Selective Adsorption in Ultrasound-Assisted Oxidative Desulfurization

2250

Energy & Fuels 2007, 21, 2250-2257

Selective Adsorption in Ultrasound-Assisted Oxidative Desulfurization Process for Fuel Cell Reformer Applications Omid Etemadi and Teh Fu Yen* Department of CiVil and EnVironmental Engineering, UniVersity of Southern California, Los Angeles, California 90089-2531 ReceiVed January 11, 2007. ReVised Manuscript ReceiVed April 9, 2007

Alumina has been used as a selective solid adsorbent for a scaleup test of a jet fuel and diesel fuel desulfurization technique. The oxidative process with ultrasound assistance has been improved for practical purposes by using solid adsorption instead of solvent extraction. Therefore, the advantages of both oxidative and adsorptive desulfurization have been put together for a modified continuous system to provide a source for portable fuel cells. Refractory sulfur compounds of benzothiophene and dibenzothiophene derivatives have been removed at 99% efficiency. The sulfur concentration in JP-8 jet fuel can be reduced from the original 850 ppm to 1 ppm at an oxidation time of 10 min at ambient temperature and atmospheric pressure. Gas chromatograms from gas chromatography-sulfur chemiluminescence detection and gas chromatographypulsed flame photometric detection evaluate the efficiency and selectivity of the ultrasound-assisted oxidative desulfurization (UAOD) process on different fuels after adsorption. gas chromatography-flame ionization detection, gas chromatography-mass spectrometry, and gas chromatography-simulated distillation were used to identify the concentration changes of sulfur compounds and hydrocarbons in fuels during the process. High sulfur adsorption capacities were obtained due to high conversion rates of the UAOD process. For marine gas oil, 12.8 mg of sulfur was removed per gram of alumina, which indicates the optimized process without the use of composite adsorbents. Acidic alumina shows promising results as an adsorbent in the UAOD process. Experiments prove that solid adsorbents are suitable for a scaleup to achieve ultralow sulfur fuel in the UAOD process.

1. Introduction Stringent emission regulations and growing environmental awareness have raised the demand for ultradeep desulfurization technologies. Rigorous emission control standards are being imposed on diesel fuel products in order to reduce the environmental impacts and eliminate the financial loss in products such as catalytic converters. Some exhaust emissions from diesel engines impede the operation of emission control devices.1 Scarce global petroleum resources demand higher efficiency for sulfur removal along with a reduction in pollutants and new emission control standards. The maximum level of sulfur compounds in the U.S. for diesel was reduced from 500 ppmw to 15 ppmw by June 2006;2,3 that is a 97% cut in the sulfur content of diesel. The existing desulfurization techniques need to be optimized, and fresh approaches are required to reach very low sulfur content in diesel fuels in a feasible way. Organic sulfur compounds (OSCs) in diesel fuels are converted to SOx in industrial and automobile waste gases. These compounds are sources of secondary pollutants that produce acid rain. Due to their higher selectivity, SOx’s are adsorbed into catalytic converters and occupy the sites that are designed for CO and NOx reduction. On the other hand, liquid hydro* Corresponding author. E-mail: [email protected]. (1) HeaVy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Requirements; U.S. EPA regulatory announcement: Washington, DC, 2000 (2) June 1 Marks Historic Milestone in Clean Diesel; U.S. EPA: Washington, DC, 2006. (3) Introduction of Cleaner-Burning Diesel Fuel Enables AdVanced Pollution Control for Cars, Trucks and Buses; U.S. EPA Program Update: Washington, DC, 2006.

carbons such as diesel and gasoline, due to their high energy density and available infrastructure, are also the primary fuels considered for automotive and portable fuel cells.4 Therefore, ultradeep desulfurization is needed to produce a fuel that can be reformed to a sulfur-free hydrogen source for fuel cell systems. Conventional methods of desulfurization in refineries are mainly through the hydrotreating of feedstocks. Hydrodesulfurization (HDS) in this manner needs severe conditions to produce ultralow sulfur fuel (ULSF). Higher temperatures and pressures, along with more catalysts and longer residence times, lead to higher operation costs in the refining process. It should be kept in mind that deep HDS will have a negative effect on the lubricity of the treated diesel. The remaining sulfur compounds in diesel fuel after HDS at sulfur levels lower than 1000 ppmw are alkyl-DBTs (DBT ) dibenzothiophene), some with alkyl substitutions at the 4 and/or 6 positions. These compounds have low reactivity in the HDS process and, therefore, are classified as the most refractory compounds in desulfurization processes. The advantages in sulfur-removal methods suggest a combined technique of selective oxidation and adsorption for producing ULSF with no consumption of hydrogen. Oxidative desulfurization (ODS) has been studied extensively in recent years.5-8 Sulfur compounds have more affinity to oxidation than their analogue hydrocarbons in diesel (4) 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, 17-49. (5) Attar, A.; Corcoran, W. H. Desulfurization of Organic Sulfur Compounds by Selective Oxidation. 1. Regenerable and Nonregenrable Oxygen Carriers. Ind. Eng. Chem. Res. 1978, 17, 102-109.

10.1021/ef0700174 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/24/2007

OxidatiVe Desulfurization Process

fuels. High conversions of sulfides to sulfones and sulfoxides provides a difference in polarity that can be used for the selective removal of OSCs with solid adsorbents. Oxidation and adsorption techniques in the ultrasound-assisted oxidative desulfurization (UAOD) process react at ambient temperature and atmospheric pressure. This process provides a clean fuel that meets the new emission control standards with minimum harm to catalytic converters.9 The activation of H2O2 in the presence of a transition metal catalyst (TMC) has been reported to produce high-efficiency oxidants for sulfur removal in diesel fuel.10 The Keggin structure in polyoxometalates such as H3PM12O40 [M ) Mo(VI), W(VI)] produces more selective oxidants. These polyoxoperoxo complexes such as PO4 [MO(µ-O2)(O2)2]43- (µ means π-bonded to the metal) oxidize nucleophile sulfur compounds in the presence of a phase-transfer agent (PTA).11-15 Major refractory compounds such as alkyl-DBT derivatives with low nucleophilicity can be oxidized to sulfones and sulfoxides in the ODS process. However, the extended reaction time and reaction safety concerns due to a high concentration of hydrogen peroxide and its excessive decomposition are major obstructions for commercializing the process.16 Ultrasound irradiation can greatly speed up the oxidation process through the biphasic transfer of oxidants.17-19 Sonication provides an emulsion that increases the concentration of reactive species and provides more of an interfacial surface for reaction.20 Cavitation is produced from the acoustic pressure wave of the ultrasound process. It propagates alternating compression and depression in microenvironments that results in bubble collapse and high instantaneous pressures and temperatures in the center (6) Zannikos, F.; Vignier, V. Desulfurization of Petroleum Fractions by Oxidation and Solvent Extraction. Fuel Process. Technol. 1995, 42, 3545. (7) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian W. H.; Ishihara, A.; Imai, T.; Kabe, T. Oxidative Desulfurization of Light Oil and Vacuum Gas Oil by Oxidation and Solvent Extraction. Energy Fuels 2000, 14, 12321239. (8) Collins, F. M.; Lucy, A. R.; Sharp, C. Oxidative Desulphurization of Oils via Hydrogen Peroxide and Heteropolyanion Catalysts. J. Mol. Catal. A: Chem. 1997, 117, 397-403. (9) Etemadi, O.; Yen, T. F. Prepr. Pap. Am. Chem. Soc., DiV. Fuel Chem. 2006, 51, 820. (10) Mei, H.; Mei, B. W.; Yen, T. F. A New Method for Obtaining UltraLow Sulfur Diesel Fuel via Ultrasound Assisted Oxidative Desulfurization. Fuel 2003, 82, 405. (11) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; Wiley: New York, 1995. (12) Bortolini, O.; Furia, F. D.; Modena, G.; Seraglia, R. Metal Catalysis in Oxidation by Peroxides. Sulfide Oxidation and Olefine Epoxidation by Dilute Hydrogen Peroxide Catalyzed by Molybdeum and Tungsten Derivatives under Phase Transfer Conditions. J. Org. Chem. 1985, 50, 26882690. (13) Venturello, C.; Aloisio, R. D.; Bart, J. J.; Ricci, M. A New Peroxotungsten Heteropoly Anion with Special Oxidizing Properties: Synthesis and Structure of Tetrahexylammonium Tetra(diperoxotungstio)phosphate(3-). Tetrahedron Lett. 1985, 107. (14) Campestrini, S., V.; Furia, F. D.; Modena, G.; Bortolini, O. Metal Catalysis in Oxidation by Peroxides. Electrophilic Oxygen Transfer from Anionic, Coordinatively Saturated Molybdeum Peroxo Complexes. J. Org. Chem. 1988, 53, 5721-5724. (15) Jorgensen, K. A. Transition-Metal-Catalyzed Epoxidations. Chem. ReV. 1989, 89, 431-458. (16) Collins, F. M.; Lucy, A. R.; Sharp, C. Oxidative Desulphurisation of Oil via Hydrogen Peroxide and Heteropolyanion Catalysis. J Mol. Cat. A: Chem. 1997, 117, 397. (17) Thompson, L. H.; Doraiswamy, L. K. Sonochemistry: Science and Engineering. Ind. Eng. Chem. Res. 1999, 38, 1250. (18) Luche, J. L. Synthetic Organic Sonochemistry; Plenum Press: New York, 1998. (19) Shah, Y. T.; Pandit, A. B.; Moholkar, V. S. CaVitation Reaction Engineering; Kluwer Academic/Plenum Publishers: New York, 1999. (20) Yen, T. F.; Gilbert, R. D.; Fendler, J. H. Membrane Mimetic Chemistry and Its Applications; Plenum Press: New York, 1994.

Energy & Fuels, Vol. 21, No. 4, 2007 2251

of the bubble. This extreme condition increases the rate of oxidation by creating active intermediates.21 Ultrasound-assisted chemical processes have been reported in previous work.10,21-25 This phenomenon is used in the UAOD process to increase the rate of oxidation in a biphasic system under catalytic reactions. The idea behind developing the UAOD process is to optimize the selectivity condition in the oxidation of OSCs. Modifications have been done in the UAOD process for identifying higherefficiency phase-transfer agents (tetraoctylammonium fluoride) and for the application of dilute H2O2.26 Sulfur compounds in diesel fuels are selectively oxidized due to their slightly greater polarity compared to other hydrocarbons. They are then removed by solid adsorbents at low temperatures and ambient pressure. Solid adsorption has been developed for desulfurizing diesel fuels at lower temperatures and pressures than those of hydrotreating technologies.27 The S Zorb sulfur removal technolgy (SRT) process can achieve ultralow levels of sulfur in lighter portions of the fluid catalytic cracker stream. Hydrogen consumption is still needed along with the sorbent in the S Zorb SRT process but in lower amounts than for the conventional hydrotreating methods. Another adsorption process was developed at Pennsylvania State University, termed PSUSARS, by using a composite metal catalyst on a porous substrate. Selective adsorption in this process is at low temperatures and atmospheric pressure without hydrogen consumption.28 The adsorption capacity and regeneration of the adsorbents are matters to be considered for scaleup techniques in refineries. The oxidative/adsorptive desulfurization method in the UAOD process is tested on different diesel fuels to target the refractory compounds that are difficult to remove by conventional techniques. This developed process is an attempt to upgrade diesel fuels with a low cost as a source for reformed fuel that can be used in fuel cells with minimum harm to the membrane catalyst. The sulfur concentration needs to be reduced to less than 1 ppmw for proton exchange membrane fuel cells and less than 10 ppmw for solid oxide fuel cells to avoid catalyst poisoning.29 A modular desulfurization unit is designed and set up with a capacity of 1 bpd to produce ULSF. A low concentration of hydrogen peroxide (as low as 0.25%) and easily regenerated catalysts, phase-transfer agents, and adsorbents are key factors for scaleup purposes in the UOAD process.30 In the UAOD process, the recalcitrant aromatic compounds of benzothiophene (BT) and DBT groups and even the most refractory (21) Tu, S. P.; Yen, T. F. The Feasibility Studies for Radical Induced Decomposition and Demetallization of Metalloporphyrins by Ultrasonication. Energy Fuels 2000, 14, 1168-1175. (22) Tu, S. P.; Kim, D.; Yen, T. F. Decolorization and Destruction of Metallophalocyannins in Aqueous Medium by Ultrasound: A Feasibility Study. J. EnViron. Eng. Sci. 2002, 1, 237-246. (23) Lin, J. R.; Yen, T. F. An Upgrading Process through Cavitation and Surfactant. Energy Fuels 1993, 7, 111-118. (24) Sadeghi, K. M.; Sadeghi, M. A.; Yen, T. F. Novel Extraction of Tar Sands by Sonication with the Use of in Situ Surfactants. Energy Fuels 1990, 4, 604-608. (25) Sadeghi, K. M.; Sadeghi, M. A.; Kuo, J.-F.; Jang, L.-K.; Yen, T. F. A New Process for Tar Sand Recovery. Chem. Eng. Commun. 1992, 117, 191-203. (26) Wan, M.-W.; Yen, T. F. Enhance Efficiency of Tetraoctylammonium Fluoride Applied to Ultrasound-Assisted Oxidative Desulfurization (UAOD) Process. Appl. Catal., A 2007, 319, 237-245. (27) Covert, C. How Philips S ZORB Sulfur Removal Technology Quickly Came to Life. World Refin. 2001, 11. (28) Ma, X. L., Sun, L; Song, C. S. 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. (29) Song, C. An Overview of New Approaches to Deep Desulfurization for Ultra-Clean Gasoline, Diesel Fuel and Jet Fuel. Catal. Today 2003, 86, 211-263.

2252 Energy & Fuels, Vol. 21, No. 4, 2007

4,6-dimethyldibenzothiophene convert to sulfone forms with high yields.34 However, the affinity of adsorption to alumina is higher among BTs compared to DBTs.9 Therefore, lower levels of sulfur are achieved in JP-8 compared to marine gas oil (MGO) that make JP-8 a suitable source for reformation to provide hydrogen for fuel cell applications. Moreover, the sulfur concentration in MGO can be reduced to less than 10 ppmw by adding a polishing stage with a solid catalyst to the continuous UAOD process.30 2. Experimental Methods 2.1. Materials. Dibenzothiophene sulfone (DBTO), 2-methylnaphthalene (2-MN), tetraoctylammonium fluoride (TOAF), phosphotungstic acid hydrate (H3PW12O40‚20H2O), aluminum oxide (activated, acidic, Brockmann I, standard grade, ∼150 mesh, 58A), and drierite (calcium sulfate, +4 mesh) were obtained from Aldrich Chemical Co. MGO with an original sulfur content of 1710 ppm was purchased from Long Beach, California, and jet fuel (JP-8) with an original sulfur content of 863 ppm was supplied by the Army Research Laboratory (ARL). Toluene, N,N-dimethyl formamide (DMF), and hydrogen peroxide (30 vol %) were obtained from VWR Inc. 2.2. Instruments. The sulfur-in-oil analyzer (SLFA-20) is manufactured by Horiba, Inc. Th ultrasound apparatus (VCX-600) is manufactured by Sonics and Materials, Inc., Newtown, CT. The UAOD setup for oxidation is discussed elsewhere.10 The gas chromatograph (Varian 3400; GC) was equipped with a pulsed flame photometric detector (PFPD) and an ion trap mass spectrometer (Saturn 2000; MS). Another gas chromatograph used was a 6890N Agilent series with a flame ionization detector (FID). This instrument is coupled with a sulfur chemiluminscence detector (SCD) Sievers model 355. The portable ultrasound unit consists of a custom-made transducer fabricated by Blatek, Inc., State College, PA. 2.3. Analysis. Total sulfur concentrations of the fuel samples were determined with a sulfur-in-oil analyzer. The measurement procedure is based on ASTM Method D4294-83, nondispersive X-ray fluorescence. This instrument measures the total sulfur concentration for samples with a sulfur content range from 0.002 to 5 wt %. For products with a total sulfur content of 20 ppm and less, the concentration of each sulfur compound was quantified with GC-PFPD and GC-SCD. For identifying sulfur compounds in fuel samples in GC-PFPD, a fused-silica capillary DB-5 MS column (30 m × 0.25 mm i.d.) with a 0.25 µm film thickness was used. The column temperature program was first retained at 100 °C for 3 min and was heated at an increasing rate of 6 °C/min to 275 °C and kept at 275 °C for 10 min. The other major peaks shown in GC-PFPD were identified using selected ion monitoring or comparing molecular ions in GC-MS. A nonlinear equation is derived for measuring samples with GC-PFPD.10,31,32 The model compound of 2-MN and DBTO in the feed and product was analyzed by GC-FID. This instrument is coupled with a SCD for simultaneous injection of the samples to identify both sulfur- and non-sulfur-containing compounds. The chromatograph was fitted with a 60 m HP-5 fused-silica column that was installed (30) Wan, M.-W.; Cheng, S.-S.; Yen, T. F. Ultrasound Assisted Oxidation Process through Solid Catalyst Polishing or Ionic Liquid Polishing. Appl. Catal., A 2007, 52, 198-199. (31) Hutte, R. S.; Ray, J. D. Sulfur-Selective Detectors. In Detectors for Capillary Chromatograph; Hill, H. H., McMinn, D. G., Eds.; John Wiley & Sons: New York, 1992; Chapter 9. (32) Ma, X. L.; Kim, J. H.; Song, C. S. Nonlinear Response and Quenching Effect in GC-PFD and GC-PFPD for Quantitative Sulfur Analysis of Low-Sulfur Hydrocarbon Fuels. Prepr. - Am. Chem. Soc., DiV. Pet. Chem. 2004, 49, 9-12 (33) Etemadi, O.; Yen, T. F. Aspects of Selective Adsorption among Oxidized Sulfur Compounds in Fossil Fuels. Energy Fuels 2007, in press. (34) Wan, M.-W. Development of a Portable, Modular Unit for the Optimization of Ultrasound Assisted Oxidative Desulfurization of Diesel. Dissertation, USC, Los Angeles, 2005.

Etemadi and Yen directly into the discharge tube. The column temperature program was first retained at 80 °C for 1 min and was heated at the increasing rate of 30 °C/min to 275 °C and kept at 275 °C for 1 min. This gave an analysis time of 7.5 min for target compounds. The burner temperature in the detector was set at 800 °C; air and hydrogen flow rates were 5.8 and 100 mL/min, respectively. The ozone generator creates a flow rate of 60 mL/min at 60 psig and 25 °C. Model compounds of 2-MN and DBTO were identified and quantified with standard samples. The advantages of GC-SCD over GC-PFPD are higher sensitivity and selectivity toward sulfur compounds.33 2.4. MGO. An appropriate volume of MGO fuel containing tetraoctylammonium fluoride (7.5 mM) and an equal volume of hydrogen peroxide (30 vol % aqueous solution) containing phosphotungstic acid (0.7 mM) were added to the glass reactor. The mixture was sonicated by ultrasound at a 20 kHz frequency for 10 min to form an emulsion, and the reactor temperature was kept at 70 °C in a water bath. The mixture was centrifuged to break the emulsion and form a layer of oil on the top. A column was packed with activated alumina (acidic, mesh ∼150) that was soaked with toluene to avoid air pores and channeling inside the media. Oxidized fuel was passed through the packed alumina column at room temperature. Original fuel, oxidized fuel, and desulfurized fuel samples were collected and subjected to GC-PFPD and GC-SCD analysis. Clarifying the total sulfur concentration in each MGO diesel sample was done by supplying a 25 ppm BT in effluents as an index. The original sulfur concentration (1710 ppmw) was confirmed by a Horiba sulfur-in-oil analyzer. Toluene and DMF were passed through the column, consecutively, after the fuel samples and effluents were collected. Sulfur concentrations of the samples were measured by a sulfur-in-oil analyzer. This instrument was calibrated for toluene and DMF due to interference in the sulfur measurements. Further on, regeneration of alumina was achieved through calcining at 550 °C for 4 h, and therefore wash out with DMF solvent was eliminated. 2.5. An Alkylnaphthalene and Sulfone Model Compound. The UAOD process is developed in many manners including the substitution of liquid/liquid extraction with solid adsorption. A comparison between acetonitrile as a polar solvent used for the extraction of sulfur compounds from a fuel sample and acidic alumina as a solid adsorbent was made in the desulfurization of MGO fuel. Both methods remove the same percentage of sulfur, but acetonitrile, as a setback, removes more than 80% of the favorable alkylnaphthalenes (which is responsible for the high octane number) in diesel fuel along with the OSCs.10 In order to recover this loss, alumina is used in the UAOD system instead of a liquid solvent for extraction. A model compound of 2-MN and DBTO with concentrations of 0.025 wt % of each compound in toluene was prepared in stock. The solution was passed through an acidic alumina column to study the selective adsorption among a hydrocarbon compound and a sulfone compound to the solid adsorbent. The effluent was measured by GC-FID for the remaining 2-MN and DBTO in the solution. 2.6. UAOD Scaleup for JP-8 Jet Fuel in a Continuous Flow System. A continuous flow system of the UAOD unit was set up with a capacity of 1 bpd to demonstrate the sulfur removal efficiency of the process. Phosphotungstic acid hydrate as a transition metal catalyst and TOAF as a phase-transfer agent are used, which are regenerated in the system through the aqueous phase. As for the solid adsorbent, acidic alumina is used in adsorption towers with one tower as a standby for substituting the exhaust columns during clacining for regeneration. The fuel that is used for the scaleup system is JP-8 jet fuel. The scaleup of the UAOD process was performed for a mixture of JP-8 fuel containing TOAF and an equal volume of hydrogen peroxide (30 vol % aqueous solution) containing phosphotungstic acid in a premixer and run in a continuous system through a sonoreactor for 10 min to oxidize the OSCs in JP-8 fuel. Jet fuel is separated from the aqueous phase in the separator and collected in a reservoir tank. Hydrogen peroxide is collected from the bottom of the separator for reusing the dissolved metal catalyst and phase-transfer agent in the continuous



Figure 1. GC-PFPD chromatograms of (top) original MGO, (middle) oxidated MGO, and (bottom) desulfurized MGO after alumina adsorbance.

system. To eliminate the remaining moisture and water, jet fuel is passed through a dewatering column that contains drierite. The adsorption tower at the last step is packed with acidic alumina to remove the OSCs in JP-8. The effluent was collected for injecting into the gas chromatographs of GC-PFPD and GC-SCD.

3. Results and Discussion 3.1. MGO. In the previous UAOD process, liquid/liquid extraction was used to remove the OSCs after oxidation. Due to its high polarity, acetonitrile can extract oxidized OSCs from diesel fuels.10 Solid adsorption through alumina powder proves to be a suitable substitute for solvent extraction due to its results in high sulfur removal.9 A comparison has been made in this study between fuel samples that undergo the same oxidation process with different sulfur removal agents, acetonitrile as a solvent for liquid/liquid extraction and alumina for solid adsorption. The results show that, for the same concentration of sulfur in the UAOD process, 33 times less alumina is needed by weight than acetonitrile to remove OSCs. This enormous difference of consumed media for sulfur removal makes it possible for further scaleup purposes for our oxidative/adsorptive method of desulfurization. Figure 1 shows the original MGO diesel before and after oxidation and also after the removal of sulfones with alumina. It is demonstrated that both BTs and DBTs are oxidized to form corresponding BTOs and DBTOs, respectively. Results from GC-PFPD and GC-SCD chromatographs show that the sulfur removal of MGO diesel in liquid/liquid extraction and that in solid adsorption are similar in both systems. After solid adsorbtion, the sulfones in the oxidized MGO were essentially removed. The total sulfur concentration is 23 ppmw; that is close to the results from liquid/liquid extraction with acetonitrile,34 which was 21 ppmw. Figure 2 shows the breakthrough curve of acidic alumina for the desulfurization of MGO diesel. The adsorption capacity of alumina for sulfur compounds in MGO diesel is 12.8 mg sulfur/g alumina. Figure 3 shows the concentration of sulfur compounds that were adsorbed into the solid adsorbent and washed out by toluene and DMF in 10 mL volume effluents. The adsorption efficiency of alumina for MGO diesel was 97% of the total OSCs. The effect of polar solvent (DMF) on solid adsorbent

Figure 2. Breakthrough curve of acidic alumina (∼150 mesh) for sulfur adsorption in MGO diesel. Amount of adsorbate can be calculated by integrating the quantity of C0-C over V between VE and VB.

recovery is that the remaining compounds adsorbed into the packed column are washed out at high rates. More than 94% of the remaining sulfur compounds are washed out by DMF. The oxidation and adsorption efficiency were clarified through the amount of sulfur compounds adsorbed into the acidic alumina adsorbent. The original concentration of MGO diesel was 1710 ppmw. About 2% of the OSCs (39 ppmw) were washed out through the toluene wash due to weak physical adsorption of the compounds into alumina. Using DMF as a polar solvent to wash out the sulfur compounds adsorbed into alumina turned out to be effective, and 93% of the remaining sulfur compounds (1590 ppmw) are washed out by DMF. The total amount of recovered sulfur compounds from the column is 94%. Considering the similar results of the remaining OSCs in MGO diesel after sulfur removal (21 ppmw from liquid extraction and 23 ppmw from solid adsorption, consecutively), a comparison is made to choose a feasible method for sulfur removal after the oxidation phase. In liquid/liquid extraction, 12 g of MGO diesel is added to 12 g of acetonitrile and shaken for 2 min. This extraction is done four times with an equal amount of remaining diesel and acetonitrile. In each extraction, 10∼12% of the diesel is lost during the separation of diesel and acetonitrile. For the remaining 8 g of diesel after four extractions, 35 g of acetonitrile is consumed. In other words, 0.23 g of diesel is desulfurized per 1 g of acetonitrile. In the above-mentioned solid adsorbent process with acidic alumina,


Etemadi and Yen

Figure 3. Consecutive washes of toluene and DMF for MGO fuel. Table 1. Relative Amounts of Sulfur Removing Agents Per Unit of MGO Diesel

Table 2. Adsorption Comparison of 2-Methylnaphthalene and DBTO to Acidic Alumina

sulfur-removing agent

a

units

diesel

acetonitrile

alumina

weight (g) volume (L)

1 1

4.34 4.57

0.13 0.12a

compound 2-methylnaphthalene dibenzothiophene sulfone

original concentration concentraction after adsorption adsorbance (ppmw) (ppmw) (%) 245 257

243 2

>1 99.2

Density including voids in packed column ) 0.89 g/cm3.

7.5 g of diesel per 1 g of alumina is desulfurized. Table 1 shows the relative amounts of acetonitrile and acidic alumina (∼150 mesh) per unit of MGO diesel. For alumina as a solid adsorbent, the weight or volume consumption is remarkably lower than that for acetonitrile solvent. Alumina is 33 times less consumed than acetonitrile by weight per gram of diesel (0.13 g of alumina corresponds to 4.34 g of acetonitrile in order to remove 99% of the OSCs in MGO diesel). This number gives a base for a feasibility study of the system to be added to the UAOD process as a sulfone removal method. Alumina is in a powder form that is much safer to handle, especially in mobile equipment such as naval ships or army tanks. It is not flammable, and it will not leak in storage rooms, and therefore it has little amount of loss during shipment. According to the consumption rates in this study, alumina is used in adsorption towers instead of acetonitrile for extraction. This comparison has been done without considering the ability of alumina as a stable adsorbent for several regenerations. Preliminary studies of regenerating alumina through calcining at 550 °C have confirmed that alumina will keep more than 98% of its capacity.32 On the other hand, for 99% desulfurization of the MGO diesel, 12.8 mg of organic sulfur is removed per 1 g of alumina. This removal quantity is achieved without using composite adsorbents. A similar removal has been reported by using a transition metal supported on activated carbon.29 An advantage of the developed UAOD process is clarified through simply using alumina for high levels of OSC removal in diesel fuel. In order to present a feasible method, the recovery of the media can be followed after separation through calcining instead of using solvents such as DMF for regeneration of the alumina. 3.2. An Alkylnaphthalene and Sulfone Model Compound. Analyzing the model compound sample (2-MN and DBTO) with GC-FID before and after alumina adsorption shows the high selectivity and affinity of the adsorbent for oxidized sulfur compounds in the presence of an alkynaphthalene with low polarity. Table 2 clarifies that only less than 1% of 2-methylnaphthalene is adsorbed into the acidic alumina. Comparing this

amount to 80% loss of the alkylnaphthalenes in MGO diesel after liquid/liquid extraction with acetonitrile10 shows that alumina as a solid adsorbent is a better option than liquid solvent for sulfur removal in the UAOD process. At the same time, more than 99% of the DBTO is adsorbed, which indicates the selective nature of alumina toward oxidized sulfur compounds in diesel fuel. It is important that hydrocarbons in diesel fuel have less affinity to oxidation than the organic sulfur compounds except the olefins. Among different groups of hydrocarbons such as n-paraffins, alkyl cyclohexanes, alkyl benzenes, alkyl naphthalenes, and so forth, the latter is extracted by polar solvents such as acetonitrile in high percentages. Figure 4 shows that, by using alumina instead of acetonnitrile, more than 99% of this compound will be maintained in diesel. Other reports have shown 6% loss of capacity by using carbon aerogels (CAs) as adsorbents for a mixed compound of DBT and 2-methylnaphthalene.35 This shows that methylnaphthalene competes for adsorbent sites in CA, but this is not the case for alumina (less than 1% adsorbance of 2-methylnaphthalene). Therefore, alumina shows better results in keeping its adsorption sites for sulfones even in the presence of 2-methylnaphthalene. This experiment shows the selectivity of alumina between the two compounds and the feasibility of adsorption. 3.3. UAOD Scaleup for JP-8 Jet Fuel in a Continuous Flow System. The results show that the UAOD process is capable of scaling up from a lab-scale and batch system to a continuous flow system followed by a fixed-bed adsorption tower and still reach a high degree sulfur removal. Major sulfur compounds in JP-8 consist of BT derivatives. The original sulfur content of JP-8 is 863 ppmw, and after treatment it reaches about 1ppmw. Ambient temperature and atmospheric pressure of the UAOD process on a large scale give the advantage of designing a simple automation system. The oxidation and adsorption processes react under mild conditions, and there is no need for costly high-pressure and high-temperature valves, and no delays (35) Jayne, D.; Zhang, Y.; Haji, S.; Erkey, C. Dynamics of Removal of Organosulfur Compounds from Diesel by Adsorption on Carbon Aerogels for Fuel Cell Applications. Int. J. Hydrogen Energy 2005, 30, 1287-1293.



Figure 4. GC-FID chromatogram of 2-methylnaphthalene + DBTO: (top) original and (bottom) after alumina adsorption.

in the system will occur due to these cases. The continuous flow can be fully automated and controlled from a personal computer.36 Figure 5 shows the results of GC-PFPD injections for original, oxidized, and desulfurized JP-8 diesel. An original and desulfurized sample of JP-8 after alumina adsorption was measured for sulfur content with a sulfur-in-oil analyzer to confirm the results from GC for total sulfur concentration before and after treatment. No sulfur was detected with the sulfur analyzer for the treated JP-8 sample. A scaleup experiment of the UAOD process in a continuous system resulted in more than 99% sulfur removal for JP-8 jet fuel. The composition of OSCs in the JP-8 pattern shows mostly BT groups, and these BT compounds have completely oxidized to sulfones under ultrasound-enhanced oxidation. Sulfur compounds in diesel fuels with retention times under 20 min are mostly BTs with alkyl carbon from 0 to 5. Compounds with retention times after 20 min are DBTs with alkyl carbons varying from 0 to 6. With the existing portable unit, a capacity of 12.5 lbs/h is achievable with high conversion efficiency. (36) Alcantara, R. Automation of a Fixed-Bed Continuous-Flow Reactor. J. Autom. Chem. 1994, 16, 187-193.

When JP-8 is under the experimental conditions of irradiation under ultrasound for 10 min, all sulfides are oxidized to sulfones, as shown in Figure 5. From the experiment, the higher affinity for BTO to adsorb into acidic alumina is observed;33 therefore, this can be a reason for better results in the UAOD process in a continuous flow system with alumina adsorption. Compared to other fuel types like MGO, JP-8 achieves sulfur levels as low as 1 ppmw in our system due to its sulfur compound being in the BT group. Also, DBTO is a larger molecule than BTO, and each DBTO molecule occupies more adsorbent sites than a BTO molecule; therefore, the number of molecules of BTO that can be adsorbed into the same adsorbent surface area is higher. 4. Conclusion Recent efforts in the UAOD process were stymied by up to an 80% loss of the alkylnaphthalenes through liquid/liquid extraction and practical problems for scaleup. The loss of alkylnaphthalenes has been overcome by using alumina as a solid adsorbent instead of acetonitrile for solvent extraction. The efficiency and selectivity of alumina in the UAOD system has


Etemadi and Yen

Figure 5. Gas chromatograms from GC-PFPD of JP-8 jet fuel before and after treatment. An asterisk (*) denotes that all sulfides are removed by adsorption.

Figure 6. Simple block diagram of the continuous UAOD process. This process is carried out mostly at ambient temperature and atmospheric pressure; however, there are optimal cases at higher temperatures and pressures. We are in the process of trying to achieve an optimum solution at lower temperatures and pressures.

been successfully proven, and at the same time, the whole process has no deleterious effect on the main hydrocarbons in MOG diesel fuel and JP-8 jet fuel. In this way, system scaleup is also feasible due to a much lesser volume of alumina powder that is needed compared to solvent for sulfur extraction. The developed UAOD process is illustrated in the block diagram of Figure 6. This is a continuous system of oxidative and adsorptive desulfurization. The sonoreactor has been tested to treat enormous amounts of fossil fuel in a short time with high oxidation rates under ambient temperature and atmospheric pressure. The modular pattern of the system enables more than one sonoreactor to be connected in series (qualitative purpose) or in parallel (quantitative purpose). After sonication, the oxidized fuels are delivered to a separator which is used for the de-emulsification of the oxidized oil from the aqueous phase. The used PTA and TMC in the aqueous phase can be regenerated and delivered back to the premixed tank. The sulfones can be removed from the oxidized fuels using

an adsorption tower. Activated alumina is used as a solid adsorbent at ambient temperature and atmospheric pressure. Alumina is a solid adsorbent that has the potential to be used commercially for separating OSCs in fossil fuels. Alumina can be regenerated by calcining, while the sulfones can be thermally destructed. Alumina-packed columns have the same efficiency of sulfone removal as liquid/liquid extraction. The recovery of alumina from sulfone adsorbents had also been proven at high rates (94%) by DMF solvent, but calcining is preferred for regeneration of the alumina, which can keep 99% of the adsorption capacity. In the UOAD continuous system, the sulfur level of 1 ppmw is achieved from treating JP-8 jet fuel that is good enough to be used as a source for producing H2 for fuel cells. The capacity of the UAOD continuous flow system scaleup is 1 bpd. For the case of MGO diesel fuel, if sulfur concentrations less than 23 ppmw are desired, the desulfurized diesel is sent to further polishing with solid catalysts and removal of possible refractory


compounds to reach ULSF as a final product. The rate of continuous reaction is accordingly enhanced and can be seen in a separate paper.37 Multiple columns as solid adsorption towers are used for sulfur removal after the sonoreactor. Screening of the adsorbent will be made at this stage with the proper adsorption isotherm test and column breakthrough test. Ambient temperature and atmospheric pressure in the UAOD process give the advantage of designing a simple automation system in the production line of ULSF. Acknowledgment. The authors are deeply grateful for the financial support of NAVSEA-ONR through Northrop Grumman and CalNova Tech, and also for the funding from ARL.

Appendix

(37) Wan, M.-W.; Cheng, S.-S.; Yen, T. F. A Portable Continuous Desulfurization Unit for Logistic Fuels. Unpublished work.


Nomenclature OSC ) organic sulfur compounds HDS ) hydrodesufurization ODS ) oxidative desulfurization UAOD ) ultrasound-assisted oxidative desulfurization FCC ) fuel catalytic cracker ULSF ) ultralow sulfur fuel SOFC ) solid oxide fuel cell PEMFC ) proton exchange membrane fuel cell MGO ) marine gas oil SCD ) sulfur chemiluminiscence detector ARL ) Army Research Laboratory PFPD ) pulsed flame photometric detector FID ) flame ionization detector SLFA ) sulfur-in-oil analyzer NDXRF ) nondispersive X-ray fluorescence BT ) benzothiophene MN ) 2-methylnaphthalene TOAF ) tetraoctylammonium fluoride SRT ) sulfur removal technology DMF ) N,N-dimethyl formamide DBTO ) dibenzothiophene sulfone CA ) carbon aerogel GC ) gas chromatography MS ) mass spectrometry T ) thiophene DBT ) dibenzothiophene PTA ) phase-transfer agents TMC ) transition metal catalyst bpd ) barrels per day ppmw ) parts per million by weight EF0700174

Selective Adsorption in Ultrasound-Assisted Oxidative Desulfurization

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