Ind. Eng. Chem. Res. 2006, 45, 7649-7655
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Desulfurization of High-Sulfur Jet Fuel by π-Complexation with Copper and Palladium Halide Sorbents Yuhe Wang, Frances H. Yang, and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109
John M. Heinzel Energy ConVersion Section, NAVSEA, Philadelphia, PennsylVania 19112
Anthony D. Nickens Ship Systems and Engineering DiVision, Office of NaVal Research, Arlington, Virginia 22203
Deep desulfurization of a high-sulfur jet fuel (for fuel cell applications) was achieved by adsorption with π-complexation sorbents. The total sulfur content of JP-5 jet fuel was removed from 1172 ppmw S to below 1 ppmw S. The following sorbents were prepared and tested: CuCl supported on activated carbon (CuCl/ AC), PdCl2/AC, PdCl2/Al2O3, and Cu(I)-Y zeolite. Comparison of these sorbents as well as other known sorbents showed that PdCl2/AC had the highest sulfur selectivity and capacity. It was found that significant breakthrough occurred at about 6.0 mL/g for desulfurization of JP-5 by PdCl2/AC. Ab initio molecular orbital computation was performed for the bond energies between different sulfur molecules and these sorbents. For methylated benzothiophene, the main sulfur molecules in jet fuels, the bond energies (and the separation factors for organosulfur/benzene) followed the order PdCl2 > CuCl > Cu-Y. This result was in agreement with the experimental breakthrough data. The spent PdCl2/AC was regenerated with benzene in a static system, and the regenerated sorbent was tested for reuse. The results showed that ∼74 wt % adsorbed sulfur could be desorbed and 72% of the sulfur capacity could be recovered for reuse. 1. Introduction Due to worldwide environmental mandates, refiners are facing the challenge of producing increasingly cleaner fuels.1 The primary focus of the new regulations is the reduction of sulfur in transportation fuels. These regulations are aimed at commercial gasoline and diesel, and jet fuels are expected to be included in the near future. The jet fuels are strategic fuels for the military and are considered by many as an excellent hydrogen source choice for future fuel cell applications.2 For fuel cell applications, the sulfur content must be reduced to below 1 ppmw because the sulfur would otherwise poison the catalysts in the fuel processor as well as the catalyst in the fuel cell. Due to the high sulfur contents in jet fuels, removal of sulfur for fuel cell applications has been a particular challenge. Removal of sulfur is an important operation in petroleum refining, and is achieved by a catalytic process operated at elevated temperatures and high pressures of hydrogen. Today, refineries rely on hydrodesulfurization (HDS) processes to reduce sulfur levels, but achieving deep desulfurization would require more severe reaction conditions, increased reactor size, and paying the penalty of increased hydrogen consumption.3 There has been great interest in developing selective sorbents for desulfurization.4-6 We have recently developed a class of sorbents that rely on a process called π-complexation to selectively remove organosulfur molecules from commercial fuels.7-13 These sorbents were prepared by using several ionexchange techniques to introduce d-block metal cations into zeolites, including Ag+, Cu+, Ni2+, and Zn2+. These ionexchanged materials are capable of producing fuels with a total * To whom correspondence should be addressed. Tel. (734) 9360771. Fax: (734) 764-7453. E-mail:
[email protected].
sulfur concentration of less than 1 ppmw. In particular, the Cu(I)-Y zeolite (vapor-phase ion exchanged, VPIE) exhibits the highest selective adsorption capacity for sulfur compounds from the transportation fuels.1,7,9 However, the Cu(I)-Y sorbent has some drawbacks: (1) Cu(I)-Y is not stable because it can be oxidized easily to Cu(II)-Y. Once oxidized, Cu2+ does not form π-complexes; hence the zeolite loses its sulfur selectivity. (2) For desulfurization of high-sulfur jet fuels, even Cu(I)-Y shows only small sulfur capacities. This leads us to develop new adsorbents with high capacity and selectivity, particularly for high-sulfur jet fuels. Preliminary experiments in our laboratory indicate that the Pd-based sorbents have higher sulfur capacities than Cu(I)-Y. In addition, our previous studies have shown that activated carbon used as a guard bed can improve the adsorptive performance of Cu(I)-Y adsorbent.12,14 Studies by other groups were also reported regarding desulfurization and denitrogenation of liquid hydrocarbon fuels by adsorption using activated carbon.4,15-20 Activated carbon impregnated with metal halides (AgCl, CuCl, CuBr, CuI, FeCl2, FeCl3, NiCl2, PdCl2, and ZnCl2) were studied for selective adsorption of CO from its mixtures by Tamon et al.21 Here, CO also forms a π-complexation bond with the impregnated metal cation. The impregnated CuCl, CuBr, CuI, and PdCl2 sorbents yielded significantly higher adsorption capacities compared with the unimpregnated carbon. In particular, the amounts of CO adsorbed on CuCl- and PdCl2impregnated carbons were around 8 and 20 times higher, respectively, than that on the unimpregnated carbon. In this work, the objective was to develop a stable and highcapacity adsorbent for removing sulfur to less than 1 ppmw S from jet fuel (e.g., JP-5), a military logistic fuel, for fuel cell applications. Several adsorbents including impregnated activated
10.1021/ie060922w CCC: $33.50 © 2006 American Chemical Society Published on Web 09/29/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006
carbon (AC) with PdCl2 and CuCl salts were investigated for the desulfurization of JP-5 jet fuel using a fixed-bed adsorber. The results show that PdCl2/AC can selectively remove sulfur from JP-5 to
