Patent GB-601978-A (GB)
Title: Process for the regeneration of caustic alkali solutions containing mercaptans
IPC: C10G19/08
Filed: 1945-04-03
Abstract: <PICT:0601978/III/1> In regenerating by oxidation a spent aqueous solution of caustic alkali containing absorbed mercaptans relatively poor in constituents of low molecular weight and dissolved organic catalyst for promoting oxidation of mercaptides to disulphides by means of oxygen or gases containing oxygen, oxygen is passed through the solution under such conditions as to leave a residual mercaptan sulphur content sufficient to inhibit the oxidation of the catalyst, and light mercaptans having less than 5 and preferably less than 4 carbon atoms, are added as such or as alkali-mercaptides to the solution either before or during oxidation.  Oxidation, such as by air blowing, may be carried out in two distinct stages and light mercaptans which may be obtained by reducing disulphides containing not more than 6 carbon atoms formed in the process and separated from the aqueous solution by fractional distillation, may be added between the stages.  The spent caustic alkali solution, which may be one obtained by alkali extraction of mercaptans from a gasoline distillate, may contain a water-soluble oil-insoluble substance to promote the solubility of the mercaptans in aqueous caustic alkali (solutizer), e.g. a lower mono- or polyhydric alcohol, lower aliphatic polyamine or alkanolamine, hydroxy or amino ether, hydrocarbon carboxylic acid such as a fatty acid of 2 to 6 carbon atoms or a dicarboxylic acid of 5 to 6 carbon atoms, or a phenol having up to 10 carbon atoms, and may be subjected to a stripping operation, e.g. by steaming, to remove a portion of the heavy mercaptans prior to oxidation.  The preferred catalysts specified are tannic acid, tannin, pyrogallol, hydroquinone, and condensed poly-hydroxy benzoic acids, though water soluble organic substances such as hydroxy and amino phenols which may contain other substituents, e.g. one or more carboxyl radicals, and those disclosed in U.S.A. Specification 2,015,038, are known to catalyse mercaptide oxidation.  In one embodiment, Fig. 2, sour hydrocarbon distillate is contacted in an extractor 50, sub-divided by a plate 52, with a minor portion of caustic alkali solution containing a solutizer from line 62 to extract preferentially relatively light mercaptans and to produce two phases which are separated, the light mercaptans entering line 66 and treated distillate entering the upper part of extractor 50, where it is contacted with the remaining larger portion of caustic alkali solution from line 53 to absorb heavier mercaptans and to produce two phases which are also separated, oxygen from line 551 being passed through the portion containing the heavier mercaptans from line 54 to produce disulphides which are skimmed-off in separator 59 and to leave a residual mercaptan sulphur content sufficient to inhibit oxidation of the catalyst; the minor separated portion containing light mercaptans in line 66 is then combined with the resulting major oxidised portion from line 61 and further oxygen from line 69 passed through the mixture in line 67 to produce disulphides which are separated in separator 72 and removed from the mixture through line 74, the remaining solution then being returned through lines 73 and 53 to the top of the extractor 50 except a smaller portion which is divided off through lines 65 and 62.  In modifications of this embodiment, (see Figs. 1 and 3, not shown), the major portion containing heavy mercaptans is either divided into two unequal parts prior to oxidation of the larger part, addition of light mercaptans and further oxidation; or is oxidised prior to division into two unequal parts and addition of light mercaptans to and further oxidation of the larger part.ALSO:In regenerating by oxidation with the formation of aliphatic disulphides, a spent aqueous solution of caustic alkali-containing absorbed mercaptans relatively poor in constituents of low molecular weight and dissolved organic catalyst for promoting oxidation of mercaptides to disulphides by means of oxygen or gases containg oxygen, oxygen is passed through the solution under such conditions as to leave a residual mercaptan sulphur content sufficient to inhibit the oxidation of the catalyst, and light mercaptans having less than 5 and preferably less than 4 carbon atoms, are added as such or as alkali-mercaptides to the solution either before or during oxidation. Oxidation, such as by air blowing, may be carried out in two distinct stages and light mercaptans which may be obtained by reducing disulphides containing not more than 6 carbon atoms formed in the process and separated from the aqueous solution by fractional distillation, may be added between the stages.  The spent caustic alkali solution, which may be one obtained by alkali extraction of mercaptans from a gasoline distillate, may contain a water-soluble oil-insoluble substance to promote the solubility of the mercaptans in aqueous caustic alkali (solutizer), e.g. a lower mono or polyhydric alcohol, lower aliphatic polyamine or alkanolamine, hydroxy or amino ether, hydrocarbon carboxylic acid such as a fatty acid of 2 to 6 carbon atoms or a dicarboxylic acid of 5 to 6 carbon atoms, or a phenol having up to 10 carbon atoms, and may be subjected to a stripping operation, e.g. by steaming, to remove a portion of the heavy mercaptans prior to oxidation. The preferred catalysts specified are tannic acid, tannin, pyrogallol, hydroquinone, and condensed polyhydroxy benzoic acids, though water-soluble organic substances such as hydroxy and amino phenols which may contain other substituents, e.g. one or more carboxyl radicals, and those disclosed in U.S.A. Specification 2,015,038, are known to catalyse mercaptide oxidation.  In one embodiment (Fig. 2), sour hydrocarbon distillate is contacted in an extractor 50, subdivided by a plate 52, with a minor portion of caustic alkali solution containing a solutizer from line 62 to extract preferentially relatively light mercaptans and to produce two phases which are separated, the light mercaptans entering line 66 and treated distillate entering the upper part of extractor 50, where it is contacted with the remaining larger portion of caustic alkali solution from line 53 to absorb heavier mercaptans and to produce two phases which are also separated, oxygen from line 55 being passed through the portion containing the heavier mercaptans from line 54 to produce disulphides which are skimmed off in separator 59 and to leave a residual mercaptan sulphur content sufficient to inhibit oxidation of the catalyst; the minor separated portion containing light mercaptans in line 66 is then combined with the resulting major oxidized portion from line 61 and further oxygen from line 69 passed through the mixture in line 67 to produce disulphides which are separated in separator 72 and removed from the mixture through line 74, the remaining solution then being returned through lines 73 and 53 to the top of the extractor 50 except a smaller portion which is divided off through lines 65 and 62.  In modifications of this embodiment (see Figs. 1 and 3, not shown), the major portion containing heavy mercaptans is either divided into two unequal parts prior to oxidation of the larger part, addition of light mercaptans and further oxidation; or is oxidized prior to division into two unequal parts and addition of light mercaptans to and further oxidation of the larger part.