Acid gas is a particular typology of

natural gas Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...

or any other gas

mixture In chemistry, a mixture is a material made up of two or more different chemical substances which can be separated by physical method. It is an impure substance made up of 2 or more elements or compounds mechanically mixed together in any proporti ...

containing significant quantities of

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

(H₂S),

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(CO₂), or similar

acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...

ic gases. A gas is determined to be

acidic An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen cation, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid. The first category of acids are the ...

or not after it is mixed with water. The pH scale ranges from 0 to 14, anything above 7 is basic while anything below 7 is acidic. Water has a neutral pH of 7 so once a gas is mixed with water, if the resulting mixture has a pH of less than 7 that means it is an acidic gas; if the pH is more than 7, that means it is an alkaline gas. The term/s ''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

gases such as

(CO₂) or hydrogen sulfide. Thus, carbon dioxide by itself is an acid gas but not a sour gas.

Dangers of acid gas

Once a process burns a gas containing an acidic mixture, that acid gas is released into the atmosphere. This causes one of manufacturing's most detrimental effects on the environment,

acid rain Acid rain is rain or any other form of Precipitation (meteorology), precipitation that is unusually acidic, meaning that it has elevated levels of hydrogen ions (low pH). Most water, including drinking water, has a neutral pH that exists b ...

. The acidic gases burned from one

power plant A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an electr ...

can travel hundreds of miles after the gas mixes with water molecules in the atmosphere. The compounds then fall to the earth again in different forms of precipitation (acid rain) and can cause respiratory health issues in humans, kill plants and wildlife, erode structures and buildings, and contaminate water sources. Acid gases are also hazardous in other ways than polluting the environment. Acid gases can be extremely

flammable A combustible material is a material that can burn (i.e., sustain a flame) in air under certain conditions. A material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort ...

and

explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An ex ...

under pressure, so must be kept away from heat, sparks, or open flames. Hydrogen sulfide is a

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

gas, it can cause breathing problems, asphyxiation and death. It also is very corrosive to metals which restricts the materials that can be used for

piping Within industry, piping is a system of pipes used to convey fluids (liquids and gases) from one location to another. The engineering discipline of piping design studies the efficient transport of fluid. Industrial process piping (and accomp ...

and other equipment for handling sour gas, as many metals are sensitive to sulfide stress cracking. Carbon dioxide at concentrations of 7% to 10.1% causes 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.

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. There are physical and chemical absorption processes to removing the toxic properties of these gases, both of which involve the

syngas Syngas, or synthesis gas, is a mixture of hydrogen and carbon monoxide in various ratios. The gas often contains some carbon dioxide and methane. It is principally used for producing ammonia or methanol. Syngas is combustible and can be used as ...

being washed with a lean solvent in an absorber to remove the H₂S. Once the toxic gas leaves the bottom of the absorber it is sent to a regenerator where the solution is further stripped with steam under extremely lower pressures to remover the

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

from the gas.{{Cite web, url=https://netl.doe.gov/research/coal/energy-systems/gasification/gasifipedia/agr#:~:text=Acid%20gases%20produced%20in%20gasification,carbon%20dioxide%20(CO2).&text=Conventional%20processes%20for%20removing%20acid,solvent%20in%20an%20absorber%20column., title=6.2.1. Acid Gas Removal (AGR) The removed H₂S is most often subsequently converted to by-product elemental

in a Claus process or alternatively converted to valuable

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

in a WSA Process unit. Processes within oil refineries or

natural-gas processing Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...

plants that remove

mercaptan In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

s 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.

References

Natural gas Oil refining Industrial gases

Dangers of acid gas

Processing and safety

See also

References