NZ206257A - A method for the purification of water containing hydrogen sulphide - Google Patents

Description

206257 Priority Date(s): Complete Specification Filed: (&//;* Class: £<*£>X?/?.*,...,?A Publication Date: .... P.O. Journal, No: NEW ZEALAND PATENTS ACT, 1953 No.: Date: COMPLETE SPECIFICATION REMOVAL OF HYDROGEN SULFIDE FROM STEAM XfWe, THE DOW CHEMICAL COMPANY, of 2030 Dow Center, Abbott Road, Midland, Michigan 4 8640, United States of America, a corporation organized and existing under the laws of the State of Delaware, United States of America, hereby declare the invention for which i / we pray that a patent may be granted to nox/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- (followed by page la) 206257 REMOVAL OF HYDROGEN SULFIDE FROM STEAM This invention relates to a process for removing hydrogen sulfide (H2S) from steam, especially from geothermal steam used to operate a turbine.

It is known from U.S. 4,123,506 dated October 31, 5 1978 and U.S. 4,202,864, dated May 13, 1980 that geothermal steam containing H2S can be purified by contacting the steam with a metal compound that forms insoluble metallic sulfides.

It is also known from U.S. 4,196,183, dated 10 April 1, 1980 that geothermal steam containing H2S can be purified by added oxygen and passing it through an activated carbon bed.

It is further known from U.S. 4,363,215, dated December 14, 1982 that geothermal steam containing 15 H2S can be purified by the reaction of the steam with hydrogen peroxide and a ferrous catalyst: Various processes for hydrogen sulfide control in geothermal steam are outlined in the U.S. Department -22r, 130D- F -!<*- -2 206257 of Energy Report #DOE/EV-0068 (March 1980) by F. B. Stephens, et al.

The removal of H2S from sour gases and sour water with ferric chelates is shown by U.S. patents 5 4,009,251, 4,091,073 and 4,076,621.

The present invention is directed to a process wherein residual steam containing H2S from a geothermal steam power pleuit is purified before release into the atmosphere.

The process of this invention has the follow- ing steps: (A) condensing said residual steam directly or indirectly with an aqueous solution in a condensing zone under a temperature sufficiently low to convert said steam into an aqueous solution containing dissolved hydrogen sulfide and its ions; (B) converting said aqueous sulfide solution to an aqueous solution containing free sulfur and ferrous chelate by contacting said aqueous sulfide solution with an aqueous solution of ferric chelate containing a sufficient amount of ferric chelate to completely oxidize said sulfide ions to sulfur in a conversion zone which may also be the condensing zone ? (C) converting said ferrous chelate solution ^ with air in a cooling zone into an aqueous solution containing ferric chelate; and 306357 J v (D) recycling said ferric chelate solution back to said conversion zone.

In the process of this invention about 20 to 80 percent of the H2S gas in the geothermal steam is 5 absorbed into the aqueous phase of a steam condensor depending on whether a direct contact or surface condensor is used whereby the dissolved H2S is treated with ferric chelate to produce finely divided sulfur. The non-condensed or non-absorbed H2S can be exhausted 10 to the atmosphere or if zero discharge is desired or required, the H2S gas can be absorbed with conventional H2S absorbers such as aqueous alkanol amine solutions or potassium carbonate solutions or caustic solutions or oxidized to sulfur using the Stretford process.

While a direct contact steam condenser with spray heads can be used to absorb about 80 percent of the H2S into the geothermal steam condensate, it has been found that it is more advantageous to use indirect condensors or surface condensors such as a shell and « . tube condensor to condense the steam when other systems are readily available for H2S abatement of the non-condensable gas stream. The advantage of using the surface condensor is that only about 20 percent of the incoming H2S is absorbed in the condensate while about 25 80 percent remains in the noncondensable gas the purification of which is more economical. Hence, less ferric chelate is needed to react with the dissolved H2S and/or its ions. Further, since the condensing fluid is not mixed with the condensate, there is less 30 total liquid volume to process. - 206257 Figure 1 illustrates a process in which this invention is applied for the oxidation of hydrogen sulfide contained in a liguid stream produced by the condensation of geothermal steam in a direct contact 5 condensor and as such eliminates the environmental pollution problems associated with the discharge of an effluent stream containing the toxic and noxious hydrogen sulfide.

Figure 2 illustrates a similar process using 10 an indirect condensor.

In Figure 1, the geothermal steam from line 2 is used to power a steam turbine 4 which is connected to an electric power generator 6. The turbine 4 exhausts through line 8 to a direct contact condenser 15 10. Cooling water containing chelated iron (ferric chelate) from line 12 is sprayed into condenser 10 for this condensation and passes from the condenser 10 through line 14 to the hot well 16 operating at 38-52°C (100-126°F). Non-condensable gases such as C02, N2, 20 02, and H2S are removed from the main condenser 10 through line 18 by two steam jet ejectors 20 and 34 and the associated condensers 26 and 42. The ejectors 20 and 34 are operated by steam supplied by lines 22 and 36 respectively. These ejectors create a partial 25 vacuum or low pressure zone. The exhaust steam from the ejector 20 is carried by line 24 to the condenser 26 and by line 32 to the second ejector 34. The exhaust steam from ejector 34 is carried by line 40 to condenser 42. Cooling water from line 12 is supplied 30 to each of the condensers 26 and 42 by lines 28 and 44 respectively. The condensed steam from condenser 26 and 42 flows by means of lines. 30 and 46 to the hot 2QZ2 well 16. The non-condensable gases and the exhaust steam are then vented to a conventional caustic scrubber, alkanol amine or potassium carbonate unit through line 48 for removal of the H2S gases.

Pump 58 is used to pump the combined condensed geothermal steam and cooling water from the hot well 16 through line 60 to the induced-draft cooling tower 62 with internal spray heads 66 where an amount of water equal to approximately 80% of the condensed steam is evaporated by the air flow through the tower which also strips all of the dissolved hydrogen sulfide from the liquid and it would be vented to the environment by means of the air stream 64 except for the use of chelated iron as described herein. The excess condensed steam which is not evaporated overflows a weir (not shown) in the base of the cooling tower 62 for disposal by line 80. The remainder of the cold water flows through line 68 to the relatively cold well 70 which operates at 24-29°C (75-85°F). Pump 50 is used to pump the cold water from the cold well 70 to the condensers 42, 26, and 10. The hot well 16 is separated from the cold well 70 by a weir 72.

In order to prevent the release of the dissolved hydrogen sulfide to the environment in the air stream 64 flowing from the top of the cooling tower 62, an amount of chelated iron is added to the circulating water which is greater than the stoichiometric amount required to oxidize the dissolved hydrogen sulfide in the hot well 16. In this manner, the dissolved hydrogen sulfide is effectively oxidized before the water enters the top of the cooling tower 62 from line 60. The air flow and time of contact between the air and water in - 2062 5 7 the cooling tower 62 is sufficiently long that the ferrous chelate which results from the oxidation of dissolved hydrogen sulfide in the hot well 16 and associated piping 56 and 60 is reoxidized to the active ferric state as it passes down through the cooling tower 62. Elemental sulfur in a finely divided solid . form produced by this process circulates freely throughout the system and may be recovered by conventional means from the overflow line 80. Such recovery methods may allow the sulfur to agglomerate into a heavy slurry after which the supernatent liquid may be removed by decantation, centrifugation, filtration, and the like.

In order to maintain at least the stoichiometric amount of iron chelate required for this process, an amount of fresh iron chelate equal to the amount lost in the overflow line 80 is added' from the storage vessel 74 by pump 76 and inlet line 78.

In Figure 2, the items and their functions are exactly the same as Figure 1 except that the condensor 110 of Figure 2 is an indirect condensor having a cooling coil diagramatically illustrated by 113. The cooling water enters the condensor 113 by line 112 and is removed by line 115.

In this modification, only the steam condensate containing H2S drains down line 114 into the hot well 116 since the cooling water does not mix with and scrub H2S from the geothermal steam. Due to this reduced amount of H2S in the condensate entering the hot well 116 from line 114, the amount of ferric chelate used or added can be much less while maintaining the required stoichiometric ratio of ferric chelate to H2S in the 2B-rl3 0B T 206257 hot well and the amount lost in the overflow 180 is less.

Geothermal steam from The Geysers Known Geothermal Resource Area (KGRA) ("The Geysers" is an area in California, U.S.A.; which is noted for its geothermal activity, including geysers) has been found to contain the following ranges and average values of hydrogen sulfide and other impurities in parts per million. gas range average h2s -1600 222 c02 290-30600 3260 ch4 13-1447 194 nh3 9-1060 194 n2 6-638 52 h2 11-218 56 One purpose of this invention is to reduce, abate, or eliminate the hydrogen sulfide (H2S) from condensed geothermal steam after the steam is used to power a turbine.

This invention can also be applied to other KGRA's that depend on liquid-dominated resources as the source of energy. In these cases, the geothermal fluids would be treated directly thus eliminating the condensation step.

In the process of this invention, it has been found that the amount of ferric chelate used for the sulfide conversion should be in the range from about 1.0 to about 6.0 moles and preferably about 1.2 to about 3.0 moles of ferric chelate per mole of H2S. 206257 The temperature range of the sulfide conversion step should be in the range from about 1.0 to about 99°C and preferably in the range from about 25 to about 60°C.

The sulfide conversion step should also be conducted at a pH in the range from about 5 to about 10 and preferably in the range from about 6.8 to about 8.3.

The temperature range of the condensing zone 10 should be in the range from about 1.0 to about 99°C and preferably in the range from about 25 to about 65°C.

The temperature range of the chelate converting zone should be in the range from about 1.0 to about 99°C and preferably in the range from about 25 to about 15 65°C.

Chelating agents useful in preparing the ferric chelate of the present invention include those chelating or complexing agents which form a water-soluble chelate. Representative of such chelating agents are 20 the aminocarboxylic acids, such as nitrilotriacetic acid, N-hydroxyethyliminodiacetic acid, ethylenediamine-tetraacetic acid, N-hydroxyethylethylenediaminetriacetic acid, diethylenetriaminepentaacetic acid, cyclohexane-diaminetetraacetic acid, triethylenetetraaminehexaacetic 25 acid and the like, including the salts thereof. Of such chelating agents, ethylenediaminetetraacetic, N-hydroxyethylethylenediaminetriacetic acid and N-hydroxyethyliminodiacetic acid, are most advantageously employed in preparing the ferric chelate used 30 herein. 23*.X30B-F 2062 5 7 Detailed examples of the invention are given below for purposes of further illustrating the invention.

Examples 1-4 ft - A fully chelated iron solution, Versenol iron (available from The Dow Chemical Company, Midland, Michigan, U.S.A.), which contained 4% iron was prepared from the trisodiura salt of N-hydroxyethylethylene-diaminetriacetic acid (Na3 HEDTA) and ferric nitrate. After baseline data were obtained on the hydrogen sulfide emissions using the apparatus of Figure 1 of the drawings when no iron chelate was present (i.e. the A control), a drum of the Versenol iron concentrate was added rapidly to the cold well 70. Thereafter, pump 76 was used to meter in additional iron chelate at a rate of about two pounds per hour of iron to balance that which was lost in the overflow 80. This resulted in the maintenance of an iron concentration in the circulating water of about 20 parts per million (ppm) which is slightly greater than the stoichiometric requirement of 14 ppm in the hot well 16. Immediately after the addition of the iron chelate to the system, the hydrogen sulfide emissions from the cooling tower became immeasurably small and could not be detected in this stream throughout the remainder of the trial.

Data obtained during this trial are presented in Table I. These data show that while the instant invention is extremely effective for the removal of hydrogen sulfide from aqueous streams, the relatively constant percentage of hydrogen sulfide in the noncon-densable gas stream 48 showed it to have little effect on gaseous streams containing hydrogen sulfide.

?A. ~non p STEAM FLOW LINE 2 M kg/hr RUN (M ft/hr) Control 81.6 (180) Example 1 83.0 (183) Example 2 81.2 <179) Example 3 80.7 (178) H2S IN H0S IN LINE 2 AIR kg/hr STREAM 64 IMll (PPm) .8 3.8 (35.0) .6 0 (34.5) .0 0 (33.1) .3 0 33.7 Example 4 80.7 15.3 0 (178) (34.2) o o TABLE I H2S IN LINE 48 kg/hr 2.9 (6.5) 2.3 (5.1) 2.5 (5.6) 2.9 (6.3) 2.7 (6.0) H,S IN LINE 48 OVERFLOW Fe FEED LINE 80 RATE M kg/hr kg/hr (»/hr) (% OF LINE 2\ (M #/hr) (#/hr) 19 15 17 19 18 17 (38) 24 (54) (44) 22 (48) 24 (54) 0.9 (2.0) 1.0 (2.3) 0.9 (2.1) 0.8 (1*8) Fe CONCENTRATION IN COLD WELL 70 (PPM> 21 21

Claims (10)

11- 206257 WHATWE CLAIM ISi

1. A process for purifying H20 containing H2S comprising (A) condensing any steam portions of said H20, directly with an aqueous solution, or indirectly, in a condensing zone at a temperature sufficiently low to convert said steam into an aqueous solution containing dissolved hydrogen sulfide and its ions; (B) converting said aqueous sulfide solution to an aqueous solution containing free sulfur and ferrous chelate by contacting said aqueous sulfide solution with an aqueous solution of ferric chelate con- . taining a sufficient amount of ferric chelate to completely oxidize said sulfide ions, to sulfur in a conversion zone which may also be the condensing zone; (C) converting said ferrous chelate solution with air in a cooling zone into an aqueous solution containing ferric chelate; and (D) recycling said ferric chelate solution back to said conversion zone. -11- -12- a.06 257

2. The process as set forth in Claim 1 wherein the amount of ferric chelate used is in the range from 1.0 to 6.0 moles of ferric chelate per mole of H2S.

3. The process as set forth in Claim 1 wherein the amount of ferric chelate used is in the range from 1.2 to 3.0 moles of ferric chelate per mole of H2S.

4. The process as set forth in any one of Claims 1 to 3 wherein ferric chelate is added as needed to maintain said sufficient amount.

5. The process of Claim 1 wherein the steam portion is condensed directly, the condensing fluid being aqueous solution of ferric chelate.

6. The process as set forth in Claim 5 wherein the amount of ferric chelate used is in the range from 1.0 to 6.0 moles of ferric chelate per mole of h2s.

7. The process as set forth in Claim 5 wherein the amount of ferric chelate used is in the range from 1.2 to 3.0 moles of ferric chelate per mole of H2S.

8. The process as set forth in any one of Claims 1 to 7 wherein said sulfide conversion is carried out at a temperature in the range from 1.0 to 99°C, and a pH in the range from 5 to 10.

9. The process as set forth in any one of Claims 1 to 7 wherein said sulfide conversion is carried -13- iwsssf out at a temperature in the range from 25 to 60®C and a pH in the range from 6.8 to 8.3.

10. The process as set forth in any one of Claims 5 to 7 wherein ferric chelate is added as needed to maintain said sufficient amount. :D THIS DAY OF <Qt C> A. J. PARK & SON Fc.n J! -S ASENTS FOR TJ !E APPLICANT?