‫الغاز إلى سوائل‬

‫هي عملية تكرير لتحويل الغاز الطبيعي أو الهيدروكربونات الغازية األخرى إلى ) ‪ ( GTL‬تحويل الغاز إلى سوائل‬
‫هيدروكربونات ذات سلسلة أطول‪ ،‬مثل البنزين أو وقود الديزل ‪ .‬يتم تحويل الغازات الغنية بالميثان إلى وقود صناعي سائل ‪.‬‬
‫توجد استراتيجيتان عامتان‪( :‬أ) االحتراق الجزئي المباشر للميثان إلى ميثانول و (ب) عمليات شبيهة بفيشر تروبش التي‬
‫تحول أول أكسيد الكربون والهيدروجين إلى هيدروكربونات‪ .‬تتبع االستراتيجية الثانية طرًق ا متنوعة لتحويل مخاليط‬
‫الهيدروجين وأول أكسيد الكربون إلى سوائل‪ .‬تم إثبات االحتراق الجزئي المباشر في الطبيعة ولكن لم يتم تكراره تجارًي ا‪ .‬تم‬
‫] ‪. [ 1 ] [ 2‬تسويق التقنيات التي تعتمد على االحتراق الجزئي بشكل أساسي في المناطق حيث يكون الغاز الطبيعي غير مكلف‬

‫‪.‬يتم استخدام ناقالت الغاز الطبيعي المسال لنقل غاز الميثان‬

‫إن الدافع وراء تحويل الغاز إلى سوائل هو إنتاج الوقود السائل‪ ،‬والذي يتم نقله بسهولة أكبر من الميثان‪ .‬يجب تبريد الميثان‬
‫إلى ما دون درجة حرارته الحرجة ‪ 82.3-‬درجة مئوية حتى يتم تسييله تحت الضغط‪ .‬وبسبب جهاز التبريد المصاحب‪،‬‬
‫ُت ستخدم ناقالت الغاز الطبيعي المسال للنقل‪ .‬الميثانول هو سائل قابل لالشتعال سهل التعامل معه‪ ،‬لكن كثافة طاقته تعادل‬
‫] ‪. [ 3‬نصف كثافة البنزين‬

‫عملية فيشر‪-‬تروبش‬

‫عملية تحويل الغاز إلى سوائل باستخدام طريقة فيشر تروبش‬

‫يمكن إنشاء عملية تحويل الغاز إلى سوائل من خالل عملية فيشر‪-‬تروبش التي تتألف من عدة تفاعالت كيميائية تعمل على‬
‫إلى هيدروكربونات طويلة السلسلة‪ .‬تكون هذه ) ‪ (H‬والهيدروجين )‪ (CO‬تحويل خليط من أول أكسيد الكربون‬
‫‪2‬‬
‫( الهيدروكربونات عادة سائلة أو شبه سائلة ولها الصيغة المثالية‬C n H n ).
2 +2

In order to obtain the mixture of CO and H2 required for the Fischer–Tropsch process, methane
(main component of natural gas) may be subjected to partial oxidation which yields a raw
synthesis gas mixture of mostly carbon dioxide, carbon monoxide, hydrogen gas (and
sometimes water and nitrogen).[4] The ratio of carbon monoxide to hydrogen in the raw synthesis
gas mixture can be adjusted e.g. using the water gas shift reaction. Removing impurities,
particularly nitrogen, carbon dioxide and water, from the raw synthesis gas mixture yields pure
synthesis gas (syngas).

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 ΔrH = +206 kJ mol−1
Water shift reaction
CO + H2O → CO2 + H2 ΔrH = -41 kJ mol−1
Synthesis
2 H2 + CO → CH3OH ΔrH = -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, Mobil developed an alternative procedure in which natural gas is converted to
syngas, and then methanol. The methanol reacts in the presence of a zeolite catalyst to form
alkanes. In terms of mechanism, methanol is partially dehydrated to give dimethyl ether:

2 CH3OH → CH3OCH3 + H2O

The mixture of dimethyl ether and methanol is then further dehydrated over a zeolite catalyst
such as ZSM-5, 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. The Mobil MTG
process is practiced from coal-derived methanol in China by JAMG. A more modern
implementation of MTG is the Topsøe improved gasoline synthesis (TiGAS).[5]

Methanol can be converted to olefins using zeolite and SAPO-based heterogeneous catalysts.
Depending on the catalyst pore size, this process can afford either C2 or C3 products, which are
important monomers.[6][7]

Syngas to gasoline plus process (STG+)

The STG+ Process

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
methyl-coenzyme M reductases. These organisms effect reverse methanogenesis. Strenuous
efforts have been made to elucidate the mechanisms of these methane-converting enzymes,
which would enable their catalysis to be replicated in vitro.[10]

Biodiesel can be made from CO2 using the microbes Moorella thermoacetica and Yarrowia
lipolytica. This process is known as biological gas-to-liquids.[11]

Commercial uses

INFRA M100 GTL Plant

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,[12] 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.

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.[13][14] 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.[15][16][17] Chevron Corporation, in a
joint venture with the Nigerian National Petroleum Corporation is commissioning the Escravos
GTL in Nigeria, which uses Sasol technology. PetroSA, South Africa's national oil company, owns
and operates a 22,000 barrels/day (capacity) GTL plant in Mossel Bay, using Sasol GTL
technology.[18]

Aspirational and emerging ventures

New generation of GTL technology is being pursued for the conversion of unconventional,
remote and problem gas into valuable liquid fuels.[19][20] GTL plants based on innovative Fischer–
Tropsch catalysts have been built by INFRA Technology. Other mainly U.S. companies include
Velocys, ENVIA Energy, Waste Management, NRG Energy, ThyssenKrupp Industrial Solutions,
Liberty GTL, Petrobras,[21] Greenway Innovative Energy,[22] Primus Green Energy,[23] Compact
GTL,[24] and Petronas.[25] Several of these processes have proven themselves with demonstration
flights using their jet fuels.[26][27]

Another proposed solution to stranded gas involves use of novel FPSO for offshore conversion
of gas to liquids such as methanol, diesel, petrol, synthetic crude, and naphtha.[28]

Economics of GTL

GTL using natural gas is more economical when there is wide gap between the prevailing natural
gas price and crude oil price on a Barrel of oil equivalent (BOE) basis. A coefficient of 0.1724
results in full oil parity.[29] GTL is a mechanism to bring down the diesel/gasoline/crude oil
international prices at par with the natural gas price in an expanding global natural gas
production at cheaper than crude oil price. When natural gas is converted in to GTL, the liquid
products are easier to export at cheaper price rather than converting in to LNG and further
conversion to liquid products in an importing country.[30][31]

However, GTL fuels are much more expensive to produce than conventional fuels.[32]

See also

Biomass to liquid
 Carbon-neutral fuel

Coal to liquid

Bibliography

Boogaard, P. J., Carrillo, J. C., Roberts, L. G., & Whale, G. F. (2017) Toxicological and
ecotoxicological properties of gas-to-liquid (GTL) products (https://www.tandfonline.com/doi/
pdf/10.1080/10408444.2016.1214676) . 1. Mammalian toxicology. Critical reviews in
toxicology, 47(2), 121-144.

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32. 2‫ب‬.‫ ص‬،2009 ‫ أكتوبر‬14 ‫ األربعاء‬،‫ وول ستريت جورنال‬،‫الخطوط الجوية القطرية تطير بطائرة تعمل بالوقود الجديد‬