Industrial gas is a group of gases that are specifically manufactured or refined for use in industry. They can be an elemental gas or a chemical compound that is either organic or inorganic. They could also be a mixture of such gases. They have value as a feedstock, in process enhancement or as a useful end product - rather than having value as a "simple" fuel.
However the oil and gas and petrochemicals industries are seen as distinct from the "industrial gas industry", so for example, whilst it is true that natural gas is a "gas" used in "industry" - often as a fuel, sometimes as a feedstock, and in this generic sense is an "industrial gas"; this term is not generally used for hydrocarbons produced by the petroleum industry directly from natural resources or in an oil refinery. Similarly gases that are petrochemicals (chemicals derived from petroleum) such as ethylene are generally not described as "industrial gases" when used in bulk, but probably would be described as such if put in a gas cylinder. The distinction here is really due to a demarcation between industries which is not always clear, rather than an exact scientific definition. However an "industrial gas" is more likely to be a pure compound or precise mixture, packaged or in small quantities, but with higher purity or tailored to a specific use (e.g. oxyacetylene welding ).
The Industrial Gases industry is a part of the wider Chemical Industry. Whilst a few products are sold to the population at large (for example balloon helium and welding gases), most are sold to other industrial enterprises. The industry came into being with the advent of the liquefaction of air allowing the separation of oxygen from air for use in steelmaking. Air separation allowed the bulk production of nitrogen and argon in addition to oxygen, and these three are often also produced as cryogenic liquid. To achieve the required low distillation temperatures an air separation unit uses a refrigeration cycle that operates by means of the Joule–Thomson effect. The other principal production technology is steam methane reforming which is used to convert natural gas and steam into a syngas containing hydrogen and carbon monoxide; although carbon dioxide is a byproduct. These two technologies are the cornerstone of the industrial gases industry and also form part of the technologies required for many fuel gasification and cogeneration, carbon capture and gas to liquids schemes. Hydrogen is touted as an alternative fuel to hydrocarbons whilst liquid hydrogen is used by NASA as a rocket fuel. Simpler gas separation technologies, such as membranes and molecular sieves, are used to produce low purity air gases.
The term “industrial gases” is also sometimes narrowly defined as just the major gases sold, which are: air, oxygen, nitrogen, argon, hydrogen, carbon monoxide and carbon dioxide. Other gases sold by the "industrial gases industry" (which might be categorised variously as "speciality gases", “medical gases”, “fuel gases” or “refrigerant gases”) are produced in much smaller quantities from a variety of processes or are sometimes simply bought from other chemical producers or can be produced from byproducts from other industries.
Air separation is however the only practical source for production of the rare noble gases neon, krypton and xenon.
Cryogenic technologies also allowed helium to be produced from natural gas, where the natural gas fields contain sufficient helium to make this economic. Cryogenics also developed the liquefaction of natural gas, hydrogen and helium.
The major industrial gases can be produced in bulk and delivered to customers by pipeline, but can also be packaged and transported. Most gases can be sold in gas cylinders and some sold as liquid in appropriate containers or as bulk liquid delivered by truck; exceptionally carbon dioxide can be produced as the cold solid dry ice. Alternatively, an industrial gas company may supply the plant and equipment to produce the gas rather than the gas itself. An example is a chemical oxygen generator. An industrial gas company may also offer to act as plant operator under an operations and maintenance contract for gases facility for a customer, since it usually has the experience of running such facilities for the production or handling of gases for itself.
Industrial gases are used in many industries including oil and gas , petrochemicals, chemical Industry, power , mining, steelmaking, metals, environmental protection, medicine, pharmaceuticals, biotechnology, fertilizers, nuclear power, electronics and aerospace.
Molecular gases 
The only chemical elements which are stable two atom homonuclear molecules at standard temperature and pressure (STP), are hydrogen (H2), nitrogen (N2) and oxygen (O2), plus the halogens fluorine (F2) and chlorine (Cl2). These gases, when grouped together with the monatomic noble gases, such as argon, in the industry are called "elemental gases" or "molecular gases" to distinguish them from molecules that are also chemical compounds.
Common industrial gases 
This list shows the most common gases sold by industrial gas companies. Many gas mixtures are also sold.
Important liquefied gases 
This list shows the most important liquefied gases.
- Bulk transport (truck, rail, ship)
- Cylinders (oxy-fuel welding and cutting, breathing, soda fountain)
Companies in the industrial gas business