CA1135481A - Method for removing hydrogen sulfide from gas streams - Google Patents

Abstract

METHOD FOR REMOVING HYDROGEN
SULFIDE FROM GAS STREAMS

Abstract of the Disclosure The problems of chemical consumption and undesirable formation of sulfur-containing salts inherent in conven-tional processes, such as the Stretford and Takahax processes, for the removal of hydrogen sulfide from gas streams are substantially reduced by the use of a novel, substantially quinone-free washing solution containing solubilized vanadium, thiocyanate ions, a carboxylate complexing agent and one or more water-soluble nonquinone aromatic compounds capable of solubilizing vanadium.
Hydrogen sulfide is removed from the hydrogen sulfide-containing gas stream (12) by contact with the washing solution (14). The absorbed hydrogen sulfide is converted to elemental sulfur by reaction with the washing solution.
The sulfur is separated (36) and recovered. The washing solution is oxidatively regenerated (24) before being recycled (30) to the absorber (10).

Description

S48~ ~

METHOD FOR ~EMO~ IG ~IYDROGEN
SULFIDE FROM GAS ST:F:EAMS

Technical Field This invention relates to the removal of hydrogen sulfide from gas streams by contacting the gas with an aqueous washing soIution, and more particularly to an im-proved method for the removal of hydrogen sulfide from gas streams in which the undesirable formation of water-soluble sulfur-containing salts and the undesirable chemi-cal consumption of the constituents of the washing solu-tion are substantially reduced.

Background Art With the increasing concern over the pollution of the atmosphere, great demands have been made on industry to produce pollution free products in a pollution-free manner. One area of particular concern has been the re-lease of sulfur and its compounds into the atmosphere as a result of the refining of petroleum, the swee-tening of sour natural gas, the processing of ores, the destructive distillation of coal and/or oil shale, the gasification or liquefaction of coal, the use of geothermal fluids to generate electricity, or other processes which produce hydrogen sulfide-containing gases.
Various processes have been employed to remove hydro-25. gen sulfide from gas streams. The Stretford process andthe Takahax process employ aqueous al~aline washing solu-tions to ahsorb the hydrogen sulfide from the gas stream and to convert the absorbed hydrogen sulfide primarily to ~.

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, sulfur. The solution is regenerated by flowing air through the solution. The product sulfur is separated from the regenerated solution before recycling the solu-tion to the absorber. The washing solution employed in the Stretford process contains a water-soluble salt of 9,10-anthraquinone disulfonic acid (ADA) and a water-soluble vanadate or other multivalent metal compound.
The washing solution employed in the Takahax process con-tains a water-soluble naphthaquinone sulfonate (N~S) alone or i~ combination with a multivalent metal compound.
While these processes are successful in substantially reducing the hydrogen sulfide content of the gas streams being treated, several problems exist which tend to in-crease their operating expenses. One such problem in-volves the chemical consumption of the washing solutionconstituents, such as ADA, NQS and/or the alkaline con-stituents of the solution. The cost of chemicals re-quired to replenish the supply of these consumable con-stituents adds significantly to the cost of the process.
Another of the problems involves the undesirable conver sion of the absorbed hydrogen sulfide and/or the product sulfur to water-soluble sulfur-c:ontaininc3 salts, such as sulfates, thiosulfates and polythionates, which accumulate in the washing solution. These accumulated salts tend to (1) increase the corrosivity of the washing solution, (2) cause precipitation of the desired constituents of the washing solution and (33 increase the rate of chemical consumption of the alkaline co~stituents of the washing solution.
In commercial plants employing these processes, attempts have been made to mitigate these problems either by periodic replacement of the washing solution or by intermittently or continuously bleeding a slipstream of the contaminated washing solution while adding make-up chemicals to replenish the circulating solution. Eoth procedures are unsatisfactory due to the expense of the .. .

~31 35~

make-up chemicals and the difficulty involved in dis-posal of the contaminated solution.
It has been proposed to modify the washing solution of the Stretford process in order to reduce the chemisal consumptio~ of ADA, such as ~y adding to the conventional Stretford solution a thiosulfate ion precursor (U.S.
Patent 3,972,989 to Fenton et al.), a thiocyanate ion precursor (U.S. Patent 4,017,594 to Fenton et al.), or a water-soluble inorganic fluoride, borate or phosphate complexing agent (U.S. Patent 4,060,594 to Fenton et al.).
It has also been proposed to modify the process-steps of the Stretford process in order to reduce the undesirable formation of ~ater-soluble sulfur-containing salts, such as by the use of mùltistage absorbers (U.S. Patent 4,125,597 to Fleck) or an oxysen-stripping device (U.S.
Patent 3,642,44~ to Beavon) to remove oxygen from the washing solution before it is contacted with the hydrogen sulfide-containing gas stream. It has also been proposed to selectively separate the undesirable water-soluble sulfur-containing salts from the washing solution in order to reduce the contaminant concentration, such as by the addition of a water-soluble alclehyde to the solution to convert the thiosulEate salts to sulfate salts which are then separated from the solution by cooling the solu-tion to selectiveiy precipitate the sulfate salts. (U.S.Patent 4/083,945 to Fenton et al.) While the aforementioned modifications have been some-what successful in reducing o~e or more of the problems associatèd with the Stretford process, these modifica-tions have not been entirely satisfactory in all in-stances. Thus a need exists for a hydrogen sulfide removal process which has all the beneficial features of the Stretford process but which substantially eliminates the aforementioned problems.
Accordingly, a primary object of this invention is to provide a method for removing hydrogen sulfide from gas .~35~

streams.
Another object of this invention is to provide ahydrogen sulfide removal method in which the hydrogen sulfide content of the gas being treated is reduced to an acceptably low level at a reasonable cost.
Still another object of this invention is to provide a method for removing hydrogen sulfide from gas streams in which the absorbed hydrogen sulfide is converted sub-stantially exclusively to sulfur and wherein the un-desirable conversion of hydrogen sulfide and/or theproduct sulfur to water-soluble sulfur-containing salts is substantially reduced or eliminated.
Yet another object of this invention is to provide a method in which hydrogen sulfide is absorbed from a gas stream by a regenerable washing solution and in which the chemical consumption of the washing solution constituents is substantially reduced or eliminated.
Further objects~ advantages and features of the inven-tion will become apparent to those skilled in the art from the following description when taken in conjunction with the accompanying drawings.
Briefly, the invention provides a method for removing hydrogen sulfide from a hydrogen sulfide-containing gas stream and converting said hydrogen sulfide to elemental sulfur, which comprises: (a) contacting said gas stream with a regenerable washing solution so as to absorb said hydrogen sulfide into said washing solution, the washing solution introduced into contact with said gas stream com-prising a substantially quinone-free aqueous solution hav-ing a pH between about 5 and about 10 and containing (1)solubilized vanadium, (2) one or more water-soluble non-quinone aromatic compounds capable of solubilizing tetra-valent vanadium, (3) thiocyanate ions and (4) a water-soluble carboxylate complexing agent; (b) allowing the absorbed hydrogen sulfide to react with constituents of said washing solution so as to convert said absorbed hydrogen sulfide substantially exclusively to elemental ~35~
-4a-sulfur; (c) oxidatively regenerating the washing solution from step (b) so as to form a regenerated washing solu-tion; and (d) separating said elemental sulfur from said washing solution.
In one preferred embodiment, the invention provides a method for removing hydrogen sulfide from a hydrogen sul-fide-containing gas stream and converting said hydrogen sulfide to elemental sulfur, which comprises: (a) contact-ing said gas stream with a regenerable washing solution so as to absorb said hydrogen sulfide into said washing solution, the washing solution introduced into contact with said gas stream comprising a substantially quinone-free aqueous alkaline solution having a pH between about 7 and about ~.5 and containing (l) between about 0.00~ and about 0.4 gram-moles of solubilized vanadium per liter, (2~ between about 0.01 and about 0.25 gram-moles of one or more water-soluble alkali metal salts of nonquinone hydroxy-aromatic sulfonates per liter, (3) between about 30 and about 150 grams of thiocyanate ions per liter, and (4) a water-soluble carboxylate complexing agent; (b) allowing the absorbed hydrogen sulfide to react with con-stituents of said washing solution so as to convert said hydrogen sulfide substantially exclusively to elemental sulfur; (c) oxidatively regenerating the washing solution from step (b) so as to form a regenerated washing solu-tion; (d) separating said elemental sulfur from said regenerated washing solution; and (e) recycling the re-generated washing solution from step (d) into contact with said gas stream in step (a).
And in another preferred embodiment, the invention provides a method for removing hydrogen sulfide from a hydrogen sulfide-containing gas stream and converting said hydrogen sulfide to elemental sulfur, which comprises:
(a) contacting said gas stream with a regenerable washing solution so as to absorb said hydrogen sulfide into said washing solution, the washing solution introduced into contact with said gas stream cornprising a quinone-free :.. ~ . . . ..

~:135~
-4b-aqueous alkaline solution having a pH between about 8 and about 9 and containing (1) between about 0.01 and about 0.2 gram-moles of solubilized vanadium per liter, (2) be-tween about 0.02 and about 0.15 gram-moles of an alkali metal 1-hydroxybenzene-4-sulfonate per liter, (3) between about 30 and about 150 grams of thiocyanate ions per liter, (4) an alkali metal citrate, and (5) a mixture of alkali metal carbonates and alkali metal bicarbonates; (b) allowing the absorbed hydrogen sulfide to react with con-stituents of said washing solution so as to convert saidhydrogen sulfide substantially exclusively to elemental sulfur; (c) oxidatively regenerating the washing solution from step (b) so as to form a regenerated washing solu-tion; (d) separating said elemental sulfur from said re-generated washing solution; and (e) recycling the regen-erated washing solution from step (d) into contact with said gas stream in step (a).
The method of this invention substantially reduces or eliminates the undesirable conversion of hydrogen sulfide and/or product sulfur to water-soluble sulfur-containing salts which would otherwise contaminate the washing solu-tion, and also substantially reduces or eliminates the chemical consumption of the constituents of the washing solution. The method also produces a sulfur product which is relatively pure and more easily washed than the sulfur produced by prior art methods.

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~5--Brief Description of The Drawings The invention will be more readily understood by reference to the drawings, wherein like numerals refer to li~e elements, and in which FIG. 1 is a schematic flow diagram illustrating the manner in which a hydrogen sulfide-containing gas is treated by one embodiment of the method of this invention;
and FIGS. 2 and 3 are schematic flow diagrams illustrat-ing preferred methods for separating the product sulfur from the sulfur sll~rry produced in the method of this in-vention.

Detailed Disclosure of Invention and Best Mode FIG. 1 schematica]ly illustrates the process eauipment employed in one embodiment of the method of this inven-tion. The hydrogen sulfide-containing gas is introduced through conduit 12 into the bottom of absorber 10 and flows upwardly through absorber 10 in countercurrent con-tact with a washing solution introduced through conduit 14 into the top of absorber 10. Substantially all of the hydrogen sulfide is absorbed from the gas by the down-wardly flowing solution to thereby form a vent gas which is su~stantially free of hydrogen sulfide. The vent gas is discharged through conduit 16 to the atmosphere.

~L35~

While an absorber employing countercurrent flow of the hydrogen sulfide-containing gas and the washing solution has been described, other known gas/liquid contacting vessels, such as a venturi scrubber employing cocurrent flow of the gas and washing solution, may be successfully employed in the method of this invention. Suitable gas/
liquid scrubbers include single and multistage contacting vessels using countercurrent and/or cocurrent flow of the gas stream and the ~ashing solution.
Once absorbed by the washing solution, the hydrogen sulfide is oxidatively converted to elemental sulfur by reaction with the constituents of the washing solution.
-- The reduced washing solution and entrained elemental sulfur are withdrawn from absorber 10 via conduit 1~ and are pumped by pump 20 through conduit 22 i-nto the bottom of oxidizer 24. Prior to introduction of -the reduced solution into oxidizer 24, sufficient time should be allowed for complete conversion of the absorbe~ hydrogen sulfide to elemental sulfur. Typically this conversion - 20 is completed in about five minut:es and sufficient resi-dence time can be provided by briefly holding the reduced solution in the bottom of absorber 10. Optionally, re-action tan~ 26 is provided between absorber 10 and oxidizer 2~, if required, in order to allow suficient reaction time for the complete conversion of the absorbed hydro~en sulfide to elemental sulfur.
ThQ reduced washing solution and entrained sulfur introduced through conduit 22 into oxidizer 24 are con-tacted with air, or other molecular~oxygen-containing gas, ~ich is introduced through conduit 28 into the bottom of oxidiæer 24. The entering air is preferably churned into small bubbles by a submerged rotating stirrer, not shown, adapted for this purpose. The reduced washing solution is oxidized by reaction with oxygen thereby regenerating the constituents of the washing solution. The air also serves to float t~e sulfur particles entering oxidizer 24 and thereby form a frothy sulfur slurry at the top of oxidizer ~1~5~

24. The regenerated washing solution is withdrawn from oxidizer 24 via conduit 30 and is recycle~ by pump 32 through condl~it 14 to the top of absorber 10. Option-ally, balance tank 34 is provided as an accumulator or surge tank for the regenerated washing solution. The sulfur slurry, which contains from about 3 to about 20 weight percent of elemental sulfur as well as entrained air and washing solution, is withdrawn from oxidizer 24 via conduit 36~ The product sulfur is then separated from the sulfur slurry.
~ IG. 2 illustrates one preferrecl method for separat-ing the product sulfur from the sulfur slurry withdrawn - from the oxidizer of FIG. 1. ~he sulfur slurry is fed through conduit 36 into separator 38, which can be a filter, a centrifuge or other device capable of separat-ing particulate sulfur from an aqueous slurry containing the same in a manner equivalent to a filter or a centri-fuge. As explained more fully hereinbelow, separator 38 is preferably not an autoclave or other device which requires excessive heating of the sulfur slurry to eEfect separation of the product sulfur. r'he separated sulfur is preferably washed with water introduced into separator 38 via conduit 40, and the filtrate comprised o~ -the eluted washing solution and wash water is withdrawn from separator 3g via ¢onduit 42 for recycle to the circulating washing solution, such as by recycle of the filtrate to ~alance tank 34. The sulfur product is withdrawn from separator 38 via conduit 44 as a relatively dry cake com-prised of finely divided sulfur particles. The particu-late sulfur product produced in this manner by the methodof this invention is relatively pure and generally is suitable for agricultural use without further purifica-tion.
. FIG. 3 illustrates another method for separating the product sulfur from the sulfur slurry, which method is preferred when a liquid sulfur product is desired. The sulfur slurry is fed through conduit 36 into separator 46, ~3~8~

which can ~e a filter, a centrifuge or the like as described above for separator 38. The separated sulEur may optionally ~e washed in separator 46 with wa-tcr introduced through conduit 48 into separator 46, and the filtrate comprised of the eluted washing solution and wash ~Jater is withdrawn from separator 46 via conduit ~0 for recycle to the circulating washing solution. Alter-natively, only the eluted washing solution and little or no wash water may be recycled to the circulating washing solution. The separated sulfur is then reslurried with additional'water introduced through conduit 48 to form a concentrated slurry having from about 3 to about 20 weight percent of sulfur, and the concentrated slurry is with-drawn from separator 46 and conducted through conduit 52 into melter 5a The concentrated slurry is heated in melter 54 to melt the sulfur and thereby form a liquid sulfur product which separates by gravity from the slurry water and is.with~rawn from melter 54 throug}
conduit 56. The aqueous effluent is withdrawn from melter 54 via conduit 5~ 'for dic,posal.
Preferably sufficient water is employed to wash the product sulfur in separator 46 so as to remove the valu-able non-sulfur constituents thereof from the sulfur and thereby reduce the loss of valuable chemicals in the aqueous effluent from melter 54. The aqueous effluent -from melter 54 is preferably not recycled to the circulat-ing washing'solution because recycle of this effluent is often detrimental to the effectiveness of the method of this invention. Dependiny upon the degree of washing achieved in separator 46, the aqueous effluent from melter 54 may contain colloidal vana~ium and/or thiosulfate and polythionate salts which tend to decrease the effective-ness of the circulating washing solution. Since these un-desirable chemical species are to some e~tent produced whenever the washing solution is treated at the hi.gh temperatures employed in an autoclave or melter, it is preferred to dispose of the aqueous autoclave effluent _ _, _ _ _,, ., _ _ .. ... . . . ...

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rather than recycle it to -Lhe washing solution.
In the method of this invention, the washing solution is circulated between the reducing atmosphere of absorber 10 and the ~xidizing atmosphere of oxidizer 24. During this circulation the composition of the washing solution will necessarily change to some extent due to the various chemical reactions involved in the method. However, the composition of the washing solution introduced into the top of absorber 10 will be substantially constant under normal operating conditions. All reference herein to the composition of the washing solution is made to the com-position of the washing solution as introduced into con-tact with the hydrogen sulfide-containing gas stream.
The washing solution employed in the method of this invention is a substantially quinone-free aqueous solu-tion containing (1) vanadium, (2) one or more water-soluble nonquinone aromatic compounds capable o~ solubil izing tetravalent vanadium, (3) -thiocyanate ions and (4) a water~soluble carboxylate co~plexing agent.
The presence of any appreciable amount of a quinone in the washing solution of this invention results in the undesirable formation of contaminant sulfate salts and therefore should be avoided. As a practical matter, the presence of very small amounts of a quinone in the solu-25 tion, such as less than about 0.0001 gram-moles of quin-one per liter, can be tolerate~ without excessive sulfate ~ formation. Accordingly for the purposes of this inventio~
a "substantlally quinone-free washing solution" is one which contains less than about 0.0001 gram-moles of quin-one per liter. Preferably the washing solution contains no quinone, i.e., the washing solution is preferably a quinone-free solution.
The washing solution contains solubiiized vanadium.
Preferably the vanadium is provided by dissolving a water-soluble vanadium-containing compound in the washing solu-tion. Suitable vanadium-containing compounds include the al~ali metal and ammonium vanadate salts, such as sodium 8~ t r,etavanadate, sodium orthovanadate, sodiur. pyrovanadate, sodium ar~onium vanadate and isopolyvanadate. The con-centration of vanadium in the washing solution, calculated in terms of grams of elemental vanadium per liter of solu-tion, is between about 0.3 and about 20 grams of vanadiumper liter, preferably between about 0.5 and about 10 grams of vanadium per liter. In terms of gram-moles of vanadium per liter of solution the corresponding concentrations are between about 0.006 and about 0.4 gram-moles of vanadium per iiter, preferably between about 0.01 and about 0.2 grar,~.-moles of vanadium per liter. Good results are obtained when the solution contains between about 2 grams (0.04 ~ram-moles) and about 8 grams (0.16 gram-moles) of vanadium per liter.
The washlng solution contains one or more water-soluble nonquinone aromatic compounds capable of solubil-izing tetravalent vanadium in the solution. As used herein, the term "nonquinone aromatic compound" means an aromatic compound~which will not be present in the wash-ing solution as a quinone, either because the quinone form of the compound is not stable in the washing solution or because the compoun~ cannot form a quinone. Generally, any water-soluble nonquinone aroratic compound capable of solubilizing tetravalent vanadium in the washing solution is suita~le for use in the method of this invention. ~he water-soluble nonquinone aromatic sulfonates are preferre~
and the water soluble nonquinone hydxoxyaromatic sulfon-ates are particularly preferred.
Suitable water-soluble nonquinone aromatic corpounds include the alkali metal and ar~oniu~ salts and the acids of the following compounds: -the l-hydroxybenzene-x-~ulfon-ates (wherein x = 2, 3 or 4); the 1 hydroxybenzene-x,y-disulfonates (wherein x = 2 or 3, y = 3j 4, 5 or 6 and ~ ~ y); the l,x-dihydroxybenzene-y-sulfonates (wherein x = 2, 3 or 4, y = 2, 3, 4 or 5 and x ~ y); the l,x-dihydroxybenzene-y,z-disulfonates (wherein x = 2, 3 or 4, y = 2, 3 or 4, z ~ 3, 4, 5 or 6 and x ~ y ~ z);

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, l-methoxybenzene-4-sulfonate; l-r.,ethylben3ene-~-sulfona-te; L
l-acetylbenzene-4-sulfonate; the l-aminobenzene-x-sulfon-ates (wherein x a 2, 3 or 4); benzene sulfonate; benzene disulfonate; l-nitrobenzene-3-sulfonate; 1,3,5-trinitro- F
5 benzene-2-sulfonate; 1,4-diaminobenzene-2-sulfonate;
1,3,5-trimethylbenzene-2 sulfonate; 1,3-clicarboxylbenzene-5-sulfonate; the l-hydroxynaphthalene-x-sulfonates (wherein x = 2, 3, 4, 5, 6, 7 or 8); the l-hydroxynaphtha-lene-x,y-disulfonat.es (wherein x = 2, 3 or 4 and y = 5, 10 6, 7 or 8); the 2-hydroxynaphthalene-x,y-disulfonates (wherein x = 3, 4, 5 or 6, y = 5, 6, 7 or 8 and x ~ y);
4,5-dihydroxynaphthalene-2,7-disulfonate; the 2,x-dihy-droxynaphthalene-3,y-disulfonates (wherein x = 6 or 7, y = 6 or 7 and x ~ y); naphthalene sulfonate; the naphtha-15 lene-x,y-disulfonates (wherein x = 1 or 2 and y = 5, 6, 7 or 8); the x-aminonaphthalene-l-sulfonates (wherein x = F
4 or 5); 2-aminonaphthalene-1,5 disulfonate; calmagite;
eriochrome black T; ~onceau 3R; indigo carmine; the ~-ethyl-5-phenylisoxazolium-x-sulfonates (wherein x = 3' or 4'); 5-formylfuran-2-sulfonate; 3-hydroxy-4-(2-hydroxy- ,~
4-sulfo-1-naphthylazo)naphthalene-2-carboxylate;
o-(2-(~-(2-hydroxy-5-sulfopheny].azo) benzylidenehydra-zino))benzoate; 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p-cisulfonate; 2,3-dioxoindoline-5-sulfonate; the pyridine-x-sulfonate (x = 2, 3 or 4); 5-carboxypyridine-

3-sulfonate; 8-hydro~yquinoline-5-sulfonate; 2-methyl-8-~ydroxyquinoline-5-sulfonate; 2-hydroxy-~-methoxy-benzophenone-5-sulfonate; the benzophenone-x,y-disulfonates (wherein x = 3 or 4 and y = 31-or 4'); diphenylether-4,4'-disulfonate; the diphenylsulfone-x,y-disulfonates (wherein x = 3 or 4 and y = 3' or 4'); methylphenylsulfone; the water-solu~le nonquinone derivatives thereof, such as the chloro-, methyl-, carboxy~, nitro-, amino- and dimetilyl-amino- derivatives thereof; and mixtures thereof.
Preferred water-soluble nonquinone aromatic compounds include the alkali metal and an~onium-salts ana the acids of the following compounds: l-hydroxybenzene-~-sulfonate;

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t ]2 1,4-dihydroxybenzene-2-sulonate; the l,~-dihydroxyben-zene-2,x-disulfonates (wherein x = 5 or 6); ~,5-dihydroxy-naphthalene-2,7-disulfonate; pyridine-3-sulfonate; 8- Ç
hydroxyquinoline-5-sulfonate; and mixtures thereof. r The concentration of the water-soluble nonquinone aromatic compound is selected in view of inter alia, the sulfur-loading of the washing solution, -the solubility of the non~uinone aromatic compound and the concentration of vanadium in the solution. Calculated in terms of gram-moles of water-soluble nonquinone aromatic compound per liter of solution, the washing solution contains between about 0.01 and about 0.25 gram-moles per lite , preferably between about 0.02 and about 0.15 gram-moles per liter.
Good results are obtained when the washing solution con-15 tains between about 0.05 and about 0.12 gram-moles of the water-soluble nonquinone aromatic compound per liter of solution. Good results are obtained when the washing solu-tion has a molar ratio of vanadium to nonquinone aromatic compounds between about 0.5 and about 2.
The thiocyanate ions in the washing solution are pro-vided by dissolving a water-soluble thiocyanate salt, such as an alkali metal or ammonium thiocyanate, in the wash-ing solution. Alternatively, one or more "thiocyanate ion precursors", such as an alkali metal or ammonium cyanide, may be dissolved in the washing solution and thereafter converted to thiocyanate ions by reaction with absorbed hydro~en sulfide. ~owever, at least for the initial ~ormulation of the washing solution it is pre-ferred that an alkali r.letal or ammonium thiocyanate be dissolved in the washing solution to provide the thio-cyanate ions. Calculated as grams of thiocyanate ions (SCN-) per liter of solution, the washing solution con-tains between about 5 and about 400 grams of thiocyanate ions per liter, preferably between about 30 and about 150 grams of thiocyanate ions per liter. Good results are obtained when the washing solution contains between about 50 and about 75 grams of thiocyanate ions per liter.

1~3591~Bl The carboxylate complexing agent in the washing solu-tion serves to maintain the solubility of vanadiu~. in the presence of thiocyanate ions. The complexing agent should contain at least two water-solubilizing groups, at least one of which is a carboxyl group. Preferably, the complexing agent contains at least one carboxyl grou and at least one hydroxy ~roup. Suitable carboxylate complex-ing agents include the water-soluble polycarboxylic acids and their salts, SUC}I as ma]eic acid, particularly those polycarboxylic acids and salts having at least one hydroxy group, such as citric acid, malic acid and/or tartaric acid, and the monohydroxy- or polyhydroxy rlono-carboxylic acids and their salts, such as glycolic acid.
Citric acid and its water-soluble salts are par-ticularly preferred carboxylate complexing agents.
The washing solution of this invention has a pH
between about 5 and about 10, more preferably between about 7 and about 9.5. Good results are obtained when the washing solution has a pH between about 8 and about 9.
The washing solution can be buffered at the desired pH by the use of conventional buffering agents, such as a weak carboxylic acid and an alkali rnetal or an~lonium salt of the wea~ carboxylic acid. Alternatively, or in addition, the p~i of the washing solution can be controlled by the addition of wea~ acids, such as carbon dioxide absorbe~
from the gas stream, and/or a base, such as sodium hydroxide. ~he al~ali metal and a~monium carbonates and bicarboIIates are preferred buffering agents, particularly mixtures of the alkali metal carbonates and bicarbonates An exel~plary washing solution has a pH between about 3 and about 9, and contains between about 5 and 10 grams of sodium carbonate per liter, between about 25 and about 30 gran!s of sodium bicarbonate per liter, and between about 10 and about 20 grams of sodium citrate per liter.
Durin$ normal operations, the washing solutions of -this invention are relatively resistant to chemical consumption of the solution constituents and do not tend to form or ', :

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accumulate contaminants at rates which would necessitate periodic replacement or bleeding of the washing solution.
Preferably the washing solution which is initially charged f-to hydrogen sulfide removal plant, i.e., the virgin solu-5 tion, contains the above-described essential constituents and is substantially free of thiosulfate salts and sul-fate salts.
The invention is further illustrated by the following examples which are illustrative of specific modes of 10 practicing the invention and are not intended as limiting r the scope of the appended claims.

Example 1 In accordance with the method of this invention, a hydrogen sulfide-containing gas is contacted with a quin-15 one-free aqueous alkaline washing solution so as to absorb substantially all of the hydrogen sulfide into the washing solution. The absorbed hydrogen sulfide is con-verted fiubstantially exclusively to elemental sulfur by reaction with the constituents of the washing solution;
20 the washing solution is oxidatively regenerated; and, after separation of the sulfur, the regenerated washing solution is recycled into contact with the hydrogen sul- i~
fide-containing gas.
The hydrogen sulfide-containing gas comprises about 25 3 mole percent of hydrogen sulfide, about 36 mole percent of carbon dioxide and about 61 mole percent of nitrogen.
The washing solution is circulated into contact with the hydrogen sulfide-containing gas at a rate sufficient to provide about 0.19 liter of washing solution per liter 30 (calculated at 60 F. and one atmosphere pressure) of hydrogen sulfide-containing gas. Over a period of about 17 days, the composition of the washing solution remains essentially constant when corrected for dilution and periodic sampling except that sodium citrate is consumed 35 and sodium thiosulfate is formed at relatively low rates.
The composition of the washing solution during this period 1 31L3~
-]5-is indicated in Table l below.

TABLE l Constituent Grams/ Rate of Gain (Loss) Concentrations Liter Grams/Liter/Day 5' Vanadium 5.9 0 Sodium l-Hydroxybenzene- 17 0

4-Sulfonate Sodium Thiocyanate 82 0 Sodium Carbonate 27 0 Equivalenta Sodium Citrate 24 to l9 (0.3) Sodiu~. Sulfateb 1.0 0 Sodium Thiosulfate 0.8 to 1.9 0.06 a- Sodium Carbonate Equivalent e~uals the sodium 15 carbonate concentration plus 0.631 times the sodium bicarbonate concentration.
b- The sodium sulfate'is introduced as an impurity in the nonquinone sulfonate constituent.

Exam~le 2 In accordance with the method of this invention, a hydrogen sulfide-containing gas is contacted with a quin-one-free aqueous alkaline washing solution so as to absorb substantially all of the hydrogen sulfide into the washing solution. The absorbed hydrogen sulfide is converted substantially exclusively to elemental sulfur by reaction with the constituents of the washin'g solution; the washing solution is oxidatively regenerated; and, after separation of the sulfur, the r`egenerated washing solution is recycled into contact with the hydrogen sulfide-containing gas.
The hydrogen sulfide-containing gas comprises about 0.98 mole percent of hydrogen sulfide, about 6.9 mole per-cent of carbon dioxide and about 92 mQle percent of nitro-gen. The washing solution is circulated into contact with the hydrogen sulfide-containing gas at a rate suEficient .

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~ -16-to provide about 0.037 liters of washing solution per liter (calculated at 60 F. and one atmosphere pressure) of hydrogen sulfide-containing gas. Over a period of about 11 days, the composition of the washing solution remains essentially constant when corrected for dilution and periodic sampling except that sodium 3-hydroxyquino-; line-5-sulfonate is consumed and sodium thiosulfate is formed at relatively low rates. The composition of the washing solution during this period is indicated in Table 2 below.

.

Constituent Grams/ Rate of Gain ~Loss) ~~ Concentrations LiterGrams/Liter/Day Vanadium 5.2 0 Sodium 8-Hydroxyquinoline-

5-Sul~onate 8 to 6 (0.16) Sodium Thiocyanate 80 0 Sodium Carbonate Equivalenta 28 0 20 Sodium Citrate 12 0 Sodium Sulfateb 1.0 0 Sodium Thiosulfate 0.2 to 0.3 0.015 .
a- Sodium Carbonate Equivalent equals tne sodium carbonate concentration plus 0.631 times the sodium bicarbonate concentration.
b- The sodium sulfate is introduced as an impurity in the nonquinone sulfonate constituent.
- .
From these examples it is apparent that tne use of the method of this invention results in essentially no sulfate formation and very- little thiosulfate formation. Accord-ingly, the quinone-free washing solutions employed in the method of this invention are substantially superior to the quinone-containing washing solutions conventionally ~3S4~3~

employed to treat hydrogen sulfide-containing gas streams.

Industrial Applicability The method of this invention is suitable for removing hydrogen sulfide from a wide variety of hydrogen sulfide-containing gas streams, including hydrogen sulfide-con-taining gas streams generated as a result of the refining of petroleum, the sweetening of sour natural gas, the processing of ores, the destructive distillation of coal and/or oil shale, the gasification or liquefaction of coal, the use of geothermal fluids to generate electricity, or other processes which produce hydrogen sulfide-contain-ing gases. The method of this invention can successfully treat gas streams containing less than one mole percent hydrogen sulfide as well as gas streams containing up to 80 mole percent hydrogen sulfide or more. One such hydro-gen sulfide-containing gas stream is the gas produced by hydrogenation and hydrolysis of a Claus process tail gas as disclosed in U.S. Patent 3,752,877 to Beavon.
Hydrogenated Claus process tail gas typically contains from about 0.5 to about 5 mole percent hydrogen sulfide~
from about 2 to about 15 mole percent carbon dioxide, from about 5 to about 30 mole percent water, and traces of methane, carbonyl sulfide and methyl mercaptan, with the remainder being nitrogen.
While particular embodiments of the invention have been described, it will be understood, of course, that the invention is not limited thereto since many obvious modifications can be made, and it is intended to include within this invention any such modifications as will fall within the scope of the appended claims.
Having now described the invention, we claim:

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Claims (21)

1. A method for removing hydrogen sulfide from a hydrogen sulfide-containing gas stream and converting said hydrogen sulfide to elemental sulfur, which com-prises:
(a) contacting said gas stream with a regenerable washing solution so as to absorb said hydrogen sulfide into said washing solution, the washing solution in-troduced into contact with said gas stream comprising a substantially quinone-free aqueous solution having a pH between about 5 and about 10 and containing (1) solubilized vanadium, (2) one or more water-soluble nonquinone aromatic compounds capable of solubilizing tetravalent vanadium, (3) thiocyanate ions and (4) a water-soluble carboxylate complexing agent;
(b) allowing the absorbed hydrogen sulfide to react with constituents of said washing solution so as to convert said absorbed hydrogen sulfide sub-stantially exclusively to elemental sulfur;
(c) oxidatively regenerating the washing solution from step (b) so as to form a regenerated washing solution; and (d) separating said elemental sulfur from said washing solution.

2. The method defined in claim 1 wherein said non-quinone aromatic compounds are alkali metal or ammonium salts or acids of one or more compounds selected from the group consisting of the l-hydroxybenzene-x-sulfonates (wherein x = 2, 3 or 4), the l-hydroxybenzene-x,y-disul-fonates (wherein x = 2 or 3, y = 3, 4, 5 or 6 and x ? y), the l,x-dihydroxybenzene-y-sulfonates (wherein x = 2, 3 or 4, y = 2, 3, 4 or 5 and x ? y), the l,x-dihydroxy-benzene-y,z-disulfonates (wherein x = 2, 3 or 4, y = 2, 3 or 4, z = 3, 4, 5 or 6 and x ? y ? z), l-methoxybenzene-4-sulfonate, 1-methylbenzene-4-sulfonate, l-acetylbenzene-4-sulfonate, the l-aminobenzene-x-sulfonate (wherein x = 2, 3 or 4), benzene sulfonate, benzene disulfonate, l-nitro-benzene-3-sulfonate, 1,3,5-trinitrobenzene-2 sulfonate, 1,4-diaminobenzene-2-sulfonate, 1,3,5-trimethylbenzene-2-sulfonate, 1,3-dicarboxylbenzene-5-sulfonate, the l-hydroxynaphthalene-x-sulfonates (wherein x = 2, 3, 4, 5,. 6, 7 or 8), the l-hydroxynaphthalene-x,y-disulfonates (wherein x = 2, 3 or 4 and y = 5, 6, 7 or 8), the 2-hydroxynaphthalene-x,y-disulfonates (wherein x = 3, 4, 5 or 6, y = 5, 6, 7 or 8 and x ? y), 4,5-dihydroxynaph-thalene-2,7-disulfonate, the 2,x-dihydroxynaphthalene-3,y-disulfonates (wherein x = 6 or 7,.y = 6 or 7 and x ? y), naphthalene sulfonate, the naphthalene-x,y-disulfonates (wherein x = 1 or 2 ancl y = 5, 6, 7 or 8), the x-aminonaphthalene-l-sulfonates (wherein x = 4 or 5), 2-aminonaphthalene-1,5-disulfonate, calmagite, erichrome black T, Ponceau 3R, indigo carmine, the N-ethyl-5-phenylisoxazolium-x-sulfonates (wherein x = 3' or 4'), 5 formylfuran-2-sulfonate, 3-hydroxy-4-(2-hydroxy-4-sulfo-1-naphthylazo)naphthalene-2-carboxylate, o-(2-[.alpha.-(2-hydroxy-5-sulfophenylazo)benzylidenehydrazino])benz-oate, 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p-disulfonate, 2,3-dioxoindoline--5-sulfonate, the pyridine-x-sulfonates (x = 2, 3 or 4), 5-carboxypyridine-3-sulfon-ate, 8-hydroxyquinoline-5-sulfonate,.2-methyl-8-hydroxy-quinoline-5-sulfonate, 2-hydroxy-4-methoxybenzophenone-5-sulfonate, the benzophenone-x,y-disulfonates (wherein x =
3 or 4 and y = 3'.or 4'), diphenylether-4,4'-disul~ona-te, diphenylamine-4,4'-disulfonate, the diphenylsulfone-x,y-disulfonates (wherein x = 3 or 4 and y = 3' or 4'), methylphenylsulfone, the water-soluble nonquinone deriva-tives thereof and mixtures thereof.

3. The method aefinea in claim 1 wherein said non-quinone aromatic compounds are alkali metal or ammonium salts or acids of one or more aromatic sulfonates selected from the group consisting of 1-hydroxybenzene-4 sulfonate, 1,4-dihydroxybenzene-2-sulfonate, the 1,4-dihydroxyben-zene 2,x-disulfonates (wherein x = 5 or 6), 4,5-dihydroxy-naphthalene-2,7-disulfonate, pyridine-3-sulfonate, 8-hydroxyquinoline-5-sulfonate, and mixtures thereof.

4. The method defined in claim 1 wherein said wash-ing solution has a molar ratio of said vanadium to said nonquinone aromatic compounds between about 0.5 and about 2.

5. The method defined in claim 1 wherein said wash-ing solution has a pH between about 7 and about 9.5.

6. The method defined in claim 1, 2, or 3 wherein said carboxylate complexing agent is an alkali metal or ammonium salt of a hydroxycarboxylic acid, and wherein said washing solution further contains a buffer-ing agent selected from group consisting of the alkali metal and ammonium carbonates, the alkali metal and ammonium bicarbonates, the alkali metal and ammonium salts of carboxylic acids and mixtures thereof.

7. The method defined in claim 1, 2, or 3 wherein said carboxylate complexing agent is an alkali metal or ammonium salt of citric acid.

8. The method defined in claim 1, 2, or 3 wherein said washing solution contains between 5 and about 400 grams of thiocyanate ions per liter, between about 0.006 and about 0.4 gram-moles of vanadium per liter, and between about 0.01 and about 0.25 gram-moles of said nonquinone aromatic compounds per liter.

9. The method defined in claim 1, 2, or 3 wherein said washing solution contains between about 30 and about 150 grams of thiocyanate ions per liter, be-tween about 0.01 and about 0.2 gram-moles of vanadium per liter and between about 0.02 and about 0.15 gram-moles of said nonquinone aromatic compounds per liter.

10. The method defined in claim 1, 2, or 3 wherein said washing solution contains between about 50 and about 75 grams of thiocyanate ions per liter, between about 0.04 and about 0.16 gram-moles of vanadium per liter, and between about 0.05 and about 0.12 gram-moles of said nonquinone aromatic compounds per liter.

11. A method for removing hydrogen sulfide from a hydrogen sulfide-containing gas stream and converting said hydrogen sulfide to elemental sulfur, which com-prises:
(a) contacting said gas stream with a regenerable washing solution so as to absorb said hydrogen sulfide into said washing solution, the washing solution in-troduced into contact with said gas stream comprising a substantially quinone-free aqueous alkaline solution having a pH between about 7 and about 9.5 and contain-ing (1) between about 0.006 and about 0.4 gram moles of solubilized vanadium per liter, (2) between about 0.01 and about 0.25 gram/moles of one or more water-soluble alkali metal salts of nonquinone hydroxy-aromatic sulfonates per liter, (3) between about 30 and about 150 grams of thiocyanate ions per liter, and (4) a water-soluble carboxylate complexing agent;
(b) allowing the absorbed hydrogen sulfide to react with constituents of said washing solution so as to convert said hydrogen sulfide substantially exclu-sively to elemental sulfur;
(c) oxidatively regenerating the washing solution from step (b) so as to form a regenerated washing solution;
(d) separating said elemental sulfur from said regenerated washing solution; and (e) recycling the regenerated washing solution from step (d) into contact with said gas stream in step (a).

12. The method defined in claim 11 wherein said one or more nonquinone hydroxyaromatic sulfonates is selected from the group consisting of l-hydroxybenzene-4-sulfonate, 1,4-dihydroxybenzene-2-sulfonate, the 1,4-dihydroxyben-zene-2,x-disulfonates (wherein x = 5 or 6), 4,5-dihydroxy-naphthalene-2,7-disulfonate, 8-hydroxyquinoline-5-sulfon-ate, and mixtures thereof.

13. The method defined in claim 11 wherein said one or more nonquinone hydroxyaromatic sulfonates is 8-hydroxyquinoline-5-sulfonate.

14. The method defined in claim 11 wherein said one or more nonquinone hydroxyaromatic sulfonates is 1-hydroxybenzene-4-sulfonate.

15. The method defined in claim 11, 12, or 14 wherein said washing solution has a molar ratio of said vanadium to said nonquinone hydroxyaromatic sulfonates between about 0.5 and about 2.

16. The method defined in claim 11, 12, or 14 wherein said carboxylate complexing agent is an alkali metal salt of a hydrocarboxylic acid, and wherein said washing solution also contains a buffering agent selec-ted from the group consisting of the alkali metal carbon-ates, the alkali metal bicarbonates and mixtures thereof.

17. The method defined in claims 11, 12, or 14 wherein said carboxylate complexing agent is an alkali metal salt of citric acid.

18. The method defined in claim 11, 12, or 14 wherein said washing solution contains between about 50 and about 75 grams of thiocyanate ions per liter, between about 0.01 and about 0.2 gram-moles of vanadium per liter, and between about 0.02 and about 0.15 gram-moles of said nonquinone hydroxyaromatic sulfonates per liter.

19. A method for removing hydrogen sulfide from a hydrogen sulfide-containing gas stream and converting said hydrogen sulfide to elemental sulfur, which comprises:
(a) containing said gas stream with a regenerable washing solution so as to absorb said hydrogen sulfide into said washing solution, the washing solution intro-duced into contact with said gas stream comprising a quinone-free aqueous alkaline solution having a pH be-tween about 8 and about 9 and containing (1) between about 0.01 and about 0.2 gram-moles of solubilized vanadium per liter, (2) between about 0.02 and about 0.15 gram-moles of an alkali metal 1-hydroxybenzene-4-sulfonate per liter, (3) between about 30 and about 150 grams of thiocyanate ions per liter, (4) an alkali metal citrate, and (5) a mixture of alkali metal carbonates and alkali metal bicarbonates;
(b) allowing the absorbed hydrogen sulfide to react with constituents of said washing solution so as to convert said hydrogen sulfide substantially exclusively to elemental sulfur;
(c) oxidatively regenerating the washing solution from step (b) so as to form a regenerated washing solution;
(d) separating said elemental sulfur from said regenerated washing solution; and (e) recycling the regenerated washing solution from step (d) into contact with said gas stream in step (a).

20. The method defined in claim 19 wherein said wash-ing solution contains between about 5 and about 10 grams of sodium carbonate per liter, between about 25 and about 30 grams of sodium bicarbonate per liter, and between about 10 and about 20 grams of sodium citrate per liter.

21. The method defined in claim 19 or 20 wherein said gas stream is hydrogenated Claus process tail gas.