Heightened concerns for cleaner air and increasingly more stringent regulations on sulfur contents in transportation fuels will make desulfurization more and more important. The problem of deep removal of sulfur has become more serious due to the lower and lower limit of sulfur content in finished gasoline and diesel fuel products by regulatory specifications, and the higher and higher sulfur contents in crude oils. The challenge for deep desulfurization of diesel fuels is the difficulty of removing refractory sulfur compounds, especially 4,6-dimethyldibenzothiophene, with traditional HDS processes. The problem is aggravated by the inhibiting effects of polyaromatics and nitrogen compounds, as well as H2S on deep HDS. The challenge for gasoline deep desulfurization is the selective HDS of thiophenic compounds without a significant loss of octane number, which can be avoided by preserving olefinic components in FCC naphtha or by isomerizing paraffinic components of naphtha. New design approaches are necessary for making affordable ultra-clean fuels.
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