Refractory sulfur compounds in gas oils

Abstract

Five heavy straight run gas oils from different crude oils were analyzed by sulfur and carbon sensitive high resolution capillary gas chromatography. Gas oils also were hydrotreated in a fixed bed gas-phase reactor. Model substances, representing refractory sulfur compounds, were converted in a CSTR slurry reactor. Sulfur concentration profiles of the original oils were calculated and showed the sulfur concentration as a function of boiling point to increase in distinct stages, each of these marking the appearance of a new family of sulfur compounds (benzothiophenes, dibenzothiophenes, and naphthobenzothiophenes). With increasing sulfur content of the oils, the concentration of the higher benzologues sulfur compounds (dibenzothiophenes in particular) did not increase proportionally but significantly less. The concentration of refractory sulfur compounds (i.e., 4-alkyl- and 4,6-dialkyl-dibenzothiophenes), accordingly, increased less than proportionally to the total sulfur content. In hydrotreating of gas oils, most of the benzothiophenic sulfur had already been removed at the 50% conversion level of the dibenzothiophenes. Families of higher benzologues sulfur compounds (e.g., naphthobenzothiophenes) seem to be even less reactive. Ordering reactivities of monomethyl-dibenzothiophenes in terms of 50% conversion temperatures shows a 25–27 K difference in afforded reaction temperature between the most and the least reactive isomers and no significant further effect of the further oil matrix on the catalyst specificity. In general terms spatial demands of the HDS reactions and spatial constraints of the catalyst active sites appear to essentially rule rates and selectivities of deep HDS of gas oils. Overall reactivities of the benzothiophene and the dibenzothiophene fractions show an almost 60 K difference. Total sulfur reactivity was found to depend strongly on the composition of the oil (with respect to sulfur compound fractions). Liquid-phase hydrotreating indicated a higher activity of novel deep HDS catalysts as compared with CoMo/Al 2O 3 and a higher specificity for refractory sulfur compounds as explainable by a change in the HDS mechanism. Activity improvements up to factor 6 on a catalyst weight base have been observed.