Gas to liquids
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Gas to liquids 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 fuels either via direct conversion or via syngas as an intermediate, for example using the Fischer Tropsch or Mobil processes.
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[edit] Fischer-Tropsch process
This method starts with partial oxidation of methane (a component in natural gas) to carbon dioxide, carbon monoxide, hydrogen and water, the carbon monoxide to hydrogen (H2) ratio is adjusted using the water gas shift reaction) and the excess carbon dioxide removal by aqueous solutions of alkanolamine (or physical solvents) the water is removed yielding synthesis gas (syngas) that is chemically reacted over an iron or cobalt catalyst to produce liquid hydrocarbons and other byproducts.
[edit] Mobil process
An alternative path starts by conversion of the natural gas to syngas, conversion of the syngas to methanol which is subsequently polymerized into alkanes over a zeolite catalyst. It was developed by Mobil in early 1970s.
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 = +206 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. First methanol is dehydrated to give dimethyl ether:
- 2 CH3OH → CH3OCH3 + H2O
This is then further dehydrated over a zeolite catalyst, ZSM-5, to give a gasoline with 80% (by weight based on the organics in the product stream) C5+ hydrocarbon products.
ZSM-5 is deactivated by a carbon build-up ("coking") over time in converting methanol to gasoline. The catalyst can be re-activated by burning off the coke in a stream of hot (500 °C (930 °F)) air; however, the number of re-activation cycles is limited.
[edit] Commercial uses
Using Gas-to-liquids processes, refineries can convert some of their gaseous waste products into valuable fuel oils, which can be sold as or blended only with diesel fuel. It may also be used for the economic extraction of gas deposits in locations where it is not economical to build a pipeline. This process will be increasingly significant as crude oil resources are depleted. Shell produces a diesel from natural gas in a factory in Bintulu, Malaysia. The world's first commercial passenger flight powered by fuel made from natural gas landed in Qatar on monday 12th Ocotber 2009. The historic journey from London Gatwick to Doha took around six hours and was operated by Qatar Airways with an Airbus A340-600 aircraft using Rolls-Royce Trent 556 engines.
Shell developed and produced the 50-50 blend of synthetic Gas to Liquids (GTL) kerosene and conventional oil-based kerosene fuel. This new fuel, to be known as GTL Jet Fuel, burns with lower sulphur dioxide and particulate emissions than pure conventional oil-based kerosene, making it attractive for improving local air quality at busy airports.[1]
