Acid gases are gas compounds 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 serves the dual purpose of purifying the natural gas while reducing harmful emissions into the atmosphere. It can be accomplished using various industrial processes known as amine gas treating or gas sweetening 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. These last two are notable for their role in producing acid rain. When one of these gases reacts with water in the atmosphere, an acid is produced which then falls as precipitation. The environmental damage caused by corrosive acid rain has spurred countries around the world to take steps to reduce or prevent emissions of sulfur dioxide and other contaminants into the air.
Although not directly involved in the production of acid rain, the common acid gases carbon dioxide and hydrogen sulfide are both corrosive, and hydrogen sulfide is toxic. Removal and disposal or repurposing of these problematic gases is a necessary part of the natural gas refining process. This is most often done through amine gas treating, a method also sometimes known as sweetening.
In amine gas treating, 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, acid gases are absorbed into a solution with the amines, leaving behind a stream of pure “sweetened” gas. The amine-acid gas solution is processed further to recycle the amines and remove the hydrogen sulfide or carbon dioxide in concentrated form.
Elemental sulfur is recovered from the concentrated hydrogen sulfide gas produced by amine treatment through a method known as the Claus process. The Claus process consists of a series of controlled chemical reactions that removes sulfur at a level as high as 99%. This recovered sulfur can then be used to manufacture 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 flared. This method, which involves incinerating the gas and releasing it into the atmosphere, is generally considered environmentally damaging. One alternative to flaring is subsurface injection. Subsurface injection, in which acid gases are injected into a deep underground reservoir, is thought to minimize environmental impact in storage and disposal of acid gas wastes.