Selective Absorption of H2S From Sour Gas

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Abstract
Selective absorption of H2S from sour gas with caustic by gas/liquid scrubbing is a new approach for oilfield operations to meet environmental and safety restrictions. This technology demonstrates that by maintaining a very short contact time in a gas/liquid scrubber it is possible for caustic to react with and remove H2S from a sour gas. Introduction Hydrogen sulfide (H2S) and carbon dioxide (CO2) are natural contaminants of gases from well casings, combustion floods, geothermal steam, or process streams. Because of safety, environmental or corrosion considerations, it normally is desirable to remove the H2S from such gases. However, these factors do not dictate the removal of CO2, and it is not desirable because of the cost involved. Safety standards set by the U.S. Occupational Safety and Health Admin. limit worker exposure to H2S at 10 ppm for an 8-hour day. Environmental guidelines ppm for an 8-hour day. Environmental guidelines require limited sulfur emissions, particularly of H2S, which has a characteristic "rotten egg" odor at levels of 3 ppm. Corrosivity of wet H2S gases is well known because of its acidic character. Many commercial methods exist to remove H2S from sour gas streams, though they all have their limitations. Some processes may be limited by the volume of gas they can handle or by the level of H2S or CO2 in the gas stream. Others may prove to be costly because of the capital investment or operating expense. Still others are not suitable for the field because of the complexity of operation, requiring highly trained personnel. In the H2S removal system described in this paper, these objections largely are overcome, as evidenced by a number of production units based on this technology. Experience has shown that this system (1) has a very low capital investment cost (2) has a simple design based on gas/liquid scrubbing (3) has straightforward installation and operation (4) will remove H2S efficiently, and (5) can treat gases widely varied in composition and volume. These units have proved effective in H2S removal from operations as widely divergent as pipeline-quality gas plants to remote locations treating vent gases with plants to remote locations treating vent gases with 10% H2S. Theory The principle of the technology presented here is based on a patent issued to The Dow Chemical Co. It states (and is reduced to practice in the method discussed here) that H2S can be absorbed selectively from sour gases also containing CO2 by maintaining a proper residence contact time between the gas and an alkaline liquid. Normally, in a typical wet scrubbing operation both acidic gases (H2S and CO2) would be absorbed. However, the absorption and reaction of H2S in a strongly alkaline solution (caustic soda) is virtually instantaneous while the absorption of CO2 is a much slower reaction. Thus, if the gas is allowed to contact the liquid for a controlled short time, the CO2 absorption and reaction with caustic will be insignificant. Because H2S does not have this limitation, it is absorbed preferentially. preferentially. JPT P. 1083

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