Acid gas is a particular typology of

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...

or any other

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

mixture In chemistry, a mixture is a material made up of two or more different chemical substances which are not chemically bonded. A mixture is the physical combination of two or more substances in which the identities are retained and are mixed in the ...

containing significant quantities of

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is poisonous, corrosive, and flammable, with trace amounts in ambient atmosphere having a characteristic foul odor of rotten eggs. The unde ...

(H₂S),

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

(CO₂), or similar

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

ic gases. A gas is determined to be

acidic In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

or not after it is mixed with water. The

pH scale In chemistry, pH (), historically denoting "potential of hydrogen" (or "power of hydrogen"), is a scale used to specify the acidity or basicity of an aqueous solution. Acidic solutions (solutions with higher concentrations of ions) are me ...

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. The term/s ''acid gas'' and ''

sour gas Sour gas is natural gas or any other gas containing significant amounts of hydrogen sulfide (H2S). Natural gas is usually considered sour if there are more than 5.7 milligrams of H2S per cubic meter of natural gas, which is equivalent to approxim ...

'' 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 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 between 6.5 and 8.5, but acid ...

. 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 generation of electric power. Power stations are generally connected to an electrical grid. Many pow ...

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 something that can burn (i.e., ''combust'') in air. A combustible material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort and a flammable mat ...

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 expl ...

under pressure so keep them 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 and asphyxiation. 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 accompan ...

and other equipment for handling sour gas, as many metals are sensitive to

sulfide stress cracking Sulfide stress cracking (SSC) is a form of hydrogen embrittlement which is a cathodic cracking mechanism. It should not be confused with the term stress corrosion cracking which is an anodic cracking mechanism. Susceptible alloys, especially steel ...

. 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 Amine gas treating, also known as amine scrubbing, gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkylamines (commonly referred to simply as amines) to remove hydrogen sulfide (H2S) and c ...

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 principly 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 (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

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 The Claus process is the most significant gas desulfurizing process, recovering elemental sulfur from gaseous hydrogen sulfide. First patented in 1883 by the chemist Carl Friedrich Claus, the Claus process has become the industry standard. Th ...

or alternatively converted to valuable

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

in a

WSA Process The wet sulfuric acid process (WSA process) is a gas desulfurization process. After Danish company Haldor Topsoe introduced this technology in 1987, it has been recognized as a process for recovering sulfur from various process gases in the form ...

unit. Processes within oil refineries or

natural-gas processing Natural-gas processing is a range of industrial processes designed to purify raw natural gas by removing impurities, contaminants and higher molecular mass hydrocarbons to produce what is known as ''pipeline quality'' dry natural gas. Natural gas ...

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 gro ...

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