Fischer–Tropsch process

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 and as synthetic fuel. The F–T 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 lead to a variety of hydrocarbons. 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 the F–T process

Several reactions are required to obtain the gaseous reactants required for F-T catalysis. First, reactant gases entering a F-T reactor must first be desulfurized to protect the catalysts, which are readily poisoned. The other major class of reactions are employed to adjust the H₂/CO ratio:

• water gas shift reaction provides a source of hydrogen:

H₂O + CO → H₂ + CO₂

• For F-T plants that start with methane, 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 net hydrogenation of CO, the hydrogenolysis 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. 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 F-T 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, 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 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. 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. 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.

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 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 F-T process. Ongoing research aims to combine biomass gasification (BG) and Fischer-Tropsch (FT) synthesis to produce renewable transportation fuels (biofuels).

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. It was commercialized in Germany in 1936. Being petroleum-poor but coal-rich, Germany used the FT-process 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.

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.

In Britain, Alfred August Aicher obtained several patents for improvements to the process in the 1930s and 1940s. Aicher's company was named Synthetic Oils Ltd. (Now based in Canada.)

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 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.

More recently with further discoveries of natural gas and particularly unconventional gas such as shale gas in North America an over supply of gas has developed in North America. This has depressed gas prices. GTL is economically viable when the gas price is relatively cheap on an energy equivalency basis to oil. Stranded gas provides relatively cheap gas but GTL is now also potentially viable in North America provided gas remains relatively cheaper than oil.

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 little 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.

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, has the world's largest Gas to Liquids complexes at Mossel Bay in South Africa. 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.

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 barrels per day (1,900 m³/d).

Ras Laffan, Qatar

The new LTFT facility scheduled to commission 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.

Rentech (Colorado, US)

A demonstration-scale F–T 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.

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.

Research developments

Choren Industries has built an F–T plant in Germany that converts biomass to syngas and fuels using the Shell F–T process.

U.S. Air Force certification

Syntroleum, a publicly traded US company has produced over 400,000 US gallons (1,500 m³) 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 U.S. Department of Energy and the U.S. 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. 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.

Carbon dioxide reuse

In 2009, chemists working for the U.S. Navy investigated Fischer-Tropsch for generating fuels, obtaining hydrogen by electrolysis of seawater. When it was combined with the dissolved carbon dioxide using a cobalt-based catalyst, the reaction produced mostly methane gas. However, use of an iron-based catalyst allowed reducing the methane produced to 30 per cent with the rest being predominantly short-chain hydrocarbons. Further refining of the hydrocarbons produced by means of solid acid catalysts such as zeolites can potentially lead to the production of kerosene-based jet fuel.

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". Robert Dorner presented the findings of his work to the American Chemical Society on 16 August 2009, in Washington DC. Of course, such a method requires an energy source – since CO₂ is a major product of combustion, converting it back into combustible material is a highly endothermic (energy-absorbing) process. In practice this would probably come from nuclear power, which is in abundant supply aboard nuclear powered ships.

Process Efficiency

The process ranges in efficiency from 25 to 50 percent.

See also

• 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

External links

• Fischer-Tropsch Archive
• Fischer-Tropsch Fuels from Coal and Biomass
• 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
• Effect of alkali metals on cobalt catalysts