Stabilized chelating agents for removing hydrogen sulfide

Abstract: Soluble chemical compounds having a high affinity for hydroxyl radicals are effective stabilizers for chelating agents used in the hydrogen sulfide removal process. The stabilizers include water-soluble aromatic compounds, bromide ions, iodide ions, cyanides, nitrites, amino acids, sugars, ascorbates, alcohols, polyols, aliphatic aldehydes, soluble organic compounds having unsaturated carbon-­ carbon bonds, dimethyl sulfoxide, organic disulfides, alkyl amines, and formates. The aromatic compounds have the additional benefit of combining with hydroxyl radicals to form chelating agents.

Description: The invention concerns the use of chelating agents to remove hydrogen sulfide from gas streams. Specifically, the invention concerns the use of stabilizers that increase the life of the chelating agents. The use of chelating agents to remove hydrogen sulfide from gas streams is known in the art. However, most chelating agents are expensive and are degraded during the hydrogen sulfide removal process. The useful life of a chelating agent solution can be extended by the addition of chemical compounds that retard the degradation of the chelating agents. Such compounds are called stabilizers and have generally been identified by experimentation. The use of stabilizers substantially improves the economics of the hydrogen sulfide removal process using chelating agents. AU-B-83126 discloses the use of salts of certain non-oxidizing acids having a low pK value to inhibit the formation of sulfur oxides. None of the agents disclosed are suggested as having any impact on improving the life of a chelating agent. EP-A-0066310 describes the use of certain stabilizers to improve the life of chelating agents. This reference does not however recognise that the removal of hydroxyl might be important to the process of stabilization, nor does it suggest that iodide and/or bromide ions might be useful for such stabilization. The present invention is concerned with the provision of solutions for removing hydrogen sulfide from gas streams, which includes a chemical stabilizer which is a compound having a high affinity for hydroxyl radicals and which is an effective stabilizer for chelating agents which are used in the hydrogen sulfide removal process. The invention provides an aqueous solution for removing hydrogen sulfide from a gas stream, comprising: metal ions in a concentration of at least about 100 ppm; a chelating agent in an amount of at least equimolar to said metal ions; A buffering compound in a concentration sufficient to provide a pH between 4 and 11; and a stabilizer in an amount sufficient to retard degradation of the chelating agent, the stabilizer being iodide ions, or bromide ions, or a combination thereof. The stabilizers of the present invention have a high affinity for hydroxyl radicals and it is believed that this affinity contributes to the effectiveness of the compounds as stabilizers. Representative metal ions are chromium, cobalt, copper, iron, lead, manganese, mercury, molybdenum, nickel, palladium, platinum, tin, titanium, tungsten, vanadium or mixtures thereof. Of the metals, iron, copper and nickel are most advantageously employed. The term "chelating agent" is well-known in the art and references are made thereto for the purposes of this invention. Chelating agents useful in the present invention include those chelating or complexing agents which form a water-soluble chelate with one or more of the afore-described metals. Representative of such chelating agents are: the aminopolycarboxylic acids, including the salts thereof; nitrilotriacetic acid; N-hydroxyethyl aminodiacetic acid; and the polyaminocarboxylic acids, such as ethylenediaminetetraacetic acid, N-hydroxyethylethylenediaminetriacetic acid, diethylenetriaminepentaacetic acid, cyclohexane diamine tetraacetic acid, and triethylene tetraamine hexaacetic acid; aminophosphonate acids, such as ethylene diamine tetra(methylene phosphonic acid), aminotri(methylene phosphonic acid), and diethylenetriamine penta(methylene phosphonic acid); phosphonic 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; polyhydroxy chelating agents, such as monosaccharides and sugars (e.g., disaccharides such as sucrose, lactose and maltose) sugar acids (e.g., gluconic or glucoheptanoic acid); and other polyhydric alcohols, such as sorbitol and manitol. Of such chelating agents, the polyaminocarboxylic acids, particularly ethylenediaminetetraacetic and N-hydroxyethylethylenediaminetriacetic acids, and the nitrolotriacetic acid are most advantageously employed. Most preferably, the metal chelate is the chelate of a ferric iron with ethylenediaminetetraacetic acid, N-hydroxyethylethylenediaminetriacetic acid, or nitrilotriacetic acid. The process for the conversion of hydrogen sulfide to sulfur involves the use of the metal chelates, e.g. ferric chelates, as oxidizing agents. During the process, the metal ions are reduced to an inactive state and can be regenerated by oxidation to an active state. However, this recycling of metal chelates is limited by the loss of metal ions from solution as a result of degradation of the chelating agents that are complexed with the metal ions to form the metal chelates. By this invention, it is established that the chelating agents used in the hydrogen sulfide removal process are significantly oxidized by hydroxyl radicals or some closely related oxygen species. Thus, compounds having a higher affinity for hydroxyl radicals then the chelating agents stabilize the chelating agent solution by preferentially combining with the hydroxyl radicals. Information concerning affinities for hydroxyl radicals was obtained from F. Ross and A. B. Ross "Selected Specific Rates of Reactions of Transients From Water in Aqueous Solution. III. Hydroxyl Radical and Perhydroxyl Radical and Their Radical Ions," Nat. Stand. Ref. Data Ser., Nat. Bur. Stand. (U.S.), 59 , Jan. 1977, PB - 263 198. The relative effectiveness of such stabilizers depends upon the relative affinity for hydroxyl radicals, the solubility of the compound in the metal chelate solution, and the concentration of the stabilizer. The typical range for metal chelate solution is generally a metal ion concentration greater than 100 parts per million (ppm) with a chelating agent to metal ion molecular ratio of greater or equal to one. The preferred ranges are a metal ion concentration of 1,000 to 10,000 ppm and a chelating agent to metal ion molecular ratio of 1 to 10. The optimum amounts depend on the chelating agent used with a ratio of 1.0 to 1.2 being preferred for the polyaminocarboxylic acids and a ratio of 1.0 to 4.0 being preferred for the nitrilotriacetic acid. These solutions must be buffered to a pH of from 4 to 11, preferably from 6 to 9. The buffering agent is preferably used in a concentration in excess of the metal ion concentration. Stabilizers for chelating agent solutions are generally effective in any concentration up to the limits of solubility, however, economics usually dictate a molar ratio of stabilizer to metal ion of from 2 to 100. Higher concentrations of stabilizers do not appreciably enhance performance and a molar ratio of from 2 to 5 is preferred. The stabilizers of the present invention have a high affinity for hydroxyl radicals as previously mentioned and many have been shown effective by the examples below. The stabilizers are sacrificed during the H₂S removal process in favor of a longer useful life for the metal chelates. The following examples are illustrative and explanatory of the present invention. Example 1 A non-stabilized ferric chelate solution was prepared by mixing the following components: To this ferric chelate solution was added 0.75 moles of potassium iodide as a stabilizer. A gas mixture containing hydrogen sulfide was sparged through the stabilized ferric chelate solution for approximately twelve hours. The sparge gas consisted of air at 1.25 m³ (44 standard cubic feet) per hour and of 5 percent hydrogen sulfide in nitrogen at 0.014 m³ (0.5 standard cubic feed) per hour. Following sparging, the ferric chelate solution was filtered and the filtrate quantitatively analyzed to determine the loss of the HEDTA (by determining the change in concentration of the trisodium salts). The filtered material was dried and quantitatively analyzed to determine the amount of sulfur recovered from the gas stream. Results are presented in a comparison Table for Example 3 below. Example 2 A non-stabilized ferric chelate solution was prepared as done for Example 1. To this solution was added 0.75 moles of potassium bromide and the solution was then sparged for twelve hours with the sparge gas of Example 1. Results are also presented in the comparison Table for Example 3 below. Example 3 (Comparison) A non-stabilized ferric chelate solution was prepared as done for Example 1 and sparged for twelve hours with the sparge gas of Example 1 without the addition of a stabilizer. The following Table demonstrates the effectiveness of the stabilizers of Examples 1 and 2 in comparison to the non-stabilized ferric chelate solution. Example 4 (Comparison) A non-stabilized ferric chelate solution was prepared as for Example 1. To this solution was added 0.75 moles of potassium chloride followed by sparging for twelve hours with the sparge gas of Example 1. In comparison to potassium iodide and potassium bromide, potassium chloride does not have a high affinity for hydroxyl radicals. The results of Examples 1 through 4 are presented for comparison in the following Table. The preceding results demonstrate the effectiveness of bromide and iodide ions as stabilizers for metal chelate solutions. The chloride ions, which do not have a high affinity for hydroxyl radicals, provided little stabilization. The sodium and ammonium salts of bromide and iodide are also expected to be suitable stabilizers based on these results.

Claims: An aqueous solution for removing hydrogen sulfide from a gas stream, comprising: metal ions in a concentration of at least about 100 ppm; a chelating agent in an amount at least equimolar to said metal ions; a buffering compound in a concentration sufficient to provide a pH between 4 and 11; and a stabilizer in an amount sufficient to retard degradation of the chelating agent, the stabilizer being iodide ions, or bromide ions, or a combination thereof. A solution as claimed in Claim 1, wherein the metal ions are chromium, cobalt, copper, iron, Iead, manganese, mercury, molybdenum, nickel, palladium, platinum, tin, titanium, tungsten, vanadium, or a mixture of two or more thereof. A solution as claimed in Claim 1 or Claim 2, wherein the chelating agent is a water-soluble chelate with metal ions. A solution as claimed in any one of the preceding claims, wherein the stabilizer is present in a molar concentration of at least two times greater than said metal ions. A solution as claimed in any one of the preceding claims, wherein bromide ions are present in the form of potassium bromide, sodium bromide or ammonium bromide. A solution as claimed in any one of the preceding claims, wherein iodide ions are present in the form of potassium iodide, sodium iodide or ammonium iodide. A method for removing hydrogen sulfide from a gas stream, comprising the steps of: contacting the gas stream containing hydrogen sulfide with an aqueous solution as claimed in any one of Claims 1 to 6 to oxidize said sulfide to elemental sulfur, and removing said elemental sulfur from said solution. The use of a bromide and/or iodide ions as a stabilizer for a chelating agent in an aqueous solution for removing hydrogen sulfide from a gas stream, comprising metal ions in a concetration of at least 100 ppm, a chelating agent in an amount at least equimolar with the said metal ions, and a buffering compound in a concentration sufficient to provide a pH of between 4 and 11.

Inventors: BEDELL STEPHEN A, BEDELL, STEPHEN A.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1987
Main classification: B01D53
IPC codes: C10K1, B01D53, C01B17