu(I)Y with a layer of (15%)
guard bed of activated carbon. The curves are similar to that in Figure 10.61.
3
Thus, sulfur-free (<0.28 ppmw) diesel of 34.3 cm and gasoline of 19.6 cm 3
can be produced per gram of total sorbent. The use of guard bed can enhance
the sulfur capacity of Cu(I)Y. As expected, a lower content of aromatics (as in
diesel) is favorable for desulfurization.

10.8. REMOVAL OF AROMATICS FROM FUELS

Purification of aliphatics by the removal of aromatics is important in the petro-
chemical industry as well as for pollution control. Current worldwide environ-
mental mandates require reduction of aromatics, particularly benzene, in the
transportation fuels. For example, the European Union standards are already in
place that require the benzene concentration in gasoline to be lowered to 1 vol %
by 2000 and to be lowered still by 2005, while the concentration of the polycyclic
aromatics in diesel fuel needs to be lowered to 11 vol % by 2002 and to lower
levels still by 2005 (Avidan et al., 2001).
In a typical benzene removal process, a combination of extraction and dis-
tillation is used (Jeanneret, 1997). Improvements by other processes have been
considered, such as pervaporation (Hao et al., 1997), liquid membranes (Li, 1968;
1971), and adsorption by temperature swing adsorption (TSA) in the liquid phase
(Matz and Knaebel, 1990). In the work of Matz and Knaebel, commercially avail-
able sorbents were used: silica gel, activated alumina, activated carbon, zeolite
13X, and polymeric resin (XAD-7). Among these sorbents, silica gel was con-
sidered the best due to its superior thermal-exchange capacity. However, the
selectivities were low.
Takahashi and Yang (2002) studied adsorption of benzene and cyclohexane
on various Y-zeolites. AgY showed superior benzene/cyclohexane selectivities
4
to NaY and H-USY. Separation factors as high as 10 were obtained with AgY
at low concentrations of benzene. The high selectivities were achieved by the
strong interaction between benzene and AgY, while the interaction with cyclohex-
ane was not influenced by cation exchange. Figure 10.62 shows the isotherms of
benzene and cyclohexane on AgY at two temperatures. The isotherms on NaY are