Claims
- 1. A process for removing hydrogen sulfide from a gas stream also containing ammonia, comprising
- establishing a recirculating liquid stream comprising a liquid solution of a scrubbing agent consisting essentially of copper sulfate, said stream being buffered to an acidic pH with ammonium sulfate, said pH being sufficiently acidic to prevent the precipitation of copper as hydroxide or basic hydrated oxides at the operating conditions,
- contacting said liquid stream with said gas stream to react said copper sulfate with said hydrogen sulfide to precipitate copper sulfide, and
- removing copper sulfide precipitate from said recirculating liquid stream.
- 2. The process of claim 1 wherein at least a portion of said ammonium sulfate is formed from the ammonia in the gas stream.
- 3. The process of claim 1 wherein additional ammonia is added to control the pH during the contacting.
- 4. The process of claim 1 wherein said gas stream is geothermal steam or geothermally derived steam.
- 5. The process of claim 1 wherein said gas stream is coke oven gas.
- 6. The process of claim 1 wherein ammonium sulfate is removed from the recirculating liquid stream.
- 7. The process of claim 1 wherein said pH is no higher than 3 measured at room temperature.
- 8. The process of claim 7 wherein said pH is between 1 and 3 measured at room temperature.
- 9. The process of claim 8 wherein additional ammonia is added to control the pH during the contacting.
- 10. The process of claim 1 further comprising hydrothermally oxidizing said copper sulfide to regenerate copper sulfate.
- 11. The process of claim 10 further comprising returning said regenerated copper sulfate to said liquid stream.
Parent Case Info
This is a continuation-in-part of our copending application Ser. No. 720,345, filed Sept. 3, 1976 now abandoned.
This invention relates to the removal of hydrogen sulfide and ammonia from gas streams and more particularly to the removal of those constituents from a variety of gas streams, including but not limited to geothermal steam and chemical process gas streams such as coke oven gases.
H.sub.2 is highly odorous, toxic and corrosive and its presence in other than negligible quantities in gas streams vented to the atmosphere presents a serious problem in terms of environmental and aesthetic norms. Hence, it is desirable to substantially eliminate H.sub.2 S gas streams vented to the atmosphere.
Geothermal fluids in the form of naturally-occurring steam or steam obtained from hot brines represent a relatively inexpensive source of electrical power. These fluids commonly contain hydrogen sulfide and a number of other gaseous constituents in varying concentrations, e.g., carbon dioxide, methane, hydrogen and nitrogen. Ammonia and boric acid are often present in naturally-occurring steam, i.e., geyser steam, and in brines. Reduction of H.sub.2 S emission is essential if geothermal fluids are to become an important source of energy in the future.
Current technology for removing H.sub.2 S from chemical process gas streams is generally based on scrubbing H.sub.2 S by a basic solution. Such scrubbing processes are generally non-specific, that is, they tend to remove to a greater or lesser extent, other acid gases, for example, CO.sub.2. When removal of only H.sub.2 S is desired, absorption of other gases is generally minimized by various modifications which generally result in considerable increase in capital and operating costs and/or a decrease of the extent to which H.sub.2 S is removed from the gas stream. Furthermore, current technology is not applicable to gases at elevated temperatures, for example to steam, since the active reagents or the products of the active reagents and H.sub.2 S, decompose or volatilize at relatively low temperatures. Cooling of the gas stream to be purified introduces additional expense and in some cases, steam for example, is not practical. There exists, therefore, a need for a practical technology which is specific to H.sub.2 S, which can be applied to hot gases, and which can purify streams containing from a few parts per million of H.sub.2 S to several percent.
Current technology for H.sub.2 S removal from gas streams is not applicable to geothermal plants for various reasons. As a general rule, the conditions characteristic of geothermal steam are unfavorable with respect to established and accepted H.sub.2 S removal techniques. For example, the Clause Process, which has been used industrially for many years, is only applicaple to gas streams containing at least 20% H.sub.2 S. Other methods (e.g., the Rectisol Process) cannot operate in steam since the active reagents are water soluble or have boiling points below 100.degree. C. Solid absorbents for H.sub.2 S, e.g., calcium carbonate (as dolomite), calcium oxide, zinc oxide, and iron oxide cannot be used either because of solubility or slow kinetics at operating temperatures. Still other technology for H.sub.2 S removal which does not permit economical and efficient cleaning of geothermal emissions is exemplified by P.A. Ferguson, Hydrogen Sulfide Removal from Gases, Air and Liquids, Pollution Technology Review No. 22, published by Noyes Data Corporation, Park Ridge, N.J.
Employment of the useful properties of cupric sulfate to remove H.sub.2 S from gaseous streams by precipitation of copper sulfide has not been practical up to now in a larger-than-laboratory-scale plant, in spite of the fact that this reaction has been known for almost 200 years. The main problems which have prevented use of this reaction in a practical fashion have to do with control of acidity generated when copper sulfide is precipitated and with regeneration of the copper sulfide so that the process can be operated as a closed loop. Operation in a closed loop fashion is mandatory, except in exceptional circumstances, if the process is to be economically viable.
The theoretical reaction for scrubbing H.sub.2 S with CuSo.sub.4 can be written as:
German Pat. No. 350,591 suggests that regeneration may be carried out by converting the cupric sulfide into sulfate by oxidation through sedimentation with oxygenous gases under pressure and with heat. An as example, it is stated that air at 100.degree. C. and 50 atm pressure will convert the sulfide to the sulfate in 1 hour. The regeneration reaction may be written as:
However, operation of a closed loop process as described by German Pat. No. 350,591 turns out to be impractical and, in fact, not possible in a continuous manner. The difficulties arise both from kinetic and thermodynamic reasons. Continuous generation of acid eventually leads to instability, in a thermodynamic sense, of the copper sulfide precipitate and reaction (1) is no longer useful as a scrubbing reaction. However, long before this occurs, kinetic effects make the scrubbing process inefficient and, consequently, of little practical value.
One possible way of curing this defect is presented by J. D. F. Marsh and J. Rich in British Pat. No. 802,284. This patent concerns a method for the simultaneous removal of H.sub.2 S and HCN from fuel gas by washing the gas with a neutral or slightly acidic solution of copper sulfate and regenerating the precipitate by roasting to produce copper oxide and a gas containing oxides of sulfur. In a particular embodiment of this reaction, the roasting of the precipitate is controlled so that the sulfur removed as oxides of sulfur in the roasted gases is just equal in quantity to the hydrogen sulfide removed from the gas as copper sulfide. The copper oxide produced in the roasting process is dissolved in the slightly acidic copper sulfate solution for reuse as the wash liquor. The combination of these steps yields a closed loop process with control of acidity which can be used to remove H.sub.2 S and HCN from gas streams.
However, the process disclosed by Marsh et al suffers from the disadvantage that oxides of sulfur are produced. In particular, when H.sub.2 S is to be removed so as to avoid environmental discharges which are unacceptable, the net result of this process is to produce sulfur oxide emissions which are equally unacceptable. Therefore, to satisfy environment protection requirements, a scrubbing process for oxides of sulfur must be operated sequentially with the copper sulfate scrubber (for removal of H.sub.2 S), with accompanying penalties in cost and operability.
Another disadvantage of the process of Marsh et al is that while it provides means for neutralizing in a stoichiometric way acid produced by absorption of H.sub.2 S, the neutralization step occurs separately from the generation of acid. Accordingly, acidity generated in the scrubbing reaction leads to an acid solution in the scrubber and hence to poor efficiency of H.sub.2 S removal when the process is operated in a continuous, closed loop fashion. To cure this defect, large volumes of wash liquor must be used so as to keep the increase of acid concentration in the scrubber at a minimum. A high ratio of scrub liquor to gas scrubbed leads to significant increases in capital equipment costs and operating expense.
The reasons outlined above seem to be responsible for the unavailability of practical systems employing copper sulfide precipitation for scrubbing H.sub.2 S from gas streams.
Accordingly the primary object of this invention is to provide an improved method and system for removing hydrogen sulfide from a variety of gaseous streams, including but not limited to geothermal steam and chemical process gas streams such as coke oven gases. Another important object is to provide an economically viable method and system for removing hydrogen sulfide from such gaseous streams.
A further important object is to provide a practical closed loop method and system for scrubbing hydrogen sulfide from gaseous streams by means of copper sulfide precipitation.
A more specific object is to provide an economical and efficient method and system for simultaneously removing hydrogen sulfide and ammonia from gaseous streams such as a stream of coke oven gases or geothermal steam.
Still another object is to provide a method of the character described which is particularly useful for removing hydrogen sulfide from steam containing less than about 100 ppm of NH.sub.3.
Another specific object of this invention is to provide an H.sub.2 S emission control system for geothermal power plants which involves a novel method of treating the steam (regardless of whether it is a dry steam, i.e., a naturally occurring geothermal steam or a flashed steam derived from a geothermal brine) directly to effect removal of H.sub.2 S. A further specific object is to provide a method and system of the character described whereby the steam is treated for H.sub.2 S removal in advance of feeding it to the power-generating turbine (or the binary-cycle heat exchanger in the case where the steam is used to heat a secondary working fluid).
Still another object is to provide a closed loop method of treating fluids to effect H.sub.2 S and ammonia removal which does not require the design of new and exotic equipment.
Further objects are to provide a method for effecting H.sub.2 S removal from gaseous streams from chemical plants and geothermal power plants which overcomes at least some of the limitations of existing technology and enables such plants to operate without discharging relatively large quantities of H.sub.2 S into the atmosphere.
Still another specific object is to provide a method which comprises removing H.sub.2 S from a gaseous stream by precipitating copper sulfide, leaching the copper sulfide to produce copper sulfate, and utilizing the sulfate to remove additional H.sub.2 S by additional metal sulfide precipitation.
US Referenced Citations (11)
Foreign Referenced Citations (2)
| Number |
Date |
Country |
| 350591 |
Oct 1919 |
DE2 |
| 802284 |
Jan 1967 |
GBX |
Continuation in Parts (1)
|
Number |
Date |
Country |
| Parent |
720345 |
Sep 1976 |
|
Patent Grant
4192854
