Gas to liquids


Gas to liquids

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 or via syngas as an intermediate, for example using the Fischer Tropsch or Mobil processes.

Contents

Fischer–Tropsch process

GTL process using the Fischer Tropsch method

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) 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.

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:

  1. Steam reforming: CH4 + H2O → CO + 3 H2 ΔrH = +206 kJ mol-1
  2. Water shift reaction CO + H2O → CO2 + H2 ΔrH = -41 kJ mol-1
  3. 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.

Commercial uses

Two companies, SASOL and Shell, have technology proven to work on a commercial scale. PetroSA (Pty) Ltd, in South Africa, completed semi-commercial demostrations of gas-to-liquids used by the company in 2011[1]. 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. 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. 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.[2][3] SASOL has recently built 34kb/d Oryx GTL facility in Ras Laffan Industrial City, Qatar.

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.[4]

One other proposed solution is to use a novel FPSO for offshore conversion of gas to liquids (methanol, diesel, petrol, syn-crude, and naphtha).[5]

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.[6][7][8]

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,[9] a resource that could be useful using GTL.

References

  1. ^ http://www.businessday.co.za/articles/Content.aspx?id=142267
  2. ^ "Pearl GTL - an overview". http://www.shell.com/home/content/aboutshell/our_strategy/major_projects_2/pearl/. 
  3. ^ "Pearl Gas-to-Liquids Plant, Ras Laffan, Qatar". http://www.chemicals-technology.com/projects/pearl-gtl/. Retrieved 2009-06-22. 
  4. ^ uiui "Qatar Airways Makes GTL History". Downstream Today. 2009-10-15. http://www.downstreamtoday.com/news/article.aspx?a_id=18626. Retrieved 2009-10-17. 
  5. ^ http://www.bpp-tech.com/index.php?option=com_content&view=article&id=60&Itemid=59
  6. ^ Fairley, Peter. Turning Gas Flares into Fuel MIT Technology Review, 15 March 2010. Retrieved: 17 March 2010.
  7. ^ Think small for associated Gas EngineerLive.com, undated. Retrieved: 17 March 2010.
  8. ^ Petrobras pilot plant CompactGTL, undated. Retrieved: 17 March 2010.
  9. ^ World Bank, GGFR Partners Unlock Value of Wasted Gas", World Bank 14 December 2009. Retrieved 17 March 2010.

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