Acid gases, including carbon dioxide and hydrogen sulfide, are common contaminants in raw natural gas and must be removed during refining. This is done through amine gas treatment, which absorbs the acid gases in a solution with amines. The concentrated hydrogen sulfide is then processed to recover elemental sulfur through the Claus process. Acid gases are also disposed of through subsurface injection to minimize environmental impact.
Acid gases are compounds of gases that create acidic solutions when added to water. They are most often encountered as contaminants in raw natural gas and must be removed during the refining process. The removal of acid gas from natural gas has the dual purpose of purifying the natural gas by reducing harmful emissions into the atmosphere. It can be made using various industrial processes known as amine gas treatment or gas softening processes, although acid gases are also sometimes disposed of through more unusual techniques, such as subsurface injection.
Carbon dioxide and hydrogen sulfide are the most common acid gases. Two other types found in industrial emissions are nitrogen oxides and sulfur dioxide. The latter two are known for their role in the production of acid rain. When one of these gases reacts with water in the atmosphere, it produces an acid which then falls out as precipitation. The environmental damage caused by corrosive acid rain has prompted countries around the world to take steps to reduce or prevent emissions of sulfur dioxide and other contaminants into the air.
While not directly involved in producing acid rain, the common acid gases carbon dioxide and hydrogen sulfide are both corrosive, and hydrogen sulfide is toxic. The removal and disposal or reuse of these problematic gases is a necessary part of the natural gas refining process. This is most often done through amine gas treatment, a method also known as sweetening.
In the treatment of amine gases, aqueous solutions of alkalamines are used to absorb hydrogen sulfide or carbon dioxide gas. Alkalamines, also called amines, are bases and can therefore neutralize acidic compounds through chemical reactions. When natural gas is treated with amines, the acid gases are absorbed in a solution with the amines, leaving behind a pure “sweetened” gas stream. The gaseous amine-acid solution is further processed to recycle the amines and remove the concentrated hydrogen sulfide or carbon dioxide.
Elemental sulfur is recovered from the concentrated hydrogen sulfide gas produced by the amine treatment through a method known as the Claus process. The Claus process consists of a series of controlled chemical reactions that remove sulfur to a level of up to 99%. This recovered sulfur can then be used to make other chemicals or sold for industrial use, preventing it from being vented into the atmosphere.
Excess acid gas from petroleum or natural gas refining that cannot be converted or recovered is sometimes burned. This method, which involves incinerating the gas and releasing it into the atmosphere, is generally considered harmful to the environment. An alternative to flaring is subsurface injection. Underground injection, where acid gases are injected into a deep underground reservoir, is believed to minimize the environmental impact in the storage and disposal of acid gas waste.
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