From Wikipedia, the free encyclopedia - View original article
|This article needs additional citations for verification. (December 2008)|
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 either via direct conversion using the new GasTechno® non-catalytic gas-to-liquids process that converts methane to methanol in one step. Or via syngas as an intermediate, for example using the Fischer Tropsch or Mobil processes.
This method starts with partial oxidation of methane (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, while the excess carbon dioxide is removed by aqueous solutions of alkanolamine (or physical solvents); finally 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. Oxygen is provided from a cryogenic air separation unit.
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:
The methanol thus formed may be converted to gasoline by the Mobil process. First methanol is dehydrated to give dimethyl ether:
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.
A third gas-to-liquids process builds on the MTG technology by converting natural gas-derived syngas directly 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:
Alternative fuel company Primus Green Energy developed the STG+ technology and is currently the only company utilizing this process. Bechtel Hydrocarbon Technology Solutions, Inc., the technology licensing and process consulting arm of Bechtel Corp., is leading development of designs for a commercial plant that will utilize the STG+ process.
At the other end of the scale from e.g. Shell's Pearl GTL facility in Qatar, the use of microchannel reactors shows promise for the conversion of unconventional, remote and problem gas into valuable liquid fuels. GTL plants based on microchannel reactors are significantly smaller than those using conventional fixed bed or slurry bed reactors, enabling modular plants that can be deployed cost effectively in remote locations and on smaller fields than is possible with competing systems.
On 1 February 2008, an Airbus A380 flew a three-hour test flight between Britain and France, with one of the A380's four Rolls-Royce Trent 900 engines using a mix of 60% standard jet kerosene and 40% gas to liquids fuel supplied by Shell. The aircraft engine needed no modification to use the GTL fuel, which was designed to be mixed with normal jet fuel. Sebastien Remy, head of Airbus SAS's alternative fuel programme, said the GTL used was no cleaner in CO2 terms than standard fuel but it had local air quality benefits because the GTL portion contains no sulphur. On 12 October 2009, a Qatar Airways Airbus A340-600 conducted the world's first commercial passenger flight using a mixture of kerosene and synthetic GTL fuel in its flight from London's Gatwick Airport to Doha.
Brazilian oil company Petrobras has ordered two small experimental GTL production facilities intended to be posted at offshore oil fields too distant or deep to justify gas pipelines to onshore GTL plant. In January 2012 Petrobras' Cenpes Research and Development Centre approved for commercial deployment the technology supplied by UK-based gas-to-liquids company CompactGTL. Petrobras is now assessing microchannel reactor technology supplied by Velocys.
The World Bank estimates that over 150 billion cubic metres of natural gas are flared or vented annually, an amount worth approximately 30.6 billion dollars, equivalent to 25 percent of the United States’ gas consumption or 30% of the European Union’s gas consumption per year, a resource that could be useful using GTL.