(753b) Desulfurization of JP-8 By Cu-Y Zeolite at Elevated Temperatures for Remote Electricity Generation Has Two Distinct Stages: Chemisorption and Surface Reaction

Because of their high energy density, ease of transportation, and ready availability, liquid hydrocarbon fuels are an attractive option for generating electricity in remote off-the-grid locations. This can be achieved with auxiliary power units (APUs) containing a reformer followed by a fuel cell [1]. However, the sulfur content of liquid fuels can quickly deactivate the reformer catalyst and fuel cell electrodes [2]. As a consequence, low-sulfur fuels (ideally with total sulfur content of around or even less than 1 ppm) are required for trouble-free operation of such power units.

Adsorptive desulfurization using metal exchanged zeolites has emerged as a promising technology for providing low-sulfur fuel because it does not require high volumes of solvents or reactive gases, which are not available in remote locations [3]. Several studies have addressed the problem of adsorptive desulfurization at near ambient temperatures [4], [5]. Benzo- and dibenzothiophenes are among the most difficult compounds to remove from jet fuels (JP-8) with high sulfur content. These sulfur compounds adsorb on the active metal sites of a zeolite via weak reversible complexation bonds. However, these sulfur compounds compete for the active sites with the aromatic hydrocarbons that are present in the fuel in much higher concentrations (10-25 %vol for aromatic hydrocarbons vs. at most a few thousand ppmw for sulfur compounds). As a result, metal active zeolites operating at ambient temperatures are not very effective for desulfurizing hydrocarbon fuels and specifically JP-8 [6], [7].

Using batch experiments with model fuels and JP-8, we recently demonstrated that elevated temperatures (130 – 180 ^oC) can help us overcome the competition between aromatic and sulfur compounds that limit the adsorption capacity of Cu exchanged Na-Y zeolites [8]. Operating at 180 °C instead of 30 °C gave a 14-fold increase in the ability of a Cu exchanged Na-Y zeolite to remove sulfur from JP-8 fuel with high sulfur concentration (2,230 ppmw). TPD experiments with spent CuNa-Y zeolites used for adsorbing sulfur compounds from model fuels convincingly showed that Cu sites retained hydrocarbons more strongly when desulfurization was carried out at temperatures above 130 °C. This was probably due to a stronger (most likely covalent) bond formed between sulfur compounds and the Cu sites in the zeolite. Desulfurization tests with JP-8 showed that both Cu sites and high temperatures (130 °C or higher) were needed for substantial sulfur removal of such JP-8 sample, with the maximum sulfur removal observed at 180 °C [8].

Adsorptive Desulfurization in a Flow System

To test the feasibility of our novel desulfurization method to produce a continuous stream of desulfurized JP-8 for remote electricity generation, we performed experiments in a continuous flow adsorber column packed with pellets of Cu-Y zeolite. The 12-inch in length by 1-inch ID column was maintained at a constant temperature of 180 °C and a pressure of 200 psi. JP-8 containing 2,230 ppmw sulfur was fed to the reactor at three flow rates (0.2, 1 and 5 mL/min) and the total outlet sulfur concentration was measured using an XRF total sulfur analyzer. We also analyzed the treated fuel with gas chromatography (GC-FID and GC-PFPD) to evaluate the overall distribution of hydrocarbon compounds and the distribution of organosulfur compounds in the treated fuel. Lastly, we characterized the spent adsorbent using SEM and studied its regeneration via thermogravimetric analysis (TGA).

Ultra-low sulfur concentrations (< 1ppmw) were reached for all flow rates. At 0.2 mL/min, ultra-low concentrations were maintained for several days and breakthrough at 1, 10 and 100 ppmw were observed after 660, 770, and 847 mL of desulfurized fuel was collected. The measured sulfur uptake at 1 ppmw breakthrough was 15.57 mg-S/g-ads, or 1.03 mol-S/mol-Cu. After 111 hours of operation at this flow rate, 1.33 L of JP-8 was processed and the sulfur uptake was 27.00 mg-S/g-ads, or 1.79 mol-S/mol-Cu, approaching the maximum values observed with our batch experiments. Ultra-clean fuel (<1 ppmw sulfur) was also obtained with the higher flow rates. As expected, however, breakthrough occurred much earlier as the JP-8 flow rate increased. While 660 mL of JP-8 with < 1 ppmw sulfur was obtained with a flow rate of 0.2 mL/min, the corresponding volumes of ultra-clean JP-8 were 162.6 mL for 1 mL/min flow rate and 96 mL for 5 mL/min.

Gas chromatography data using a flame ionization detector (GC-FID) showed a hydrocarbon matrix in the treated JP-8 similar to that of untreated JP-8, with the main peaks corresponding to paraffinic hydrocarbons ranging from C9 to C15. GC-FID chromatograms of treated samples at higher flow rates (1 and 5 mL/min) showed similar results, suggesting that the overall hydrocarbon matrix in the fuel did not change upon treatment. These results agree with our earlier report in which JP-8 was desulfurized in batch reactors using Cu-Y zeolite at the same temperature [8]. Additionally, the organosulfur compounds found during breakthrough using gas chromatography coupled with a pulsed flame photometric detector (GC-PFPD) corresponded to the alkylated benzothiophenes originally present in the untreated JP-8.

The most significant observation, however, involved a dramatic deviation from the breakthrough behavior one would expect from an adsorption system. After reaching a level that strongly depended on the flow rate, the outlet sulfur concentration did not continue to increase as expected. Instead, the outlet sulfur concentration leveled off and we detected the release of hydrogen sulfide. This is a strong indication that the mode of sulfur removal shifted from chemisorption on the partially saturated adsorbent to surface reactions of the organosulfur compounds that involved cleavage of C-S bonds [9]. These reactions gave us 60 to 70 wt% sulfur removal that was maintained for a significant period of time until the release of hydrogen sulfide forced us to end the experiments for safety reasons. In further agreement with literature data [9], the catalytic decomposition reactions of organosulfur compounds deposited significant amounts of carbon on the Cu-Y zeolite. Elemental analysis of the spent adsorbent showed that it contained 27-31%wt carbon, while the initial total carbon content in the pellets before desulfurization was 0.012%wt, as determined by ASTM E1019-18.

Regeneration tests were performed using thermogravimetric analysis (TGA). Because carbon was deposited on the surface of the adsorbents, the regeneration procedure was carried out in air to convert the carbon and sulfur depositions to carbon dioxide and sulfur dioxide. High regeneration temperatures (550 or 600 °C) were necessary to restore the adsorbent to its initial color. Regeneration under these conditions restored the adsorptive desulfurization performance of our zeolite to about 90% of its initial value.

We conclude that desulfurization of JP-8 by Cu-Y zeolite at elevated temperature proceeds in two distinct stages. Sulfur compounds are first removed by chemisorption and the utilization of the Cu active sites reaches values that may be significantly higher than 1 mol of S per mol of Cu (1.79 mol-S/mol-Cu for 0.2 mL/min). When the saturation of the adsorbent becomes very high, however, the mode of sulfur removal shifts from chemisorption to surface reaction that cleaves C-S bonds, releases hydrogen sulfide and deposits significant amount of carbon on the zeolite.

Our study also demonstrates that continuous flow adsorbers packed with Cu-Y zeolite and operating at elevated temperatures can be used for adsorptive desulfurization of JP-8 for remote electricity generation using fuel cells. We will present calculations showing how production requirements of clean fuel can be met by careful selection of column lengths, flow rates and by using parallel column bundles. Finally, the catalytic reactions we observed at the end of the chemisorption stage open new possibilities for employing adsorber/reactor cascades to meet low sulfur content and production rate specifications.

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