GB654152A - Improvements relating to the catalytic desulphurisation of naphthas - Google Patents

Abstract

Straight-run naphtha is desulphurized by passing it in vapour form in admixture with hydrogen over a sulphur-resistant catalyst, which catalyses not only the dehydrogenation of naphthenes to aromatics but also the conversion of organically-combined sulphur into hydrogen sulphide, at a temperature and pressure such that the dehydrogenation of naphthenes contained in the naphtha is effected to an extent not substantially in excess of that required to provide sufficient hydrogen to convert the organically-combined sulphur in the naphtha into hydrogen sulphide and to maintain the vapour phase in the reaction zone, a hydrogen-rich gas mixture being separated from the treated naphtha and recycled to the reaction zone to constitute the whole of the hydrogen supplied thereto. In general, the desulphurization is carried out at 650-800 DEG F. and 25-500 pounds per square inch pressure and results in a product which, except for sulphur elimination, has properties and boiling range similar to the feed-stock. The hydrogen-rich gases may be separated by cooling the treated naphtha at reaction pressure. The hydrogen sulphide in the separated gases builds up to an equilibrium concentration, after which the gases may be recycled to the reaction zone without further increase in the content of hydrogen sulphide which thereafter remains dissolved in the treated naphtha until such time as it is depressurized. If desired, however, the hydrogen sulphide may be removed from the gases and the H2S-free gases recycled to the reaction zone. Before recycling, the gases may be submitted to treatment for increasing the relative proportion of hydrogen therein, for example by passage through an oil tower, whereby light hydrocarbons in the gas are preferentially adsorbed. Catalysts mentioned are cobalt molybdate deposited on or incorporated with a support such as alumina, a mixture of oxides of cobalt and molybdenum, especially a pelleted catalyst comprising these oxides with alumina and graphite, molybdenum oxide on alumina, a mixture of sulphides of tungsten and nickel, molybdenum sulphide. Under the conditions of the process, cobalt and molybdenum oxides and cobalt molybdate react with hydrogen sulphide so that the oxygen is partly displaced by sulphur but this does not detract from the activity of the catalyst. In an example, a straight-run Iranian naphtha of 150 DEG C. end point is passed over cobalt molybdate supported on alumina at 750 DEG F. and 50 pounds per square inch pressure with recycle of hydrogen-rich gases. The products are cooled at the operating pressure and the desulphurized naphtha is given a light caustic soda wash to remove dissolved hydrogen sulphide.