Gas to liquids (GTL) is a refinery process to convert natural gas or other gaseous hydrocarbons into longer-chain hydrocarbons, such as gasoline or diesel fuel. Methane-rich gases are converted into liquid synthetic fuels. Two general strategies exist: (i) direct partial combustion of methane to methanol and (ii) Fischer–Tropsch-like processes that convert carbon monoxide and hydrogen into hydrocarbons. Strategy ii is followed by diverse methods to convert the hydrogen-carbon monoxide mixtures to liquids. Direct partial combustion has been demonstrated in nature but not replicated commercially. Technologies reliant on partial combustion have been commercialized mainly in regions where natural gas is inexpensive.
The motivation for GTL is to produce liquid fuels, which are more readily transported than methane. Methane must be cooled below its critical temperature of -82.3 °C in order to be liquified under pressure. Because of the associated cryogenic apparatus, LNG tankers are used for transport. Methanol is a conveniently handled combustible liquid, but its energy density is half of that of gasoline.
Fischer–Tropsch process
A GtL process may be established via the Fischer–Tropsch process which comprises several chemical reactions that convert a mixture of carbon monoxide (CO) and hydrogen (H2) into long chained hydrocarbons. These hydrocarbons are typically liquid or semi-liquid and ideally have the formula (CnH2n+2).
In order to obtain the mixture of CO and H2 required for the Fischer–Tropsch process, methane (main component of natural gas) may be subjected to partial oxidation which yields a raw synthesis gas mixture of mostly carbon dioxide, carbon monoxide, hydrogen gas (and sometimes water and nitrogen). The ratio of carbon monoxide to hydrogen in the raw synthesis gas mixture can be adjusted e.g. using the water gas shift reaction. Removing impurities, particularly nitrogen, carbon dioxide and water, from the raw synthesis gas mixture yields pure synthesis gas (syngas).
The pure syngas is routed into the Fischer–Tropsch process, where the syngas reacts over an iron or cobalt catalyst to produce synthetic hydrocarbons, including alcohols.
Methane to methanol process
Methanol is made from methane (natural gas) in a series of three reactions: Steam reforming CH4 + H2O → CO + 3 H2 ΔrH = +206 kJ mol−1 Water shift reaction CO + H2O → CO2 + H2 ΔrH = -41 kJ mol−1 Synthesis 2 H2 + CO → CH3OH ΔrH = -92 kJ mol−1
The methanol thus formed may be converted to gasoline by the Mobil process and methanol-to-olefins.
Methanol to gasoline (MTG) and methanol to olefins
In the early 1970s, Mobil developed an alternative procedure in which natural gas is converted to syngas, and then methanol. The methanol reacts in the presence of a zeolite catalyst to form alkanes. In terms of mechanism, methanol is partially dehydrated to give dimethyl ether: 2 CH3OH → CH3OCH3 + H2O The mixture of dimethyl ether and methanol is then further dehydrated over a zeolite catalyst such as ZSM-5, which in practice is polymerized and hydrogenated to give a gasoline with hydrocarbons of five or more carbon atoms making up 80% of the fuel by weight. The Mobil MTG process is practiced from coal-derived methanol in China by JAMG. A more modern implementation of MTG is the Topsøe improved gasoline synthesis (TiGAS).
Methanol can be converted to olefins using zeolite and SAPO-based heterogeneous catalysts. Depending on the catalyst pore size, this process can afford either C2 or C3 products, which are important monomers.
Syngas to gasoline plus process (STG+)
A third gas-to-liquids process builds on the MTG technology by converting natural gas-derived syngas into drop-in gasoline and jet fuel via a thermochemical single-loop process.
The STG+ process follows four principal steps in one continuous process loop. This process consists of four fixed bed reactors in series in which a syngas is converted to synthetic fuels. The steps for producing high-octane synthetic gasoline are as follows: