Patent GB-667368-A (GB)
Title: Process for the removal of aromatic mercaptans from catalytically cracked gasoline
Filed: 1948-07-23
Abstract: The spent aqueous alkali metal hydroxide, which results from the refining of catalytically cracked gasoline distillates in the absence of oxygen (see Group III), is acidified with a mineral acid such as hydrochloric acid to liberate the impurities, particularly aromatic mercaptans, removed from the gasoline. Aliphatic mercaptans and phenols are substantially completely released from the hydroxide solution on acidification to a pH of approximately 9, and separate as an oily layer which may be withdrawn.  On further acidification of the hydroxide solution to approximately pH 2, the aromatic mercaptans are substantially completely released and also separate as an oily layer which may be withdrawn.  Some olefins may be present in the hydroxide solution, so that it is desirable to exclued oxygen during the acidification.  According to an alternative method, the hydroxide solution be acidified in one stage to a pH of about 2, another extracted with a solvent such as ether, oxygen during the acidification.  According to an alternative method, the hydroxide solution may be acidified in one stage to a pH of about 2, another extracted with a solvent such as ether, benzene, pentane or hexane. The extract is freed from solvent and treated with alkali hydroxide solution.  There is thus obtained a solution of aromatic mercaptans, aliphatic mercaptans and phenols, free of any residual oil.  This new hydroxide solution is then partially acidified to remove the aliphatic mercaptans and phenols.  In the examples: (1) a spent caustic soda solution from the gaoline refining is acidified with hydrochloric acid to pH 6 and extracted with ether; the extract after evaporation of the solvent is mixed with caustic soda solution and the new caustic solution is then acidified, first to pH 11, then to pH 9, and finally to pH 2; at each acidification an oily layer separates and is removed, that removed on acidification to pH 2 being the aromatic mercaptan fraction composed mainly of thiophenol; (2) a spent caustic soda solution from the gasoline refining is acidified with hydrochloric acid, first to pH 9 and then to pH 2; at each acidification an oily layer separates and is removed, that removed on acidification to pH 2 consisting principally of thiocresols. The aromatic mercaptans which may be present in the gasoline are thiophenol, methyl thiophenol, dimethyl thiophenol, ethyl thiophenol, diethyl thiophenol, propyl thiophenol, and dipropyl thiophenol.ALSO:A gasoline distillate containing aromatic mercaptans is prepared by a catalytic cracking process and is maintained out of contact with oxygen until after it has been sweetened by contact with an aqueous solution of an alkali metal hydroxide.  The aqueous alkali solution removes the aromatic mercaptans and in this way excessive gum formation in the gasoline is avoided, since it is stated that, in the presence of oxygen, the aromatic mercaptons catalyze the oxidation of olefins and diolefins p  present in the gasoline.  The olefins and diolefins, in such circumstances, are oxdized to peroxides, and these in turn react with the aromatic mercaptons with the formation of gum. In an example, a West Texas gas oil boiling in the range of about 400 DEG -700 DEG F. is cracked at 900 DEG -1000 DEG F. and 5-25 p.s.i. pressure in the presence of a fluidized silica-alumina gel catalyst containing approximately 15 per cent silica.  A gasoline fraction having an end point of about 400 DEG  F. is distilled from the cracked products and, before this gasoline comes into contact with oxygen, it is washed with aqueous caustic soda.  The caustic soda solution may have been freed of oxygen before use by purging with an intert gas.  The treatment with caustic soda solution may take place in an inert atmosphere, and any phenols or aliphatic mercaptans present will also be removed by the caustic treatment.  The Specification as open to inspection under Sect. 91 refers to the sweetening, in the absence of oxygen, according to the process, of any gasoline prepared by a cracking process.  This subject-matter does not appear in the Specification as accepted.