IMPROVED PROCESS FOR REMOVAL OF H2S FROM GEOTHERMAL STEAM

Abstract: A process for the removal of H₂S from a fluid stream whereby the H₂S is converted to soluble sulfur compounds by treatment with a solution of ferric chelates containing an oxidizing agent and a water soluble cationic polymeric catalyst such as poly(dimethyldiallyl ammonium chloride).

Description: The invention concerns a process wherein a fluid stream containing hydrogen sulfide (H₂S) is contacted with an aqueous solution containing a poly­ valent metal chelate and the hydrogen sulfide in said stream is converted to soluble sulfur compounds in a relatively short time. U.S. Patents 4,123,506 and 4,202,864 teach that geothermal steam containing H₂S can be purified by contacting the steam with a metal compound that forms insoluble metallic sulfides. U.S. Patent 4,196,183 discloses that geo­ thermal steam containing H₂S can be purified by adding oxygen and passing it through an activated carbon bed. Various processes for hydrogen sulfide control in geothermal steam are outlined in the U.S. Department of Energy Report #DOE/EV-0068 (March 1980) by F. B. Stephens, et al. U. S. Patent 4,009,251 discloses the removal of hydrogen sulfide from gaseous streams with metal chelates to form sulfur substantially without the formation of sulfur oxides. U.S. Patent 4,451,442 discloses a process for the removal of hydrogen sulfide from geothermal steam. This process generates free sulfur or sulfur solids which are then converted to soluble sulfur compounds. The present invention is directed to a process wherein fluid streams containing H₂S are purified by rapidly converting the H₂S to soluble sulfur compounds by using a polyvalent metal chelate and a water soluble oxidizing agent in the presence of a polymeric catalyst. The process of this invention concerns the removal of H₂S from a fluid stream containing H₂S which comprises: A) contacting said H₂S stream in a first reaction zone with an aqueous solution at a pH range suitable for removing H₂S whereby said solution contains (1) an effective amount of at least one ferric chelate, (2) an effective amount of oxidizing agent capable of oxidizing elemental sulfur to soluble sulfur compounds, and (3) an effective amount of one or more water soluble cationic polymeric catalysts whereby the conversion of H₂S to free sulfur and then to soluble sulfur compounds is accelerated by the presence of said catalysts and said ferric chelate is reduced to a ferrous chelate; (B) contacting said solution containing ferrous chelate in a second reaction zone with an oxygen containing gas stream whereby said chelate is reoxi­ dized; and (C) recirculating said reoxidized solution back to said first reaction zone. Advantages of the process described herein are the rapid elimination of sulfur solids which foul piping, heat-exchanger surfaces, cooling tower basins and the like. Such fouling of equipment in geothermal power plants leads to costly down-time for maintenance and loss of power production. The figure of the drawing illustrates a process in which this invention is applied for the oxidation of hydrogen sulfide contained in a liquid stream produced by the condensation of geothermal steam. In the drawing, the geothermal steam from line 2 is used to power a steam turbine 4 which is connected to an electric power generator 6. Branch lines 18, 20, and 22 directly supply steam from line 2 to the steam turbine 4, steam ejector 24, and steam ejector 26, respectively. The turbine 4 exhausts through line 8 to a direct contact condenser 10. Cooling water containing chelated iron (ferric chelate) from line 28 is sprayed into condenser 10 for this condensation and passes from the condenser 10 through line 14 to the hot well 16 operating at 100-125°F ( 38 - 52 °C). Non-condensable gases such as CO₂, H₂, CH₄, N₂, O₂, and part of the H₂S are removed from the main condenser 10 through line 36 by two steam jet ejectors 24 and 26 and the associated condensers 12 and 13. The ejectors 24 and 26 are operated by steam supplied by lines 20 and 22, respectively. These ejectors create a partial vacuum or low pressure zone. The exhaust steam from the ejector 24 is carried by line 38 to the condenser 12 and by line 40 to the second ejector 26. The exhaust steam from ejector 26 is carried by line 42 to condenser 13. Cooling water from line 28 is supplied to each of the condensers 12 and 13 by lines 32 and 30, respectively. The condensed steam from condensers 12 and 13 flows by means of lines 44 and 46 to the hot well 16. The non-condensable gases including the remaining H₂S and the exhaust steam are then fed through line 48 to an incinerator or SO₂ generator 54 for oxidation of the H₂S to SO₂. An oxygen containing gas such as air, oxygen, or mixtures thereof is supplied to the generator 54 by line 55. The SO₂ generator 54 is a conventional catalytic inciner­ ator, however, a thermal incinerator may be used if desired. The SO₂ generated in the incinerator 54 is fed by line 52 to the cold water line 58 from the pump 60. The water and gaseous SO₂ are thoroughly mixed in an inline mixer 50. The water containing dissolved SO₂ is then fed to the condensors 10, 12, and 13 by lines 28, 32, and 30. Sufficient amounts of iron chelate is added after start-up to the cold well 66 by line 56 to make up for the amounts lost by continuous blowdown through line 76. In a similar manner, caustic solutions such as aqueous sodium hydroxide are added by line 78 to the cold well 66 to adjust or maintain the pH of the recircu­ lating solution within the desired range of 5 to 11 and preferably 7 to 9. The aqueous solution in the cold well 66 is withdrawn by line 68 into pump 60 and pumped by line 58 to the static mixer 50 and thence to the condensors 10, 12, and 13. The aqueous solution in the hot well 16 is withdrawn by line 64 into pump 62 and pumped through line 70 to the cooling tower 72 where the solution is sprayed into the tower and oxidized by air circulation. Line 76 is provided for continuous solution withdrawal. About 10 percent of the steam from line 2 is continuously withdrawn from line 76 which is typically reinjected into the underground steam-bearing formation. Line 74 is provided to allow the cooled solution to recycle back to the cold well 66. The cooling tower 72 is vented to the atmosphere at 80 with substantially no H₂S being present. The iron chelates used herein are water soluble iron chelates. The term "chelating agent" is well-known in the art. Chelating agents useful in preparing the ferric iron chelate of the present invention include those chelating or complexing agents which form a water-soluble chelate with iron. Representative of such chelating agents are the aminopolycarboxylic acids, including the salts thereof, nitrilotriacetic acid, N-hydroxyethyl aminodiacetic acid and the poly­ aminocarboxylic acids including ethylenediaminetetra­ acetic acid, N-hydroxyethylenediaminetriacetic acid, diethylenetriaminepentaacetic acid, cyclohexene diamine tetraacetic acid, triethylene tetraamine hexa­ acetic acid and the like; aminophosphonate acids such as ethylene diamine tetra(methylene phosphonic acid), aminotri(methylene phosphonic acid), diethylenetriamine penta(methylene phosphonic acid); phosphonate acids such as 1-hydroxy ethylidene-1,1-diphosphonic acid, 2-phosphono acetic acid, 2-phosphono propionic acid, and 1-phosphono ethane 1,2-dicarboxylic acid; poly­ hydroxy chelating agents such as monosaccharides and sugars (e.g., disaccharides such as sucrose, lactose and maltose), sugar acids (e.g., gluconic or glucohep­ tanoic acid); other polyhydric alcohols such as sorbitol and manitol; and the like. Of such chelating agents, the polyaminocarboxylic acids, particularly ethylene­ diaminetetraacetic and N-hydroxyethylethylenediamine­ triacetic acids, are most advantageously employed in preparing the iron chelate used herein. Most prefer­ ably, the iron chelate is the chelate of ferric iron with a polyaminocarboxylic acid, with the most prefer­ red polyaminocarboxylic acids being selected on the basis of the process conditions to be employed. Ethyl­ enediaminetetraacetic acid and N-hydroxyethylethylene­ diaminetriacetic acid are generally particularly prefer­ red. The oxidizing agent used herein to oxidize elemental sulfur to soluble sulfur compounds is prefer­ ably sulfur dioxide which can be generated by oxidizing a side stream of hydrogen sulfide. Other oxidizing agents that can be used are the alkali metal salts of inorganic oxidizing acids such as perchloric, chloric, hypochlorous and permanganic acids. Examples of useful cationic polymeric catalysts to be used in this invention are polyethyleneamines, poly(2-hydroxypropyl-1-N-methylammonium chloride) and the 1,1-dimethyl analogue, poly[N-dimethylaminomethyl)­ -acrylamide, poly(2-vinylimidazolinum bisulfate), poly(diallyldimethyl ammonium chloride) and poly(N-di­ methyl aminopropyl)-methacrylamide. These cationic polymers are well known and are commercially available under various tradenames. See, for example, "Commercial Organic Flocculants" by J. Vostricil et al. Noyes Data Corp. 1972. Other useful cationic catalysts are set forth in J. Macromal. Science - Chem A4 pages 1327-1417 (1970). For the purposes of this invention, a suitable pH range for the aqueous solution in the first reaction zone is 5.5 to 10.5. A more preferred range is 6.0 to 9.0 and the most preferred range is 7.0 to 8.0. It has been found that the ferric chelate can be used in an effective amount which is generally at least two moles of ferric iron for each mole of H₂S in the feed gas. A preferred mole ratio range is 2.2:1 to 2.4:1. The oxidizing agent can be used in an effective amount greater than one mole of oxidizing agent per mole of H₂S in the feed gas. The preferred range is 1.2:1 to 1.5:1. The cationic polymer is used in small but effective amounts. It has been found than an effective amount is generally any amount greater than one part per million (ppm) based on the weight of the condensate recycle stream. A preferred range is 10 to 25 ppm. It is to be understood that greater amounts than these levels can be used, but they are uneconomical in that larger amounts have about the same effect as 10 ppm. Detailed examples of the invention are given below for the purposes of further illustrating the invention. Control At zero time, 1.075 g of 2000 ppm sulfide (as Na₂S.9H₂O) stock solution (0.00215g S⁻⁻or 6.7 x 10⁻⁵ moles S⁻⁻) was added to 250 ml of geothermal condensate having a pH of about 8 and a temperature of 50°C. The condensate contained 30 ppm ferric iron (0.0075g Fe⁺³ or 1.39 x 10⁻⁴ moles Fe⁺³) as the ferric chelate of N-hydroxyethylethylenediaminetriacetic acid. The sodium sulfide furnished sulfide ions and is a simulant for the addition of H₂S to an iron chelate solution. The geothermal condensate solution changes from yellow to orange with the addition of the sodium sulfide, then back to yellow and finally became cloudy­ -white as the sulfur particles precipitated. After three minutes, 0.013 grams (1.03 x 10⁻⁴ moles) of Na₂SO₃ was added to simulate the addition of base­ -absorbed sulfur dioxide as an oxidizing agent. The time required for clarification of the solution by visual inspection was noted . The above procedure was repeated five times and the average time for solution clarification was about 19 seconds. Examples 1-6 The procedure of the control experiment was repeated with various water soluble cationic polymeric catalysts at various concentrations. The results are set forth in Table I. Examples 7-12 The procedure of the control was repeated with various amounts of Agefloc WT-40. The results are set forth in Table II. Examples 13-18 The procedure of the control was repeated with various amounts of derivatized PAPA polymer. The results are set forth in Table III. Table I shows that the use of various cati­ onic polymer catalysts can substantially reduce the time for clarification of the solution. This indi­ cates that the free sulfur is being rapidly converted to sulfur compounds by the polymer catalysts. Tables II and III illustrate the fact that with increasing amounts of the cationic polymers, the decrease in time for clarification levels off.

Claims: 1. A process for removing H₂S from a geothermal fluid stream containing H₂S which comprises contacting said H₂S stream in a first reaction zone with an aqueous solution at a pH range suitable for removing H₂S, characterised in that said solution contains (1) an effective amount of at least one ferric chelate, (2) an effective amount of an oxidizing agent capable of oxidizing elemental sulfur to soluble sulfur compounds, and (3) an effective amount of one or more water soluble cationic polymeric catalysts whereby the conversion of H₂S to free sulfur and then to soluble sulfur compounds is accelerated by the presence of said catalysts and said ferric chelate is reduced to a ferrous chelate; said solution containing ferrous chelate is contacted in a second reaction zone with an oxygen containing gas stream whereby said chelate is reoxidized; and said reoxidized solution is recirculated back to said first reaction zone. 2. A process as claimed in Claim 1, wherein in the first reaction zone the pH range is from 5.5 to 10.5; at least two moles of ferric chelate per mole of H₂S are present; at least one mole of oxidizing agent is present; and greater than one ppm of one or more water soluble cationic polymeric catalysts is present. 3. A process as claimed in either of the preceding claims, wherein said pH range is 6.0 to 9.0 and the amount of said polymeric catalyst is 10 to 25 ppm. 4. A process as claimed in any one of the preceding claims, wherein the mole ratio range of said ferric chelate is 2.2:1 to 2.4:1. 5. A process as claimed in any one of the preceding claims, wherein the mole ratio of said oxidizing agent is 1.2:1 to 1.5:1. 6. A process as claimed in any one of the preceding claims, wherein said ferric chelate is the ferric chelate of a polyaminocarboxylic acid. 7. A process as claimed in Claim 6, wherein the ferric chelate is ethylenediaminetetraacetic acid or N-hydroxyethylethylenediaminetriacetic acid. 8. A process as claimed in any one of the preceding claims, wherein said oxidizing agent is sulfur dioxide. 9. A process as claimed in any one of the preceding claims, wherein said cationic polymer is poly(dimethyldiallyl ammonium chloride).

Inventors: HOPKINS II, THOMAS M., LAMPTON, ROBERT D., JR.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1986
Main classification: C01B17
IPC codes: C01B17, B01J31, B01D53