Article pubs.acs.org/EF
Response of Different Types of Sulfur Compounds to Oxidative Desulfurization of Jet Fuel Michael T. Timko,*,† Ezequiel Schmois,‡ Pushkaraj Patwardhan,‡ Yuko Kida,‡ Caleb A. Class,‡ William H. Green,‡ Robert K. Nelson,§ and Christopher M. Reddy§ †
Department of Chemical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, Massachusetts 01609, United States ‡ Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States § Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole, Massachusetts 02543, United States S Supporting Information *
ABSTRACT: Oxidative desulfurization (ODS) removes organic sulfur compounds from liquid transportation fuels (including diesel and jet fuels) in a two-step process: (1) chemical oxidation to form sulfones and (2) adsorption (or extraction) of the sulfones onto a polar adsorbent such as alumina. Continued development of ODS is limited in part by a lack of understanding of how different sulfur types in real fuels respond to its constituent oxidation and extraction steps. We treated two JP-8 jet fuels (described by here as 3773 and 4177, respectively) using the two-step ODS process. These two fuels had similar physical properties and hydrocarbon compositions but differing sulfur contents: the 3773 fuel was 720 ppmw, while that of the 4177 fuel sulfur content was 1400 ppmw. For the two-step ODS process, we used activated carbon-promoted performic acid as the oxidant and activated alumina as the adsorbent. The complete ODS treatment reduced the sulfur content of the 3773 fuel to a level below the detection limits of our total sulfur analyzer (40 ppmw), implying >94% sulfur removal. However, ODS treatment reduced the sulfur content of the 4177 fuel to 350 ppmw, or 75% sulfur removal. To investigate this discrepancy at the molecular level, we targeted sulfur compounds in the stock and treated fuels using one-dimensional gas chromatography and comprehensive two-dimensional gas chromatography with both sulfur selective detection and time-of-flight mass spectrometry. Initially, the 4177 fuel was dominated by a suite of compounds identified as sulfides, disulfides, and thiophenes (SDT), whereas the 3773 fuel was dominated by its benzothiophene (BT) content. The SDT compounds were easily oxidized, but the corresponding sulfones were not efficiently removed using the alumina adsorbent. The BT compounds were more resistant to oxidation than the SDT compounds, but the oxidized BT compounds were more efficiently removed using the adsorbent than either the BT compounds or oxidized SDT compounds. Development of ODS technologies should account for the different responses of different sulfur compounds to the oxidation and adsorption treatments.
1. INTRODUCTION Oxidative desulfurization (ODS) has the potential to be an energy-efficient alternative or complement to hydrodesulfurization (HDS) for the removal of sulfur from hydrocarbon fuels, especially for remote, portable, or mobile niche applications.1−11 Whereas HDS is reduction by hydrogen, ODS consists of two steps: (1) chemical oxidation into organic sulfones and (2) adsorption or extraction of the sulfones.1−3,11−14 Figure 1 provides structures for some common sulfur compound petroleum and petroleum fractions and their sulfone products resulting from oxidation. After oxidation, the majority of the sulfones can be removed with high capacities (>10 mg g−1) using common hydrophilic adsorbents, such as silica or alumina.15 ODS has several potential advantages over HDS: (1) operation under conditions milder than those of HDS (200 °C and >100 bar for HDS); (2) no need for hydrogen, thereby reducing the overall system size, reducing safety risks, and potentially reducing operating costs; and (3) effectiveness in removing thiophene and especially dibenzothiophene (DBT) © 2014 American Chemical Society
compounds that have been shown to be refractory to HDS. Otsuki et al.4 and later Te and Fairbridge5 reported that compounds shown to be refractory to HDS, specifically dibenzothiophene and its alkylated derivatives, were readily oxidized in the presence of a hydrogen peroxide. ODS compares favorably to adsorption desulfurization (ADS), another frequently tested sulfur removal technology. Because the polarities of the parent sulfur compounds are very similar to those of the fuel hydrocarbon compounds, ADS requires expensive adsorbents, high-temperature operation (>100 °C), and difficult regeneration steps. 2,16−35 In comparison, sulfur compounds are readily oxidized to more highly polar sulfone compounds,1,3 and ODS can be effective with catalysts and adsorbents that are less expensive than those required for ADS and conditions milder than those required for ADS. Received: January 21, 2014 Revised: April 23, 2014 Published: April 24, 2014 2977
dx.doi.org/10.1021/ef500216p | Energy Fuels 2014, 28, 2977−2983
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Article
Table 1. Physical and Chemical Properties of the JP-8 Jet Fuels molecular composition paraffins (wt %) alkylbenzenes (wt %) alkylnaphthalenes (wt %) indans and tetralins (wt %) bulk composition total sulfur content (ppmw) hydrogen content (wt %) distillation properties initial boiling point (°C) 10% recovered (°C) 20% recovered (°C) 50% recovered (°C) 90% recovered (°C) end point (°C) residual (vol %) physical properties density (g cm−3) freeze point (°C) heat of combustion (MJ kg−1)
Figure 1. Benzothiophene (top) and dibenzothiophene (bottom) structures with carbon and sulfur atoms labeled by position. Benzothiophene-sulfone and dibenzothiophene-sulfone, formed after oxidation of the native thiophene compounds, are also shown.
While ODS was described many years ago and a wide range of catalysts and oxidants have been evaluated,1,2,4,5,7−14,36−42 most of this work has been performed on e.g., DBT in isooctane, and data for real transportation fuels are sparse. In part because of their lower overall usage rates, jet fuels in particular have received scant previous attention. In fact, even the molecular distribution of sulfur compounds present in jet fuels has not been comprehensively studied.43,44 Nonetheless, jet fuels are critical products, especially for the military as JP-8 jet fuel is a primary logistics fuel. Progress developing ODS techniques for real fuels, and jet fuel in particular, has been limited in part by a lack of data about the performance of ODS on a molecular level. Specifically, the oxidation and adsorption tendencies of different types of sulfur compounds are unknown, limiting current practices for optimizing individual steps and thereby reducing the overall efficiency of the ODS process. To investigate how ODS affects different sulfur-containing compounds, we treated JP-8 military jet fuel using a well-known ODS protocol3,13,41 and tracked the changes in sulfur speciation using gas chromatography and sulfur chemiluminescence detection (GC−SCD), comprehensive two-dimensional gas chromatography with sulfur chemiluminescence detection (GC×GC−SCD), and comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry detection (GC×GC−ToFMS). Specifically, we used comprehensive GC×GC−SCD to monitor the reactivity of benzothiophenes (BT), sulfides, disulfides, and thiophenes (SDT), and sulfones (SU) to the oxidation and alumina adsorption steps in ODS. GC×GC is a powerful analytical technique that allows separation of compounds first on the basis of their volatility (as is typically done using GC) and then on the basis of their polarity (hence, the term “two-dimensional GC”); the term “comprehensive” is used to denote that all compounds injected into the system are conserved and separated on both columns. This study provides invaluable insights into the ODS process by investigating at the molecular level, thus providing knowledge for modifying and improving future efforts for science-based design of ODS processes for transportation fuels.
POSF 3773
POSF 4177
80.0 13.6 2.3 4.1
79.6 12.6 1.5 6.3
720 13.9
1400 13.7
152 173 179 198 239 260 1.1
162 183 190 207 237 265 1.2
0.799 −50 43.0
0.815 −58 43.0
exception of the 4177 fuel (1400 ppmw) containing roughly twice the amount of sulfur as the 3773 fuel (720 ppmw). Otherwise, the two fuels have nearly identical paraffin and alkylbenzene contents, hydrogen contents, distillation properties, and other physical properties. Hydrogen peroxide (30 wt % aqueous solution), formic acid (99% pure), and calcium chloride (99.99% on a trace metal basis) were acquired from Sigma-Aldrich and used as received. HP-120, a phosphoric acid wood-based activated carbon, was a gift from Pica and was used as received. Activated alumina (Brockman I, acidic) was obtained from Sigma-Aldrich and baked overnight at 220 °C in air to remove water and used shortly after being removed from the oven. 2.2. ODS Treatment. For demonstration purposes, we elected to use an ODS protocol that utilizes formic acid and hydrogen peroxide as a two-component oxidant and activated carbon as a promoter.3 A broad range of ODS technologies have been reported in the literature; this method was chosen on the basis of previous studies of JP-8 jet fuel.3 As described previously,3 the acid/peroxide/carbon system has the advantage of not requiring hazardous or expensive chemicals, such as organohydroperoxides. A disadvantage, especially for mobile systems or military applications, is the volume of aqueous reagents required as the chosen ODS method requires approximately 1 part oxidant phase per 3−4 parts treated fuel phase, on a volume basis. A 500 mL glass batch reactor that was closed to the atmosphere was used for all experiments. The reactor was loaded with 180 mL of fuel, and then the contents were equilibrated at 60 °C for 10 min. The aqueous phase was prepared by mixing 35 mL of hydrogen peroxide and 10 mL of formic acid. To that mixture was added in one portion 120 mg of activated carbon. The oxidant/slurry two-phase mixture was added to the reaction flask. A PTFE-coated motor-driven impeller, stirring at approximately 1000 rpm, was positioned at the boundary between the two phases to improve interfacial contact. A thermocouple (type T) placed directly in the fuel/water mixture monitored the reaction temperature within ±1 °C. On the basis of previous work,3 the reaction proceeded for 90 min and 15 mL samples were removed every 45 min. To maintain the initial volume fractions throughout the experiment, the organic/water ratio of samples was approximately 180/45. Withdrawn samples were increasingly yellow with reaction time. The organic phase (upper layer) was decanted to remove the aqueous phase (lower phase). To remove any residue water in the organic phase, a small quantity of calcium chloride was added to each organic sample and the sample
2. MATERIALS AND METHODS 2.1. Materials. Two JP-8 jet fuels were obtained from the Air Force Research Laboratory (AFRL) and used for all tests. We have adopted the AFRL identifiers to describe these fuels as samples “3773” and “4177”. Table 1 summarizes the AFRL bulk physical and chemical properties of the two jet fuels. Overall, they were very similar, with the 2978
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placed in a rotary shaker for 30 min. After drying, the samples were filtered with syringe filters (0.22 μm pore size) to remove calcium chloride. When contained in 30 mL sample flasks, the final samples were clear and faintly yellow. For the extraction of the polar sulfur compounds formed during oxidation, aluminum oxide was added to each sample at a 10/1 fuel/ alumina weight ratio, and the samples were shaken in a rotary shaker for 30 min. The mixtures were filtered through a cellulose membrane to remove any suspended aluminum oxide. Samples were retained for analysis (1) before any treatment, (2) after only aluminum oxide adsorption, (3) after only oxidation treatment, (4) after the combined alumina oxide and oxidation treatment, and (5) after a single oxidation and two adsorption treatments. The Supporting Information provides a schematic of the procedure. 2.3. Sample Analysis. Fuel samples were analyzed using a combination of techniques, including gas chromatography (GC, Agilent 7890) using a sulfur chemiluminescence detector (SCD, Agilent/Sievers 355) and a RTX-1 column (30 m length, 250 μm internal diameter, 4 μm film thickness); comprehensive twodimensional gas chromatography with SCD (GC×GC−SCD, Leco) with an HP-5MS primary column (30 m, 250 μm internal diameter) and a Rxi-17Sil-MS secondary column (2 m, 250 μm internal diameter); and GC×GC with a time-of-flight mass spectrometer detector (GC×GC−ToFMS, Leco). The two GC×GC columns were selected to have complementary chemistries to afford simultaneous separation based on vapor pressure and polarity. The primary HP-5MS column consists of dimethyl siloxane (95%) with 5% diphenyl polysiloxane and is regarded as one of the least polar polysiloxane columns commonly available. Accordingly, the RTX-1 column provides separation based primarily on differences in vapor pressure among the compounds present in the mixture. The secondary Rxi-17Sil-MS is composed of equal amounts of dimethyl polysiloxane and diphenyl polysiloxane and is regarded as having “midpolar” separation characteristics. Accordingly, the Rxi17Sil-MS separates compounds on the basis of both vapor pressure and polarity. SCD is a sulfur-specific detection technique that converts sulfur to SO2 using ozone and then quantifies the SO2 using chemiluminescence. By virtue of the selective chemical oxidation step and subsequent quantification via chemiluminescence, SCD achieves an extremely high sulfur sensitivity (106), and a linear range (>3 orders of magnitude). In comparison, another common sulfur detection method, flame photometric detection, achieves inferior sulfur sensitivity (20 pg s−1), a selectivity (∼105), and a linear range (∼3 orders of magnitude). These features proved to be necessary for the detection and quantification of the full range of sulfur compounds present of the neat and treated JP-8. Samples were analyzed using GC×GC (Leco) equipped with a time-of-flight mass spectrometer (ToFMS), closely following the protocols described previously.45,46 As with GC×GC−SCD, the GC×GC−ToFMS technique utilized a nonpolar (5% diphenyl polysiloxane) and midpolar column (50% diphenyl polysiloxane) in series to obtain a volatility and polarity separation. Unlike SCD, ToFMS was capable of providing unit mass resolution chemical composition information using electron impact ionization and fragmentation followed by time-of-flight mass spectrometry. Because of molecular differences in ionization efficiency and fragmentation, the estimated detection limit for ToFMS is less well-defined than for SCD. The vendor specifies the detection limit as a 10/1 signal-to-noise ratio for 2 pg of a hexachlorobenzene standard at a mass of 284, with a linear response over 4 orders of magnitude. In practice, we find that ToFMS is ∼10 times less sensitive than the SCD technique, though we have not fully quantified the comparison. In addition to GC analysis, X-ray fluorescence (XRF, Horiba SLFA1800H) was used to quantify the total sulfur content.
3. RESULTS AND DISCUSSION Table 2 provides total sulfur analysis data for the neat and treated fuels. Clearly, the two-stage ODS process is far more Table 2. Total Sulfur Contents and Sulfur Distribution Data of Stock and Treated Fuels sample descriptiona
totalb (ppmw)
BTc,d (%)
SDTc,e (%)
SUc,f (%)
720 700
60 3
40
