PROCESS FOR THE REMOVAL OF H2S FROM A SOUR H2S-CONTAINING GASEOUS STREAM

Abstract: Process for the removal of H₂S from a sour H₂S-contain­ ing gaseous stream comprising: (a) contacting the sour gaseous stream in a contacting zone with an aqueous reaction solution at a temperature below the melting point of sulphur, the solution comprising an oxidizing polyvalent metal chelate and both phosphate and thiosulphate ions or precursors thereof, and producing a purified gaseous stream and an aqueous solution contain­ ing crystalline sulphur and reduced polyvalent metal chelate; (b) removing crystalline sulphur from the aqueous solution and regenerating latter solution in a regenerating zone to produce a regenerated solution containing oxidizing polyvalent metal chelate; and (c) returning the regenerated solution to the contacting zone.

Description: PROCESS FOR THE REMoVAL OF H2S FR3M A SOUR H2S-CCNTAINING GASEOUS STREAM The present invention relates to a process for the removal of H2S from a sour H2S-containing gaseous stream. The presence of significant quantities of H2S and CO, 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 gas is contacted, preferably with a solvent-reactant system which comprises a regenerable reactant, to produce solid free sulphur which is recovered either prior to or subsequent to regeneration. Suitable reactant materials include polyvalent metallic ions, 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. A problem associated with such processes is that the solid sulphur produced, if special measures are not taken, may be of poor quality, i.e., it may be finely divided and difficult to separate from the aqueous reactant solution. The invention seeks to overcome this problem. Accordingly, the invention relates to a process for the removal of H2S from a sour H2S-containing gaseous stream comprising: (a) contacting the sour gaseous stream in a contacting zone with an aqueous reaction solution at a temperature below the melting point of sulphur, the solution comprising an oxidizing polyvalent metal chelate and both phosphate and thiosulphate ions or precursors thereof, and producing a purified gaseous stream and an aqueous solution containing crystalline sulphur and reduced polyvalent metal chelate; (b) removing crystalline sulphur from the aqueous solution and regenerating latter solution in a regenerating zone to produce a regenerated solution containing oxidizing poly valent metal chelate; and (c) returning the regenerated solution to the contacting zone. The sulphur crystals obtained in step (b), due to the presence of the combination of phosphate and thiosulphate ions, are of imprcved quality, i.e., they have increased size, and, thus, improved filterability. The reduced polyvalent metal chelate is regenerated, preferably by contacting the aqueous solution in a regeneration zone with oxygen. The term "oxygen", as used herein, includes oxygen#contnning gases, such as air or air-enriched with oxygen. The oxygen oxidizes the reduced metal ions of the chelate to the higher valence state, and the regenerated mixture is returned to the contact zone. Electrochemical regeneration may also be employed. A sour gaseous stream containing H2S and CO, can be subjected to the process according to the invention, too. Broadly, this is accomplished by the use of an absorbent mixture containing an absorbent for 002 (and preferably for H2S, as well), an effective amount of a polyvalent metal chelate and an amount of the ions described. A purified or "sweets gaseous stream is produced which meets general industrial and commerciai H2S and CO2 specifications. The CO, is absorbed and the H2S is immediately converted to sulphur by the polyvalent metal chelate. In the process, the polyvalent metal chelate is reduced, and the sulphur may be treated, as described, supra. As in the previous enoodment, the sulphur crystals may be removed prior to or subsequent to regeneration of the aqueous solution, and the crystals produced will be of increased size. Preferably, if the volume of C02 absorbed is large, the reduced chelate-containing solution is 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). Alternately, or if small quantities of CO, are absorbed, the C02 may simply be stripped in the regeneration zone. Preferably, the loaded absorbent mixture and the reduced polyvalent metal chelate are regenerated by contacting the mixture in a regeneration zone or zones with an oxygencontaining gas. The oxygen-containing gas may be air, oxygen, or air-enriched with oxygen. The oxygen-containing gas accomplishes two functions, the stripping of the CO2 from the loaded absorbent mixture, and the oxidation of the reduced reactant to a higher oxidation state. The oxygen (in whatever form supplied) is supplied in a stoichiometric equivalent or excess with respect to the amount of reactant present in the mixture. Preferably, the oxygen-containing gas is supplied in an amount of from about 1.2 to 3 times excess. Good mixing of the gaseous stream and reaction solution in the contact zone may be provided by conventional apparatus currently available, such as tray type units, in-line mixers (at quick liquid and gas velocities) and stirred or agitated vessels. Poor or inefficient contacting will negate the results of the invention, in some instances, if contacting is quite inefficient, producing sulphur of inferior quality. The particular type of sour gaseous stream treated is not critical, as will be evident to those skilled in the art. Streams particularly suited to removal of H2S and CO2 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 hydro carbon streams, especially those streams of this type having a low ratio of H2S to C02, and other gaseous hydrocarbon streams. The term "hydrocarbon streams", 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 hydrocarbon. Again, streams containing principally a single hydrocarbon e.g., ethane, are eminently suited to the practice of the invention. Streams derived from the gasification and/or partial oxidation of gaseous or liquid hydrocarbon may be treated by the invention. The eH2S 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. CO, content may also vary, and may range from about 0.5 per cent to over 99 per cent by volume. Obviously, the amounts of H2S and C02 present are not generally a limiting factor in the practice of the invention. The temperatures employed in the contacting or absorptioncontact zone are not generally critical, except that the reaction is carried out at a temperature below the melting point of sulphur, and, if an absorbent is used, they must permit acceptable absorption of 002. In many commercial applications, such as the removal of H2S and cho2 from natural gas to meet pipeline specifications, contacting 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 of from 10 C to 80 C are suitable, and temperatures from 20 C to 45 0C are preferred. Contact times may range from about 1 second to about 270 seconds or longer, with contact times of 2 seconds to 120 seconds 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 absorption zone. If heat is added to assist regeneration, cooling of the absorbent mixture is required before return of the absorbent mixture to the absorption zone. In general, temperatures of from about 10 C to 80 OC, preferably 20 C to 45 C may be employed. Pressure conditions in the contacting zone may vary widely, depending on the pressure of the gas to be treated. For example, pressures in the contacting zone may vary from one bar up to two hundred bar. Pressures of from one bar to about one hundred bar are preferred. In the regeneration zone or zones, pressures may be varied considerably, and will preferably range from about 0.5 bar to about three or four bar. The pressure-temperature relationships involved are well understood by those skilled in the art, and need not be detailed herein. Preferably the pH in the process according to the invention ranges fram 6 to 7.5. The procedure is preferably conducted continuously. As indicated, the H2S, when contacted, is quickly converted by a solution of the polyvalent metal chelate to elemental sulphur. The amount of chelate compound supplied is an effective amount, i.e., an amount sufficient to convert all or substantially all of the H2S in the gas stream, and will generally be on the order of at least about two mols per mol of H2S. Ratios of from about 1 mol to about 15 mols of chelate per mol of H2S may be used, with ratios of from about 2 mols per mol to about 5 mols of chelate per mol of H2S being preferred. The manner of preparing the aqueous solution containing an absorbent is a matter of choice. For example, the chelate may be added to the absorbent, and, if necessary, then water added. The amount of water added will normally be just that amount necessary to achieve solution of the chelate, and can be determined by routine experimentation. Since the chelate may have a significant solubility in the solvent, and since water is produced by the reaction of the H2S and the chelate, precise amounts of water to be added cannot be given. In the case of absorbents having a low solubility for the 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 chelate is added as an aqueous solution to the liquid absorbent. Where the chelate 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 contacting zone. The chelate solution will suitably be employed as an aqueous solution having a concentration of from 0.01 molar to 3 molar, preferably a concentration of 0.1 to 3.0 molar, a concentration of about 1.0 molar being in particular preferred. Any oxidizing polyvalent metal chelate, or mixtures thereof, may be used, but those of iron, copper, and manganese are preferred, particularly iron. The polyvalent metal should be capable of oxidizing hydrogen sulphide, while being reduced itself from a higher to a lower valence state, and should then be oxidizable from the lower valence state to the higher valence state in a typical redox reaction. Other polyvalent metals which can be used include lead, mercury, palladium, platinum, tungsten, nickel, chromium, cobalt, vanadium, titanium, tantalum, zirconium, molybdenum, and tin. The absorbents employed in this invention are those absorbents which have a high degree of selectivity in absorbing CO, (and preferably H2S as well) from the gaseous streams. Any of the known absorbents conventionally used which do not affect the activity of the reactant 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 sour gaseous stream. The particular absorbent chosen is a matter of choice, given these qualifications, and selection can be made by routine experimentation. For example, diethylene glycol monoethyl ether, propylene carbonate, tetraethylene glycol#dimethyl ether, N-methyl pyrrolidone, sulpholane, methyl isobutyl ketone, 2,4-pentanedione, 2,5-hexanedione, diacetone alcohol, hexyl acetate, cyclohexanone, mesityl oxide, and 4-methyl- 4 -methoxy-pentone- 2 may be used. Suitable temperature and pressure relationships for different 002 selective absorbents are knawn, or can be calculated by those skilled in the art. The phosphate and thiophosphate ions are suitably present in the aqueous reaction solution in a molar ratio of from 0.5 to 3:1. The solution containing the ions may be prepared in any suitable fashion. Preferably, the polyvalent metal compounds or chelate (s) may be dissolved and the phosphorus-containing material and the thiosulphate, or precursor thereof, may be added. Alternatively, the phosphorus-containing material may be provided in water, and the chelate, etc., may be added. The phosphate ions and thiosulphate ions, are supplied in an amount effective to improve the quality of the sulphur produced. Within the ratios mentioned the amount to be employed may be determined by experimentation, it being observed that the ions will preferably be present in a range of from about 0.05 to 1.0 moles of phosphate per litre, and from about 0.05 to 1.0 moles of thiosulphate per litre. Any suitable sources or precursors of phosphate and thiosulphate ions may be employed. Preferred materials are sodium, potassium, and lithium phosphate, polyphosphate, -hydrogen or -dihydrogen phosphates, and mixtures thereof, and sodium, potassium, or lithium thiosulphate, bisulphite, dithionite, or tetrathionite, and mixtures thereof. The term "polyphosphate", as used herein, is understood to include ortho-, and metaphosphate, and mixtures thereof, and the various phosphoric acids may be employed. The ratios of the various components are as provided, supra. Preferred chelating agents of the oxidizing polyvalent metal chalates are selected from nitrilotriacetic acid and an acid having the formula EMI8.1 wherein - from two to four of the groups Y are selected from acetic acid and propionic acid groups; - fran zero to two of the groups Y are selected from 2 hydrcxyethyl and 2-hydrcxypropyl groups; and - R represents ethylene, propylene, isopropylene, or cyclo hexane or benzene wherein the two hydrogen atoms replaced by nitrogen atans are in the 1,2-position. Particular preference is given to oxidizing polyvalent metal chelates which contain nitrilotriacetic acid or N-(2 hydrcxyethyl) -ethylenediamine triacetic acid as chelating agent. When iron is used as the polyvalent metal and nitrilotriacetic acid as the chelating agent the molar ratio of the nitrilotriacetic acid to the iron is preferably frcm 1.2 to 1.4. The concentration of the iron chelate of nitrilotriacetic acid -in the aqueous solutions is preferably from 0.1 to 0.8 moles per litre. The amount of thiosulphate ions is then preferably from 0.05 to 0.5 moles per litre. The invention also relates to a composition for use in aqueous solutions for removing H2S from a sour gaseous stream in a process according to the present invention which composition comprises (A) a polyvalent metal chelate, (B) an alkali metal or ammonium phosphate, polyphosphate, hydrogen- or dihydrogenphosphate and (C) an alkali metal thiosulphate, bisulphite, dithionite or tetrathionite, the molar ratio of (A) to (B) to (C) being 1-6:0.5-3:1. Preferably, iron is the polyvalent metal and nitrilotriacetic acid is the chelating agent. In case of the iron chelate of nitrilotriacetic acid, the molar ratio of (A) to (B) to (C) is preferably 1-2:0.5-3:1. The composition may be utilized in preparing the aqueous solution, it being understood that such a composition can simply be added to water. The composition may contain the oxidized or the reduced chelates (or mixtures thereof), those skilled in the art recognizing that the reduced chelate may be employed simply by "regenerating" or oxidizing the solution. If a mixture of the components of the composition of the invention is dissolved in water, the water can be evaporated to dryness by heating and/or reduced pressure leaving the claimed composition as a dry solid. This dry solid redissolves and possesses the same properties as the original claimed composition. The manner of recovering the sulphur crystals is a matter of choice. For example, the crystals may be recovered by settling, filtration, liquid flotation, or by suitable devices such as a hydroclone, etc. In order to describe the invention in greater detail, reference is made to the accompanying schematic drawing. The values given herein relating to temperatures, pressures, compositions, etc., are calculated or merely exemplary and should not be taken as delimiting the invention. Figure 1 illustrates the first #todiment of the invention, wherein sulphur is removed prior to regeneration, while Figure 2 illustrates removal of sulphur after regeneration. In Figure 1, sour gas, e.g., natural gas containing about 0.5 per cent by volume H2S, in line (1) enters contactor (2) (tray type) into which also enters, from line (12), an aqueous reaction solution comprising e.g. an aqueous 0.3 M solution of the Fe(III) chelate of nitrilotriacetic acid and having a pH of 7 or an aqueous 1.0 M solution of the Fe(III) chelate of N-(2 hydroxyethyl)ethylene diarnine triacetic acid having a pH of 6, the solution further containing phosphate ion, added as 0.2 or 0.5 moles per litre of disodium hydrogen phosphate, and thiosulphate ion, added as 0.3 or 0.5 moles per litre of sodium thiosulphate pentahydrate. The pressure of the feed gas is about 84 bar, and the temperature of the aqueous solution is about 35 OC. A contact time of about 120 seconds is employed in order to react all the H2S. Purified or sweet" gas leaves contactor (2) through line (3). The "sweet" gas is of a purity sufficient to meet standard requirements. In the solution, the H2S is converted to elemental sulphur by the Fe(III) chelate, the Fe(III) chelate in the process being converted to the Fe(II) chelate. The aqueous solution containing elemental sulphur is removed continuously and sent through line (4) to a depressurization and degassing unit (5), and then through line (6) to sulphur recovery unit (7). Sulphur recovery unit (7) may be of any suitable type. Preferably, unit (7) comprises a filtration unit. It is not necessary that all the sulphur be removed in the sulphur recovery step, and some sulphur retention may be beneficial. The type of unit chosen is thus flexible. Preferably, the amount of sulphur removed in the separation step is simply balanced with the rate of sulphur intake in contactor (2), which is of course, dependent on the amount of H2S in gas stream in line (1). Those skilled in the art may adjust the appropriate rates of withdrawal of the streams. From unit (7), the sulphurfree or substantially sulphur-free solution is sent via line (8) to regeneration zone (9). In regeneration zone (9), the solution is contacted with excess air from line (10) to convert Fe(II) chelate in the solution to the Fe(III) chelate. The temperature of the regeneration column is about 35 OC, and pressure in the column is maintained at about 2 bar. Spent air is removed from zone (9) through line (11), while regenerated aqueous solution is returned via line (12) to contactor (2). As indicated, Figure 1 illustrates the aspect of the invention wherein the sulphur recovery is carried out prior to regeneration. Removal of the sulphur after regeneration may be preferred in sane instances, and may be accomplished by positioning of the sulphur recovery unit "after" the regeneration zone. Thus, in a separate #tibodiment, regenerated liquid, still containing sulphur, may be passed to units analogous or equivalent to unit (7), sulphur recovered, and regenerated sulphur-free solution returned to contactor (2). Accordingly, in Figure 2, the sulphur-containing solution is passed, after degassing in (5), via line (13) to regenerator (14) where it is regenerated, as previously described. The regenerated sulphur containing solution is removed via line (15), and passed to sulphur recovery unit (16). Spent air is removed via line (17). Regenerated reactant solution is returned via line (18) to contactor (2). 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" includes, where suitable, the use of segmented equipment operated in series, or the division of one unit into multiple units because of size constraints, etc. For example, a contacting column might comprise two separate columns in which the solution fran the lower portion of the first column would be introduced into the upper portion of the second column, 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. The following experiments illustrate the effect of the additives of the invention. The values given herein relating to temperatures, pressures, compositions, etc., should be considered merely exemplary and not as delimiting the invention. Experiment 1 H2S was passed into a contact vessel into which also an aqueous solution containing 1.5 per cent by weight Fe (based on the total weight of the mixture as the Fe(III) chelate of nitrilotriacetic acid (NTA) was introduced. The ligand was supplied in 40 per cent molar excess, basis the iron, and the pH of the system was 7. The pressure of the feed gas was atmospheric and the temperature of the mixture was about 35 OC. A contact time of about 120 seconds was employed. Good gas-liquid contact was achieved by stirring the mixture vigorously. In the mixture, the H2S was converted to elemental sulphur by the Fe(III) chelate, Fe(III) chelate in the process being converted to the Fe(II) chelate. The sulphur produced was very fine and difficult to separate from the solution, and had a mean volume diameter (urn) by Coulter Counter of 5.8. nq#e#iment 2 A procedure similar to Experiment 1 was followed, except that about 1.3 per cent of Fe as the Fe(III) chelate of nitrilotriacetic acid was used, and the solution also contained 2.6 per cent thiosulphate added as sodium thiosulphate pentahydrate. Both percentages were by weight, based on the total weight of the mixture. The sulphur produced had a mean diameter (m) by Coulter Counter of 6.8. Experiment 3 A procedure similar to Experiment 1 was followed, except that about 1.3 per cent of Fe as the Fe(III) chelate of nitrilotriacetic acid was used, and the solution also contained 3.3 per cent phosphate added as an equimolar mixture of disodium hydrogen phosphate and potassium dihydrogen phosphate. Both percentages were by weight, based on the total weight of the mixture. The sulphur produced had a mean diameter (urn) by Coulter Counter of 7.9. FxperiirEnt4 A procedure similar to Experiment 1 was followed, except that the aqueous solution contained 5.4 per cent by weight Fe (based on the total weight of the mixture) as the Fe(III) chelate of N- (2-hydroxyethyl) ethylenediamine triacetic acid. The ligand was supplied in 10 per cent molar excess, basis the iron, and the pH of the system was 6. The sulphur produced was very fine and difficult to separate fuam the solution, and had a mean volume diameter (gum) by Coulter Counter of 2.3. Experiment 5 A procedure similar to Experiment 1 was followed, except that about 1.3 per cent of Fe as the Fe(III) chelate of nitrilotriacetic acid was used, and the solution also contained 3.0 per cent phosphate added as an eqnmolar mixture of disodium hydrogen phosphate heptahydrate and potassium dihydrogen phosphate, and 2.6 per cent thiosulphate added as sodium thiosulphate pentahydrate. All percentages were by weight, based on the total weight of the mixture. The sulphur produced had a mean diameter (urn) by Coulter Counter of 10.4. Experiment 6 A procedure similar to Experiment 2 was followed, except that about 4.4 percent of Fe as the Fe(III) chelate of N-(2hydroxyethyl) ethylenediamine triacetic acid was used, and the solution also contained 3.4 per cent phosphate, added as disodium hydrogen phosphate heptahydrate, and 4.0 per cent thiosulphate added as sodium thiosulphate pentahydrate (all percentages by weight, based on the total weight of the mixture). The sulphur produced had a mean diameter (pom) by Coulter Counter of 6.9. From the results of the above experiments it appears that the sulphur obtained in solutions which contained both thiosulphate and phosphate ions has a greater mean diameter and is therefore easier to separate than sulphur obtained in solutions which contained none (cf. experiments 1 and 4) or only one of the above ions (cf. experiments 2 and 3).

Claims: CLAIMS 1. Process for the removal of H2S from a sour H,S-containing gaseous stream comprising: (a) contacting the sour gaseous stream in a contacting zone with an aqueous reaction solution at a temperature below the melting point of sulphur, the solution comprising an oxidizing polyvalent metal chelate and both phosphate and thiosulphate ions or precursors thereof, and producing a purified gaseous stream and an aqueous solution containing crystalline sulphur and reduced polyvalent metal chelate; (b) removing crystalline sulphur from the aqueous solution and regenerating latter solution in a regenerating zone to produce a regenerated solution containing oxidizing poly valent metal chelate; and (c) returning the regenerated solution to the contacting zone. 2. Process according to claim 1, in which phosphate and thiosulphate ions are present in the aqueous reaction solution in a molar ratio of from 0.5 to 3:1. 3. Process according to claim 1 or 2, in which the ions are present in the aqueous reaction solution in an amount of from 0.05 to 1.0 mole of phosphate per litre and from 0.05 to 1.0 moles of thiosulphate per litre. 4. Process according to any one of claims 1-3 in which as polyvalent metal iron is employed. 5. Process according to any one of claims 1-4, in which the chelating agent of the oxidizing polyvalent metal chelate is selected from nitrilotriacetic acid and an acid having the formuLa: EMI14.1 wherein - from two to four of the groups Y are selected from acetic and propionic acid groups; - from zero to two of the groups Y are selected from 2 hydroxyethyl and 2-hydroxypropyl groups; and - R represents ethylene, propylene, isopropylene, or cyclo hexane or benzene wherein the two hydrogen atoms replaced by nitrogen atoms are in the 1,2-position. 6. Process according to claim 5, in which the oxidizing polyvalent metal chelate contains nitrilotriacetic acid or N- (2-hydroxyethyl) -ethylenediamine triacetic acid as chelating agent. 7. Process according to any one of claims 1-6, in which as precursors for phosphate, polyphosphate, hydrogen phosphate or dihydrogen phosphate and as precursor for thiosulphate bisulphite, dithionite or tetrathionite are used. 8. Process according to any one of claims 1-7, in which the regeneration is carried out by contacting the aqueous solution with an oxygen-containing gas. 9. Composition for use in aqueous solutions for removing H2S from a sour gaseous stream in a process according to any one of claims 1-8, the composition comprising (A) a polyvalent metal chelate, (B) an alkali metal or ammonium phosphate, polyphosphate, hydrogen- or dihydrogen phosphate and (C) an alkali metal thiosulphate, bisulphite, dithionite or tetrathionite, the molar ratio of (A) to (B) to (C) being 1-6:0.5-3:1. 10. Composition as claimed in claim 9, in which (A) is an iron chelate of nitrilotriacetic acid, the molar ratio of (A) to (B) to (C) being 1-2:0.5-3:1.

Inventors: DIAZ, ZAIDA
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1984
Main classification: B01D53
IPC codes: C01B17, B01D53