Gas converts to liquid via a multi-step process that starts with isolating pure methane from natural gas, reacting the methane with oxygen at high temperature, and passing the resulting gas over a catalyst, forming liquid hydrocarbons. This basic set of steps is called the Fischer-Tropsch process, in honor of the two scientists who discovered it in the 1920s.
The first step in the process is isolating methane. When energy companies extract natural gas from within the Earth, it is not a pure product. Natural gas contains sulfur, water and other impurities that interfere with the liquefaction process, but these impurities have other uses. Waste sulfur goes into making fertilizers, water gets reclaimed and facilities burn other impurities for power generation.
After being mixed with pure oxygen, the purified methane reacts in a gasification chamber at a high temperature. This process produces synthetic gas, which is known in the industry as syngas.
The syngas then enters one or more reaction vessels. These vessels contain a catalyst, traditionally iron or cobalt, but many energy companies use proprietary synthetic catalysts. In the reaction vessels, the syngas converts to liquid hydrocarbons.
After going through the Fischer-Tropsch process, the resulting hydrocarbons are either mixed into crude oil or cracked. Cracking is a chemical process that converts the long-chain hydrocarbons generated in the process into shorter, more useful chemicals.