Gas to liquids

A GtL process may be established via the Fischer–Tropsch process which comprises several chemical reactions that convert a mixture of carbon monoxide (CO) and hydrogen (H₂) into long chained hydrocarbons. These hydrocarbons are typically liquid or semi-liquid and ideally have the formula (C_nH_2n+2).

In order to obtain the mixture of CO and H₂ required for the Fischer–Tropsch process,

The pure syngas is routed into the Fischer–Tropsch process, where the syngas reacts over an iron or cobalt catalyst to produce synthetic hydrocarbons, including alcohols.

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 and methanol-to-olefins.

Methanol to gasoline (MTG) and methanol to olefins

In the early 1970s,

A third gas-to-liquids process builds on the MTG technology by converting natural gas-derived syngas into drop-in gasoline and jet fuel via a thermochemical single-loop process.^[8]

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:^[9]

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.

Biological gas-to-liquids (Bio-GTL)

With methane as the predominant target for GTL, much attention has focused on the three enzymes that process methane. These enzymes support the existence of methanotrophs, microorganisms that metabolize methane as their only source of carbon and energy. Aerobic methanotrophs harbor enzymes that oxygenate methane to methanol. The relevant enzymes are methane monooxygenases, which are found both in soluble and particulate (i.e. membrane-bound) varieties. They catalyze the oxygenation according to the following stoichiometry:

CH₄ + O₂ + NADPH + H⁺ → CH₃OH + H₂O + NAD⁺

Anaerobic methanotrophs rely on the bioconversion of methane using the enzymes called