GB794576A - Improved hydrodesulfurization process - Google Patents

Abstract

In the hydrodesulphurization of a petroleum feedstock containing more than 1.5 per cent by weight of sulphur in the presence of a cobalt molybdate catalyst, the catalyst is presulphided by treatment at a temperature at least 50 DEG F. lower than the desulphurization temperature with a petroleum fraction which contains no more sulphur than the feedstock. The process may be used for the treatment of high boiling (e.g. 350-1050 DEG F.) petroleum fractions containing relatively large amounts (e.g. 2-6 per cent by weight) of sulphur. The feedstock may be a straight run distillate or a fraction from thermal or catalytic cracking, coking, or deasphalting. The presulphiding treatment may be carried out with the feedstock to be desulphurized, at a temperature preferably at least 100 DEG F. lower than that of desulphurization; conditions are, for example, a temperature of 550-650 DEG F., a pressure of 200-800 p.s.i.g., a feed rate of 0.5-2 vol./vol./hr., and a recycle rate of 500-3000 S.C.F. of hydrogen per barrel of feed. Hydrodesulphurization conditions are a temperature of 650-800 DEG F., a pressure of 100-1000 p.s.i.g., a feed rate of 0.5-5 vol./vol./hr., and a recycle rate of 500-5000 S.C.F. of hydrogen per barrel of feed. An alternative presulphiding method is to treat the catalyst with a petroleum fraction having a sulphur content of 0.2 to 1 per cent and a boiling point of e.g. 300-700 DEG F. The fraction should preferably contain 90-110 mgs. of mercaptan sulphur per 100 mls. Temperature, feed rate and recycle rate for presulphiding may be the same as in the first embodiment, but the pressure is preferably 100-250 p.s.i.g. The presulphiding treatment is continued until the amount of sulphur in the liquid product reaches an equilibrium; at this stage the catalyst may have a sulphur content corresponding to a 15-65 per cent conversion of cobalt molybdate to cobalt thiomolybdate. The original catalyst may consist of cobalt and molybdenum oxides deposited on alumina, bauxite, silica gel, or activated magnesia; a small amount of silica may be present as a stabilizer. A fixed, moving, or fluidized bed of catalyst may be used; with a fixed bed the catalyst particle size is preferably 1/16 to 1/2 inch. Gas separated from the product stream obtained in the presulphiding or desulphurization steps is scrubbed with an aqueous solution of caustic soda or di- or tri-ethanolamine to remove hydrogen sulphide and other sulphur compounds, and recycled, after heating, if required. The recycle gas should preferably contain at least 40 per cent by volume of hydrogen and less than 5 per cent by volume of hydrogen sulphide; part of the gas may be withdrawn and replaced by fresh hydrogen-containing gas obtained from a hydroforming process, from contacting light hydrocarbons with a nickel catalyst, or from the dehydrogenation of secondary butyl alcohol. When regeneration of the catalyst is necessary, it is desirable to carry out the presulphiding step before the catalyst is used again. In examples: (a) a heavy straight run gas oil containing 2.1 per cent of sulphur is first passed over a cobalt molybdate on alumina catalyst at 600 DEG F. to presulphide the catalyst and then at 700 DEG F. for hydrodesulphurization; the product contains 0.13 per cent sulphur; (b) a similar catalyst is presulphided with a light straight run heating oil containing about 0.5 per cent of sulphur and then used to desulphurize a coker gas oil from a sulphur content of about 4 per cent of 0.4 per cent; (c) a gas oil as in (a) is desulphurized using a similar catalyst which has been presulphided as in (b), the sulphur content being reduced to 0.11 per cent.