Acid gas removal is an important process in various branches of the hydrocarbon processing industry, primarily in natural gas processing and refining. Acid gas removal is also an essential part of other processes, such as coal gasification where carbon dioxide, hydrogen sulfide, carbonyl sulfides, mercaptans, and other contaminants need to be removed.
Acid gas is defined as gas containing significant amounts of contaminants, such as hydrogen sulfide (H2S), carbon dioxide (CO2), and other acidic gases. Sour gas is gas contaminated with H2S. This term comes from the rotten smell due to sulfur content. Thus, "gas sweetening" refers to H2S removal, because it improves the odor of the gas being processed, while "acid gas removal" refers to the removal of both CO2 and H2S.
Acid gases need to be removed in order to comply with sales gas quality regulations. These regulations are in place to minimize environmental impact and ensure gas transport pipeline integrity, avoiding undesired occurrences, such as corrosion caused by H2S and CO2 in the presence of water. Acid gases also need to be removed due to the toxicity of compounds, such as H2S, and the lack of the heating value of CO2. Typically, "pipeline quality" or sales gas is required to be sweetened to contain concentrations of H2S that's no more than 4 parts per million (ppm), and a heating value of no less than 920 to 1150 Btu/SCF, depending on the final consumer requirements.