From Wikipedia, the free encyclopedia

The Fischer–Tropsch process (or Fischer–Tropsch Synthesis) is a set of chemical reactions that convert a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. The process, a key component of gas to liquids technology, produces a petroleum substitute, typically from coal, natural gas, or biomass for use as synthetic lubrication oil or as synthetic fuel.^[1] The F-T process has received intermittent attention for a variety of reasons, i.e. as a source of low-sulfur diesel fuel or to address the supply or cost of petroleum-derived hydrocarbons.

Process chemistry

The Fischer–Tropsch process involves a variety of chemical reactions, which lead to a series of both desirable and undesirable byproducts. Useful reactions give alkanes:

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

where ‘n’ is a positive integer. The formation of methane (n = 1) is generally unwanted. Most of the alkanes produced tend to be straight-chain alkanes, although some branched alkanes are also formed. In addition to alkane formation, competing reactions result in the formation of alkenes, as well as alcohols and other oxygenated hydrocarbons. Usually, only relatively small quantities of these non-alkane products are formed, although catalysts favoring some of these products have been developed.

Other reactions relevant to FT

Several reactions are required to obtain the gaseous reactants required for F-T catalysis. All feedstocks entering a FT reactor must be desulfurized, for example. The water gas shift reaction gives the gases that are fed into the reactors:

H₂O + CO → H₂ + CO₂

This reaction is also used to adjust the H₂:CO ratio of the gas that is fed to the reactor.

For F-T plants that start with methane, another important reaction is steam reforming, which converts the methane into CO and H₂:

2 H₂O + CH₄ → CO + 3 H₂

Chemical mechanisms

The conversion of CO to alkanes involves net hydrogenation and hydrogenolysis of the CO. Such reactions are assumed to proceed via initial formation of surface-bound metal carbonyls. Subsequently, the CO ligand undergoes dissociation to give oxide and carbide centers.^[2]. Other potential intermediates in the reduction of CO feature C-1 fragments including formyl (CHO), hydroxycarbene (CH(OH), hydroxymethyl (CH₂OH), methyl (CH₃), methylidene (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 FT catalysts are poorly developed.

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. As a result 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.

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 significantly 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,^[3] which can be expressed as:

W_n/n = (1-α)²α^n-1

Where W_n 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; 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 seems to be the most active catalyst, 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. Catalysts are supported on high-surface-area binders/supports such as silica, alumina, or zeolites.^[4] 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 chemical 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 notoriously 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.

LTFT and HTFT

The low temperature F-T process rely on cobalt-based catalysts. Illustrative operations are the original German plants and the Shell Middle Distillate process in Malaysia. LTFT produces blending stock for fuels, not fuels themselves. In contrast, high temperature F-T catalysis, which operates at 300-345 °C and relies on iron-based catalysts, produces fuel grade diesel directly.^[5]

Gasification

F-T 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 CO/H₂ ratio of ~0.7 compared to the ideal ratio of ~2. This ratio is adjusted via the water-gas shift reaction. Gasification is a dirty and expensive (endergonic) process. Coal-based Fischer–Tropsch plants can produce significant amounts of CO₂, in part due to the high energy demands of the gasification process.

Combining [biomass gasification]] (BG) and Fischer-Tropsch (FT) synthesis is a possible route to produce renewable transportation fuels (biofuels).^[6]

History

Since the invention of the original process by Franz Fischer and Hans Tropsch, working at the Kaiser Wilhelm Institute in the 1920s, many refinements and adjustments have been made. The term “Fischer-Tropsch” now applies to a wide variety of similar processes (Fischer-Tropsch synthesis or Fischer-Tropsch chemistry). Fischer and Tropsch filed a number of patents, e.g., US patent no. 1,746,464, applied 1926, published 1930.^[7] It was commercialized in Germany in 1936. Being petroleum-poor but coal-rich, in Germany the FT-process was used by Nazi Germany and Japan during World War II to produce ersatz (German: substitute) fuels. F-T production accounted for an estimated 9% of German war production of fuels and 25% of the automobile fuel.^[5]

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.^[8][5]

In Britain, Alfred August Aicher obtained several patents for improvements to the process in the 1930s and 1940s.^[9] Aicher’s company was named Synthetic Oils Ltd. (There is no connection with the Canadian company of the same name.)

Commercialization

Fluidized Bed Gasification with FT-pilot in Güssing, Burgenland, Austria

The F-T process has been applied on a large scale in some industrial sectors, although its popularity is hampered by 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 only becomes practical when using “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.

Sasol

The largest scale implementation of F-T technology are in a series of plants operated by Sasol in South Africa, a country with large coal reserves but lacking in oil. 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.^[10]

Shell Middle Distillate Synthesis

One of the largest implementations of F-T 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 barrel/day.

Ras Laffan, Qatar

The new LTFT facility scheduled to commission in 2010 at Ras Laffan, Qatar is based on the Sasol technology, using cobalt catalysts at 230 °C. It includes the “Pearl GTL” plant, converting natural gas to petroleum liquids at a rate of 140,000 barrels/day, with additional production of 120,000 barrels of oil equivalent in natural gas liquids and ethane.

UPM (Finland)

In October 2006, Finnish paper and pulp manufacturer UPM announced its plans to produce biodiesel by Fischer–Tropsch process alongside the manufacturing processes at its European paper and pulp plants, using waste biomass resulted by paper and pulp manufacturing processes as source material.^[11]

Other

In the US, some coal-producing states have invested in F-T plants. In Pennsylvania, Waste Management and Processors Inc. was funded by the state to implement F-T technology licensed from Shell and Sasol to convert so-called waste coal (leftovers from the mining process) into low-sulfur diesel fuel.^[12][13]

Research developments

Choren Industries has built an FT plant in Germany.^[14][15] A small US-based company, Rentech, focuses on converting nitrogen-fertiliser plants from using a natural gas feedstock to using coal or coke, and producing liquid hydrocarbons as a co-product.

U.S. Air Force certification

Syntroleum, a publicly traded US company (Nasdaq: SYNM) has produced over 400,000 gallons 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 US, 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 US Department of Energy, the Department of Transportation. Most recently, Syntroleum has been working with the U. S. 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 U.S. military’s largest user of fuel, began exploring alternative fuel sources in 1999. On December 15, 2006, a B-52 took off from Edwards AFB, 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.^[16] 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.^[16][17]

Carbon dioxide reuse

In 2009, chemists working for the U.S. Navy investigated Fischer-Tropsch for generating fuels using hydrogen by electrolysis of seawater. When combined with the dissolved carbon dioxide using a cobalt-based catalyst, this study produced mostly methane gas, but 30 per cent methane with the rest being short-chain hydrocarbons. Further refining of the hydrocarbons produced could potentially lead to the production of kerosene-based jet fuel.^[18]

The abundance of CO₂ makes seawater an attractive alternative fuel source. Scientists at the U.S. Naval Research Laboratory stated that, “although the gas forms only a small proportion of air – around 0.04 per cent – ocean water contains about 140 times that concentration”.^[18] Dorner presented the findings to the American Chemical Society on 16 August 2009, at the Marriott Metro Center in Washington DC.^[19]

See also

Energy portal

• Algae fuel
• Bergius process
• Biogasoline
• Biomass to liquid
• Fischer Assay
• Future energy development
• Hydrogenation, a generic term for this type of process
• Hubbert peak
• Karrick process
• Synthetic Liquid Fuels Program
• Unconventional oil
• Wood gas

References

External links

• Fischer-Tropsch Archive
• Abiogenic Gas Debate 11:2002 (EXPLORER)
• Unconventional Ideas About Unconventional Gas (Society of Petroleum Engineers)
• Process of synthesis of liquid hydrocarbons – Great Britain patent GB309002 – Hermann Plauson
• Clean Diesel from Coal by Kevin Bullis
• Implementing the “Hydrogen Economy” with Synfuels (pdf)
• Carbon-to-Liquids Research