Title of Invention

"A PROCESS FOR RECOVERING SULFUR FROM HYDROGEN SULFIDE"

Abstract

A process for the preparation and recovery of sulfur from a hydrogen sulflde containing sour gas which comprises contacting hydrogen sulfide containing sour gas with a catalyst comprising of an aqueous solution of chelated iron having concentration' of iron in the range of 0.01% to 2% by weight, a chelating agent of the kind herein described, the ratio of said chelating agent to iron being in the range of 0.25: 1 to 2: 1, from 0.1 to 15.0% by weight of a ligand stabilizer of the kind such as herein described, from 0.01 to 5.0% by weight of a regeneration promoter of the kind such as herein described, a conventional surfactant, and the balance if any consisting of a conventional defoamer, at a temperature below the melting point of sulfur and pH in the range of 6 to 10, to produce an aqueous mixture containing sulfur and reduced catalyst composition removing sulfur from said aqueous mixture in any conventional manner and regenerating the catalyst composition by contacting said reduced catalyst with oxygen or an oxygen containing gas.

Full Text

The present invention relates to a process for recovering sulfur from hydrogen sulfide. The present invention is a divisional out of Indian Patent Application number 877/DEL/93 dated August 16, 1993. The present invention relates to a catalyst composition for use in recovery of sulfur from hydrogen sulfide containing sour gas streams. More particularly, the present invention relates to a catalytic process for recovery of sulfur from hydrogen sulfide containing sour gas. It is well known in the art that when hydrogen sulfide comes in contact with a suitably chelated polyvalent metal ion in its oxidized form, the metal ion oxidizes hydrogen sulfide to elemental sulfur, metal ion itself getting converted to the reduced state. Continuous working of the process therefore, required replenishment of oxidized form of the metal ion on a continuous basis. Therefore, there has been a need to design a system for enabling rapid regeneration of oxidized form of the metal ion to enable the process to work on a continuous catalytic basis. The prior art also faced a serious problem of completing side reactions which resulted in undesirable by-products as well as deactivation of the catalyst. Accordingly, it is an object of the present invention to provide an improved catalyst composition for use in recovery of sulfur from hydrogen sulfide containing sour gas streams. It is further object of the present invention to provide a catalyst composition which is capable of rapid regeneration. It is yet a further object of the present invention to provide a catalyst composition which evinces a long term performance and which can suppress the formation of by-products. It is yet another object of the present invention to recover sulfur In high yield from hydrogen sulfide containing sour gas streams. The applicants have succeeded in meeting the above objects of the present invention and obviating the difficulties encountered by the prior art by formulating a catalyst composition based on ferric ion. The applicants have surprisingly discovered that when instead of any polyvalent metal ion, ferric ion is employed in combination with polyamino carboxylic acid and their alkali metal salts as chelating agents, not only high yield of sulfur is obtained but the catalyst is also rapidly regenerated enabling the process to work on a continuous basis. The reactions involved will be as follows:- H2 S (g) > H2S (aq) (I) H2S (aq) + 2 OH' >S2 + 2H2O (2) S2 + 2Fe3+ 2Fe2+ + S° (3) 2Fe2+ +1/2O2 + H2O-2Fe3+ + 2 OH' (4) The applicants have further discovered that when metal-ligand compounds described above are combined with a polyhydroxy sugar the catalyst shows remarkable stability. Typical polyhydroxy sugar that may be employed in the improved catalyst of the present invention are sucrose, sorbitol and mannitol. However to aid in the regeneration of the catalyst, further additives can be employed. Typically, the applicants have discovered that addition of thiocyanates or soluble phosphates in the catalyst composition enhance the regeneration of the catalyst. These additives aid in the regeneration of ferric ions by aeration. These additives are such that their complexes with ferrous iron are unstable and easily get oxidised. Hence, when ferrous ions are formed during the formation of sulfur, they bind to these regeneration additives and their ferrous complex gets rapidly oxidised to the ferric ions. Suitable thiocyanates are thiocyanates of sodium, pottasium or ammonium and a particularly preferred phosphate is disodium phosphate. In order to remove the product sulfur from the scrubbing solution and prevent froth formation with the gases, it is found necessary to add suitable surfactants which can wet the sulfur particles, aid its flocculation and settling at the bottom of the reactor. The compounds which can be used include conventional cationic, anionic or non-ionic surfactants, although, low foaming, nonionic surfactants are desirable. The foaming generated due to addition of surfactants can be suppressed by the use of suitable conventional defoamers. Use of a surfactant alongwith a defoamer results in the smooth plant operation on a continous basis as the product sulfur is effectively separated from the reaction zone. Formation of by-products is also minimised. Thus, the catalyst composition consisting of a physical admixture of an iron salt, chelating agent, regeneration promotor, ligand stabilizer, surfactant and defoamer show an unexpected synergistic activity resulting in highly improved performance in sulfur recovery. These components of the composition, however, do not react with each other. Accordingly the present invention provides a catalyst composition for use in recovery of sulfur from hydrogen sulfide containing sour gas stream which comprises from 0.01 to 2.0% of iron by weight of said composition from 0.25:1 to 2:1 ratio of chelating agent of the kind herein described, with iron, from 0.1 to 15.0% by weight of a ligand stabilizer of the kind such as herein described, from 0.01 to 5.0% by weight, a regeneration promoter of the kind such as herein described, a conventional surfactant and the balance, if any, being constituted by a conventional defoamer. The applicants have particularly found that the catalyst composition can effectively recover sulfur from gas streams containing extremely low hydrogen sulfide levels (even less than 100 ppm) to a very high, i.e. 100% hydrogen sulfide concentrations. According to the present invention there is provided a process for recovering sulfur from hydrogen sulfide containing sour gas which comprises contacting hydrogen sulfide containing sour gas with a catalyst comprising of an aqueous solution of chelated iron having concentration of iron in the range of 0.01% to 2% by weight, a chelating agent of the kind herein described, the ratio of said chelating agent to iron being in the range of 0.25: 1 to 2: 1, from 0.1 to 15.0% by weight of a ligand stabilizer of the kind such as herein described, from 0.01 to 5.0% by weight of a regeneration promoter of the kind such as herein described, 2ppm to 50ppm by weight of a conventional surfactant, and the balance if any consisting of a conventional defoamer, at a temperature between 10 to 80°C and pH in the range of 6 to 10 and at ambient pressure conditions, to produce an aqueous mixture containing sulfur and reduced catalyst composition removing sulfur from said aqueous mixture in any conventional manner and regenerating the catalyst composition by contacting said reduced catalyst with oxygen or an oxygen containing gas. Thus, the invention provides a process for almost completely removing H2S from a sour gas stream such as naturally occurring gases, syntheses gases, process gases and fuel gases. The particular type of sour gas stream treated is not critical, as will be evident to those skilled in the art. Broadly, the process comprises of: (a) contacting the H2S containing sour gas with the aforementioned catalyst in a conventional contacting zone, at a temperature below the melting point of sulfur, preferably between 10 and 80°C and more preferably around 30 to 50°C pH is kept in the range between 6 and 10, preferably between 7 and 9. Sufficient residence time is provided, for complete removal of FhS from the acid gas by the catalyst. Best results are obtained at a pressure between 0.1 to 20MPa. (b) Removing sulfur from the aqueous mixture to produce aqueous mixture having reduced sulfur content. (c) Regenerating the deactivated catalyst by contacting the reduced catalyst with oxygen or any oxygen containing gas such as air. The catalyst composition and process conditions are adjusted to limit the ligand degradation as much as possible. Sufficient residence time is provided to achieve required amount of conversion of Fe (II) - chelate to Fe (III) - chelate. The temperature employed is not critical and may be widely varied. The regeneration temperature is preferably maintained at substantially the same temperature as the absorption zone. The H2 S containing gas may be contacted with the catalyst in a suitable contactor which can be easily selected by those skilled in the art. For example, bubble columns, a backmixed reactor or packed bed may be used. The gas and liquid may be contacted either cocurrently or countercurrently. The residence time should be sufficient to convert all the H2S in the gas. The reduced catalyst containing Fe (Il)-chelate and Fe (Ill)-chelate is withdrawn from the first contacting zone, and transported to another contacting zone. Sulfur particles may be removed from the aqueous mixture before the regenerator. The aqueous mixture is taken to a settler where the slurry is allowed to settle. From the top of the settler, the catalyst is taken to the second contact zone. From the settler bottom, the thickened slurry is taken to a filter. The filter cake so produced may be washed and dried, or it may be melted in an autoclave to obtain 99.9% pure sulfur. The filtrate from the filter is recycled to the second contact zone. The second contact zone may consist of any of the gas-liquid contactors, known to the engineer, for example, bubble column, backmixed reactor etc. Oxygen is passed co-current or counter current to the reduced catalyst when the Fe (II)-chelate is converted to Fe (III) - chelate. The residence time is so maintained that Fe (II) - chelate is converted to Fe (III) -chelate but Fe (II) remains at all times. The product obtained has been found to be of high purity when it is suitably and effectively separated from the catalyst components. The sulfur can be separated from the catalyst by filtration or centrifugation. The filter cake is reslurried with water and melted to get crystalline bright yellow product of purity higher than 99.6%. Alternatively, the sulfur cake can be washed and dried to get precipitated sulfur of purity high than 99.6%. From this it is clear that the present invention produces sulfur of saleable quality, useful in a variety of downstream industries. The composition of the catalyst and its use in sulfur recovery from hydrogen sulfide containing gases can be illustrated by the following examples: Example 1 A coordinated complexed iron solution useful for recovering sulfur from hydrogen sulfide containing gases was prepared by dissolving 150g. ethylene diamine tetra acetic acid tetrasodium salt in 500 ml. water alongwith 20g sodium thiocyanate and 170g of 70% sorbitol solution followed by the addition and dissolution of lOOg. ferric sulfate while maintaining the pH alkaline (pH 7.5 - 9) using sodium hydroxide solution. The dark red solution obtained is made to 1 liter and has an iron concentration of approx. 2% by weight. Example 2 The solution as obtained in Example 1 diluted with water to get an iron concentration of 0.4% by weight. This was used to recover sulfur from hydrogen sulfide contained in a gas stream from a fuel oil based fertilizer plant. The gas contained carbondioxide (more than 80%) and hydrogen sulfide (2-12%), the rest being nitrogen and traces of hydrocarbons. The solution was charged in a laboratory scale glass reactor with liquid holding capacity of 8 liters and two reaction zones. The hydrogen sulfide containing gas was bubbled into one reaction zone where the coordinated ferric solution oxidised it to elemental sulfur. In the other reaction zone, the ferrous ions formed during the previous reaction are oxidised to ferric ions by a stream of air regenerating the solution. Thus the conversion of hydrogen sulfide to elemental sulfur can be carried out in a continuous catalytic way and the solution as prepared in Example 1 is a catalyst for sulfur recovery. Example 3 The sulfur recovery procedure as given in Example 2 was found to produce a thick layer of sulfur froth on the top of the reactor. To overcome this problem, a non-ionic surfactant was incorporated in the concentration of 50 ppm by weight in the solution. It was observed that the incorporation of the surfactant caused the sulfur to become we by the catalyst and it settled to the bottom of the reactor where setting zone has been provided. The sulfur slurry could be easily removed periodically and solid sulfur recovered by filtration or centrifugation to get a sulfur cake. The surfactant solution was added at regular intervals to effect sulfur settling. It was observed that the addition of surfactant for sulfur settling resulted in the formation of soap like foaming of the catalyst solution. In order to overcome this problem a solution of a defoamer was added at regular intervals. The suitable defoamer was a mineral oil based formulation stable at alkaline pH. Example 4 Following the procedure as given in Examples 2 and 3 product sulfur was isolated as a wet cake after removal of catalyst solution. A part of this cake was dried in the oven at 100°C to get an amorphous powder which had a sulfur content of 95% by weight. A second portion of the sulfur cake was reslurried with water and melted in an autoclave at 120° to get bright yellow crystalline sulfur product, the sulfur content of this was found to be 99.7%by weight, indicating that the sulfur recovered is of saleable quality., A third portion of the cake was washed with water and dried in the oven at 110°C. The powdered sulfur was found to have a purity of 99.7% by weight. Example 5 A concentrated aqueous catalyst solution was prepared following procedure given in Example 1 wherein ethylene diamine tetraacetic acid disodium salt was used as chelating agent alongwith appropriate quantity of sodium hydroxide to maintain pH between 7 and 9. The resulting solution after dilution with water to 4000 ppm by weight in iron content was effective in recovering sulfur from hydrogen sulfide containing gases as illustrated in Examples 2 thro 4. Example 6 The procedure of Example 1 was followed in preparing a concentrated solution wherein nitrilotriacetic acid trisodium salt was used as the chelating agent. The resulting solution when diluted with water to 4000 ppm in iron content was found to be an effective catalyst for recovering sulfur from hydrogen sulfide containing gases as illustrated in Example 2 thro 4. Example 7 Following procedure as in Example 3, a cationic surfactant was incorporated in the solution at a concentration of 50 ppm by weight alongwith an oil based defoamer at 10ppm levels. With regular dosing of these additives, the system was found to work without frothing and foaming throughout the course of the experimental run of several weeks. Example 8 Following the procedure as given in Example 2, wherein the catalyst concentration used with 2000 ppm in iron content, sulfur could be recovered from hydrogen sulfide containing gases with high efficiency where the H2S content of the gas varied between 1 to 7% by volume. The off gases from the reactor had a hydrogen sulfide content of less than 1 ppm throughout the course, of these experiments. Example 9 A bench scale reactor having a catalyst holding capacity of 250 liters was use for sulfur recovery as given in Example 2. The catalyst solution incorporating surfactant and defoamer as illustrated in Example 3 was used. The efficiency of sulfur recovery was found to be more than 99.9% when gases containing hydrogen sulfide of up to 10% by volume were treated in this reactor. Example-10 Using a bench scale reactor as given in Example-9, a catalyst of 2000 ppm was effective in recovering sulfur up to 99.9% conversion when the gas being treated had a hydrogen sulfide concentration of up to 7% by volume. Example -11 A concentrated aqueous catalyst solution was prepared following procedure as given in Example 1, wherein ethylene diamine tetraacetic acid disodium salt was used as chelating agent alongwith appropriate quantity of sodium hydroxide to maintain pH between 7 and 9. The resulting solution after dilution with water to 1000 ppm by weight in iron content was effective in recovering sulfur from hydrogen sulfide containing gases as illustrated by examples 2 thru 4. Example-12 Using a bench scale reactor as given in Example-9, a catalyst of 1000 ppm, produced as in Example-11, was effective in recovering sulfur up to 99.9% conversion when the gas being treated had a hydrogen sulfide concentration of up to 6% by volume. Example-13 A concentrated aqueous catalyst solution was prepared following procedure as given in Example -1. The resulting solution after dilution with water to 500 ppm by weight in iron content was effective in recovering sulfur from hydrogen sulfide containing gases as illustrated by Examples 2 thru 4. Example -14 Using a bench scale reactor as given in Example-9, a catalyst of 500 ppm, produced as in Example-13, was effective in recovering sulfur up to 99.9% conversion when gas being treated had a hydrogen sulfide concentration of up to 4% by volume. Example - 15 A conventional catalyst solution consisting of ferric iron concentration of 500 ppm, an amine chelating agent and iron in a molar ratio of 0.8:1 and a polyhydroxy chelating agent and iron in a molar ratio of 0.8:1 was prepared as per US Patent No. 4,189,462. This solution was used for treating hydrogen sulfide containing gas as given in Example - 2. The catalyst composition as claimed in this invention of same concentration in iron was used for treating the same gas as above. It was observed that the build up of ferrous ion concentration was 50-100% less in the present invention. WE CLAIM-; 1. A process for recovering sulfur from hydrogen sulfide containing sour gas which comprises contacting hydrogen sulfide containing sour gas with a catalyst comprising of an aqueous solution of chelated iron having concentration of iron in the range of 0.01% to 2% by weight, a chelating agent of the kind herein described, the ratio of said chelating agent to iron being in the range of 0.25: 1 to 2: 1, from 0.1 to 15.0% by weight of a ligand stabilizer of the kind such as herein described, from 0.01 to 5.0% by weight of a regeneration promoter of the kind such as herein described, 2ppm to 50ppm by weight of a conventional surfactant, and the balance if any consisting of a conventional defoamer, at a temperature between 10 to 80°C and pH in the range of 6 to 10 and at ambient pressure conditions, to produce an aqueous mixture containing sulfur and reduced catalyst composition removing sulfur from said aqueous mixture in any conventional manner and regenerating the catalyst composition by contacting said reduced catalyst with oxygen or an oxygen containing gas. 2. A process as claimed in claim 1, wherein said catalyst and hydrogen sulfide containing sour gas are contacted with each other at a temperature between 30 to 50°C. 3. A process as claimed in claim 2, wherein said pH is between 7 and 9. 4. A process as claimed in any one of claims 1 to 3 wherein said hydrogen containing sour gas is contacted with said catalyst at a pressure in the range of from 0.1 to 20 Mpa. 5. A process as claimed in any one of claims 1 to 3 wherein said ferric salt is selected from ferric sulphate, ferric nitrate ferric chloride. 6. A process as claimed in any one of claims 1 to 4 wherein said chelating agent is selected from polyamino polycarboxylic acids and their alkali metal salts. 7. A process as claimed in claim 5 wherein said chelating agent is selected from ethylene diamine tetra acetic acid, nitrilo triacetic acid and their alkali metal salts. 8. A process as claimed in any one of claims 1 to 6 wherein said ligand stabilizer is a poyhydroxy sugar. 9. A process as claimed in claim 7 wherein said polyhydroxy sugar is selected from sucrose, sorbitol and mannitol. 10. A process as claimed in any one of claims 1 to 8 wherein said regeneration promotor is selected from thiocyanates and soluble phosphates. 11. A process as claimed in claim 8 wherein said thiocyanate is selected from sodium, potassium and ammonium thiocyanate. 12. A process as claimed in claim 8 wherein said phosphate comprises disodium phosphate. 13. A process as claimed in any one of claims 1 to 10 wherein said surfactant is a cationic, anionic or a non-ionic surfactant. 14. A process for the preparation and recovery of sulfur from hydrogen sulfide containing sour gas stream substantially as herein described with reference to the foregoing examples.

Documents:

491-del-2000-abstract.pdf

491-del-2000-claims.pdf

491-del-2000-correspondence-others.pdf

491-del-2000-correspondence-po.pdf

491-del-2000-description (complete).pdf

491-del-2000-form-1.pdf

491-del-2000-form-13.pdf

491-del-2000-form-2.pdf

491-del-2000-form-3.pdf

491-del-2000-form-4.pdf

491-del-2000-gpa.pdf

491-del-2000-petition-138.pdf