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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 non-catalytic processes that convert methane to methanol in one step—or via syngas as an intermediate, such as in the Fischer Tropsch, Mobil and syngas to gasoline plus processes.
The Fischer–Tropsch process starts with partial oxidation of methane (natural gas) to carbon dioxide, carbon monoxide, hydrogen gas and water. The ratio of carbon monoxide to hydrogen is adjusted using the water gas shift reaction, while the excess carbon dioxide is removed with aqueous solutions of alkanolamines (or physical solvents). Removing the water yields synthesis gas (syngas) which 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.
Methanol is made from methane (natural gas) in a series of three reactions:
This is then further dehydrated over a zeolite catalyst, ZSM-5, to give a gasoline with hydrocarbons of five or more carbon atoms making up 80% of the fuel by weight. ZSM-5 is deactivated by a build-up of carbon in a process known as "coking". 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:
Using gas-to-liquids processes, refineries can convert some of their gaseous waste products (flare gas) into valuable fuel oils, which can be sold as is or blended only with diesel fuel. The World Bank estimates that over 150 billion cubic metres (5.3×1012 cu ft) of natural gas are flared or vented annually, an amount worth approximately $30.6 billion, equivalent to 25% of the United States' gas consumption or 30% of the European Union's annual gas consumption, a resource that could be useful using GTL. Gas-to-liquids processes 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.
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.
Two companies, SASOL and Royal Dutch Shell, have technology proven to work on a commercial scale. PetroSA completed semi-commercial demonstrations of gas-to-liquids used by the company in 2011. Royal Dutch Shell produces a diesel from natural gas in a factory in Bintulu, Malaysia. Another Shell GTL facility is the Pearl GTL plant in Qatar, the world's largest GTL facility and there are reports that Shell is looking at the feasibility of a GTL facility in Louisiana, US. SASOL has recently built the Oryx GTL facility in Ras Laffan Industrial City, Qatar and together with Uzbekneftegaz and Petronas builds the Uzbekistan GTL plant. Chevron Corporation, in a joint venture with the Nigerian National Petroleum Corporation is commissioning the Escravos GTL in Nigeria, which uses Sasol technology.
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. The fuel 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 STG+ technology is currently operating at pre-commercial scale in Hillsborough, New Jersey at a plant owned by alternative fuels company Primus Green Energy. The plant produces approximately 100,000 gallons of high-quality, drop-in gasoline per year directly from natural gas. Further, the company announced the findings of an independent engineer’s report prepared by E3 Consulting, which found that STG+ system and catalyst performance exceeded expectations during plant operation. The pre-commercial demonstration plant has also achieved 720 hours of continuous operation.
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