TheFischer–Tropsch process (FT) is a collection ofchemical reactions that converts a mixture ofcarbon monoxide andhydrogen, known assyngas, into liquidhydrocarbons. These reactions occur in the presence of metalcatalysts, typically at temperatures of 150–300 °C (302–572 °F) and pressures of one to several tens of atmospheres. The Fischer–Tropsch process is an important reaction in bothcoal liquefaction andgas to liquids technology for producing liquid hydrocarbons.[1]
In the usual implementation, carbon monoxide and hydrogen, the feedstocks for FT, are produced fromcoal,natural gas, orbiomass in a process known asgasification. The process then converts these gases intosynthetic lubrication oil andsynthetic fuel.[2] This process has received intermittent attention as a source of low-sulfur diesel fuel and to address the supply or cost of petroleum-derived hydrocarbons. Fischer–Tropsch process is discussed as a step of producing carbon-neutral liquid hydrocarbon fuels from CO2 and hydrogen.[3][4][5]
The process was first developed byFranz Fischer andHans Tropsch at theKaiser Wilhelm Institute for Coal Research inMülheim an der Ruhr, Germany, in 1925.[6]
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The Fischer–Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbons, ideally having the formula (CnH2n+2). The more useful reactions producealkanes as follows:[7]
wheren is typically 10–20. The formation of methane (n = 1) is unwanted. Most of the alkanes produced tend to be straight-chain, suitable asdiesel fuel. In addition to alkane formation, competing reactions give small amounts ofalkenes, as well asalcohols and other oxygenated hydrocarbons.[8]
The reaction is a highlyexothermic reaction due to a standardreaction enthalpy (ΔH) of −165 kJ/mol CO combined.[9]
Converting a mixture of H2 and CO intoaliphatic products is a multi-step reaction with several intermediate compounds. The growth of the hydrocarbon chain may be visualized as involving a repeated sequence in which hydrogen atoms are added to carbon and oxygen, the C–O bond is split and a new C–C bond is formed.For one –CH2– group produced by CO + 2 H2 → (CH2) + H2O, several reactions are necessary:
The conversion of CO to alkanes involveshydrogenation of CO, thehydrogenolysis (cleavage with H2) of C–O bonds, and the formation of C–C bonds. Such reactions are assumed to proceed via initial formation of surface-boundmetal carbonyls. The COligand is speculated to undergo dissociation, possibly into oxide andcarbide ligands.[10] Other potential intermediates are various C1 fragments includingformyl (CHO), hydroxycarbene (HCOH), hydroxymethyl (CH2OH),methyl (CH3), methylene (CH2),methylidyne (CH), and hydroxymethylidyne (COH). Furthermore, and critical to the production of liquid fuels, are reactions that form C–C bonds, such asmigratory insertion. Many related stoichiometric reactions have been simulated on discretemetal clusters, but homogeneous Fischer–Tropsch catalysts are of no commercial importance.
Addition of isotopically labelled alcohol to the feed stream results in incorporation of alcohols into product. This observation establishes the facility of C–O bond scission. Using14C-labelledethylene andpropene over cobalt catalysts results in incorporation of these olefins into the growing chain. Chain growth reaction thus appears to involve both 'olefin insertion' as well as 'CO-insertion'.[11]
Fischer–Tropsch plants associated withbiomass or coal or related solid feedstocks (sources of carbon) must first convert the solid fuel into gases. These gases include CO, H2, and alkanes. This conversion is calledgasification.[12]Synthesis gas ("syngas") is obtained from biomass/coal gasification is a mixture of hydrogen and carbon monoxide. The H2:CO ratio is adjusted using thewater-gas shift reaction. Coal-based FT plants produce varying amounts of CO2, depending upon the energy source of the gasification process. However, most coal-based plants rely on the feed coal to supply all the energy requirements of the process.
Carbon monoxide for FT catalysis is derived from hydrocarbons. Ingas to liquids (GTL) technology, the hydrocarbons are low molecular weight materials that often would be discarded or flared. Stranded gas provides relatively cheap gas. For GTL to be commercially viable, gas must remain relatively cheaper than oil.
Several reactions are required to obtain the gaseous reactants required for FTcatalysis. First, reactant gases entering a reactor must bedesulfurized. Otherwise, sulfur-containing impurities deactivate ("poison") thecatalysts required for FT reactions.[8][7]
Several reactions are employed to adjust the H2:CO ratio. Most important is thewater-gas shift reaction, which provides a source ofhydrogen at the expense of carbon monoxide:[8]
For FT plants that usemethane as thefeedstock, another important reaction isdry reforming, which converts the methane into CO and H2:
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Generally, the Fischer–Tropsch process is operated in the temperature range of 150–300 °C (302–572 °F). Higher temperatures lead to faster reactions and higher conversion rates but also tend to favor methane production. For this reason, the temperature is usually maintained at the low to middle part of the range. Increasing the pressure leads to higher conversion rates and also favors the formation of long-chainedalkanes, both of which are desirable. Typical pressures range from one to several tens of atmospheres. Even higher pressures would be favorable, but the benefits may not justify the additional costs of high-pressure equipment, and higher pressures can lead to catalyst deactivation viacoke formation.
A variety of synthesis-gas compositions can be used. For cobalt-based catalysts the optimal H2:CO ratio is around 1.8–2.1.Iron-based catalysts can tolerate lower ratios, due to their intrinsicwater-gas shift reaction activity. This reactivity can be important for synthesis gas derived from coal or biomass, which tend to have relatively low H2:CO ratios (< 1).
Efficient removal of heat from the reactor is the basic need of FT reactors since these reactions are characterized by high exothermicity. Four types of reactors are discussed:
In general the product distribution of hydrocarbons formed during the Fischer–Tropsch process follows anAnderson–Schulz–Flory distribution,[14] which can be expressed as:
whereWn is the weight fraction of hydrocarbons containingn carbon atoms, andα is the chain growth probability or the probability that a molecule will continue reacting to form a longer chain. In general, α is largely determined by the catalyst and the specific process conditions.
Examination of the above equation reveals that methane will always be the largest single product so long asα is less than 0.5; however, by increasingα close to one, the total amount of methane formed can be minimized compared to the sum of all of the various long-chained products. Increasingα increases the formation of long-chained hydrocarbons. The very long-chained hydrocarbons are waxes, which are solid at room temperature. Therefore, for production of liquid transportation fuels it may be necessary to crack some of the FT products. In order to avoid this, some researchers have proposed using zeolites or other catalyst substrates with fixed sized pores that can restrict the formation of hydrocarbons longer than some characteristic size (usuallyn < 10). This way they can drive the reaction so as to minimize methane formation without producing many long-chained hydrocarbons. Such efforts have had only limited success.
Four metals are active ascatalysts for the Fischer–Tropsch process: iron, cobalt, nickel, and ruthenium. Since FT process typically transforms inexpensive precursors into complex mixtures that require further refining, FT catalysts are based on inexpensive metals, especially iron and cobalt.[15][16] Nickel generates too much methane, so it is not used.[7]
Typically, suchheterogeneous catalysts are obtained through precipitation from iron nitrate solutions. Such solutions can be used to deposit the metal salt onto thecatalyst support (see below). Such treated materials transform into active catalysts by heating under CO, H2 or with the feedstock to be treated, i.e., the catalysts are generated in situ. Owing to the multistep nature of the FT process, analysis of the catalytically active species is challenging. Furthermore, as is known for iron catalysts, a number of phases may coexist and may participate in diverse steps in the reaction. Such phases include various oxides andcarbides as well aspolymorphs of the metals. Control of these constituents may be relevant to product distributions. Aside from iron and cobalt, nickel and ruthenium are active for converting the CO/H2 mixture to hydrocarbons.[11] Although expensive,ruthenium is the most active of the Fischer–Tropsch catalysts in the sense that It works at the lowest reaction temperatures and produces higher molecular weight hydrocarbons. Ruthenium catalysts consist of the metal, without any promoters, thus providing relatively simple system suitable for mechanistic analysis. Its high price preclude industrial applications. Cobalt catalysts are more active for FT synthesis when the feedstock is natural gas. Natural gas has a high hydrogen to carbon ratio, so the water-gas shift is not needed for cobalt catalysts. Cobalt-based catalysts are more sensitive than their iron counterparts.
Illustrative of real world catalyst selection, high-temperature Fischer–Tropsch (HTFT), which operates at 330–350 °C, uses an iron-based catalyst. This process was used extensively bySasol in theircoal-to-liquid plants (CTL). Low-temperature Fischer–Tropsch (LTFT) uses an iron- or cobalt-based catalyst. This process is best known for being used in the first integrated GTL-plant operated and built byShell inBintulu, Malaysia.[17]
In addition to the active metal (usually Fe or Co), two other components comprise the catalyst: promoters and thecatalyst support. Promoters are additives that enhance the behavior of the catalyst. For F-T catalysts, typical promoters including potassium and copper, which are usually added as salts. The choice of promoters depends on the primary metal, iron vs cobalt.[18] Iron catalysts need alkali promotion to attain high activity and stability (e.g. 0.5 wt%K2O). Potassium-doped α-Fe2O3 are synthesized under variable calcination temperatures (400–800 °C).[19] Addition of Cu for reduction promotion, addition ofSiO
2,Al
2O
3 for structural promotion and maybe some manganese can be applied for selectivity control (e.g. high olefinicity). The choice of promoters depends on the primary metal, i.e., iron vs cobalt.[18] While group 1 alkali metals (e.g., potassium), help iron catalysts, they poison cobalt catalysts.
Catalysts are supported on high-surface-area binders/supports such assilica,alumina, orzeolites.[16]
The F-T process attracted attention as a means ofNazi Germany to produce liquid hydrocarbons. The original process was developed byFranz Fischer andHans Tropsch, working at theKaiser-Wilhelm-Institut for Chemistry in 1926. They filed a number of patents,e.g.,U.S. patent 1,746,464, applied 1926, published 1930.[20] It was commercialized byBrabag in Germany in 1936. Being petroleum-poor but coal-rich, Germany used the process duringWorld War II to produceersatz (replacement) fuels. FT production accounted for an estimated 9% of German war production of fuels and 25% of the automobile fuel.[21] Many refinements and adjustments have been made to the process since Fischer and Tropsch's time.
TheUnited States Bureau of Mines, in a program initiated by theSynthetic Liquid Fuels Act, employed sevenOperation Paperclipsynthetic fuel scientists in a Fischer–Tropsch plant inLouisiana, Missouri in 1946.[21][22]
In Britain, Alfred August Aicher obtained severalpatents for improvements to the process in the 1930s and 1940s.[23] Aicher's company was namedSynthetic Oils Ltd (not related to a company of the same name in Canada).[citation needed]
Around the 1930s and 1940s, Arthur Imhausen developed and implemented an industrial process for producing edible fats from these synthetic oils throughoxidation.[24] The products were fractionally distilled and the edible fats were obtained from theC
9-C
16 fraction[25] which were reacted withglycerol such as that synthesized from propylene.[26]"Coal butter" margarine made from synthetic oils was found to be nutritious and of agreeable taste, and it was incorporated into diets contributing as much as 700 calories per day.[27][28] The process required at least 60 kg of coal per kg of synthetic butter.[26]
The LTFT facilityPearl GTL atRas Laffan, Qatar, is the second largest FT plant in the world afterSasol's Secunda plant in South Africa. It usescobalt catalysts at 230 °C, converting natural gas to petroleum liquids at a rate of 140,000 barrels per day (22,000 m3/d), with additional production of 120,000 barrels (19,000 m3) of oil equivalent innatural gas liquids andethane.
Another plant in Ras Laffan, called Oryx GTL, has been commissioned in 2007 with a capacity of 34,000 barrels per day (5,400 m3/d). The plant utilizes the Sasol slurry phase distillate process, which uses a cobalt catalyst. Oryx GTL is a joint venture betweenQatarEnergy andSasol.[29]
The world's largest scale implementation of Fischer–Tropsch technology is a series of plants operated bySasol inSouth Africa, a country with large coal reserves, but little oil. With a capacity of 165000 Bpd at itsSecunda CTL plant.[30] The first commercial plant opened in 1952.[31] Sasol uses coal and natural gas as feedstocks and produces a variety of synthetic petroleum products, including most of the country'sdiesel fuel.[32]
PetroSA, another South African company, operates a refinery with a 36,000 barrels a day plant that completed semi-commercial demonstration in 2011, paving the way to begin commercial preparation. The technology can be used to convert natural gas, biomass or coal into synthetic fuels.[33]
One of the largest implementations of Fischer–Tropsch technology is inBintulu, Malaysia. ThisShell facility convertsnatural gas into low-sulfurDiesel fuels and food-grade wax. The scale is 12,000 barrels per day (1,900 m3/d).
Velocys operated a demonstration plant with Envia in Oklahoma City during 2017 and 2018. The Joint Venture was closed down and reactor returned to Velocysfollowing a leak and insurance settlement.
Starting as a biomass technology licensor[34] In Summer of 2012 SGC Energia (SGCE) successfully commissioned a pilot multi tubular Fischer–Tropsch process unit and associated product upgrading units at the Pasadena, Tx Technology Center. The technology center focused on the development and operations of their XTLH solution which optimized processing of low value carbon waste streams into advanced fuels and wax products.[35] This unit also serves as an operations training environment for the 1100 BPDJuniper GTL facility constructed inWestlake LA.
In October 2006,Finnish paper and pulp manufacturerUPM announced its plans to produce biodiesel by the Fischer–Tropsch process alongside the manufacturing processes at its European paper and pulp plants, using wastebiomass resulting from paper andpulp manufacturing processes as source material.[36]
Texas based Arcadia eFuels in conjunction with Sasol andTopsoe is constructing a sustainable aviation fuel plant inVordingborg, Denmark that will use Fischer-Tropsch process to convertsyngas derived fromwater electrolysis andcarbon capture into an e-diesel fuel foraviation.[37][38] The plant will begin production in 2028 with additional plants in development inTeesside, United Kingdom and the United States.[39][40]
A demonstration-scale Fischer–Tropsch plant was built and operated by Rentech, Inc., in partnership with ClearFuels, a company specializing in biomass gasification. Located inCommerce City CO, the facility produces about 10 barrels per day (1.6 m3/d) of fuels from natural gas. Commercial-scale facilities were planned forRialto, California;Natchez, Mississippi;Port St. Joe, Florida; andWhite River, Ontario.[41] Rentech closed down their pilot plant in 2013, and abandoned work on their FT process as well as the proposed commercial facilities.
In 2010,INFRA built a compact PilotPlant for conversion of natural gas into synthetic oil. The plant modeled the full cycle of the GTL chemical process including the intake of pipeline gas, sulfur removal, steam methane reforming, syngas conditioning, and Fischer–Tropsch synthesis. In 2013 the first pilot plant was acquired by VNIIGAZGazprom LLC. In 2014 INFRA commissioned and operated on a continuous basis a new, larger scale full cycle Pilot Plant. It represents the second generation of INFRA's testing facility and is differentiated by a high degree of automation and extensive data gathering system. In 2015, INFRA built its own catalyst factory inTroitsk (Moscow, Russia). The catalyst factory has a capacity of over 15 tons per year, and produces the unique proprietary Fischer–Tropsch catalysts developed by the company's R&D division. In 2016, INFRA designed and built a modular, transportable GTL (gas-to-liquid) M100 plant for processing natural and associated gas intosynthetic crude oil inWharton TX. The M100 plant is operating as a technology demonstration unit, R&D platform for catalyst refinement, and economic model to scale the Infra GTL process into larger and more efficient plants.[42]
In the United States and India, some coal-producing states have invested in Fischer–Tropsch plants. In Pennsylvania, Waste Management and Processors, Inc. was funded by the state to implement FT technology licensed from Shell and Sasol to convert so-calledwaste coal (leftovers from the mining process) intolow-sulfur diesel fuel.[43][44]
Choren Industries has built a plant inGermany that converts biomass to syngas and fuels using the Shell FT process structure. The company went bankrupt in 2011 due to impracticalities in the process.[45][46]
Biomass gasification (BG) and Fischer–Tropsch (FT) synthesis can in principle be combined to produce renewable transportation fuels (biofuels).[47]
In partnership with Sunfire,Audi producesE-diesel in small scale with two steps, the second one being FT.[48]
Syntroleum, formerly a publicly traded United States company, has produced over 400,000 U.S. gallons (1,500,000 L) of diesel and jet fuel from the Fischer–Tropsch process using natural gas at its demonstration plant nearTulsa, Oklahoma. Using natural gas as a feedstock, the ultra-clean, low sulfur fuel has been tested extensively by theUnited States Department of Energy and theUnited States Department of Transportation. Syntroleum worked to develop a synthetic jet fuel blend that will help the Air Force to reduce its dependence on imported petroleum. The Air Force, which is the United States military's largest user of fuel, began exploring alternative fuel sources in 1999. On December 15, 2006, aB-52 took off fromEdwards Air Force Base,California for the first time powered solely by a 50–50 blend ofJP-8 and Syntroleum's FT fuel. The seven-hour flight test was considered a success. The goal of the flight test program is to qualify the fuel blend for fleet use on the service's B-52s, and then flight test and qualification on other aircraft. The test program concluded in 2007. This program was part of theDepartment of Defense Assured Fuel Initiative, an effort to develop secure domestic sources for the military energy needs. The Pentagon had hoped to reduce its use of crude oil from foreign producers and obtain about half of its aviation fuel from alternative sources by 2016.[49] More recently in 2021,another batch of synthetic jet fuel was manufactured for the Air Force by Twelve and Emerging Fuels Technology - the latter being Syntroleum's successor company which was established by the founders and management team of Syntroleum and having bought its laboratory in Tulsa.
Carbon dioxide is not a typical feedstock for FT catalysis. Hydrogen and carbon dioxide react over a cobalt-based catalyst, producing methane. With iron-based catalysts unsaturated short-chain hydrocarbons are also produced.[50] Upon introduction to the catalyst's support,ceria functions as a reverse water-gas shift catalyst, further increasing the yield of the reaction.[51] The short-chain hydrocarbons were upgraded to liquid fuels over solid acid catalysts, such aszeolites.
Using conventional FT technology the process ranges in carbon efficiency from 25 to 50 percent[52] and a thermal efficiency of about 50%[53] for CTL facilities idealised at 60%[54] with GTL facilities at about 60%[53] efficiency idealised to 80%[54] efficiency.
A Fischer–Tropsch-type process has also been suggested to have produced a few of the building blocks ofDNA andRNA withinasteroids.[55] Similarly, the hypotheticalabiogenic petroleum formation requires some naturally occurring FT-like processes.
Biological Fischer-Tropsch-type chemistry can be carried out by the enzymenitrogenase at ambient conditions.[56][57]
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