Fischer-Tropsch synthesis

Franz Fischer (1911)

The Fischer-Tropsch synthesis (also Fischer-Tropsch process , FT synthesis for short ) is a large-scale process for liquefying coal by indirect hydrogenation of coal. The process was developed by the German chemists Franz Fischer and Hans Tropsch in 1925 at the Kaiser Wilhelm Institute for Coal Research in Mülheim an der Ruhr .

The conversion of the coal into liquid products takes place in several stages. The gasification of coal initially produces synthesis gas , a mixture of carbon monoxide and hydrogen . In addition to coal, natural gas , crude oil and biomass are available as raw material sources for the production of synthesis gas. The synthesis gas is then converted into a wide range of gaseous and liquid hydrocarbons using heterogeneous catalysis. Contacts based on cobalt or iron serve as catalysts . Depending on the catalyst and target product, the process works at temperatures of around 160 to 300 ° C. and pressures of up to 25  bar .

The products are liquid, low-sulfur synthetic fuels , synthetic motor oils and hydrocarbons , so-called paraffin slack , which serves as a raw material base for the chemical industry. Oxygen-containing hydrocarbons such as ethanol and acetone as well as ethene , propene and higher olefins and alcohols are produced as by- products ; the coproduct are water and carbon dioxide .

history

First work

Paul Sabatier

The first work on the hydrogenation of carbon monoxide with hydrogen was carried out by Paul Sabatier and Jean Baptiste Senderens in 1902. With nickel and cobalt catalysts, they obtained water and methane as the main products at atmospheric pressure and temperatures between 200 and 300 ° C. The process would have increased the calorific value of town gas and removed the toxic carbon monoxide it contained, but did not catch on for reasons of cost. Sabatier received the Nobel Prize in Chemistry in 1912 for his work on hydrogenation.

The BASF reported in 1913, a patent that the production of saturated and unsaturated aliphatic hydrocarbons by hydrogenation of carbon monoxide at a pressure of about 100 bar and temperatures of 400 ° C using nickel, cobalt, zinc and other metals or their oxides described as catalysts.

Franz Fischer , founding director of the Kaiser Wilhelm Institute for Coal Research in Mülheim an der Ruhr , which was established in 1912 and whose maintenance was largely provided by the hard coal industry, put his research focus on the utilization of coke oven gas in the early 1920s . This is a mixture of hydrogen, methane, nitrogen and carbon monoxide that was produced when the coal was coked and that was available in excess to heavy industry.

In 1921, together with Hans Tropsch , he developed a process based on alkalized iron-containing catalysts for the production of mixtures of oxygen-containing compounds from synthesis gas, the so-called Synthol process . At a pressure of 100 to 150  bar and a temperature of about 400 ° C. they received mixtures of aldehydes , ketones , carboxylic acid esters , alcohols and carboxylic acids , so-called synthol .

The following year, the chemist Matthias Pier succeeded the industrial production of methanol by hydrogenation of carbon monoxide at high pressure with alkali-free zinc oxide - chromium oxide - catalysts . Since the presence of iron led to methane formation, the reactors were lined with copper . The use of alkalized contacts yielded higher alcohols, especially isobutanol . As a result of these successes, BASF focused its research on carbon monoxide hydrogenation on methanol production and isobutyl oil synthesis .

Working at the Kaiser Wilhelm Institute for Coal Research

Kaiser Wilhelm Institute for Coal Research , today the Max Planck Institute for Coal Research

The shortage of crude oil and the associated bottleneck in the fuel supply during the First World War triggered a politically and militarily motivated search for alternative solutions in the German Reich, primarily based on local coal.

Fischer commissioned Otto Roelen , who had been Tropsch's assistant since 1921, at the Mülheim Institute, starting in 1924 with the further investigation of the synthol process. After the oxygen-containing mixtures produced with them had proven unsuitable for use as motor fuel, Tropsch changed the reaction conditions and carried out the tests under normal pressure. Although only low yields were achieved at the beginning, the work of Fischer, Tropsch and Roelen finally led to the industrial production of aliphatic hydrocarbons using cobalt or iron catalysts.

Large-scale process

Locations of the Fischer-Tropsch plants in the German Reich
lignite, hard coal.

During the Second World War , the FT synthesis gained (wartime) economic importance in Germany. With it, the demand for liquid fuels, so-called Kogasin as the synthesis product after its production could result from co ks, gas , Benz in , was called, be met from domestic coal. By the end of the Second World War, the chemical industry built a total of nine plants that worked with FT synthesis and had a capacity of 0.6 million t / a.

A method by Arthur Imhausen used the higher molecular weight fraction, the paraffin slack, for fatty acid synthesis through paraffin oxidation . The fatty acids were used as raw materials in the Märkische Seifenindustrie , but from 1939 they were also used to produce a synthetic edible fat .

Production of synthetic fuel using the Fischer-Tropsch process

Location	operator	Feedstock	Production capacity 1943/44 in tons per year
Holten	Ruhrchemie	Hard coal	60,000
Rauxel	Victor Union	Hard coal	40,000
Wanne-Eickel	Croup	Hard coal	55,000
Bergkamen	Essen coal mines	Hard coal	85,000
Dortmund	Hoesch	Hard coal	55,000
Moers	Rhine Prussia	Hard coal	75,000
Schwarzheide	BRABAG	Brown coal	180,000
Lützkendorf	Central German fuel and oil works AG	Brown coal	30,000
Deschowitz	Schaffgotsch Petrol GmbH	Brown coal	40,000
	Total of all FT systems		620,000

post war period

Wood gasifier Güssing (2006)

The products of FT synthesis were never competitive with petroleum-based fuels , so that the industry almost completely abandoned the process after the war. The facilities were dismantled according to the Washington resolution of the Western powers . After the oil crisis , however, research was resumed in the 1970s and a pilot plant was built in Bottrop . This was discontinued at the end of the 1980s, as production was only profitable if the price of petrol was above 2.30  German marks .

Procedure of Sasol

Sasol's CtL filling station in Bobsburg, South Africa

In the Republic of South Africa , which also had sufficient coal resources and had to import crude oil, the first modern coal-to-liquid (CtL) plant in South Africa was put into operation in 1955 for political reasons . It was built by the specially founded Suid Afrikaanse Steenkool en Olie (Sasol) with the participation of the German Lurgi AG . The Sasol 1 pilot plant was designed for around 6,000 barrels of fuel per day. From 1980 onwards, capacities were significantly expanded due to the political developments in South Africa.

In 1980 and 1982, Sasol put Sasol II and Sasol III into operation. This gave a total capacity of 104,000 barrels / day. With the political opening, the program was expanded to include natural gas as a raw material source. In 1995 and 1998, Sasol created additional capacities for 124,000 barrels / day of CtL and GtL fuel (gas-to-liquid) . Since hard coal can be obtained relatively cheaply in open-cast mining , the country still covered around 40% of its fuel requirements from coal petroleum in 2006.

Sasol became world market leader in the field of XtL technologies thanks to the South African developments and in 2006 built a modern GtL plant in Qatar with a capacity of 34,000 barrels / day. This is a high-temperature process with process temperatures of 350 ° C (Synthol and Advanced Synthol), in which petrol and alkenes are produced as platform chemicals . Together with Foster Wheeler , Sasol also planned a Fischer-Tropsch plant in China . This plant with an annual capacity of 60,000 barrels uses a low temperature process with 250 ° C and is used to produce diesel fuel and waxes .

Turnaround in raw materials

In 1993 the oil company Royal Dutch Shell put its first GTL plant into operation. The plant in Bintulu in Malaysia has a capacity of 12,000 barrels / day and is operated in a specially developed Fischer-Tropsch process, the Shell Middle Distillate Synthesis (SMDS process). Together, Shell and Sasol want to build up additional GtL capacities of around 60,000 barrels of GtL / day.

The United States has large seams of coal close to the surface that are relatively easy to mine in open pit mining. On September 19, 2006, the United States Air Force tested a Boeing B-52H with a 50:50 mixture of ordinary JP-8 fuel and synthetic fuel derived from coal at Edwards Air Force Base in view of the increased fuel prices and simultaneously high demand . The test flight should clarify the question of how this fuel has proven itself in practice and whether it can be operated reliably and economically. An accompanying research project concluded that Fischer-Tropsch fuels offered an alternative as a source of JP-8 for the US Air Force.

In the course of the turnaround in raw materials , biofuels in particular moved into the focus of fuel production. The Fischer-Tropsch synthesis again received the interest of research and development. It is true that biomass to liquid fuels especially as European biofuels promoted the second generation, but no BtL production was put into operation. Individual pilot projects have started, the meanwhile insolvent Choren Industries wanted to produce the BtL fuel they called SunFuel and SunDiesel in a plant in Freiberg , Saxony .

Raw materials in the process

Coal as a raw material

Scheme of the Lurgi pressure gasifier

To provide the synthesis gas for the Fischer-Tropsch synthesis, coal was originally used initially at temperatures of over 1000 ° C in the coal gasification , for example in the Lurgi pressure gasifier , Winkler generator or Koppers-Totzek reactor , with steam and air or oxygen converted to synthesis gas . Since this conversion only achieves a hydrogen-to-carbon monoxide ratio of 0.7 in the first step, part of the carbon monoxide is reacted with water in a water-gas shift reaction to form carbon dioxide and hydrogen until a ratio of 2: 1 is reached. The synthesis gas is cooled, with phenol and ammonia being separated off, and subjected to rectisol scrubbing , with carbon dioxide, hydrogen sulfide , hydrocyanic acid and organic components being removed. The catalysts are sensitive to sulfur, the hydrogen sulfide content is usually reduced to a volume content of less than 30  ppb . The clean gas still contains about 12% methane , ethane , nitrogen and noble gases as well as about 86% carbon monoxide and hydrogen in a ratio of 1: 2.

Natural gas, biomass and waste as raw materials

The great advantage of the Fischer-Tropsch process is that any energy-rich raw material is basically suitable for the process. In addition to coal and natural gas, this also includes biogas , wood , agricultural waste and household waste . The world's first plant for the use of solid biomass was built in 2005 in Choren near Freiberg . In 2011 she became insolvent.

In 2009, the general approval of Fischer-Tropsch fuels (FT-SPK) by ASTM as aviation fuel took place. In 2014, airlines such as British Airways and Cathay Pacific preferred to make FT fuels from household waste and had begun building facilities in London and Hong Kong .

Procedure

Pressure and temperature

The purified raw gas, which has a hydrogen to carbon monoxide ratio of about 2 to 2.2, is converted into hydrocarbons such as paraffins , olefins and alcohols in a heterogeneous catalytic manner in a build-up reaction . End products are gasoline ( synthetic gasoline ), diesel , heating oil and raw materials for the chemical industry. The reaction takes place even at atmospheric pressure and at a temperature of 160 to 200 ° C; technically, higher pressures and temperatures are used depending on the process. The synthesis proceeds according to the following reaction scheme:

${\displaystyle n\,\mathrm {CO} +(2n+1)\,\mathrm {H} _{2}\leftrightharpoons \mathrm {C} _{n}\mathrm {H} _{2n+2}+n\,\mathrm {H} _{2}\mathrm {O} }$( Alkanes )

${\displaystyle n\,\mathrm {CO} +(2n)\,\mathrm {H} _{2}\leftrightharpoons \mathrm {C} _{n}\mathrm {H} _{2n}+n\,\mathrm {H} _{2}\mathrm {O} }$( Alkenes )

${\displaystyle n\,\mathrm {CO} +(2n)\,\mathrm {H} _{2}\leftrightharpoons \mathrm {C} _{n}\mathrm {H} _{2n+1}\mathrm {OH} +(n-1)\,\mathrm {H} _{2}\mathrm {O} }$( Alcohols )

Around 1.25 kg of water are produced per kilogram of fuel, and around half of the hydrogen used is used to produce it. Iron-containing catalysts catalyze the water-gas shift reaction , so that instead of water and the toxic carbon monoxide, carbon dioxide and hydrogen are produced:

${\displaystyle \mathrm {CO+H_{2}O\;{\overrightarrow {\leftarrow }}\;CO_{2}+H_{2}} \qquad \Delta H_{R\ 298}^{0}=-41{,}2\ \mathrm {kJ/mol} }$

Catalysts

A large number of catalysts are used in the Fischer-Tropsch synthesis. The most commonly used are based on the transition metals cobalt , iron , nickel and ruthenium . Porous metal oxides with large specific surfaces such as diatomaceous earth , aluminum oxide , zeolites and titanium dioxide are used as carriers .

The catalysts can be prepared by impregnating the porous metal oxides with metal salt solutions and then calcining them. The catalyst activity is increased by promoters, which are not themselves catalytically active catalyst components such as alkali metals or copper . Furthermore, the pore size distribution of the support, the calcination and reduction conditions and the resulting particle sizes of the active catalyst metal influence the catalytic activity. Substances such as alkali metals, which are good promoters for iron catalysts, act as catalyst poisons in the case of cobalt catalysts, for example. Cobalt, nickel and ruthenium remain in the metallic state during the reaction, while iron forms a number of oxides and carbides. It is assumed, however, that cobalt oxides, which remain due to incomplete reduction of the salt used, play a promoter role.

Iron- and cobalt-containing catalysts are usually obtained by precipitation , often together with other metals and other promoters. The original Fischer and Tropsch catalyst was produced by coprecipitation of cobalt , thorium and magnesium nitrate , with kieselguhr added to the freshly precipitated catalyst. The further steps such as shaping, drying and reduction of the cobalt salt have a decisive influence on the activity of the catalyst. Cobalt catalysts show only low activity in the water-gas shift reaction, while iron catalysts catalyze this.

Conduct of proceedings

The procedure is determined by the need to dissipate the large heat of reaction of around 3000  kilojoules per cubic meter of converted synthesis gas. The temperature is removed by water, the temperature of which is regulated by adjusting the pressure. Temperatures that are too high lead to methane formation and rapid coking of the catalyst.

Products

The typical Fischer-Tropsch product contains around 10-15% liquefied gases ( propane and butane ), 50% petrol, 28% kerosene , 6% soft paraffin (paraffin slack) and 2% hard paraffins. The process is important for the large-scale production of gasoline and oils from coal , natural gas or biomass . The chain length distribution of the hydrocarbons formed during the reaction follows a Schulz-Flory distribution . The chain length distribution can be described by the following equation:

${\displaystyle \ W_{n}=n(1-\alpha )^{2}\alpha ^{(n-1)}}$,

where W _{n is} the weight fraction of the hydrocarbon molecules with n carbon atoms and α is the chain growth probability. In general, α is determined by the catalyst and the specific process conditions. By varying the process conditions and the design of the catalyst, the selectivity for various products, such as olefins as raw materials for the chemical industry, can be controlled.

Reaction mechanism

Originally it was assumed that the formation of hydrocarbons occurs via the hydrogenation of surface-bound metal carbide species. By mechanistic studies with ¹⁴ C-labeled carbon monoxide could be shown that this mechanism could make only a small contribution to the overall reaction. In the period that followed, various mechanisms were proposed and investigated, the incorporation of ¹⁴ C-labeled components and the subsequent investigation of the ¹⁴ C distribution in the products being a frequently used investigation method.

The reaction mechanism can be divided into the steps of chemisorption of carbon monoxide and dissociative chemisorption of hydrogen, chain growth, hydrogen transfer, hydrogenolysis and desorption . Analogously to hydroformylation , it is assumed that surface-bound metal carbonyls are part of the catalytically active system. The chain growth step could proceed via the formation of acyl complexes and their subsequent hydrogenation to the alkyl complex . Another molecule of carbon monoxide could insert into the metal-alkyl bond.

Kinetic studies in tubular reactors , with which the rate-determining step in other heterogeneously catalyzed reactions, such as the chemical reaction, the diffusion through the boundary layer or the pore diffusion , could be determined, did not lead to a clear result when investigating the Fischer-Tropsch synthesis. The reaction network consists of a series of complex, partly reversible chemical and transport reactions . It is also assumed that the catalytically active center is formed under reaction conditions by chemisorption of the reactants and changes over the length of the catalyst bed. Investigations in gradient-free reactors showed an activation energy of 93 to 95 kJ mol ⁻¹ and an inhibiting influence of the carbon monoxide concentration. In kinetic studies in gradient-free spinning basket reactors, the formation of a surface-bound methylene species , which results from the hydrogenation of chemisorbed carbon monoxide, was identified as the rate-determining step. One such step in the water-gas shift reaction is the formation of a surface- bound formyl species .

Anderson-Emmett Mechanism

In investigations by Anderson and Paul Hugh Emmett it was found that carbon monoxide chemisorbed on metal centers is hydrogenated by hydrogen to an enolic primary complex of the type M = CH (OH) (M = metal). The chain growth occurs through the carbon-carbon linkage of two neighboring enols with elimination of water. The hydrogenation of this intermediate stage creates a methylhydroxy carbene complex , which in turn is available for the formation of a carbon chain with neighboring enol complexes with elimination of water. It was found that ¹⁴ C-labeled 1-propanol is quickly incorporated into the hydrocarbon formed. This was taken as an indication of intermediate enol complexes.

Pichler-Schulz mechanism

In the Pichler-Schulz mechanism, chain growth is determined by the insertion of carbon monoxide into a metal-alkyl bond with subsequent hydrogenation to form the alkyl radical that has grown around a CH ₂ group. This mechanism implies that the insertion and subsequent hydrogenation are fast compared to the chain termination reaction. This mechanism is supported by the disappearance of the infrared band of adsorbed carbon monoxide during the Fischer-Tropsch reaction.

Sachtler-Biloen mechanism

Recent studies seem to support a mechanism through the breakdown of chemisorbed carbon monoxide into adsorbed C1 species and oxygen. Indications of this are based on the easy incorporation of pre-adsorbed marked carbons into the resulting hydrocarbon chain. The chain growth takes place according to a Gaube – Maitlis model via surface-bound alkylidene or alkylidene species.

Process variants

Arge synthesis

Fischer-Tropsch column

The procedure is carried out in several variants. In addition to the normal pressure process developed by Fischer and Tropsch, the medium pressure process developed by Pichler, also called high-load or consortium synthesis, was commercialized by a working group of the Ruhrchemie and Lurgi companies . The conversion of the coal gasification products on iron contacts doped with copper and potassium carbonate takes place in the fixed bed process at temperatures around 220 to 240 ° C and pressures up to 25 bar. The carbon monoxide to hydrogen ratio is 1 to 1.7. Paraffin / olefin mixtures, so-called slack, are obtained as products.

The reaction is exothermic with 158 kilojoules per mole of CH ₂ group formed at 250 ° C:

${\displaystyle n\,\mathrm {CO} +(2n)\,\mathrm {H} _{2}\leftrightharpoons {-}(\mathrm {CH} _{2})_{n}{-}+\ n\,\mathrm {H} _{2}\mathrm {O} \,;\,H_{\mathrm {R} }=-158\,\mathrm {kJ} /\mathrm {mol} }$

One problem is the removal of the high heat of hydrogenation in order to ensure that the reaction is carried out as isothermally as possible. The Arge reactor originally had a diameter of three meters and was equipped with 2052 catalyst tubes that hold around 35 tons or 40 cubic meters of catalyst. The catalytic converter is arranged in narrow pipes surrounded by water. The heat of reaction is dissipated by boiling water under pressure. Insufficient heat dissipation leads to a temperature gradient across the catalyst bed and can lead to increased methane production or coking of the contacts. Decreasing catalytic activity of the contacts is compensated for by increasing the reaction temperature.

The catalyst volume in modern reactors is around 200 m ³ . A Fischer-Tropsch plant with several reactors requires around 1,500,000 m ³ of synthesis gas per hour  under standard conditions and produces around 2,000,000 t of hydrocarbons per year. The synthesis is carried out in three stages with a total conversion of approx. 94%. In addition to being carried out in a fixed bed reactor, there are process variants with fluidized bed processes (Hydrocol process), as fly ash synthesis, in which the catalyst is present as fluidized fly ash, or in an oil suspension ( Rheinpreußen - Koppers process).

Synthol process

One reaction variant is the synthol synthesis developed by the companies Sasol and Kellogg. It should not be confused with the process of the same name developed by Fischer and Tropsch. The process is a fly ash synthesis; with him the catalyst is metered in as a powder with the reaction gas. The process works at 25 bar and temperatures above 300 ° C. This means that low molecular weight hydrocarbons are preferably formed. The ratio of carbon monoxide to hydrogen is approximately 1: 2.

See also

• Bergius Pier Procedure
• XtL fuel
• Power-to-Liquid

literature

• Thorsten Gottschau: Biomass-to-Liquid (BtL) fuels. Overview and perspectives. In: Rainer Schretzmann, Jörg Planer (ed.): Power plant field and forest. Bioenergy for Germany. , AID, Bonn 2007, ISBN 978-3-8308-0680-6 , (proceedings for the AID Agriculture Forum on November 10, 2006 in Bonn)
• Steffen Bukold : Oil in the 21st Century. Volume 2: Alternatives and Strategies. Oldenbourg, Munich 2009, ISBN 978-3-486-58898-9
• Friedrich Benthaus u. a .: raw material coal. Properties, extraction, refinement , 1st edition, Verlag Chemie, Weinheim 1978, ISBN 3-527-25791-8
• Manfred Rasch: History of the Kaiser Wilhelm Institute for Coal Research 1913–1943 , Weinheim 1989.

Web links

Commons : Fischer-Tropsch synthesis  - collection of images, videos and audio files

• The Fischer-Tropsch Archive

Individual evidence

1. ↑ P. Sabatier, JB Senderens: Hydrogenation of CO over Nickel to Produce Methane. In: J. Soc. Chem. Ind. 21, 1902, pp. 504-506.
2. ^ Emil Fischer: The tasks of the Kaiser Wilhelm Institute for coal research. In: Steel and Iron. 32, 1912, pp. 1898-1903.
3. ↑ The Method of Direct Hydrogenation by Catalysis , lecture by Paul Sabatier on the occasion of the award of the Nobel Prize, December 11, 1912 (accessed on November 12, 2016)
4. ↑ BASF, DRP Patent 293.787 (1913)
5. ↑ Franz Fischer, Hans Tropsch: About the production of synthetic oil mixtures (Synthol) by building up from carbon oxide and hydrogen. In: fuel chem. 4, 1923, pp. 276-285.
6. ↑ Kai-Olaf Hinrichsen, Jennifer Strunk: Basic chemical methanol. In: News from chemistry. 54, 2006, pp. 1080-1084, doi : 10.1002 / nadc.20060541109 .
7. ^ Friedrich Asinger: methanol, chemical and energy raw material . Akademie-Verlag, Berlin, 1987, ISBN 3-05-500341-1 , p. 122.
8. ↑ Franz Fischer, Hans Tropsch: About the direct synthesis of petroleum hydrocarbons under normal pressure. (First communication). In: Reports of the German Chemical Society (A and B Series). 59, 1926, pp. 830-831, doi : 10.1002 / cber.19260590442 .
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10. ↑ ^{a b c d e f g h i j k} F. Benthaus u. a .: raw material coal. Properties, extraction, refinement , Verlag Chemie, Weinheim, 1st edition, 1978, ISBN 3-527-25791-8 , p. 43 ff.
11. ↑ Stealing coal for twenty minutes, but double fat filtration . In: Der Spiegel . No. 46 , 1947 ( online ).
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14. ^ Cheap gasoline from Bottrop, at Welt.de. Retrieved October 15, 2013 . 
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27. ^ JT Kummer, TW DeWitt, PH Emmett: Some Mechanism Studies on the Fischer-Tropsch Synthesis Using ¹⁴ C In: Journal of the American Chemical Society . 70, 1948, pp. 3632-3643, doi : 10.1021 / ja01191a029 .
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30. ↑ Gerard P. van der Laan, Antonie ACM Beenackers: Intrinsic kinetics of the gas-solid Fischer-Tropsch and water gas shift reactions over a precipitated iron catalyst. In: Applied Catalysis A: General. 193, 2000, pp. 39-53, doi : 10.1016 / S0926-860X (99) 00412-3 .
31. ^ W. Keith Hall, RJ Kokes, PH Emmett: Mechanism Studies of the Fischer-Tropsch Synthesis: The Incorporation of Radioactive Ethylene, Propionaldehyde and Propanol In: Journal of the American Chemical Society. 82, 1960, pp. 1027-1037, doi : 10.1021 / ja01490a005 .
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This version was added to the list of articles worth reading on October 25, 2013 .