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GTL Gas to LiquidBy Kerry Pritchard2011

Gas to Liquid.

What is GTL ?Gas-to-Liquid fuels are fuels that can be produced from naturalgas, coal and biomass using a Fischer-Tropsch chemical reactionprocess. The liquids produced include naphtha, diesel, andchemical feedstock's. The resulting GTL diesel can be used neator blended with today's diesel fuel and used in existing dieselengines and infrastructure. These fuels provide an opportunityto reduce dependence on petroleum-based fuels and reducetailpipe emissions

GTL Gas to LiquidNatural gas can be use to produce bulkpetrochemicals, including methanol andammonia, but these are relatively small users ofthe gas reserves with limited markets. Liquid andother petroleum products are cheaper totransport, market, distribute to large markets.These can be moved in existing pipelines orproducts tankers and even blended with existingcrude oil or product streams. Further, no specialcontractual arrangements are required for theirsale with many suitable domestic and foreignmarkets.

GTL Gas to LiquidNew technology is being developed and applied to convertnatural gas to liquids in gas to liquids technology (GTL). Theprojects are scalable, allowing design optimisation andapplication to smaller gas deposits. The key influences ontheir competitiveness are the cost of capital, operating costsof the plant, feedstock costs, scale and ability to achievehigh utilisation rates in production. As a generalisationhowever, GTL is not competitive against conventional oilproduction unless the gas has a low opportunity value and isnot readily transported.

GTL Gas to LiquidGTL not only adds value, but capable of producing products that could besold or blended into refinery stock as superior products with lesspollutants for which there is growing demand. Reflecting its origins as agas, gas to liquids processes produces diesel fuel with an energy densitycomparable to conventional diesel, but with a higher cetane numberpermitting a superior performance engine design. The Cetane Numberindicates how quickly the fuel will auto-ignite, and how evenly it willcombust. Most countries require a minimum cetane number of around45 to 50: A higher cetane number represents a lower flame temperature,providing a reduction in the formation of oxides of nitrogen (NOx) thatcontributes to urban smog and ground level ozone. Fischer-Tropschdiesel has a cetane number in excess of 70. Naphtha produced is sulfurfree and contains a high proportion of paraffinic material suitable ascracker feedstock or the manufacture of solvents.

GTL Gas to LiquidAnother “problem” emission associated with diesel fuel isparticulate matter, which is composed of unburnt carbonand aromatics, and compounds of sulfur. Fine particulatesare associated with respiratory problems, while certaincomplex aromatics have been found to be carcinogenic. Lowsulfur content, leads to significant reductions in particulatematter that is generated during combustion, and the lowaromatic content reduces the toxicity of the particulatematter reflecting in a worldwide trend towards thereduction of sulfur and aromatics in fuel.

TechnologyIt is technically feasible to synthesise almost anyhydrocarbon from any other; and in the past five decadesseveral processes have been developed to synthesise liquidhydrocarbons from natural gas.There are two broad technologies for gas to liquid (GTL) toproduce a synthetic petroleum product, (syncrude): a directconversion from gas, and an indirect conversion via synthesisgas (syngas)

TechnologySynthesis gas is produced by reacting methane (or carbon) withsteam at elevated temperatures to yield a useful mixture of carbonoxides and hydrogen. It can be produced by a variety of processesand feedstock's. It may require the indicated compositionaladjustment and treatment before use in the following majorapplications:Directly used for methanol synthesis. The dried syngas can be usedwithout further adjustment since there is a net conversion of bothCO and CO2 to methanol.Ammonia synthesis gas, requiring maximum hydrogen productionand removal of oxygen-bearing compounds.

TechnologyOxo synthesis gas, requiring composition adjustment and CO2removal to give a 1:1 H2:CO synthesis gas.Industrial gases, as a source of high purity CO, CO2 or H2,Reducing gas, a mixture of CO and H2 requiring CO2 removalbefore being used to reduce oxides in ores to base metals.Fuels either as a substitute fuel gas from a liquid or solidfeedstock, or as an intermediate for Fischer-Tropsch or zeolitebased alternative liquid fuel technologies.

TechnologyThe direct conversion of methane, (typically 85 to 90 percent of natural gas), eliminates the cost of producingsynthesis gas but involves a high activation energy and isdifficult to control. Several direct conversion processes havebeen developed but none have been commercialized beingeconomically unattractive.

Fischer-TropschThe discovery of F-T chemistry in Germany dates back to the1920s and its development has been for strategic rather thaneconomic reasons, as in Germany during World War II and inSouth Africa during the apartheid era. Mobil developed the"M-gasoline" process to make gasoline from methanolimplemented in 1985 in a large integrated methanol-togasoline plant in New Zealand. The New Zealand plant was atechnical success but produced gasoline at costs above 30per barrel and required large subsidies from the NewZealand government.

SyngasThe syngas step converts the natural gas to hydrogen andcarbon monoxide by partial oxidation, steam reforming or acombination of the two processes. The key variable is thehydrogen to carbon monoxide ratio with a 2:1 ratiorecommended for F-T synthesis. Steam reforming is carriedout in a fired heater with catalyst-filled tubes that producesa syngas with at least a 5:1 hydrogen to carbon monoxideratio. To adjust the ratio, hydrogen can be removed by amembrane or pressure swing adsorption system. Helpingeconomics is if the surplus hydrogen is used in a petroleumrefinery or for the manufacture of ammonia in an adjoiningplant.

TechnologyThe partial oxidation route provides the desired 2:1 ratio andis the preferred route in isolation of other needs.There aretwo routes: one uses oxygen and produces a purer syngaswithout nitrogen; the other uses air creating a more dilutesyngas. However, the oxygen route requires an air separationplant that increases the cost of the investment.The steam reforming process produces a syngas of H2:COratio of about 3:1 with the surplus H2 that can be separatedby a hollow fiber membrane process. Evaluations suggest thepartial oxidation would be the preferred route when thesurplus H2 from the steam reforming process has to bedisposed of at fuel value. Under these conditions, the productvalue of syngas by partial oxidation is lower than steamreforming. The partial oxidation process is also slightly lesscapital intensive.

ConversionConversion of the syngas to liquid hydrocarbon is a chaingrowth reaction of carbon monoxide and hydrogen on thesurface of a heterogeneous catalyst. The catalyst is eitheriron- or cobalt-based and the reaction is highly exothermic.The temperature, pressure and catalyst determine whethera light or heavy syncrude is produced. For example at 330Cmostly gasoline and olefins are produced whereas at 180 to250C mostly diesel and waxes are produced.

ConversionThere are mainly two types of F-T reactors. The vertical fixedtube type has the catalyst in tubes that are cooled externallyby pressurized boiling water. For a large plant, severalreactors in parallel may be used presenting energy savings.The other process is uses a slurry reactor in which preheated synthesis gas is fed to the bottom of the reactor anddistributed into the slurry consisting of liquid wax andcatalyst particles. As the gas bubbles upwards through theslurry, it is diffused and converted into more wax by the F-Treaction. The heat generated is removed through thereactor's cooling coils where steam is generated for use inthe process.

Commercial examplesSasol is a synfuel technology supplier established to providepetroleum products in coal-rich but oil-poor South Africa.The firm has built a series of Fischer-Tropsch coal-to-oilplants, and is one of the world's most experienced syntheticfuels organization and now marketing a natural-gas-to-oiltechnology. It has developed the world's largest syntheticfuel project, the Mossgas complex at Mossel Bay in SouthAfrica that was commissioned in 1993 and produces a smallvolume of 25 000 barrels per day. To increase the proportionof higher molecular weight hydrocarbons, Sasol hasmodified its Arge reactor to operate at higher pressures.

Commercial examplesSasol has commercialized four reactor types with the slurryphase distillate process being the most recent. Its productsare more olefinic than those from the fixed bed reactors andare hydrogenated to straight chain paraffin's. Its Slurry PhaseDistillate converts natural gas into liquid fuels, most notablysuperior-quality diesel using technology developed from theconventional Arge tubular fixed-bed reactor technology. Theresultant diesel is suitable as a premium blendingcomponent for standard diesel grades from conventionalcrude oil refineries. Blended with lower grade diesels itassists to comply with the increasingly stringentspecifications being set for transport fuels in North Americaand Europe.The other technology uses the Sasol Advanced Synthol (SAS)reactor to produce mainly light olefins and gasolinefractions. Sasol has developed high performance cobaltbased and iron based catalysts for these processes.

Commercial examplesThe company claims a single module or the Sasol SlurryPhase Distillate plant, that converts 100 MMscfd(110 terajoules per day of gas) of natural gas into 10 000barrels a day of liquid transport fuels, that can be built at acapital cost of about US 250 million. This cost equates to acost per daily barrel of capacity of about US 25 000including utilities, off-site facilities and infrastructure units.If priced at US 0.50/MMBtu, the gas amounts to a feedstockcost of US 5 per barrel of product. The fixed and variableoperating costs (including labour, maintenance and catalyst)are estimated at a further US 5 per barrel of product,thereby resulting in a direct cash cost of production of aboutUS 10 a barrel (excluding depreciation). These costs shouldhowever be compared with independent assessments.

Commercial examplesIn June 1999, Chevron and Sasol agreed to an alliance tocreate ventures using Sasol's GTL technology. The twocompanies have conducted a feasibility study to build a GTLplant in Nigeria that would begin operating in 2003. Sasolreportedly also has been in discussions with Norway'sStatoil, but no definitive announcements have been made.

Gasoline productionThere are two methanol-based routes to gasoline. Mobil'smethanol-to-gasoline (MTG) process based on the ZSM-5zeolite catalyst was commercialized in 1985 in a plant nowowned by Methanex in New Zealand. Commercialapplications of the MTG process are now anticipated to usea fluid bed reactor with their higher efficiency and lowercapital cost.

Useful .orghttp://www.spe.org/elibinfo/eLibrary www.fischertropsch.org/primary documents/patents/GB/gb309002.pdf

By Kerry Pritchard 2011. Gas to Liquid. Gas-to-Liquid fuels are fuels that can be produced from natural gas, coal and biomass using a Fischer-Tropschchemical reaction process. The liquids produced include naphtha, diesel, and chemic