Acid gas

Acid gas is natural gas, petroleum byproduct gas or any other gas mixture containing significant amounts of acidic gases.

The terms acid gas and sour gas are often incorrectly treated as synonyms. Strictly speaking, a sour gas is any gas containing hydrogen sulphide (H₂S) in significant amounts whereas an acid gas is any gas containing significant amounts of acidic substances such as gaseous carbon dioxide (CO₂), hydrogen sulfide or mercaptans (RSH). Thus, carbon dioxide by itself is an acid gas but not a sour gas.

Discussion

Before a raw natural gas containing hydrogen sulfide or carbon dioxide can be used, the raw gas must be treated to reduce those impurities to acceptable levels and this is commonly done with an amine gas treating process. The removed H₂S is most often subsequently converted to by-product elemental sulphur in a Claus process. Any mercaptans present are commonly removed with a sweetening process.

Processes within oil refineries or natural gas processing plants that remove mercaptans and/or hydrogen sulfide are commonly referred to as sweetening processes because they result in products which no longer have the sour, foul odors of mercaptans and hydrogen sulfide.

Hydrogen sulfide is a toxic gas. It also restricts the materials that can be used for piping and other equipment for handling sour gas, as many metals are sensitive to sulfide stress cracking.

Carbon dioxide at concentrations of 7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour. Concentrations above 17% are lethal with exposure of 1 minute or more.