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Abstract
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Desulfurization of ultra-low sulfur species remaining in natural gas (NG) is important because of its critical effects on both chemical and clean energy applications. Environmental regulations and fuel cell guidelines require NG with a sulfur content of ppm or less. In this study, the adsorptive behavior of representative trace-level sulfur compounds in CH4 balance, H2S, and tetrahydrothiophene (THT) at 4 and 50 ppm, was investigated on zeolite 5A and 13X pellets. Breakthrough experiments were conducted in various binary systems (one sulfur compound in CH4), and surface analysis of the used zeolites was performed to elucidate the adsorption mechanism. The influence of the CO2 impurity level in CH4 on sulfur removal was also investigated in ternary systems (one sulfur compound with 1.5% CO2 in a CH4 balance). The main pathway of H2S adsorption on zeolite 13X was catalytic oxidation, whereas it was presumed to be dissociative adsorption on zeolite 5A. In contrast, physisorption was dominant for THT removal in both zeolites. The lower contribution of chemisorption in THT led to less adsorptive amount than that in H2S removal at ultralow concentrations. Furthermore, the molecular structure of THT (higher kinetic diameter) resulted in a faster breakthrough for zeolite 5A than for zeolite 13X. Therefore, more amount of H2S was removed than THT in both zeolites, and zeolite 13X demonstrated better performance for both sulfur compounds than zeolite 5A. Notably, the presence of 1.5% CO2 in the feed promotes the catalytic oxidation of H2S.
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