(559ad) Adsorptive Desulfurization of Liquid Fuels at Elevated Temperatures Using Metal Exchanged Zeolite Y

Authors: 
Dias da Silva, P., Rice University
Zygourakis, K., Rice University
Wong, M. S., Rice University
Samaniego, S., Rice University
Introduction

The high energy density, ease of transportation, and ready availability of liquid hydrocarbon fuels make them an attractive option for generating electricity remotely with auxiliary power units (APUs) containing a reformer and fuel cells [1]. This approach remains problematic, however, since the sulfur originally in the liquid fuels can quickly deactivate the fuel cell electrodes [2]. As a consequence, ultra-low sulfur fuels (ideally with total sulfur content of less than 1 ppm) are required for the trouble-free operation of fuel cells. Among the technologies available for desulfurizing fuels, adsorptive desulfurization has emerged as a promising approach because it does not require high volumes of solvents or reactive gases, which are not available in remote locations [3]. Therefore, this process has been under investigation for remote desulfurization of liquid fuels for fuel cell applications. More specifically, desulfurization of JP-8 has gained attention because JP-8 is the main fuel used by the military of NATO nations. Generating electricity with fuel cells in remote areas is a primary need for the military, particularly for silent watch and ground soldier applications [1]. JP-8 is composed primarily of paraffinic, aromatic and naphthenic hydrocarbons. Sulfur concentrations in JP-8 can reach up to 3,000ppmw, present predominantly in alkylated benzothiophenes [4].

Several studies have reported using metal exchanged zeolites for adsorptive desulfurization at near ambient temperatures [5], [6]. Since sulfur removal at low temperature involves reversible complexation, spent adsorbents can be easily regenerated [7]. Such weak reversible complexation bonds, however, are not sufficiently specific to remove substantial amounts of sulfur from real fuels because of the co-extraction of aromatic compounds. Aromatics are similar in size and structure to the predominant sulfur species found in jet fuels (alkylated benzothiophenes) and compete with them for the active sites [8]. And since jet fuels usually contain 10%vol to 25%vol of aromatics [9], only a small fraction of the organosulfur compounds present in the fuel can be removed.

Therefore, alternative approaches should be considered to address the problem of competitive adsorption and improve the selectivity of sulfur adsorption. Some theoretical studies have suggested that elevated temperatures may improve the adsorption selectivity of organosulfur compounds on zeolites [10]. However, very little is known about the effect of temperature on the adsorption of organosulfur compounds from liquid fuels on zeolitic materials beyond near ambient temperatures. Our understanding about the effect of temperature on adsorptive desulfurization is incomplete.

Elevated Temperatures Significantly Improve the Sulfur-Removal Capacity of CuNa-Y Zeolite

In an effort to overcome the limitations imposed by the co-extraction of aromatics in liquid fuels, we studied adsorptive desulfurization at temperatures substantially higher than ambient temperature. Na-Y and Cu-exchanged Na-Y (CuNa-Y) zeolites were used to remove sulfur first from model fuels and then from a JP-8 fuel at temperatures up to 180oC.

Batch desulfurization and TPD experiments with model fuels containing 3-methyl-benzothiophene (3-MBT), dodecane and toluene showed that 3-MBT removal was strongly dependent on treatment temperature and involved weak physisorption bonds with Na sites, weak complexation bond with Cu sites at 30 or 80oC and strong chemisorption S-Cu bonds at 130 or 180oC. Overcoming the competition that favors the adsorption of aromatics at low temperatures, the formation of S-Cu bonds at high temperatures shifted the balance and allowed significant levels of 3-MBT removal even when toluene was present at high concentrations.

We then proceeded to carry out desulfurization experiments with a JP-8 fuel containing 2,230 ppmw of total sulfur. While our CuNa-Y zeolite removed very little sulfur from the JP-8 at 30 or 80oC, the use of elevated temperatures dramatically improved the desulfurization efficacy of CuNa-Y zeolite, increasing its sulfur-removal capacity from 2.5mg-S/g-adsorbent at 30oC to 36mg-S/g-adsorbent at 180oC.

To determine if CuNa-Y zeolite at 180oC can remove all sulfur compounds present in JP-8, we submitted the fuel to a sequence of batch desulfurization experiments at this temperature. Treated JP-8 fuel (recovered from the first desulfurization step) was desulfurized again with fresh adsorbent in the batch reactor and the same process was repeated two more times . These sequential batch experiments simulate a continuous desulfurization process. We found that the first desulfurization step removed the least refractory organosulfur compounds, that is those with the lowest boiling points. Subsequent steps progressively adsorbed the more recalcitrant organosulfur compounds until all the sulfur had been removed from the JP-8 fuel.

Our results demonstrate the feasibility of using zeolitic adsorbents and elevated temperatures for ultra-deep desulfurization of JP-8 fuels. This technology could potentially enable the use of liquid fuels such as JP-8 on remotely located integrated auxiliary power units containing a reformer and fuel cells.

REFERENCES

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