Desulfurization of JP-8 Jet Fuel by Selective Adsorption over a Ni

Apr 12, 2005 - Sobrante Way, Sunnyvale, California 94086. Received August 9, 2004. Revised Manuscript Received March 4, 2005. This work focuses on ...
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Energy & Fuels 2005, 19, 1116-1125

Desulfurization of JP-8 Jet Fuel by Selective Adsorption over a Ni-based Adsorbent for Micro Solid Oxide Fuel Cells Subramani Velu,†,‡ Xialiang Ma,† Chunshan Song,*,† Mehdi Namazian,§ Sivakumar Sethuraman,§ and Guhanand Venkataraman§ Clean Fuels and Catalysis Program, The Energy Institute, and Department of Energy and Geo-Environmental Engineering, The Pennsylvania State University, 209 Academic Projects Building, University Park, Pennsylvania 16802, and Altex Technologies Corporation, 244 Sobrante Way, Sunnyvale, California 94086 Received August 9, 2004. Revised Manuscript Received March 4, 2005

This work focuses on optimizing the performance of a Ni/SiO2-Al2O3 adsorbent for the adsorptive desulfurization of jet fuel, including JP-8 jet fuel containing around 736 ppmw sulfur and a light JP-8 jet fuel having around 380 ppmw sulfur, obtained by fractionation for micro solid oxide fuel cell applications. The fractionated light JP-8 jet fuel had only traces of heavier alkylated benzothiophenes (C3-BTs). The removal of C3-BTs such as sterically hindered 2,3, 7-trimethylbenzothiophene from the JP-8 jet fuel by fractionation improved the adsorbent capacity by 2.5 times. The particle sizes of the adsorbent and its bed dimensions were examined and optimized to achieve targeted sulfur adsorption capacity of over 10 mg of S/g of adsorbent without encountering pressure drop across the bed. The adsorptive desulfurization of fractionated light JP-8 over the Ni/SiO2-Al2O3 adsorbent having particle sizes between 0.15 and 0.25 mm offered a sulfur breakthrough adsorption capacity of about 11.5 mg of S/g of adsorbent without developing any significant pressure drop across the beds. High sulfur-adsorption capacity could be achieved with Ni/SiO2-Al2O3 when the overall aspect ratio, axial aspect ratio, and radial aspect ratio of the adsorbent bed were around 60, 3000, and 100, respectively.

Introduction Fuel cells are promising as more efficient, convenient, and ultra-low-emission power generation systems for portable devices, automobiles, and stationary power plants.1-3 Due to the high energy density and the existing infrastructure for storage, transportation, and distribution, liquid hydrocarbon fuels such as gasoline, diesel, and jet fuel are considered to be promising fuels for mobile and stationary fuel cell applications.1-6 Jet fuel is particularly attractive as a logistic fuel for some portable power applications. An integrated micro fuel * Author to whom correspondence should be addressed. Fax: 814-865-3248. E-mail address: [email protected]. † The Pennsylvania State University. ‡ Current address: Research Triangle Institute, Research Triangle Park, NC 27709. § Altex Technologies Corporation. (1) Song, C. Fuel Processing for low-temperature and high-temperature fuel cells; Challenges, and opportunities for sustainable development in the 21st Century. Catal. Today 2002, 77, 17. (2) Song, C.; Ma, X. New Design Approaches to Ultra-Clean Diesel Fuels by Deep Desulfurization and Deep Dearomatization. Appl. Catal. B: Environ. 2003, 41, 207. (3) Song, C. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel and jet fuel. Catal. Today 2003, 86, 211. (4) Krause, T.; Kopasz, J.; Rossignol, C.; Carter, J.; Krumpelt, M. Catalytic Autothermal Reforming of Hydrocarbon Fuels for Fuel cell systems. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2002, 47, 542. (5) Fukunaga, T.; Katsuno, H.; Matsumoto, H.; Takahashi, O.; Akai, Y. Development of kerosene fuel processing system for PEFC. Catal. Today 2003, 84, 197. (6) Zheng, J.; Strohm, J. J.; Hoehn, M.; Song, C. Pre-reforming of jet fuels on modified Rh/CeO2-Al2O3 catalysts for micro solid oxide fuels cell. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 2004, 49, 21.

processor in combination with a micro solid oxide fuel cell (SOFC) using jet fuel has been viewed as an attractive portable power source as an alternative to the many packages of conventional batteries needed for portable electronic operations for several days. SOFC can take synthesis gas (from reforming) as the fuel, unlike proton-exchange membrane fuel cell, which requires further conversion of reformate in multiple stages including water gas shift and preferential oxidation. Therefore, a fuel processor in combination with SOFC could be made into a portable power device. However, the real jet fuel contains various organic sulfur compounds with total sulfur content ranging from 300 to 3000 parts per million by weight (ppmw), and this needs to be removed to well below 30 ppmw before it can be catalytically reformed into a H2-rich syngas for SOFC, because sulfur is a poison to both reforming and electrode catalysts.5,6 Hydrodesulfurization (HDS) is a conventional method that is being employed by the refineries to produce lowsulfur gasoline and diesel in order to meet environmental regulations.3,8 However, this method is highly inconvenient for fuel cells and not applicable for reducing (7) (a) Velu, S.; Ma, X.; Song, C. Selective adsorption for removing sulfur from jet fuel over zeolite-based adsorbents. Ind. Eng. Chem. Res. 2003, 42, 5293. (b) Ma, X.; Sun, L.; Song, C. A New Approach to Deep Desulfurization of Gasoline, Diesel Fuel and Jet Fuel by Selective Adsorption for Ultra-Clean Fuels and for Fuel Cell Applications. Catal. Today 2002, 77, 107.

10.1021/ef049800b CCC: $30.25 © 2005 American Chemical Society Published on Web 04/12/2005

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Figure 1. Schematic representation of compact logistic fuel preprocessor for micro solid oxide fuel cell system.

sulfur level in logistic fuels in a portable fuel cell system, as the process requires severe conditions such as high temperature (300-400 °C) and high H2 pressure (30-80 kg/cm2). Alternative methods such as adsorptive desulfurization, oxidative and extractive desulfurization, biodesulfurization, etc., are being explored in recent years to produce ultra-low-sulfur liquid transportation fuels.3,7-11 Among various alternative methods, adsorptive desulfurization is considered to be more promising. ConocoPhillips Petroleum Company has recently commercialized a new process known as S Zorb for producing low-sulfur gasoline.8,12 The process is based on reactive adsorption occurring over a solid adsorbent at elevated temperatures up to 400 °C and pressures up to 400 psi by use of a relatively small H2 stream. Similarly, the Research Triangle Institute reported a TReND (transport reactor naphtha desulfurization) process for the deep desulfurization of naphtha over a metal oxide-based adsorbent around 400 °C with minimal hydrogen consumption.13 Our laboratory at the Pennsylvania State University has been exploring a new process concept called selective adsorption for removing sulfur (PSU-SARS) without the use of H2 gas or high temperature or high pressure for producing ultraclean gasoline, diesel, and jet fuel for fuel cell and future refinery applications.1-3,7,14-16 A wide variety of materials such as zeolites, supported metals, mixed oxides, metal sulfides, and carbon materials are being evaluated for the PSU-SARS processes (8) Babich, I. V.; Moulijn, J. A. Science and technology of novel process for deep desulfurization of oil refinery streams: a review. Fuel 2003, 82, 607. (9) Mckinley, S. G.; Angelici, R. J. Deep desulfurization by selective adsorption of dibenzothiophenes on Ag+/SBA-15 and Ag+/SiO2. Chem. Commun. 2003, 2620. (10) Sano, Y.; Choi, K.; Korai, Y.; Mochida, I. Adsorptive removal of sulfur and nitrogen species from a straight run gas oil over activated carbons for its deep hydrodesulfurization. Appl. Catal. B: Environ. 2004, 49, 219. (11) Li, C.; Jiang, Z.; Gao, J.; Yang, Y.; Wang, S.; Tian, F.; Sun, F.; Sun, X.; Ying, P.; Han, C. Ultra-deep desulfurization of diesel: Oxidation with a recoverable catalyst assembled in emulsion. Chem. Eur. J. 2004, 10, 2277. (12) ConocoPhillips’ S Zorb process is published on the Internet at http://www.conocophillips.com/news/nr/rel398.asp. (13) Turk, B. S.; Gupta, R. P. RTI’s TReND process for deep desulfurization of naphtha. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2001, 46, 392. (14) Ma, X.; Sun, L.; Song, C. Adsorptive desulfurization of diesel fuel over a metal sulfide-based adsorbent. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2003, 48, 522. (15) Velu, S.; Watanabe, S.; Ma, X.; Song, C. Regenerable adsorbents for the adsorptive desulfurization of transportation fuels for fuel cell applications. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2003, 48, 526. (16) Watanabe, S.; Velu, S.; Ma, X.; Song, C. New ceria-based selective adsorbents for removing sulfur from gasoline. Prepr. Pap.s Am. Chem. Soc., Div. Fuel. Chem. 2003, 48, 695.

under ambient conditions. Among them, nickel adsorbent supported on silica-alumina (Ni/SiO2-Al2O3) exhibited an excellent performance in removing sulfur from jet fuel.17 Sulfur compounds in this approach are selectively removed by a direct sulfur-adsorbent interaction rather than π-complexation.18 While the PSU-SARS approach is applicable to various hydrocarbon liquid and gaseous fuels, the present work deals specifically with one type of liquid fuels, jet fuel. Pennsylvania State University (PSU) and Altex Technologies Corporation (Altex) have been collaborating to develop a compact logistic fuel preprocessor and reformer (LFPPR) for SOFC for portable power applications.17,19 The LFPPR approach is shown schematically in Figure 1. In this approach, a real JP-8 jet fuel is first fractionated by use of a specially designed fractionator being developed at Altex in part on the basis of detailed chemical analysis of jet fuels at PSU.19 The heavy end of the fractionated fuel is used for providing heat to the fuel processor by means of a micro burner. The light fraction of the fuel containing around 380 ppmw sulfur is first desulfurized by use of an organic sulfur trap (OST) by adsorption, and the desulfurized fuel is reformed to produce syngas suitable for SOFC. To have a compact fuel processor, and also to easily integrate the OST with the reformer, the adsorptive desulfurization needs to be performed at elevated temperature, around 200-220 °C. The challenge here is to develop a new adsorbent and adsorption process to achieve a high sulfur adsorption capacity of over 10 mg of S/g of adsorbent without developing any significant pressure drop across the adsorption bed. The required level of desulfurization for micro-SOFC under consideration is below 30 ppmw for jet fuels that may contain as high as 3000 ppmw sulfur. Lower sulfur levels to below 10 ppmw or below 1 ppmw can also be achieved as needed for other fuel cell applications. In this paper we focus our attention on developing an OST to reduce the sulfur in the JP-8 jet fuel to below (17) Ma, X.; Velu, S.; Sun, L.; Song, C.; Mehdi, N.; Siva, S. Adsorptive desulfurization of JP-8 jet fuel and its light fraction over nickel-based adsorbents for fuel cell applications. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2003, 48, 688. (18) Velu, S.; Ma, X.; Song, C. Mechanistic investigations on the adsorption of organic sulfur compounds over solid adsorbents in the adsorptive desulfurization of transportation fuels. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 2003, 48, 693. (19) (a) Namazian, M.; Sethuraman, S.; Wan, W.; Kelly, J.; Ma, X.; Velu, S.; Zheng, J.; Strohm, J.; Song, C. Portable petroleum distillate fuel processor and reformer for fuel cell applications, Abstracts of Fuel Cell Seminar 2003; Nov 3-7, 2003, Miami Beach, FL; p 696. (b) Lai, W.-C.; Song, C. S. Temperature-Programmed Retention Indices for GC and GC-MS Analysis of Coal- and Petroleum-derived Liquid Fuels. Fuel 1995, 74, 1436.

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Figure 2. Schematic representation of adsorption desulfurization system designed for the PSU-SARS study.

30 ppmw by use of a Ni/SiO2-Al2O3 adsorbent. The effects of the adsorbent particle sizes and the bed dimensions and the influence of the JP-8 jet fuel fractionation are examined in order to achieve the optimum performance of the OST. In heterogeneous catalytic reactions on packed-bed reactors, the variation of particle size and the dimensionless parameters such as axial, radial, and overall aspect ratios influence the catalytic performance.20-23 The effects of these parameters on the adsorptive desulfurization of light JP-8 are examined and discussed in this paper for the first time. Experimental Section The Ni/SiO2-Al2O3 adsorbent employed in the present study was prepared by the wet impregnation method and contained about 55 wt % Ni in metallic state. The BET surface area of the sample was estimated by the N2 adsorption-desorption method at liquid N2 temperature, and it is around 160 m2/g. The adsorbent in bulk (20-30 g) was prereduced in H2 gas at 500 °C for 5-6 h, passivated by use of sulfur-free n-hexane, and stored in the same solvent in an airtight sample bottle. The adsorbent under this condition (in the presence of solvent) is essentially nonpyrophoric and can be stored for several months without significant degradation in activity of the material. Adsorption experiments were performed on a fixedbed flow apparatus designed in our laboratory as shown in Figure 2. The adsorbent with different particle sizes was packed in custom-made stainless steel columns with different lengths and diameters. The packed columns were placed in a multichannel convection oven. The temperature of the oven was measured with a digital temperature display (Omega). To ensure that Ni in the Ni-based adsorbents is in the reduced (20) Carberry, J. J. Chemical and Catalytic Reaction Engineering; McGraw-Hill: New York, 1976; Chapter 10. (21) Balakotaiah, V.; Christoforatou, E. L.; West, D. H. Transverse concentration and temperature nonuniformities in adiabatic packedbed catalytic reactors. Chem. Eng. Sci. 1999, 54, 1725. (22) Sie, S. T. Advantages, possibilities, and limitations of smallscale testing of catalysts for fixed-bed processes. In Deactivation and Testing of hydrocarbon-processing catalysts; O’Connor, P., Takatsuka, T., Woolery, G. L., Eds.; ACS Symposium Series 634; American Chemical Society: Washington, DC, 1996. (23) Tesser, R.; Serio, M. D.; Santacesaria, E. Catalytic oxidation of methanol to formaldehyde: an example of kinetics with transport phenomena in a packed-bed reactor. Catal. Today 2003, 77, 325.

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Figure 3. GC-PFPD chromatograms of commercially available whole JP-8 jet fuel and the fractionated light JP-8 jet fuel. form, the adsorbent bed was pretreated with H2 gas at a flow rate of 50-60 mL/min at ambient pressure, heated slowly (2 °C/min) up to 230 °C, and kept at this temperature for about 1 h in H2 flow. The oven temperature was then decreased to the desired adsorption temperature. The H2 flow was stopped and the jet fuel was passed through the adsorption bed by use of an HPLC pump at the desired flow rate. The pressure drop across the adsorption bed was measured with an accuracy of better than 0.5 psi by using a special pressure meter (McMaster-Carr) designed for measuring the pressure of the hydrocarbon fuel up to 300 °C. The effluent from the adsorption column was collected every 10-20 min and analyzed on an Antek 9000S/N total sulfur analyzer with a detection limit of 0.5 ppmw sulfur. The nature of sulfur compounds in the treated and untreated fuels was identified by using a Hewlett-Packard gas chromatograph equipped with sulfur-selective pulsed flame photometric detector (GC-PFPD).7 The JP-8 fuel used in the present study (Batch 00-POSF-3804) was obtained from the U.S. Air Force Wright Patterson Laboratory and contained about 736 ppmw sulfur. A specially designed fractionator at Altex Technologies Corp. was used to produce several liters of light fuel from this JP-8 fuel.19 Computer-aided molecular orbital calculations of the sulfur compounds were performed by a semiempirical quantum chemistry method (the PM3 method) in computer-aided chemistry (CAChe) molecular orbital package (MOPAC), version 94.24

Results and Discussion The PFPD chromatograms of real and light JP-8 fuel obtained after fractionation around 230 °C are shown in Figure 3. The fuel properties and the nature of sulfur compounds present in the commercial jet fuels are described elsewhere.7,25 It is interesting to note that the sulfur content in the light JP-8, after removal of 30% of the heavy ends, is reduced from about 736 to 380 ppmw. A significant reduction in the intensity of (24) Song, C.; Ma, X.; Schmitz, A. D.; Schobert, H. H. Shape-selective isopropylation of naphthalene over mordenite catalysts: Computational analysis using MOPAC. Appl. Catal. A 1999, 182, 175. (25) Andresen, J. M.; Strohm, J. J.; Sun, L.; Song, C. Relationship between the formation of aromatic compounds and solid deposition during thermal degradation of jet fuels in the pyrolytic regime. Energy Fuels 2001, 15, 714.

Jet Fuel Desulfurization over a Ni-Based Adsorbent

Figure 4. Breakthrough curves for the adsorptive desulfurization of the whole JP-8 jet fuel and fractionated light JP-8 jet fuel over Ni/SiO2-Al2O3 adsorbent under identical conditions. Particle sizes 0.025-0.075 mm; bed dimensions 4.6 mm i.d. × 150 mm length.

peaks for C3-BT reveals that fractionation mainly removes the heavier C3-BT molecules, especially the 2,3,7-trimethylbenzothiophene (2,3,7-TMBT), as the intensity of this peak is reduced significantly. The boiling point of benzothiophene (BT) without any methyl substituent is 221 °C, and that for 2-methyl benzothiophene (2-MBT) is around 240 °C. The boiling point of C3-BT, that is, BT having three methyl substituents, can be expected to be well above 240 °C. Since the fractionation is performed at 230 °C, most of the C3-BT having boiling points over 240 °C stays in the heavy fraction of the JP-8. To investigate whether the fractionation of JP-8 jet fuel improves the adsorption performance, the adsorptive desulfurization of commercial JP-8 and the fractionated light JP-8 has been performed over Ni/SiO2Al2O3 adsorbent under identical experimental conditions, and the results are displayed in Figure 4. In the adsorptive desulfurization of the real jet fuel, the sulfur content in the treated fuel reaches around 30 ppmw after treating about 19 mL of fuel/g of adsorbent. On the other hand, the sulfur content in the treated light fuel reaches about 30 ppmw after treating approximately 53 mL of fuel/g of adsorbent. In other words, while 1 g of the adsorbent is capable of treating 53 mL of light JP-8, the same amount of adsorbent can treat only 19 mL of real JP-8. The corresponding 30 ppmw sulfur breakthrough capacities are approximately 6 and 16 mg of sulfur/g of adsorbent for the parent and the light JP-8, respectively. This indicates that fractionation of the jet fuel improves the sulfur adsorption performance by over 2.5 times, although the sulfur content has been reduced by about 50% through the fractionator (from about 736 ppmw in the parent JP-8 to 380 ppmw in the light JP-8). It should be noted that, as shown in Figure 3, the fractionated light JP-8 contains relatively lower amounts of C3-BT such as 2,3,7-TMBT compared to their contents in the commercial jet fuel. The improved sulfur adsorption performance observed in the desulfurization of the light jet fuel could therefore be attributed to the removal of significant amounts of

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C3-BT such as 2,3,7-TMBT, which exhibits steric hindrance because of the presence of methyl groups at the C-2 and C-7 positions of the BT ring. The GC-PFPD chromatograms of commercial JP-8 and light JP-8 fractions collected during adsorptive desulfurization are shown in Figures 5 and 6, respectively. In the adsorptive desulfurization of JP-8, the sulfur content in the treated fuel reaches about 3.5 and 19 ppmw after treatment of about 11.6 and 16.5 mL/g of adsorbent, respectively. However, the volume of light JP-8 (LJP-8) treated to achieve the same sulfur level is about 30 and 48.2 mL/g of adsorbent, respectively. This indicates that the desulfurization of light JP-8 is over 2.5 times easier than the whole JP-8 even in the production of ultralow sulfur jet fuels with sulfur contents well below 5 ppmw. Another interesting observation from Figures 5 and 6 is that the chromatograms of fuels containing 3.5 ppmw sulfur show a single tiny peak corresponding to 2,3,7-TMBT, and the intensity of this peak increases with increasing sulfur content to about 19 ppmw in the treated fuel. At this point, 2,3,7-TMBT is the major sulfur compound present in both commercial and light JP-8 fuel. It can be inferred from these results that 2,3,7-TMBT is the most difficult sulfur compound present in even the light JP-8, although its content is less compared to that in whole JP-8 fuel. The electron density isosurfaces of BT and 2,3, 7-TMBT generated by computer simulation are compared in Figure 7. The computer simulation clearly demonstrates the existence of steric hindrance due to the presence of methyl groups at 2 and 7 positions of the BT ring. Steric hindrance in the substituted benzothiophenes and dibenzothiophenes is a well-known factor that strongly affects the kinetics of the hydrodesulfurization reactions.26,27 On the basis of the results observed in the present study, it can be stated that fractionation of transportation fuels has the significant advantage of removing the sterically hindered sulfur compounds, thereby improving the sulfur adsorption capacity of the adsorbent. The improved sulfur adsorption performance of light jet fuel in the present study also reveals that the sulfur compounds over the Ni/SiO2-Al2O3 adsorbent are removed by a direct sulfur-adsorbent interaction. In fact, adsorptive desulfurization of a model fuel containing a mixture of thiophene, tetrahydrothiophene, benzene, and 1,5-hexadiene in hexane solvent over a similar Ni-based adsorbent indicated that the adsorbent removed thiophene and tetrahydrothiophene selectively without adsorbing benzene and 1,5-hexadiene.18 These results further support that the sulfur compounds in the present study are removed by a direct sulfuradsorbent interaction rather than via π-complexation, as reported recently over Cu- and Ag-exchanged zeolites.28-31 Since the π-complexation involves aromatic π electrons for making weak chemical interactions with (26) Whitehurst, D. D.; Isoda, T.; Mochida, I. Present State of the Art and Future Challenges in the Hydrodesulfurization of Polyaromatic Sulfur compounds. Adv. Catal. 1998, 42, 345. (27) Hashimoto, K.; Matzuo, K.; Kominami, H.; Kera, Y. Cerium oxides incorporated into Zeolite cavities and their reactivity. J. Chem. Soc., Faraday Trans. 1997, 93, 3729. (28) Yang, R. T.; Hernandez-Maldonado, A. J.; Yang, F. H. Desulfurization of Transportation Fuels with Zeolites Under Ambient Conditions. Science 2003, 301, 79.

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Figure 5. GC-PFPD chromatograms of commercially available whole JP-8 jet fuel fractions collected after adsorptive desulfurization over Ni/SiO2-Al2O3 adsorbent.

the adsorbent, the method generally removes heavier sulfur compounds more selectively than the lighter ones. However, jet and diesel fuels contain large amounts (10-30 wt %) of non-sulfur-bearing aromatics such as naphthalenes, which can also strongly compete with sulfur-bearing aromatics. Another challenge in developing a desulfurizer for the compact and light LFPPR was to develop an OST with less than 1 psi pressure drop but still achieving high adsorption capacity of over 10 mg of S/g of adsorbent at a 220 °C operating temperature. To achieve this goal, the bed dimension and the particle sizes of the adsorbent have been varied. The results of varying these parameters are summarized in Tables 1-3. Table 1 compares the results obtained by using a short column having dimensions of 16.6 mm i.d. × (29) Hernandez-Maldonado, A. J.; Stamatis, S. D.; Yang, R. T.; He, A. Z.; Cannella, W. New Sorbents for Desulfurization of Diesel Fuels via π-Complexation: Layered Beds and Regeneration. Ind. Eng. Chem. Res. 2004, 43, 769. (30) Hernandez-Maldonado, A. J.; Yang, R. T. Desulfurization of Liquid fuels by Adsorption via π-Complexation with Cu(I)-Y and Ag-Y zeolites. Ind. Eng. Chem. Res. 2003, 42, 123. (31) Hernandez-Maldonado, A. J.; Yang, R. T. Desulfurization of Diesel Fuels via π-Complexation with Nickel(II)-Exchanged X- and Y-Zeolites. Ind. Eng. Chem. Res. 2004, 43, 1081.

29.8 mm length, a standard column of 4.6 mm i.d. × 150 mm length, and a column having 7.8 mm i.d. × 150 mm length. Both the inlet pressure and the sulfur adsorption capacity show a strong dependence on the column dimensions. The inlet pressure increases with increasing length and with decreasing diameter of the column. The variation of the pressure drop with diameter is due to variation of the WHSV. Very low sulfur adsorption capacity of around 2.6 mg of S/g of adsorbent is obtained by using short column (16.6 mm i.d. × 29.8 mm length) and the capacity increases to about 7.2 mg of S/g of adsorbent when the length of the column is increased to 150 mm and the diameter is decreased to 7.8 mm. The adsorption capacity is approximately doubled (13.5 mg of S/g of adsorbent) when the diameter of the column is further reduced to 4.6 mm from 7.8 mm by keeping the same length (150 mm). However, the inlet pressure across the bed rose to 40 psi. To reduce the pressure drop across the standard bed of 4.6 mm i.d. × 150 mm length, the same adsorbents with different particle sizes, namely, 0.025-0.075, 0.075-0.150, 0.150-0.250, and 0.25-0.40 mm, were examined and the results are summarized in Table 2. Interestingly, the inlet pressure dropped to below 1 psi

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Figure 6. GC-PFPD chromatograms of fractionated light JP-8 jet fuel fractions collected after adsorptive desulfurization over Ni/SiO2-Al2O3 adsorbent.

Figure 7. Electron density isosurfaces corresponding to molecular structures of benzothiophene and 2,3,7-trimethylbenzothiophene by molecular simulation with MOPAC.

Figure 8. Effect of Ni/SiO2-Al2O3 adsorbent particle size on the adsorptive desulfurization of light JP-8 jet fuel at 220 °C. Experimental conditions are summarized in Table 3.

upon increasing the particle size from 0.025-0.075 to 0.075-0.15 mm range. This also reduced the sulfur adsorption capacity from 13.5 to about 6.6 mg of S/g of adsorbent. The variation of sulfur adsorption capacity

with particle sizes of the adsorbent shown in Figure 8 indicates that the measured sulfur adsorption capacity decreases with increasing particle size. The changes in the pressure drop and the sulfur adsorption capacity are

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Table 1. Effect of Bed Dimensions on the Adsorptive Desulfurization of Light JP-8 over Ni/SiO2-Al2O3 Adsorbent Having Smaller Particle Sizes bed dimensions parameter column i.d. (mm) column length (mm) volume of adsorbent (mL) weight of adsorbent (g) particle size range (mm) fuel tested sulfur content in fuel (ppmw) flow rate of feed (mL/min) WHSVa (h-1) temperature (°C) inlet pressure (psi) breakthrough capacity at 30 ppmw sulfur level (mg of S/g of adsorbent) overall aspect ratio axial aspect ratio radial aspect ratio a

16.6 mm i.d. × 29.8 mm length

4.6 mm i.d. × 150 mm length

7.8 mm i.d. × 150 mm length

16.6 29.8 6.45 6.65 0.025-0.075 light JP-8 380 0.1 0.68 220