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Fischer–Tropsch process

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The Fischer–Tropsch process is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. It was first developed by Franz Fischer and Hans Tropsch at the "Kaiser-Wilhelm-Institut für Kohlenforschung" in Mülheim an der Ruhr (Germany) in 1925. The process, a key component of gas to liquids technology, produces a synthetic lubrication oil and synthetic fuel, typically from coal, natural gas, or biomass.[1] The Fischer–Tropsch process has received intermittent attention as a source of low-sulfur diesel fuel and to address the supply or cost of petroleum-derived hydrocarbons.

Process chemistry

The Fischer–Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbon molecules according to the formula (CnH(2n+2)). The more useful reactions produce alkanes as follows:

(2n + 1) H2 + n CO → CnH(2n+2) + n H2O

where n is a positive integer. The formation of methane (n = 1) is generally unwanted as methane is gaseous at standard temperature and pressure. Most of the alkanes produced tend to be straight-chain, suitable as diesel fuel. In addition to alkane formation, competing reactions give small amounts of alkenes, as well as alcohols and other oxygenated hydrocarbons.[2]

Feedstocks: Gasification

Fischer–Tropsch plants associated with coal or related solid feedstocks (sources of carbon) must first convert the solid fuel into gaseous reactants, i.e., CO, H2, and alkanes. This conversion is called gasification. Synthesis gas obtained from coal gasification tends to have a H2/CO ratio of ~0.7 compared to the ideal ratio of ~2. This ratio is adjusted via the water-gas shift reaction. Coal-based Fischer–Tropsch plants can 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 Fischer–Tropsch process. Ongoing research aims to combine biomass gasification (BG) and Fischer–Tropsch (FT) synthesis to produce renewable transportation fuels (biofuels).[3]

Feedstocks: GTL

Carbon monoxide for FT catalysis is derived from hydrocarbons. In gas to liquids (GTL) technology, the hydrocarbons are low molecular weight materials that often would be discarded or flared. GTL is economically viable when the gas price is relatively cheap on an energy equivalency basis to oil. Stranded gas provides relatively cheap gas. GTL is viable provided gas remains relatively cheaper than oil.

Several reactions are required to obtain the gaseous reactants required for Fischer–Tropsch catalysis. First, reactant gases entering a Fischer–Tropsch reactor must be desulfurized. Otherwise, sulfur-containing impurities deactivate ("poison") the catalysts required for Fischer–Tropsch reactions.[2]

Several reactions are employed to adjust the H2/CO ratio. Most important is the water gas shift reaction, which provides a source of hydrogen at the expense of carbon monoxide:[2]

H2O + CO → H2 + CO2

For Fischer–Tropsch plants that use methane as the feedstock, another important reaction is steam reforming, which converts the methane into CO and H2:

H2O + CH4 → CO + 3 H2

Chemical mechanisms

The conversion of CO to alkanes involves hydrogenation of CO, the hydrogenolysis (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-bound metal carbonyls. The CO ligand is speculated to undergo dissociation, possibly into oxide and carbide ligands.[4] Other potential intermediates are various C-1 fragments including formyl (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 as migratory insertion. Many related stoichiometric reactions have been simulated on discrete metal clusters, but homogeneous Fischer–Tropsch catalysts are poorly developed and of no commercial importance.

Process conditions

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 formation of long-chained alkanes, 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 via coke 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 promote the water-gas-shift reaction and thus can tolerate lower ratios. This reactivity can be important for synthesis gas derived from coal or biomass, which tend to have relatively low H2:CO ratios (<1).

Product distribution

In general the product distribution of hydrocarbons formed during the Fischer–Tropsch process follows an Anderson–Schulz–Flory distribution,[5] which can be expressed as:

Wn/n = (1 − α)2αn−1

where Wn is the weight fraction of hydrocarbon molecules containing n carbon atoms. α 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 as alpha is less than 0.5; however, by increasing α close to one, the total amount of methane formed can be minimized compared to the sum of all of the various long-chained products. Increasing α 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 Fischer–Tropsch 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 (usually n<10). This way they can drive the reaction so as to minimize methane formation without producing lots of long-chained hydrocarbons. Such efforts have met with only limited success.

Fischer–Tropsch catalysts

A variety of catalysts can be used for the Fischer–Tropsch process, but the most common are the transition metals cobalt, iron, and ruthenium. Nickel can also be used, but tends to favor methane formation ("methanation").

Cobalt-based catalysts are highly active, although iron may be more suitable for low-hydrogen-content synthesis gases such as those derived from coal due to its promotion of the water-gas-shift reaction. In addition to the active metal the catalysts typically contain a number of "promoters," including potassium and copper. Group 1 alkali metals, including potassium, are a poison for cobalt catalysts but are promoters for iron catalysts. Catalysts are supported on high-surface-area binders/supports such as silica, alumina, or zeolites.[6] Cobalt catalysts are more active for Fischer–Tropsch 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. Iron catalysts are preferred for lower quality feedstocks such as coal or biomass.

Unlike the other metals used for this process (Co, Ni, Ru), which remain in the metallic state during synthesis, iron catalysts tend to form a number of phases, including various oxides and carbides during the reaction. Control of these phase transformations can be important in maintaining catalytic activity and preventing breakdown of the catalyst particles.

Fischer–Tropsch catalysts are sensitive to poisoning by sulfur-containing compounds. The sensitivity of the catalyst to sulfur is greater for cobalt-based catalysts than for their iron counterparts.

Promotors also have an important influence on activity. Alkali metal oxides and copper are common promotors, but the formulation depends on the primary metal, iron vs cobalt.[7] Alkali oxides on cobalt catalysts generally cause activity to drop severely even with very low alkali loadings. C5+ and CO2 selectivity increase while methane and C2-C4 selectivity decrease. In addition, the olefin to parafin ratio increases.

LTFT and HTFT

High-temperature Fischer–Tropsch (or HTFT) is operated at temperatures of 330°C–350°C and uses an iron-based catalyst. This process was used extensively by Sasol in their coal-to-liquid plants (CTL). Low-Temperature Fischer–Tropsch (LTFT) is operated at lower temperatures and uses a cobalt based catalyst. This process is best known for being used in the first integrated Gas-to-Liquid (GTL) plant operated and built by Shell in Bintulu, Malaysia.[8]

History

Since the invention of the original process by Fischer and Tropsch, working at the Kaiser-Wilhelm-Institut for Chemistry in the 1920s, many refinements and adjustments have been made. Fischer and Tropsch filed a number of patents, e.g., U.S. patent 1,746,464, applied 1926, published 1930.[9] It was commercialized by Brabag in Germany in 1936. Being petroleum-poor but coal-rich, Germany used the Fischer–Tropsch process during World War II to produce ersatz fuels. Fischer–Tropsch production accounted for an estimated 9% of German war production of fuels and 25% of the automobile fuel.[10]

The United States Bureau of Mines, in a program initiated by the Synthetic Liquid Fuels Act, employed seven Operation Paperclip synthetic fuel scientists in a Fischer–Tropsch plant in Louisiana, Missouri in 1946.[10][11]

In Britain, Alfred August Aicher obtained several patents for improvements to the process in the 1930s and 1940s.[12] Aicher's company was named Synthetic Oils Ltd. (Not related to a company of the same name in Canada)[13]

Commercialization

Fluidized bed gasification with FT-pilot in Güssing, Burgenland, Austria

The Fischer–Tropsch process has been applied in large-scale gas–liquids and coal–liquid facilities such as Shell's Pearl GTL facility in Ras Laffan, Qatar. Such large facilities are susceptible to high capital costs, high operation and maintenance costs, the uncertain and volatile price of crude oil, and environmental concerns. In particular, the use of natural gas as a feedstock becomes practical only with use of "stranded gas", i.e., sources of natural gas far from major cities which are impractical to exploit with conventional gas pipelines and LNG technology; otherwise, the direct sale of natural gas to consumers would become much more profitable. Several companies are developing the process to enable practical exploitation of so-called stranded gas reserves.

Conventional FT reactors have been optimized for massive coal-to-liquids and gas–liquid facilities such as Shell's Pearl GTL facility. These slurry bed and fixed-bed reactors are much larger than the sizes needed for biofuel facilities or for smaller-scale natural-gas fields. The use of microchannel reactors scales down the size of the reaction hardware and overcomes the heat and mass transport problems associated with conventional FT technology. Enhanced heat transfer inside the microchannels reactor allows for optimal temperature control, which maximizes catalyst activity and life. While no smaller scale plant is currently in commercial operation, indications show capital costs, operating costs and size could all be reduced relative to conventional FT facilities.[14][15] An order has reportedly been placed for a 1400-bbl/day modular GTL plant using the technology of a company called Velocys.[16]

In Australia, Linc Energy commenced construction in 1999 of the world's first gas–liquid plant operating on synthesis gas produced by underground coal gasification.[17] The GTL plant uses the F-T process, and produced liquids in 2008.[18]

Sasol

The largest scale implementation of Fischer–Tropsch technology are in a series of plants operated by Sasol in South Africa, a country with large coal reserves, but little oil. The first commercial plant opening in 1952, 40 miles south of Johannesburg.[19] Sasol uses coal and now natural gas as feedstocks and produces a variety of synthetic petroleum products, including most of the country's diesel fuel.[20]

In December, 2012 Sasol announced plans to build a 96,000 barrels a day plant in Westlake, Louisiana using natural gas from tight shale formations in Louisiana and Texas as feedstock. Costs are estimated to be between 11 and 12 billion dollars with $2 billion in tax relief being contributed by the state of Louisiana. The planned complex will include a refinery and a chemical plant.[21]

PetroSA

PetroSA, a South African company which, in a joint venture, won project innovation of the year award at the Petroleum Economist Awards in 2008,[22] has the world's largest Gas to Liquids complexes at Mossel Bay in South Africa.[23] The refinery is 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.[24]

Shell middle distillate synthesis

One of the largest implementations of Fischer–Tropsch technology is in Bintulu, Malaysia. This Shell facility converts natural gas into low-sulfur Diesel fuels and food-grade wax. The scale is 12,000 barrels per day (1,900 m3/d).

Ras Laffan, Qatar

The new LTFT facility Pearl GTL which began operation in 2011 at Ras Laffan, Qatar, uses cobalt 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 in natural gas liquids and ethane. The first GTL plant in Ras Laffan was commissioned in 2007 and is called Oryx GTL and has a capacity of 34 000 bbl/day. The plant utilizes the Sasol slurry phase distillate process which uses a cobalt catalyst. Oryx GTL is a joint venture between Qatar Petroleum and Sasol.

UPM (Finland)

In October 2006, Finnish paper and pulp manufacturer UPM announced its plans to produce biodiesel by the Fischer–Tropsch process alongside the manufacturing processes at its European paper and pulp plants, using waste biomass resulting from paper and pulp manufacturing processes as source material.[25]

Rentech (Colorado, U.S.)

A demonstration-scale Fischer–Tropsch plant is owned and operated by Rentech, Inc., in partnership with ClearFuels, a company specializing in biomass gasification. Located in Commerce City, Colorado, the facility produces about 10 barrels per day (1.6 m3/d) of fuels from natural gas. Commercial-scale facilities are planned for Rialto, California; Natchez, Mississippi; Port St. Joe, Florida; and White River, Ontario.[26] Rentech closed down their pilot plant in 2013, and does not appear to be continuing work on their FT process and the proposed commercial facilities.

Other

In the United States, some coal-producing states have invested in Fischer–Tropsch plants. In Pennsylvania, Waste Management and Processors, Inc. was funded by the state to implement Fischer–Tropsch technology licensed from Shell and Sasol to convert so-called waste coal (leftovers from the mining process) into low-sulfur diesel fuel.[27][28]

Research developments

Choren Industries has built an Fischer–Tropsch plant in Germany that converts biomass to syngas and fuels using the Shell Fischer–Tropsch process.[29][30]

U.S. Air Force certification

Syntroleum, 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 and coal at its demonstration plant near Tulsa, Oklahoma. Syntroleum is working to commercialize its licensed Fischer–Tropsch technology via coal-to-liquid plants in the United States, China, and Germany, as well as gas-to-liquid plants internationally. Using natural gas as a feedstock, the ultra-clean, low sulfur fuel has been tested extensively by the United States Department of Energy (DOE) and the United States Department of Transportation (DOT). Most recently, Syntroleum has been working with the United States Air Force 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, a B-52 took off from Edwards Air Force Base, California for the first time powered solely by a 50–50 blend of JP-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 is part of the Department of Defense Assured Fuel Initiative, an effort to develop secure domestic sources for the military energy needs. The Pentagon hopes to reduce its use of crude oil from foreign producers and obtain about half of its aviation fuel from alternative sources by 2016.[31] With the B-52 now approved to use the FT blend, the C-17 Globemaster III, the B-1B, and eventually every airframe in its inventory to use the fuel by 2011.[31][32]

Carbon dioxide reuse

Carbon dioxide is not a typical feedstock for F-T catalysis. Hydrogen and carbon dioxide react over a cobalt-based catalyst, producing methane. With iron-based catalysts short-chain, unsaturated hydrocarbons are also produced.[33] Upon introduction to the catalyst's support, ceria functions as a reverse water gas shift catalyst, further increasing the yield of the reaction.[34] The short chain hydrocarbons were upgraded to liquid fuels over solid acid catalysts, such as zeolites.

Process efficiency

Using conventional FT technology the process ranges in carbon efficiency from 25 to 50 percent[35] and a thermal efficiency of about 50%[36] for CTL facilities idealised at 60%[37] with GTL facilities at about 60%[36] efficiency idealised to 80%[37] efficiency.

See also

References

  1. ^ US Fuel Supply Statistics Chart
  2. ^ a b c Takao Kaneko, Frank Derbyshire, Eiichiro Makino, David Gray and Masaaki Tamura "Coal Liquefaction" in Ullmann's Encyclopedia of Industrial Chemistry, 2001, Wiley-VCH, Weinheim. doi:10.1002/14356007.a07_197
  3. ^ Oliver R. Inderwildi, Stephen J. Jenkins, David A. King (2008). "Mechanistic Studies of Hydrocarbon Combustion and Synthesis on Noble Metals". Angewandte Chemie International Edition. 47 (28): 5253–5. doi:10.1002/anie.200800685. PMID 18528839.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. ^ Bruce C. Gates “Extending the Metal Cluster-Metal Surface Analogy” Angewandte Chemie International Edition in English, 2003, Volume 32, pp. 228 – 229. doi:10.1002/anie.199302281
  5. ^ http://www.fischer-tropsch.org/DOE/DOE_reports/510/510-34929/510-34929.pdf P.L. Spath and D.C. Dayton. "Preliminary Screening — Technical and Economic Assessment of Synthesis Gas to Fuels and Chemicals with Emphasis on the Potential for Biomass-Derived Syngas", NREL/TP510-34929,December, 2003, pp. 95
  6. ^ Andrei Y. Khodakov, Wei Chu, and Pascal Fongarland “Advances in the Development of Novel Cobalt Fischer–Tropsch Catalysts for Synthesis of Long-Chain Hydrocarbons and Clean Fuels” Chemical Reviews, 2007, volume 107, pp 1692–1744. doi:10.1021/cr050972v
  7. ^ Christine M. Balonek, Andreas H. Lillebø, Shreyas Rane, Erling Rytter, Lanny D. Schmidt, Anders Holmen “Effect of Alkali Metal Impurities on Co–Re Catalysts for Fischer–Tropsch Synthesis from Biomass-Derived Syngas” Catalysis Letters 2010, volume 138, pp 8–13. doi:10.1007/s10562-010-0366-4.
  8. ^ pages 33–41
  9. ^ http://www.fischer-tropsch.org/primary_documents/patents/US/us1746464.pdf
  10. ^ a b Leckel, D., "Diesel Production from Fischer–Tropsch: The Past, the Present, and New Concepts", Energy Fuels, 2009, volume 23, 2342–2358. doi:10.1021/ef900064c
  11. ^ German Synthetic Fuels Scientist
  12. ^ For example, British Patent No. 573,982, applied 1941, published 1945"Improvements in or relating to Methods of Producing Hydrocarbon Oils from Gaseous Mixtures of Hydrogen and Carbon Monoxide" (PDF). January 14, 1941. Retrieved 2008-11-09.
  13. ^ "Oil from Coal and Other Synthetic Fuels". IGG. Retrieved 29 April 2012.
  14. ^ Smedley, Mark. "Small GTL's Market Reach as Great as Opec's, UK Firm Says" (PDF). World Gas Intelligence. Retrieved 19th Dec 2012. {{cite web}}: Check date values in: |accessdate= (help)
  15. ^ Jamieson, Andrew. "Keeping the Options Open" (PDF). Petroleum Economist. Retrieved LNG 2012. {{cite web}}: Check date values in: |accessdate= (help)
  16. ^ Lane, Jim (Nov. 20, 2012). "Little Big Tech: Can Fischer-Tropsch technology work at smaller scale?". BiofuelsDigest. Retrieved 26 April 2013. {{cite web}}: Check date values in: |date= (help)
  17. ^ Linc gears up for Chinchilla GTL.
  18. ^ First production of liquids
  19. ^ "Construction of World's First Synthesis Plant" Popular Mechanics, February 1952, p. 264, bottom of page.
  20. ^ "technologies & processes" Sasol
  21. ^ Clifford Krauss (December 3, 2012). "South African Company to Build U.S. Plant to Convert Gas to Liquid Fuels". The New York Times. Retrieved December 18, 2012.
  22. ^ "PetroSA wins innovation award". SouthAfrica.info. 2008-10-10. Retrieved 2013-06-05.
  23. ^ "PetroSA – South Africa's National Oil Company". Petrosa.co.za. Retrieved 2013-06-05.
  24. ^ "PetroSA technology ready for next stage | Archive | BDlive". Businessday.co.za. 2011-05-10. Retrieved 2013-06-05.
  25. ^ "UPM-Kymmene says to establish beachhead in biodiesel market". NewsRoom Finland. Archived from the original on 2007-03-17. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  26. ^ http://www.rentechinc.com/ (official site)
  27. ^ "Governor Rendell leads with innovative solution to help address PA energy needs". State of Pennsylvania. Archived from the original on 2008-12-11. {{cite web}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  28. ^ "Schweitzer wants to convert Otter Creek coal into liquid fuel". Billings Gazette. August 2, 2005. Archived from the original on 2009-01-01. {{cite news}}: Unknown parameter |deadurl= ignored (|url-status= suggested) (help)
  29. ^ http://www.choren.com Choren official web site
  30. ^ Fairley, Peter. Growing Biofuels – New production methods could transform the niche technology. MIT Technology Review November 23, 2005
  31. ^ a b Zamorano, Marti (2006-12-22). "B-52 synthetic fuel testing: Center commander pilots first Air Force B-52 flight using solely synthetic fuel blend in all eight engines". Aerotech News and Review.
  32. ^ "C-17 flight uses synthetic fuel blend". 2007-10-25. Archived from the original on 2012-12-12. Retrieved 2008-02-07.
  33. ^ Dorner, Robert (2010). "Heterogeneous catalytic CO2 conversion to value-added hydrocarbons". Energy Environ. Sci. 3: 884–890. doi:10.1039/C001514H. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  34. ^ Dorner, Robert. "Catalytic Support for use in Carbon Dioxide Hydrogenation Reactions".
  35. ^ Dominik Unruh, Kyra Pabst and Georg Schaub "Fischer−Tropsch Synfuels from Biomass: Maximizing Carbon Efficiency and Hydrocarbon Yield" Energy Fuels, 2010, 24, pp 2634–2641. doi:10.1021/ef9009185
  36. ^ a b Fischer-Tropsch Refining by Arno de Klerk
  37. ^ a b http://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/48_1_New%20Orleans__03-03_0567.pdf

Further reading

  • Klerk, Arno de (2011). Fischer–Tropsch refining (1st ed.). Weinheim, Germany: Wiley-VCH. ISBN 9783527326051.
  • Klerk, Arno de., Edward Furimsky (2010). Catalysis in the refining of Fischer–Tropsch syncrude. Cambridge: RSC Publishing.{{cite book}}: CS1 maint: multiple names: authors list (link)
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