Purifying Geothermal Steam

This invention relates to a process wherein exhaust steam from a steam turbine operated by geothermal steam is condensed with an aqueous solution containing ferric chelate and the hydrogen sulfide in said geothermal 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.sub.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.sub.2 S can be purified by added 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 Mar. 1980) by F. B. Stephens, et al.

The removal of H.sub.2 S from sour gases and sour water with ferric chelates is shown by U.S. Pat. Nos. 4,091,073 and 4,076,621 respectively.

The present invention is directed to a process wherein residual steam containing H.sub.2 S from a geothermal steam power plant is purified before release into the atmosphere.

The process of this invention has the following steps:

(A) condensing said residual steam with an aqueous solution containing a ferric chelate in a condensing zone under temperature sufficiently low to convert said steam and said solution into an aqueous solution containing H.sub.2 S and said ferric chelate;

(B) converting said aqueous solution to an aqueous solution containing free sulfur and ferrous chelate;

(C) converting said solution containing ferrous chelate with air in a cooling zone into an aqueous solution containing ferric chelate; and

(D) recycling said ferric chelate solution back to said condensing zone.

In the process of this invention about 80 percent of the H.sub.2 S gas in the geothermal steam is absorbed into the aqueous phase of a direct contact steam condensor whereby the H.sub.2 S is treated with ferric chelate to produce finely divided sulfur. The non-condensed or non-absorbed H.sub.2 S can be exhausted to the atmosphere or if zero discharge is desired or required, the H.sub.2 S gas can be absorbed with conventional H.sub.2 S absorbers such as aqueous alkanol amines solutions or potassium carbonate solutions.

The FIGURE 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 and as such eliminates the environmental pollution problems associated with the discharge of an effluent stream containing the toxic and noxious hydrogen sulfide. A simiar application could be made to streams resulting from industrial processes.

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. The turbine 4 exhausts through line 8 to a direct contact condenser 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 100.degree.-125.degree. F. Non-condensable gases such as CO.sub.2, N.sub.2, O.sub.2, and H.sub.2 S 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 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 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 well 16. The non-condensable gases and the exhaust steam are then vented to a conventional caustic scrubber or alkanol amine unit through line 48 for removal of the H.sub.2 S 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 75.degree.-85.degree. 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 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 chalate equal to the amount lost in the overflow line 80 is added from the storage vessel 74 by pump 76 and inlet line 78.

Detailed examples of the invention are given below for purposes of further illustrating the invention.

A fully chelated iron solution, Versenol.RTM. iron, which contained 4% iron was prepared from the trisodium salt of N-hydroxyethylethylenediaminetriacetic acid (Na.sub.3 HEDTA) and ferric nitrate. After baseline data were obtained on the hydrogen sulfide emissions when no iron chelate was present (i.e. the control), a drum of the Versenol.RTM. 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 noncondensable gas stream 48 showed it to have little effect on gaseous streams containing hydrogen sulfide.