Abstract:
Fluid streams containing hydrogen sulfide from a steam tubine or from a sour gas stream are contacted with an aqueous solution of a polyvalent metal chelate and a bisulfite whereby the hydrogen sulfide is converted to free sulfur and then to soluble sulfur compounds. The metal chelate is reduced to a lower oxidation state metal chelate and reduced metal chelate is subsequently oxidized with air back to the higher oxidation state and reused. The bisulfite is formed by combustion of a portion of the fluid stream and subsequent absorption of the sulfur dioxide formed thereby in a two-stage countercurrent scrubber operating at conditions favorable for high bisulfite and low sulfite formation and selective away from carbon dioxide absorption.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process wherein a fluid stream containing hydrogen sulfide is contacted with an aqueous solution containing a polyvalent metal chelate and the hydrogen sulfide in said steam is removed. 
     It is known from U.S Pat. No. 4,123,506 dated Oct. 31, 1978 and U.S. Pat. No. 4,202,864, dated May 13, 1980 that geothermal steam containing H 2  S can be purified by contacting the steam with a metal compound that forms insoluble metallic sulfides. 
     It is also known from U.S. Pat. No. 4,196,183, dated Apr. 1, 1980 that geothermal steam containing H 2  S 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 #DOW/EV-0068 (March, 1980) by F. B. Stephens, et al. 
     U.S. Pat. No. 4,009,251, dated Feb. 22, 1977 discloses the removal of hydrogen sulfide from gaseous streams with metal chelates to form sulfur substantially without the formation of sulfur oxides. 
     In U.S. Pat. No. 4,414,817 dated Nov. 15, 1983, there is disclosed a process for the removal of hydrogen sulfide from geothermal steam. However, this process generates free sulfur or sulfur solids which must be removed. The instant process is superior in that the sulfur solids are minimized by being converted to soluble sulfur compounds. 
     In U.S. Pat. No. 4,451,442, dated May 29, 1984, there is disclosed a process for the removal of hydrogen sulfide from geothermal streams with minimum solid sulfer production. In this process, hydrogen sulfide is removed from fluid streams containing the same using a polyvalent metal chelate and an oxidizing agent. The oxidizing agent is preferably sulfur dioxide which can be generated by oxidizing a side stream of the hydrogen sulfide. However, in this process, the production of SO 2  also forms CO 2  which results in the formation of insoluble carbonates. These insoluble salts are troublesome and costly in geothermal power plants and other applications where solids free operation is necessary or desirable. 
     In U.S. Pat. No. 4,622,212, dated Nov. 11, 1986, there is described a hydrogen sulfide removal method using a chelating solution containing thiosulfate as a stabilizer. 
     In U.S. Pat. No. 3,446,595, dated May 27, 1969, there is described a gas purification process in which hydrogen sulfide is absorbed with bisulfite to form elemental sulfur and sulfite. This sulfite is regenerated to form bisulfite by contact with sulfur dioxide which in turn is formed by combustion of the elemental sulfur. 
     U.S. Pat. No. 3,859,414, dated Jan. 7, 1975, describes a process in which sulfite is reacted with hydrogen sulfide in a gas stream at thiosulfate forming conditions, e.g. a pH between 6 and 7, to form soluble sulfur compounds. 
     Other references which may be relevant to the instant disclosure include U.S. Pat. Nos. 4,629,608; 3,447,903; and 3,851,050. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a process wherein fluid streams containing H 2  S are purified by converting the H 2  S to soluble sulfur compounds by using a polyvalent metal chelate and a sulfite oxidizing agent. 
     The process of this invention has the following steps: 
     (a) incinerating hydrogen sulfide to form sulfur dioxide; 
     (b) selectively absorbing said sulfur dioxide without substantial carbon dioxide absorption in a basic aqueous solution to form sulfites in said solution essentially free of insoluble carbonates; 
     (c) contacting said fluid stream in a first reaction zone with aqueous solution at a pH range suitable for hydrogen sulfide removal wherein said solution contains an effective amount of polyvalent metal chelate to convert said hydrogen sulfide to sulfur and to reduce said polyvalent metal chelate to a lower oxidation state; 
     (d) contacting said sulfur with said sulfites to form soluble sulfur compounds; 
     (e) contacting said reduced polyvalent metal chelate in a second reaction zone with oxygen to reoxidize said metal chelate; and 
     (f) recirculating said reoxidized solution back to said first reaction zone. 
     Advantages of the process described herein are the substantial elimination of sulfur solids and insoluble carbonate salts which foul piping, heat-exchanger surfaces, cooling tower basins and the like. Such fouling of equipment in geothermal power plants, for example, leads to costly downtime for maintenance and loss of power production. Advantages of the process, when used for gas scrubbing are elimination of the need for expensive mechanical equipment such as settlers, frothers, filters, centrifuges, melters and the like for sulfur removal. This is particularly advantageous when treating streams having low sulfur content and recovery of the sulfur does not warrant the equipment required for its removal from the process. 
     Furthur advantages of the process described herein include the minimization of sulfur emissions and the ability to optimize the hydrogen sulfide removal process by formation of a sulfur-solubilizing agent (sulfites) under controlled conditions to further assure complete sulfur solubilization and to minimize the use of makeup reagents such as chelating solution and caustic. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 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. 
     FIG. 2 illustrates a process in which this invention is applied to the removal of hydrogen sulfide form a sour gas stream such as a natural gas stream, refinery gas, synthesis gas, or the like. 
    
    
     In FIG. 1 the geothermal steam from line 2 is used to power a steam turbine 4 which is connected to an electric power generator 6. Line 18 directly supplies steam from line 2 to the steam turbine 4. The turbine 4 exhausts through line 8 to a condenser 10. Cooling water containing chelated iron (ferric chelate) and sulfites 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. Non-condensable gases such as CO 2 , H 2 , CH 4 , N 2 , O 2  and part of the H 2  S are removed from the main condenser 10 through line 36. If desired, a conventional steam ejector or ejectors may be employed in line 36 to create a partial vacuum or low pressure zone. The exhaust steam from line 36, including the H 2  S and non-condensable gas is fed to an incinerator or SO 2  generator 54 for oxidation of the H 2  S to SO 2 . An oxygen-containing gas such as air, oxygen, or mixtures thereof is supplied to the generator 54 by line 55. The SO 2  generator 54 is a conventional catalytic incinerator, however, a thermal incinerator may be used if desired. 
     Sufficient amounts of polyvalent metal chelate is added after start-up to the cold well 66 by line 56 to make up for the amounts lost by continuous blow down through line 76. In a similar manner, caustic solutions such as aqueous sodium hydroxide are added, if needed, by line 78 to the cold well 66 to adjust or maintain the pH of the recirculating 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 63 into pump 60 and pumped through line 58 to the static mixer 50 and thence to condenser 10 via line 28. 
     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 10-20 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 H 2  S being present. 
     The SO 2  generated in the incinerator, along with the non-condensable gases and combustion products thereof, is fed via line 52 to optional quench vessel 81 and thence through line 82 to a first-stage scrubbing vessel 84 where it is absorbed by contact with alkali metal and sulfite/bisulfite solution at a pH of 4-7 circulated via pump 83 and recirculation loop 85. Unabsorbed gases from scrubber 84 are fed through line 86 to second-stage scrubber 88 where residual SO 2  is absorbed to less than 10 ppm in the gas which is then vented through line 87. A solution of alkali metal, bisulfite and sulfite at a pH of 8.5-9.5 is circulated through scrubber 88 by means of pump 89 and second-stage recirculation loop 90. Make-up alkali metal hydroxide is added through line 91 to recirculation loop 90 to maintain the desired pH and also to ensure that the alkali metal is reacted with sulfite in the recirculation loop 90 to form bisulfite, so that absorption of Co 2  in scrubber 88 and the resultant formation of carbonates therein is substantially avoided. Absorption solution is fed from recirculation loop 90 through line 92 to recirculation loop 85 to maintain the desired pH and scrubbing liquor level in scrubber 84. Scrubbing liquor containing sulfite and/or bisulfite is fed from recirculation loop 85 through line 93 to line 58 in a sufficient amount to maintain soluble sulfur-forming conditions in condenser 10. 
     In FIG. 2, a sour gas feed is led by line 110 where it is combined with the aqueous solution from line 158 and thence to a static mixer 112 for good gas-liquid contact. The combined streams are fed into the first separator 114. The gaseous effluent from the separator 114 is led overhead by line 116 where it is combined with the recycled aqueous solution in line 126 and fed by line 118 to a static mixer 120 and then to a second gas-liquid separator 122. The overhead gas from the second separator 122 which is the purified or sweetened gas product of this process is removed by line 124 while the liquid bottoms are removed by line 156, pump 154, and recycled by line 158 to the first separator 114. 
     The bottoms from the first separator 114 are removed by line 164 to the pump 160 and pumped through line 162 where it is mixed, with or without static mixer 150, with aqueous solution from line 184. The mixed bottoms and liquid effluent from lines 162 and 184 respectively are passed through line 152 into an oxidation rector 146. An oxygen-containing gas is supplied to the oxidizer 146 by the line 144 so that the polyvalent metal chelate is oxidized to its higher state of oxidation. The non-absorbed gases are purged overhead by line 148. The bottoms from the oxidizer 146 are removed by line 143 to pump 142. A purge line 135 is provided for the continuous removal of a portion of the aqueous solution from the pump line 136. 
     The pump line 136 feeds into a mixing tank 132 where a mixer 134 stirs the chemicals that are added. Line 138 is provided for the addition of aqueous caustic solution to the tank 132 so that the pH can be adjusted within the desired range. Line 140 is provided for the addition of make up polyvalent metal chelate. The contents of the mixing tank 132 are removed by line 130 to the pump 128 for recycle back to the second separator 122 by line 126. 
     Hydrogen sulfide is fed from any convenient source such as a pressurized tank or the like (not shown) through line 166, with an oxygen-containing gas such as air, oxygen, or a mixture thereof supplied through line 168, to SO 2  generator or incinerator 178. The SO 2  is routed through line 172 into an optional quench vessel 183 and thence through line 187 to a first scrubber 180. Scrubbing solution is circulated through scrubber 180 for contact with and absorption of the SO 2  by means of pump 179 and recirculation loop 181. Partially scrubbed SO 2  -containing gas is taken overhead by line 184 to a second scrubbing vessel 182 through which a scrubbing solution is circulated by means of pump 185 and recirculation loop 186. The scrubbed gas (less than 10 ppmv SO 2 ) is purged overhead from scrubber 182 by line 194. Makeup caustic or other alkali metal or ammonium hydroxide is introduced from line 190 into the recirculation loop 186 at a sufficient rate to maintain a pH in the range of about 8.6-9.5, and so that carbonate formation in the scrubbers 180,182 is substantially avoided by reaction of the alkali metal to form sulfite and/or bisulfite before being placed in contact with the SO 2  -containing gas which may also contain CO 2 . Scrubbing solution from scrubber 182 is introduced to recirculation loop 181 through line 192 from recirculation loop 186 at a sufficient rate to maintain a pH of about 4-7 in the scrubbing solution in first scrubber 180. Scrubbing solution containing sulfite and/or bisulfite is fed to line 152 through line 184 from recirculation loop 181 to maintain soluble sulfur-forming conditions in oxidizer 146 as described above. 
     Alternatively, the sulfite and/or bisulfite solution or the the metal chelate solution may be fed to the process at points other than described above. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The polyvalent metal chelates used herein are aqueous soluble, polyvalent metal chelates of a reducible polyvalent metal, i.e., a polyvalent metal which is capable of being reduced and a chelating or complexing agent capable of holding the metal in solution. As used herein, the term polyvalent metal includes those reducible metals having a valence of two or more. Representative of such polyvalent metals are chromium, cobalt, copper, iron, lead, manganese, mercury, molybdenum, nickel, palladium, platinum, tin, titanium, tungsten and vanadium. Of said polyvalent metals, iron, copper and nickel are most advantageously employed in preparing the polyvalent metal chelate, with iron being most preferred. 
     The term &#34;chelating agent&#34; is well-known in the art and references are made thereto for the purposes of this invention. Chelating agents useful in preparing the polyvalent metal chelate of the present invention include those chelating or complexing agents which form a water-soluble chelate with one or more of the aforedescribed polyvalent metals. Representative of such chelating agents are the aminopolycarboxylic acids, including the salts thereof, nitrilotriacetic acid, N-hydroxyethyl aminodiacetic acid and the polyaminocarboxylic acids including enthylenediaminetetraacetic acid, N-hydroxyethylethylenediaminetriacetic acid, diethylenetriaminepentaacetic acid, cyclohexene diamine tetraacetic acid, triethylene tetraamine hexaacetic 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-phosphonoacetic 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); other polyhydric alcohols such as sorbitol and mannitol; and the like. Of such chelating agents, the polyaminocarboxylic acids, particularly ethylenediaminetetraacetic and N-hydroxyethylethylenediaminetriacetic acids, are most advantageously employed in preparing the polyvalent metal chelate used herein. Most preferably, the polyvalent metal chelate is the chelate of a ferric iron with a polyaminocarboxylic acid, with the most preferred polyaminocarboxylic acids being selected on the basis of the process conditions to be employed. Ethylenediaminetetraacetic acid and N-hydroxyethylethylenediaminetriacetic acid are generally particularly preferred. 
     For the purpose of this invention, an effective amount of a polyvalent metal chelate is that amount ranging from about a stoichiometric amount based n the hydrogen sulfide absorbed to the amount represented by the solubility limit of the metal chelate in the solution. In like manner, an effective amount of an oxidizing agent (sulfite and/or bisulfite) is that amount ranging from about a stoichiometric amount based on the free sulfur formed to about five times the stoichiometric amount. 
     Sulfite and/or bisulfite (collectively referred to herein as &#34;sulfites&#34;) is employed as an oxidizing agent in the present process to maintain conditions in at least the second (oxidation-regeneration) reaction zone, and preferably also the first reaction zone, suitable for the formation of soluble sulfur compounds, e.g. thiosulfate, and to avoid the formation of solid elemental sulfur therein. The source of the sulfites employed is preferably the aqueous absorption effluent of H 2  S combustion products, and the combustion products are preferably obtained by combustion or catalytic incineration of a portion of the H 2  S-containing stream treated by the process. The aqueous absorption is preferably effected in a two-stage countercurrent scrubber using basic alkali metal hydroxide or ammonium hydroxide at conditions selective away from CO 2  absorption. This is accomplished, for example, by adding the makeup alkali metal hydroxide to a recirculation line or loop so that the alkali metal is contacted with the SO 2  containing gas in the form of sulfites so the absorption solution is essentially free of alkali metal hydroxide which could absorb CO 2  and concomitantly form carbonates which are undesirable in a desirably solidsfree system, and which are particularly undesirable where the aqueous chelating solution is cooled in a cooling tower. In such a two-stage scrubbing system, the first stage scrubber is preferably operated at a pH of about 4.5, e.g. about 4-5, while that of the second stage is about 9, e.g. about 8.5-9.5. This two-stage scrubbing is thus preferred because of no excess alkalinity in the sulfite/bisulfite effluent, i.e. a high proportion of bisulfite relative to sulfite which is economical by virtue of less makeup caustic being used, very low SO 2  slippage (usually less than 10 ppm) and substantially no alkali metal carbonates in the sulfite/bisulfite effluent due to the selectivity away from CO 2 . 
     CONTROL 1 
     To a 1-liter agitated reactor in a constant temperature bath was added about 500 water, 14.8 (0.0448 mole) ferric iron-N(hydroxyethyl)-ethylene diaminetriacetic acid chelate (FE +2 .HEDTA), and 1.15 (0.0148 mole) of sodium sulfide as a stimulant for the absorption of 0.0148 mole of H 2  S. The pH was adjusted to 7.0 with NH 4  OH or HCl. The reaction was carried out for 30 minutes at 20° C during which time substantially all of the sulfide was oxidized by the ferric iron to elemental sulfur. The iron was reduced to the ferrous state. 
     The total reaction solution was then weighed and filtered onto a tared filter paper for gravimetric determination of weight percent sulfur solids. The tared filter paper was dried and weighed. The weight percent sulfur solds, based on solution weights, was calculated. The filtrate was analyzed for weight percent thiosulfate (S 2  O 3   = ) and sulfate (SO 4   = ) by ion chromatography. 
     Analytical results showed 966 ppm sulfur solids and 164 ppm sodium thiosulfate (Na 2  S 2  O 3 ). Sulfate (SO 4   = ) was below detectable limits, i.e., less than 110 ppm. 
     EXAMPLE I 
     The reaction was carried out using the method and conditions of Control 1 except that 2.95 of sodium sulfite was added. This represents a stoichiometric amount of 50% excess with respect to the sodium sulfide of Control 1. 
     Analytical results showed 149 ppm sulfur solids and 3440 ppm sodium thiosulfate. 
     EXAMPLE II &amp; CONTROL 2 
     The reaction was carried out using the method and conditions of control 1 except the pH was controlled at 8.0. With no sulfite addition (Control 2) analysis showed 953 ppm sulfur solids and 232 ppm sodium thiosulfate. With sulfite addition, (Example II) analysis showed only 53 ppm sulfur solids and 3412 ppm sodium thiosulfate. 
     EXAMPLE III &amp; CONTROL 3 
     The reaction was again carried out using the method and conditions of Control 1 except the pH was controlled at 6.0. 
     With no sulfite addition, (Control 3 ) analysis showed 968 ppm sulfur solids and 149 ppm sodium thiosulfate. With sulfite addition, (Example III) analysis showed 163 ppm sulfur solids and 3370 ppm sodium thiosulfate. 
     CONTROL 4 
     The reaction was again carried out using the method and conditions of Control 1, except that pH was not controlled. The pH fell to about 3.6 resulting in nearly complete loss of H 2  S abatement efficiency and loss of SO 2  absorption. Most of the Na 2  S 2  O 3  was probably formed initially at the higher pH. 
     Results of the Examples and Controls are shown in Table 1. 
     
                       TABLE I______________________________________       ppm     ppm  pH   Solids  Na.sub.2 S.sub.2 O.sub.3                        Remarks______________________________________Control 1    7.0    966      164   No sulfite additionExample I    7.0    149     3440   With sulfite additionControl 2    8.0    953      232   No sulfite additionExample II    8.0     53     3412   With sulfite additionControl 3    6.0    968      149   With sulfite additionExample III    6.0    163     3370   With sulfite additionControl 4     3.6-   58     2054   No pH contr/with SO.sub.2    8.0                   feed______________________________________ 
    
     EXAMPLES IV 
     A pilot scale two-stage countercurrent scrubber was used to scrub CO 2  and SO 2  -containing gas streams. The raw gas stream was fed consecutively through the first stage scrubber and then through the second stage scrubber. Makeup caustic was added to the recirculation line of the second stage scrubber to maintain a pH of approximately 9.0. Scrubbing solution from the second-stage scrubber was in turn added to the first stage scrubber to control the pH at approximately 4.5. The gases scrubbed contained 1% SO 2 , 10% CO 2 , 4.5% O 2  and the balance N 2 , saturated with water at 140° F. (Example IV) and at 180° F. (Example V); and 5% SO 2 , 10% CO 2 , 4.5% O 2  (Example VI). All streams were scrubbed to less than 1 ppmv SO 2 , and the aqueous effluent of the first stage scrubber contained a high proportion of NaHSO 3 , and no detectable free NaOh which is required for efficient solids control.