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The term acid gas covers any type of gas or gaseous mixture which forms an acidic compound when mixed with water. The most common types of acid gases are hydrogen sulfide (H2S) and carbon dioxide (CO2). Acidic gases are found in natural gas and must be removed through a process known as amine gas treating before the natural gas is usable. While the terms acid gas and sour gas are often mistakenly used to describe acid gases, the terms are not the same thing. A sour gas contains high amounts of hydrogen sulfide, while an actual acid gas contains any of the acidic gases.
When found in natural gas, hydrogen sulfide can be considered both a sour gas and an acid gas. Carbon dioxide, however, is an acidic gas but not technically a sour gas. Gases need to have these acidic materials removed before they are used. This is accomplished using the amine gas treating process, referred to as sweetening. The process takes certain liquids and uses them to remove the harmful hydrogen sulfide and carbon dioxide, or other combination of acidic gases, from the natural gas.
Sweetening causes the hydrogen sulfide and the carbon dioxide to separate from the natural gas. This is key because, being types of acid gas, these two substances will become acidic when they come into contact with water. Any building running a gas pipeline will experience problems from these acidic gases if not removed. The acid will begin to eat through the pipes and cause leakage. The occurrence of these gases also limits the types of materials that can be used when installing gas lines.
To sweeten the natural gas using the amine gas treating process, an ethanolamine is added. Ethanolamine is a solvent that scrubs or cleans the acidic gases; it absorbs carbon dioxide and hydrogen sulfide but leaves the other gases untouched. Several types of the organic compound ethanolamine exist. Common compounds used in the sweetening process include monoethanolamine (MEA) and methyldiethanolamine (MDEA).
Aside from the risk of damaging the pipeline, hydrogen sulfide is highly toxic and dangerous, even deadly, to animals or people who are exposed to it. It is also flammable and produces a foul stench, thus where the name "sour gas" comes from. Carbon dioxide is not a sour gas and, in contrast to hydrogen sulfide, is an odorless gas.