Catalytic hydro‐desulphurization of gas oil: Contribution of hydrofining studies to the clarification of the structure of the sulphur compounds present in gas oil

Abstract

AbstractIn the course of a study on desulphurization of mineral oils by catalytic hydrogenation, it was recognized that this hydro‐desulphurization reaction may also contribute to the clarification of the nature of the sulphur compounds present.To this end, by means of model experiments with pure synthetic sulphur compounds, an analysis was first made of the gross type of reactions occurring when sulphur compounds are decomposed by hydrogenation into H2S and hydrocarbon fragment(s). The results conclusively indicate that under the conditions chosen and with the type of catalyst used the picture definitely is not that known from hydrocracking: the reaction is virtually limited to a rupture of C‐S‐bonds and hydrogen‐saturation of free valency. Our type of catalyst did effect hardly, if at all, the hydrogenation of aromatic rings present in the sulphur compounds. Only in the case of dibenzothiophene was partial hydrogenation of the rings observed. Parallel experiments with more aggressive catalysts invariably showed considerable aromatic saturation.The results of these model tests were then used in the interpretation of hydrogenation experiments made with a full range gas oil of Middle East origin.In a large number of experiments carried out on this gas oil, the reaction product was scrutinized for the presence of low boiling hydrocarbons. As in no case more than traces of gaseous hydrocarbons or liquid hydrocarbons distilling below 200°C could be found, it is evident that no sulphides and di‐ or polysulphides can have been present in the feed stock. As it was known from chemical analysis that no mercaptans are present either, the sulphur compounds in our specific gas oil must preponderantly be of the thiophene (or thiophane) type.By hydrogenation of a concentrate of the sulphur compounds (obtained by extraction of the gas oil with liquid anhydrous hydrogen fluoride), it was possible to study the corresponding hydrocarbons more closely. By means of careful chromatographic analysis it was shown that these were for about 60 % of aromatic and for 40 % of naphthenic nature, and that only few, if any, non‐cyclic hydrocarbons were present. Consequently, the original thiophene or thiophane compounds present in the gas oil investigated are not of the alkyl thiophene or alkyl thiophane type. They presumably consist as to about 30‐40 % of thiophenes (thiophanes) which contain one homocyclic ring and in about 70‐60 % two homocyclic rings which are not in the condensed configuration are present next to the thiophene (thiophane) ring.No conclusion can be drawn as to the relative proportion of naphthene and aromatic rings in the original sulphur compounds.