Acid gas

Acid gas is a particular typology of natural gas or any other gas mixture containing significant quantities of hydrogen sulfide (H₂S), carbon dioxide (CO₂), or similar acidic gases.

The terms acid gas and sour gas are often incorrectly treated as synonyms. Strictly speaking, a sour gas is any gas that specifically contains hydrogen sulfide in significant amounts; an acid gas is any gas that contains significant amounts of acidic gases such as carbon dioxide (CO₂) or hydrogen sulfide. Thus, carbon dioxide by itself is an acid gas but not a sour gas.

Processing and safety

Before a raw natural gas containing hydrogen sulfide and/or carbon dioxide can be used, the raw gas must be treated to reduce 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 sulfur in a Claus process or alternatively converted to valuable sulfuric acid in a WSA Process unit.

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 when exposed for more than one minute.

See also

• Natural gas processing
• Oil refinery
• Rectisol
• Selexol
• Sour gas
• WSA Process