CA1193828A - H.sub.2s removal process - Google Patents

Abstract

A B S T R A C T

Process for the removal of H2S and CO2 from a gas by: -a) contacting the gas with an aqueous solution of an oxidizing polyvalent metal chelate, optionally in an absorbent, containing a stabilizer selected from N,N-diethylhydroxylamine, thiourea, thiosemicarbazide, 2,2-thiodiethanol and zinc isopropylxanthate, b) removing sulphur from the aqueous admixture, c) regenerating the aqueous admixture, and d) returning regenerated aqueous admixture to step (a).

Description

~93~

H2S REMOVAL ~ROCESS

The presence of signiEicant quantities of H2S and CO2 in various "sour" industrial gaseous streams poses a persistent problem. Although various procedures have been developed to remove and recover these contaminants, most such processes are deficient, for a variety of reasons.
In one cyclic method currently attracting attention, the sour gaseous stream is contacted, preferably with a solvent which contains a regenerable reactant, to produce solid elemental sulphur which is recovered either prior or subsequent to regeneration. Suitable reactants include ions of polyvalent metals, such as iron, vanadium, copper, manganese, and nickel, and include polyvalent metal chelates. Preferred reactants are coordination complexes in which the polyvalent metals form chelates with specified organic acids.
In yet another process, e.g., that disclosed in U.S. patent 4,091,073, issued May 23, 1978, to Winkler, CO2 present in the gaseous stream is also removed by the use of a suitable absorbent selective for C~2.
Because these "clean up" processes generally represent significant costs to manufacturing operations~ any improvements in such processes which increase their efficiency may have great economic importance. For example, where ligands or chelates of polyvalent metals are employed, degradation or decomposition of the polyvalent metal chelates represents an important cost in the process and requires measures for decomposition product bleed or removal and addition of fresh solution. Evan in the case of preferred chelates such as those of N-(2-hydroxyethyl)-ethylenediaminetriacetic acid and nitrilotriacetic acid, ligand decomposition, over a period of time, requires attention to prevent .~
i;

build-up of decomposition products and consequent loss of effi-ciency. The invention addresses this problem.
Accordingly, the invention comprises, in one embodiment, a process for the removal of H2S from a sour gaseous stream com-prising contacting the sour gaseous stream in a contacting zone with an aqueous reactant solution at a temperature below the melt-ing point of sulphur, the solution containing an effective amount of a reactant selected from tne group consisting of oxidizing polyvalent metal chelate compounds and mixtures thereof, and a stabilizing amount of a stabilizing composition selected from the group consisting of N,N-diethylhydroxylamine, thiourea, thiosemi-carbazide, 2,2-thiodiethanol, zinc isopropylxanthate, and mixtures thereof. A sweet gaseous stream is produced, and an aqueous admixture containing crystalline sulphur and a reduced reactant is removed from the contacting zone. At least a portion of the sulphur crystals may be removed before regenerating the reactant, or at least a portion of the sulphur crystals may be removed after regeneration. Alternatively, the sulphur may be recovered, as described in United States patent Serial No. 4,401,642 entitled Froth Process, by G. Blytas and Z. Diaz, filed May 26, 19~1. The reduced polyvalent metal chelate compound or mixture of such com-pounds is regenerated, preferably by contacting the aqueous solu-tion in a regeneration zone or zones with oxygen. As used herein, , ~
~- i 2a ~1~3~

the term "oxygen" includes oxygen-containing gases, such as air or air enriched wlth oxygen. The oxygen oxidizes the reduced metal ions of the chelate or chelates to a higher valence state, and the regenerated mixture is returned to the contacting zone.
The stabilizing composition employed herein is supplied to reduce the rate o~ ligand or chelate degradation.
In another embodiment of the invention, a sour gaseous stream containing H2S and C02 is contacted with a selective ~3~

absorbent-aqueous reactant mixture at a temperature below the melting point of sulphur, the reactant solution and procedure being similar to that described hereinbefore. Broadly, this is accomplished by the use of an absorbent mixture containing a selective absorbent for C02 (and preferably for H2S, as well), an effective amount of an oxidizing polyvalent metal chelate compound or mixtures thereof, and a stabilizing amount of the stabilizing composition(s) described. A purified or "sweet"
gaseous stream is produced which meets general industrial and commercial H2S and C02 specifications. The C02 is absorbed and the ~2S is immediately converted to sulphur by the oxidizing polyvalent metal chelate compound or mixture of such compounds.
In the process, the reactant is reduced, and the sulphur may be treated, as described hereinbefore. The sulphur crystals may be removed prior or subsequent to regeneration of the admixture.
The invention also provides, in this embodiment, for the regeneration of the reactant and the absorbent. Preferably, the loaded absorbent mixture and the reduced polyvalent metal chelate, or mixtures thereof, are regenerated by contacting the mixture in a regeneration zone or zones with oxygen. As used herein, the term "oxygen" includes oxygen-containing gases. The oxygen is preferably supplied present in air, or air enriched with oxygen.
If slgnificant quantities of C02 have been absorbed the reactant-containing solution is preferably treated, such as by heating or pressure reduction, to remove the bulk of the C02 before regeneration of the reactant (either prior or subsequent to sulphur removal). Alternatively, or if small quantities of C02 are absorbed, the C02 may simply be stripped ln the regeneration zone.
As indicated, the regeneration of the reactant is preferably accomplished by the utilization of oxygen, preferably as present in air. The oxygen will accomplish two functions, first the oxidation of the reactant to its higher valence state, and second the stripping of any residual C02 (if originally present) from the absorbent mixture. The oxygen (in whatever form supplied) is ~33~

supplied in a stoichiometric equivalent or excess with respect to the amount of polyvalent metal in the polyvalent metal chelate compound or compounds present in the mixture. Preferably, the oxygen is supplied in an amount in the range of from about 1.2 to 3 times excess.
The particular type of gaseous stream treated is not critical, as will be evident to those skilled in the art. Streams particu-larly suited to removal of H2S and C02 by the practice of the invention are, as indicated, naturally occurring gases, synthesis gases, process gases, and fuel gases produced by gasification procedures, e.g. gases produced by the gasification of coal~ petroleum, shale, tar sands, etc. Particularly preferred are coal gasification streams, natural gas streams and refinery feedstocks composed of gaseous hydrocarbon streams, especially those streams of this type having a low ratio of H2S to C02, and other gaseous hydro-carbon streams. The term "hydrocarbon stream(s)", as employed herein, is intended to include streams containing significant quantities of hydrocarbon (both paraffinic and aromatic), it being recognized that such streams contain significant "impurities"
not technically defined as a hydrocarbon. Streams containing principally a single hydrocarbon, e.g., ethane, are eminently suited to the practice of the inventionO Streams derived from the gasification and/or partial oxidation of gaseous or liquid hydrocarbon may be treated according to the invention. The H2S
content of the type of streams contemplated will vary extensively, but, in general, will range from about 0.1 per cent to about 10 per cent by volume. C02 content may also vary, and may range from about 0.1 per cent to about 99 per cent or greater by voluma.
Obviously, the contents of H2S and C02 present are not generally a limiting factor in the practice of the invention.
The temperatures employed in the contacting or absorption-contact zone are not generally critical, except that the reaction is carried out below the melting point of sulphur, and, if an absorbent is used, the temperatures employed must permit acceptable 3~

absorption of CO2. In many commercial applications, such as the removal of ~I2S (and, if desired, CO2) from natural gas to mee-t pipeline specifications, absorption at ambient temperatures is desired, since the cost of refrigeration would exceed the benefits obtained due to increased absorption at the lower temperature. In general, temperatures in the range of from 10C to 80~C are suit-able, and temperatures in the range of from 20C to 45C are pre-ferred. Contact times may be in the range of from about 1 s to about 270 s or longer, with contact times in the range of from

2 s to 120 s being preferred.
Similarly, in the regeneration or stripping zone or zones, temperatures may be varied widely. Preferably, the regeneration zone should be maintained at substantially the same temperature as the contacting zone. If heat is added to assist regeneration, cooling of the aqueous admixture is required before return of the admixture to the contacting zone. In general, tem-peratures in the range of f-om about 10C to 80C, preferably 20C
to ~5C~ may be employed.
Pressure conditions in the contacting zone may vary widely, depending on the pressure of the gaseous stream to be treated. For example, pressures in the contacting zone may vary from 1 bar up to 152 or even 203 bar. Pressures in -the range from 1 bar to about 101 bar are preferred. In the regeneration or desorption zone or zones, pressures also may be varied consider-ably, and will preferably range from abou-t 0.5 bar to about 3 or ~3B;~
5a 4 bar. The pressure-temperature relationships involved are well understood by those skilled in the art, and need not be detailed herein. Other conditions of operation for this type of reaction process, e.g., pH, etc., are further described in United States patent 3,068,065 to Hartley, et al, dated December 11, 1962, and United States patent 4,009,251 to Meuly, issued February 22, 1977.
Preferably, if the iron chelate of nitrilotriacetic acid is used, pH in the process of the invention will range from about 6 to about 7.5. I'he process is preferably conducted

3~

continuously.
As indicated, the H2S, when contacted, is rapidly converted in the process of the invention by the oxidizing polyvalent metal chelate~ or mixtures thereof, to elemental sulphur. Since many polyvalent metal chelates have limited solubility in many solvents or absorbents, if an absorbent is used, the polyvalent metal chelates are preferably supplied in admixture with the liquid absorbent and water. The amount of oxidizing polyvalent metal chelate, or mixtures thereof, supplied is an effective amount, i.e., an amount sufficient to convert all or substantially all of the H2S in the gaseous stream, and will generally be on the order of at least about 1 mol per mol of H2S. Ratios in the range of from about 1 or 2 mol to about 15 mol of polyvalent metal chelate per mol of H2S may be used, with ratios in the range of from about 2 mol per mol`to about 5 mol of polyvalent metal chelate per mol of H2S being preferred. The manner of preparing the admixture containing an absorbent is a matter of choice. For example, the polyvalent metal chelate may be added to the absorbent, and, if necessary, then ~ater added. The amount of water added will normally be just that amount necessary to achieve solution of the polyvalent metal chelate, and can be determined by routine experimentation. Since the polyvalent metal chelate may have a significant solubility in the solvent, and since water is produced by the reaction of the H2S and the ions of the chelate, precise amounts of water to be added cannot be given. In the case of absorbents having a low solubility for the polyvalent ~etal chelate, approximately 5 per cent to 10 per cent water by volume, based on the total volume of the absorbent mixture, will generally provide solvency. Preferably, however, the polyvalent metal chelate or chelates are added as an aqueous solution to the liquid absorbent. Where the reactant is supplied as an aqueous solution, the amount of solution supplied may be about 20 per cent to about 80 per cent by volume of the total absorbent admixture supplied to the absorption zone.

2~1 A polyvalent metal chelate solution will generally be supplied as an aqueous solution having a concentration in the range of from about 0.1 molar to about 3 molar, and a concentration of about 1.0 molar is preferred. If iron is used, the ligand to iron molar ratio may range from 1.1 to 1.6, preferably 1.2 to 1.4.
Any oxidizing polyvalent metal chelate may be used, but those of iron, copper and manganese are preferred, particularly iron.
The polyvalent metal of the chelate should be capable of oxidizing hydrogen sulphide, while being reduced itself from a higher to a lower valence state and should then be oxidizable by oxygen from the lower valence state to the higher valence state in a typical redox reaction. Other polyvalent metal chelates which may be used include those of lead, mercury, palladium, platinum, tungsten, nickel, chromium, cobalt, vanadium, titatium, tantalum, zirconium, molybdenum, and tin.
Preferred reactants are coordination complexes in which oxidizing polyvalent metals form chelates with an acid having the formula (X~ 3_n~N (Y)n wherein n is an integer from 1 to 3; Y represents a carboxy-methyl or 2-carboxyethyl group and X a 2-hydroxyethyl or 2-hydroxypropylgroup or an alkyl group having from 1 to 4 carbon atoms; or Y ,Y
/
/ N-R-N

y Y wherein:

- from 2 to 4 of the groups Y represent carboxymethyl or 2-carboxy-ethyl groups;
- from 0 to 2 of the groups Y represent a 2-hydroxyethyl or 2-hydroxypropyl group, and ~93~

- R represents an ethylene, a trime-thylene, l-methylethylene, 1,2-cyclohexylene or 1,2-benzylene group or with a mixture of such acids.
The polyvalent metal chelates are readily formed in aqueous solution by reaction of an appropriate salt, oxide or hydroxide of the polyvalent metal and the chelating agent in the acid form or an alkali metal or ammonium salt thereof. Exemplary chelating agents include aminoacetic acids derived from ammonia or 2-hydroxyalkyl amines, such as glycine (aminoacetic acid), digly-cine (iminodiacetic acid), NTA (nitrilotriacetic acid~, a 2-hydroxyalkyl glycine; a dihydroxyalkylglycine, and hydroxyethyl-or hydroxypropyldiglycine; aminoacetic acids derived from ethylene-diamine, diethylenetriamine, 1,2-propylenediamine, and 1,3-propylenediamine, such as EDTA (ethylenediaminetetraacetic acid), HEDTA (2-hydroxyethylethylenediaminetriacetic acid), DETPA
(diethylenetriaminepentaacetic acid); aminoacetic acid derivatives of cyclic 1,2-diamines, such as 1,2-diaminocyclohexane-N,N-tetra-acetic acid, and 1,2-phenylenediamine-~,N-tetraacetic acid, and the amides of polyaminoacetic acids disclosed in sersworth United States Patent No. 3,580,950. The iron chelates of NTA and 2-hydroxyethylethylenediaminetriacetic acid are preferred.
The absorbents employed in this invention are those absorbents which have a high degree of selectivity in absorbing C2 (and preferably H2S as well) from the gaseous streams. Any of the known absorbents conventionally used which do not affect the activity of the polyvalent chelate, or mixtures thereof, and which exhibit sufficient solubility for the reactant or reactants may be employed. As indicated, the absorbent preferably has good absorbency for H2S as well, in order to assist in the removal of any H2S present in the gaseous streams.
The particular absorbent chosen is a matter of choice, given these qualifications, and selection can be made by routine experimentation. For example, 3,6-dioxaoctanol (also referred to as "Carbitol" or "diethylene glycol mono ethyl-ether"), propylene carbonate, 2,5,8,11,14-pentaoxapentadecane (also referred to as "tetraethylene glycol dimethyl ether"), N-methylpyrrolidone, tetrahydrothiophene l,l-dioxide (also referred to as 7'sulfolane"), methyl isobutyl ketone, 2,4-pentanedione, 2,5-hexanedione, 2-hydroxy-2-methyl-4-pentanone (also referred to as "diacetone alcohol"), hexyl acetate, cyclohexanone, 4-methyl-3-penten-2-one (also referred to as "mesityl oxide"), and 4-methyl-4-methoxypentanone-2 may be used. Suitable temperature and pressure relationships for different C02-selective absorbents are known, or can be calculated by those skilled in the art.
As indicated, the compounds designated may be used in reducing the rate of degradation of the chelates employed.
More particularly, N,N-diethylhydroxylamine, thiourea, thiosemicarbazide, 2,2-thiodiethanol, zinc ~sopropylxanthate, and mixtures thereof, may be used. The formulae of 2,2-thiodiethanol and zinc isopropylxanthate are HOCH2CH2SCa2CH2QH
and iC3H70C(S)S-Zn-S-(S)GOiC3H7, respectively. The composition is supplied in a stabilizing amount, i.e., an amount sufficient to reduce or inhibit the rate of degradation. The term 'Istabilizing amount" thus also includes those minor amounts of the stabilizers employed, which, while perhaps not exhibiting an especially discernible reduction of ligand rate of reduction, are effective, nonetheless, in conjunction with amounts of other non-interfering and non-reactive ligand stabilizing compositions, such as those described in my copending application No. 402901 entitled Sulphur Recovery Process, filed 13th May, ]982 ;~ 1L9~21~

in reducing the ligand rate of degradation or decompo-sition. This amount may be determined by experimentation.
In general, the amount employed will range from about 0.001 to about 0.5 and particularly from 0.005 to 0.5 mol per litre of solution, with an amount of 0.03 to about 0.3 mol per litre being preferred. Those skilled in the art may adjust the amount added to produce optimum results, a primary consideration being simply the cost of the stabilizer added.
The manner of recovering the sulphur is a matter of choice.
For example, the sulphur may be recovered by settling, filtration, liquid flotation, or by suitable devices, such as a hydrocyclone. It is not necessary that all sulphur be removed on a contimlous basis in the process; the process may suitably be operated with a minor inventory or significantly reduced content of sulphur in the system.
In order to disclose the invention in greater dætail, the following experiments were run. The values given herein relating to temperatures, pressures, compositions, etc., should be considered merely exemplary and not as delimiting the invention.
Comparative Experiment An aqueous solution (150 ml) of the Fe chelate of nitrilotriacetic acid was placed in a vessel, and a stream of pure H2S was sparged into the solution with rapid stirring.
Temperature oE the solution was 35 C, and the solution contained 0.27 mol per litre iron as Fe . Nitrilotriacetic acid ligand was present in 40 per cent mol excess, basis the iron. l-Decanol (0,060 ml, 300 parts per million by weight3 was also added to the solution. The pH of the solution was 7, and pressure was atmospheric. Addition of the H2S (360 ml) was continued until approximately 70 per cent of the Fe was converted to the Fe state, which took about 2 to 3 min. The flow of the ~2S and the stirring action were then discontinued, oxygen in excess was sparged into the solution, and stirring resumed for 15 min, thus regenerating the Fe , and completing one cycle. The procedure was repeated for 5 cycles, the time of regeneration varying up to 30 min, at which time the solution was removed from the vessel and filtered. The sulphur produced wa~ washed, dried, and weighed. A small amount (3 ml) of solution was removed for analysis of nitrilotriacetic acid. The remainder of the solution was then returned to the vessel, and a small amount of H2S04 was added to bring the pH back to 7. The general procedure was followed for 5 cycles, and the filtration, acid addition (if necessary), analysis, etc. was repeated. A total of 15 cycles was run, and 1-decanol (0.030 ml) was added after the 6th and 12th cycles.
The difference in weight between initial weight of nitrilotriacetic acid ligand and that r~m~;ning after 15 cycles was calculated, and is a measure of loss of ligand per unit weight of sulphur produced. The result is shown in table I.
Example I
A procedure similar to that of the Comparative Experiment was followed, except that the solution also contained O.IM
N~N-diethylhydroxylamine. The result is shown in table I.
Example II
A procedure similar to that of the Comparative Experiment was followed, except that the solution also contained O.lM
thiourea. The result is shown in table I.
Example III
A procedure similar to that of the Comparative Experiment was followed, except that the solution also contained 0.05M
thiosemicarbazide. The result is shown in table I.

~ 3~

TABLE I

Stabilizing Grams of ligand lost per composition gram of sulphur produced Comparative Experiment none 0.14 Example I N,N-diethyl- 0.09 hydroxylamine Example II thiourea 0.07 Example III thiosemicarbazide 0.06 Example IV
A procedure similar to that of the Comparative Experiment was followed, except that the solution also contained O.IM
2,2-thiodiethanol. The result is shown in table II.
Example V
A procedure similar to that of the Comparative Experiment was followed, except that the solution also contained 0.008M
zinc isopropylxanthate. The result is shown in table II.
Example VI
A procedure similar to that of the Comparative Experiment was followed, but the solvent employed consisted of 33 per cent by volume water and 67 per cent by volume carbitol, and 0.03M
zinc isopropylxanthate was utilized in the solvent. The result is shown in table II.

TABLE II

Stabilizin~ Grams of ligand lost per composition ~ram of sulphur produced Comparative Experiment nane 0.14 Example IV 2,2-thiodiethanol 0.07 Example V zinc isopropylxanthate 0.11 Example VI zinc isopropylxanthate 0.05 ~33~

While the invention has been illustrated with particular apparatus, those skilled in the art will appreciate that, except where specified, other equivalent or analogous units may be employed. The term "zones", a~ employed in the specification and claims, includes, where suitable, the use of segmented equipment operated in series, or the devision of one Imit into multiple units because of size constraints, etc. For example, an absorption column might comprise two separate columns in which the solution from the lower portion of the first column would be introduced into the upper portion of the second col~mmn, the gaseous material from the upper portion of the first column being fed into the lower portion of the second column. Parallel operation of units is, of course, well within the scope of the invention.
Again, as will be understood by those skilled in the art, the solutions or mixtures employed may contain other materials or additives for given purposes. For example, U.S. pa~ent 3,933,993 discloses the use of bufrering agents, such as phosphate and carbonate buffers. Similarly, U.S. patent 4,009,251 describes various additives, such as sodium oxalatè, sodium forma~e, sodium thiosulphate, and sodium acetate, which may be beneficial.

Claims (24)

C L A I M S

1. A process for the removal of H2S from a sour gaseous stream, which process comprises the following steps: -a) contacting the sour gaseous stream in a contacting zone with an aqueous reactant solution at a temperature below the melting point of sulphur, the solution containing an effective amount of a reactant selected from the group consisting of oxidizing polyvalent metal chelate compounds, and mixtures thereof, and a stabilizing amount of a stabilizing composition selected from the group consisting of N,N-diethylhydroxylamine, thiourea, thiosemicarbazide, 2,2-thiodiethanol, zinc isopropylxanthate and mixtures thereof, and producing a sweet gas stream and an aqueous admixture containing crystalline sulphur and a reduced reactant;
b) removing at least a portion of the crystalline sulphur from the aqueous admixture;
c) regenerating the aqueous admixture in a regeneration zone to produce a regenerated reactant;
d) returning aqueous admixture containing regenerated reactant from the regeneration zone to the contacting zone.

2. A process as claimed in claim 1, in which the stream from which the H2S is removed is selected from naturally-occurring gases, synthesis gases, process gases and fuel gas.

3. A process as claimed in claim 1, in which the sour gaseous stream is selected from natural gas, a stream derived from the gasification of coal, or a hydrocarbon stream.

4. A process for the removal of H2S from a sour gaseous stream, which process comprises the following steps: -a) contacting the sour gaseous stream in a contacting zone with an aqueous reactant solution at a temperature below the melting point of sulphur, the solution containing an effective amount of a reactant selected from the group consisting of oxidizing polyvalent metal chelate compounds and mixtures thereof, and a stabilizing amount of a stabilizing composition selected from the group consisting of N,N,-diethylhydroxylamine, thiourea, thiosemi-carbazide, 2,2-thiodiethanol, zinc isopropylxanthate, and mixtures thereof, and producing a sweet gas stream and an aqueous admixture containing crystalline sulphur and a reduced reactant;
b) regenerating the aqueous admixture in a regeneration zone to produce an aqueous admixture containing a regenerated reactant;
c) removing at least a portion of the crystalline sulphur from aqueous admixture from step (b) to produce an aqueous admixture having reduced sulphur content;
d) returning aqueous admixture having reduced sulphur content to the contacting zone.

5. A process as claimed in claim 4, in which the stream from which the H2S is removed is selected from naturally-occurring gases, synthesis gases, process gases and fuel gas.

6. A process as claimed in claim 4, in which the sour gaseous stream is selected from natural gas, a stream derived from the gasification of coal, or a hydrocarbon stream.

7. A process as claimed in claim I or 4, in which the stabilizing composition is present in the reactant solution in an amount in the range of from 0.001 to 0.5 mol per litre of solution.

3. A process as claimed in claim 1 or 4, in which the stabilizing composition is present in the reactant solution in an amount in the range of from 0.03 to 0.3 mol per litre of solution.

9. A process as claimed in claim 1 or 4, in which the reactant is a coordination complex of a polyvalent metal with an acid having the general formula:
(X)3-n-N-(Y)n wherein n is an integer in the range of from 1 to 3, Y represents a carboxymethyl or 2-carboxyethyl group and X a 2-hydroxyethyl or 2-hydroxypropyl group, or an alkyl group having in the range of from 1 to 4 carbon atoms;
or wherein:
from 2 to 4 of the groups Y represent carboxymethyl or 2-carboxyethyl groups;
from O to 2 of the groups Y represent a 2-hydroxyethyl or 2-hydroxypropyl group, and R represents an ethylene, a trimethylene, 1-methylethylene, 1,2-cyclohexylene or 1,2-benzylene group;
or with a mixture of such acids.

10. A process as claimed in claim 1 or 4, in which the reactant is a coordination complex of a polyvalent metal with an aminoacetic acid derived from ethylenediamine.

11. A process as claimed in claim 1 or 4, in which the reactant is a coordination complex of a polyvalent metal with an amino-acetic acid derived from ammonia.

12. A process as claimed in claim 1 or 4, in which the polyvalent metal is iron.

13. A process as claimed in claim 1 or 4, in which the reactant is a coordination complex of iron with 2-hydroxyethyl-ethylenediaminetriacetic acid.

14. A process as claimed in claim 1 or 4, in which the reactant is a coordination complex of iron with nitrilotriacetic acid.

15. A process for the removal of H2S and CO2 from a sour gaseous stream, which process comprises the following steps:-a) contacting the sour gaseous stream in a contacting zone at a temperature below the melting point of sulphur with a lean CO2-selective absorbent mixture containing an effective amount of a reactant selected from the group consisting of oxidizing polyvalent metal chelate compounds and mixtures thereof, and a stabilizing amount of a stabilizing composition selected from the group consisting of N,N-diethyl-hydroxylamine, thiourea, thiosemicarbazide, 2,2-thiodiethanol, zinc isopropylxanthate, and mixtures thereof, and producing a sweet gaseous stream and an absorbent admixture containing absorbed CO2, crystalline sulphur and reduced reactant;
b) removing at least a portion of the crystalline sulphur from the absorbent admixture, and leaving a solution containing absorbed CO2 and reduced reactant, c) stripping the solution containing absorbed CO2 and said reduced reactant to remove CO2, and regenerating said solution, producing a lean CO2-selective absorbent solution containing regenerated reactant, and d) returning lean CO2-selective absorbent solution containing regenerated reactant to the contacting zone.

16. A process for the removal of H2S and CO2 from a sour gaseous stream, which process comprises the following steps:
a) contacting the sour gaseous stream in a contacting zone at a temperature below the melting point of sulphur with a lean CO2-selective absorbent mixture containing an effective amount of a reactant selected from the group consisting of oxidizing polyvalent metal chelate compounds and mixtures thereof, and a stabilizing amount of a stabilizing composition selected from the group consisting of N,N-diethylhydroxylamine, thiourea, thiosemicarbazide, 2,2-thiodiethanol, zinc isopropylxanthate, and mixtures thereof, and producing a sweet gaseous stream and an absorbent mixture containing absorbed CO2, crystalline sulphur, and reduced reactant;
b) stripping the absorbent mixture containing absorbed CO2, crystalline sulphur, and said reduced reactant, and regenerating said absorbent mixture, producing a lean CO2-selective absorbent solution containing a regenerated reactant and sulphur, c) removing at least a portion of the crystalline sulphur from the lean CO2-selective absorbent solution containing the sulphur and the regenerated reactant, and leaving a lean CO2-selective absorbent solution containing regenerated reactant, and d) returning lean CO2-selective absorbent solution containing regenerated reactant to the contacting zone.

17. A process as claimed in claim is or 16, in which the stabilizing composition is present in the reactant solution in an amount in the range of from 0.001 to 0.5 mol per litre of solution.

18. A process as claimed in claim 15 or 16 in which the stabilizing composition is present in the reactant solution in an amount in the range of from 0.03 to 0.3 mol per litre of solution.

19. A process as claimed in claim 15 or 16, in which the reactant is a coordination complex of a polyvalent metal with an acid having the general formula:
(X)3-n-N-(Y)n wherein n is an integer in the range of from 1 to 3, Y represents a carboxymethyl or 2-carboxyethyl group and X a 2-hydroxyethyl or 2-hydroxypropyl group or an alkyl group having in the range of from 1 to 4 carbon atoms;
or wherein:
- from 2 to 4 of the groups Y represent carboxymethyl or 2-carboxyethyl groups;
- from 0 to 2 of the groups Y represent a 2-hydroxyethyl or 2-hydroxypropyl group, and - R represents an ethylene, a trimethylene, 1-methylethylene, 1,2-cyclohexylene or 1,2-benzylene group;
or with a mixture of such acids.

20. A process as claimed in claim 15 or 16, in which the reactant is a coordination complex of a polyvalent metal with an aminoacetic acid derived from ethylenediamine.

21. A process as claimed in claim 15 or 16, in which the reactant is a coordination complex of a polyvalent metal with an aminoacetic acid derived from ammonia.

22. A process as claimed in claim 15 or 16, in which the polyvalent metal is iron.

23. A process as claimed in claim 15 or 16, in which the reactant is a coordination complex of iron with 2-hydroxy-ethylethylenediaminetriacetic acid.

24. A process as claimed in claim 15 or 16, in which the reactant is a coordination complex of iron with nitrilotriacetic acid.