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—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.

Fischer–Tropsch process

Main article: Fischer–Tropsch process

GTL process using the Fischer Tropsch method

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.

Methane to methanol process

Methanol is made from methane (natural gas) in a series of three reactions:

Steam reforming

CH₄ + H₂O → CO + 3 H₂   Δ_rH = +206 kJ mol⁻¹

Water shift reaction

CO + H₂O → CO₂ + H₂   Δ_rH = -41 kJ mol⁻¹

Synthesis

2 H₂ + CO → CH₃OH   Δ_rH = -92 kJ mol⁻¹

The methanol thus formed may be converted to gasoline by the Mobil process.

Methanol to gasoline process (MTG)

In the early 1970s, Mobil developed an alternative procedure in which natural gas is converted to syngas, and then methanol. The methanol polymerized over a zeolite catalyst to form alkanes.

First methanol is dehydrated to give dimethyl ether:

2 CH₃OH → CH₃OCH₃ + H₂O

This is then further dehydrated over a zeolite catalyst such as ZSM-5, which would theoretically yield ethylene:

CH₃OCH₃ → C₂H₄ + H₂O

but 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.

Syngas to gasoline plus process (STG+)

Main article: Syngas to gasoline plus

The STG+ Process

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:

1. Methanol Synthesis: Syngas is fed to Reactor 1, the first of four reactors, which converts most of the syngas (CO and H2) to methanol (CH3OH) when passing through the catalyst bed.
2. Dimethyl Ether (DME) Synthesis: The methanol-rich gas from Reactor 1 is next fed to Reactor 2, the second STG+ reactor. The methanol is exposed to a catalyst and much of it is converted to DME, which involves a dehydration from methanol to form DME (CH3OCH3).
3. Gasoline synthesis: The Reactor 2 product gas is next fed to Reactor 3, the third reactor containing the catalyst for conversion of DME to hydrocarbons including paraffins (alkanes), aromatics, naphthenes (cycloalkanes) and small amounts of olefins (alkenes), mostly from C6 (number of carbon atoms in the hydrocarbon molecule) to C10.
4. Gasoline Treatment: The fourth reactor provides transalkylation and hydrogenation treatment to the products coming from Reactor 3. The treatment reduces durene (tetramethylbenzene)/isodurene and trimethylbenzene components that have high freezing points and must be minimized in gasoline. As a result, the synthetic gasoline product has high octane and desirable viscometric properties.
5. Separator: Finally, the mixture from Reactor 4 is condensed to obtain gasoline. The non-condensed gas and gasoline are separated in a conventional condenser/separator. Most of the non-condensed gas from the product separator becomes recycled gas and is sent back to the feed stream to Reactor 1, leaving the synthetic gasoline product composed of paraffins, aromatics and naphthenes.

Commercial uses

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 (5.3×10^¹² 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, such as those developed by Velocys, 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. Construction is underway for one such plant: ENVIA Energy's Oklahoma City GTL plant being built adjacent to Waste Management's East Oak landfill site. The project is being financed by a joint venture between Waste Management, NRG Energy, Ventech and Velocys. The feedstock for this plant will be a combination of landfill gas and pipeline natural gas.

On August 1, 2014, Biofuels Power Corporation (BFLS)signed a letter of intent with ThyssenKrupp Industrial Solutions and Liberty GTL, Inc. to build a small scale gas-to-liquid demonstration facility in Houston,Texas. The parties have established a non-binding target date to complete installation and commissioning of the GTL Pilot Plant on or before December 31, 2014. The purpose of the GTL Pilot Plant is to commercially demonstrate converting stranded natural gas resources to synthetic crude oil. BFLS will operate the GTL Pilot Plant for the 2-year demonstration. ThyssenKrupp will provide technical services and contribute a previously operating auto-thermal reformer pilot plant of proven design (“ATR”), which will be used to generate synthesis gas feedstock for the production of synthetic crude oil. Liberty will provide intellectual property and operating know-how regarding crude oil synthesis along with the relevant catalyst supply. The Liberty technical team is also credited for designing the FT (Fischer Tropsch) Reactor which will convert the synthetic gas to synthetic crude oil. The GTL Pilot Plant will be assembled at the Houston Clean Energy Park, which is an industrial estate owned by BFLS. The Houston site is located between the Eagle Ford Natural Gas Field and numerous refineries.

One other proposed solution is to use a novel FPSO for offshore conversion of gas to liquids such as methanol, diesel, petrol, synthetic crude, and naphtha.

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 has also completed its assessment of 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.

See also

• Biomass to liquid
• Carbon-neutral fuel
• Coal to liquid

External links

• Airbus conducts A380 alternative-fuel demonstration flight
• Dutch Government site on Royal TESO GTL project
• University of California San Diego webpage
• The production of methanol and gasoline