The Fischer–Tropsch process is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, typically at temperatures of 150–300 °C (302–572 °F) and pressures of one to several tens of atmospheres. The process was first developed by Franz Fischer and Hans Tropsch at the Kaiser-Wilhelm-Institut für Kohlenforschung in Mülheim an der Ruhr, Germany, in 1925.
As a premier example of C1 chemistry, Fischer–Tropsch process is an important reaction in both coal liquefaction and gas to liquids technology for producing liquid hydrocarbons. In the usual implementation, carbon monoxide and hydrogen, the feedstocks for F-T, are produced from coal, natural gas, or biomass in a process known as gasification. The Fischer–Tropsch process then converts these gases into a synthetic lubrication oil and synthetic fuel. 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.
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 produce alkanes as follows:
(2n + 1) H2 + n CO → CnH2n+2 + n H2O
where n is typically 10–20. The formation of methane (n = 1) is unwanted. 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.
Fischer–Tropsch intermediates and elemental reactions
Converting a mixture of H2 and CO into aliphatic products obviously should be a multi-step reaction with several sorts of intermediates. 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:
Associative adsorption of CO
Splitting of the C–O bond
Dissociative adsorption of 2 H2
Transfer of 2 H to the oxygen to yield H2O
Desorption of H2O
Transfer of 2 H to the carbon to yield CH2
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. Other potential intermediates are various C1 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.
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. Using 14C-labelled ethylene and propene 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’.
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 and the product is called synthesis gas (“syngas”). 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 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.
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. 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.
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:
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
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 can tolerate lower ratios, due to intrinsic Water Gas Shift Reaction activity of the Iron catalyst. This reactivity can be important for synthesis gas derived from coal or biomass, which tend to have relatively low H2:CO ratios (< 1). Design of the Fischer–Tropsch process reactor Efficient removal of heat from the reactor is the basic need of Fischer–Tropsch reactors since these reactions are characterized by high exothermicity. Four types of reactors are discussed: Multi tubular fixed-bed reactor This type of reactor contains a number of tubes with small diameter. These tubes contain catalyst and are surrounded by boiling water which removes the heat of reaction. A fixed-bed reactor is suitable for operation at low temperatures and has an upper temperature limit of 530 K. Excess temperature leads to carbon deposition and hence blockage of the reactor. Since large amounts of the products formed are in liquid state, this type of reactor can also be referred to as a trickle flow reactor system. Entrained flow reactor An important requirement of the reactor for the Fischer–Tropsch process is to remove the heat of the reaction. This type of reactor contains two banks of heat exchangers which remove heat; the remainder of which is removed by the products and recycled in the system. The formation of heavy waxes should be avoided, since they condense on the catalyst and form agglomerations. This leads to fluidization. Hence, risers are operated over 570 K. Slurry reactors Heat removal is done by internal cooling coils. The synthesis gas is bubbled through the waxy products and finely-divided catalyst which is suspended in the liquid medium. This also provides agitation of the contents of the reactor. The catalyst particle size reduces diffusional heat and mass transfer limitations. A lower temperature in the reactor leads to a more viscous product and a higher temperature (> 570 K) gives an undesirable product spectrum. Also, separation of the product from the catalyst is a problem.
Fluid-bed and circulating catalyst (riser) reactors
These are used for high-temperature Fischer–Tropsch synthesis (nearly 340 °C) to produce low-molecular-weight unsaturated hydrocarbons on alkalized fused iron catalysts. The fluid-bed technology (as adapted from the catalytic cracking of heavy petroleum distillates) was introduced by Hydrocarbon Research in 1946–50 and named the ‘Hydrocol’ process. A large scale Fischer–Tropsch Hydrocol plant (350,000 tons per annum) operated during 1951–57 in Brownsville, Texas. Due to technical problems, and lacking economy due to increasing petroleum availability, this development was discontinued. Fluid-bed Fischer–Tropsch synthesis has recently been very successfully reinvestigated by Sasol. One reactor with a capacity of 500,000 tons per annum is now in operation and even larger ones are being built (nearly 850,000 tons per annum). The process is now used mainly for C2 and C7 alkene production. This new development can be regarded as an important progress in Fischer–Tropsch technology. A high-temperature process with a circulating iron catalyst (‘circulating fluid bed’, ‘riser reactor’, ‘entrained catalyst process’) was introduced by the Kellogg Company and a respective plant built at Sasol in 1956. It was improved by Sasol for successful operation. At Secunda, South Africa, Sasol operated 16 advanced reactors of this type with a capacity of approximately 330,000 tons per annum each. Now the circulating catalyst process is being replaced by the superior Sasol-advanced fluid-bed technology. Early experiments with cobalt catalyst particles suspended in oil have been performed by Fischer. The bubble column reactor with a powdered iron slurry catalyst and a CO-rich syngas was particularly developed to pilot plant scale by Kölbel at the Rheinpreuben Company in 1953. Recently (since 1990) low-temperature Fischer–Tropsch slurry processes are under investigation for the use of iron and cobalt catalysts, particularly for the production of a hydrocarbon wax, or to be hydrocracked and isomerised to produce diesel fuel, by Exxon and Sasol. Today slurry-phase (bubble column) low-temperature Fischer–Tropsch synthesis is regarded by many authors as the most efficient process for Fischer–Tropsch clean diesel production. This Fischer–Tropsch technology is also under development by the Statoil Company (Norway) for use on a vessel to convert associated gas at offshore oil fields into a hydrocarbon liquid.
Raw materials in the proceedings
Coal as raw material
To provide the synthesis gas for the Fischer-Tropsch synthesis was originally coal alone at temperatures above 1000 ° C in the coal gasification, for example in Lurgi pressure gasifier, Winkler generator or Koppers-Totzek reactor, with water vapor and air or oxygen converted to synthesis gas. Since only a hydrogen to carbon monoxide ratio of 0.7 is achieved in this reaction in the first step, a portion of the carbon monoxide is converted with water in a water gas shift reaction to carbon dioxide and hydrogen, up to a ratio of 2: 1 is reached. The synthesis gas is cooled, takingPhenol and ammonia are separated, and subjected to Rectisolwäsche, wherein carbon dioxide, hydrogen sulfide, hydrogen cyanide and organic constituents are removed. The catalysts are sensitive to sulfur, the hydrogen sulfide content is usually to a volume content of less than 30 ppb reduced. 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 1: 2.
Natural gas, biomass and waste as raw material
The big advantage of the Fischer-Tropsch process is that every high-energy raw material is basically suitable for the process. In addition to coal and natural gas, these include biogas, wood, agricultural waste or household waste. The world’s first solid biomass plant was built in 2005 in Choren near Freiberg. In 2011 she became insolvent.
In 2009, the general approval of the Fischer-Tropsch fuels (FT-SPK) by the ASTM as aviation fuel. In 2014, airlines such as British Airways and Cathay Pacific preferred the production of FT fuels from household waste and had begun building such facilities in London and Hong Kong.
In general the product distribution of hydrocarbons formed during the Fischer–Tropsch process follows an Anderson–Schulz–Flory distribution, which can be expressed as:
Wn/n = (1 − α)2αn−1
where Wn is the weight fraction of hydrocarbons containing n carbon atoms, and 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 α 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 had only limited success.
Pressure and temperature
The purified crude gas, which has a ratio of hydrogen to carbon monoxide of about 2 to 2.2, is reacted heterogeneously catalytically in a synthesis reaction to hydrocarbons such as paraffins, olefins and alcohols. End products are gasoline (synthetic gasoline), diesel, heating oil and raw materials for the chemical industry. The reaction takes place already 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:
About 1.25 kilograms of water are produced per kilogram of fuel, about half of the hydrogen used is used for its production. Iron-containing catalysts catalyze the water-gas shift reaction, resulting in carbon dioxide instead of water:
In the Fischer-Tropsch synthesis, a variety of catalysts is used. The most commonly used are based on the transition metals cobalt, iron, nickel and ruthenium. The carriers used are porous metal oxides with large specific surface areas such as kieselguhr, aluminum oxide, zeolites and titanium dioxide.
The catalysts can be prepared by impregnation of the porous metal oxides with metal salt solutions and subsequent calcination. The catalyst activity is by promoters, these are not self-catalytic active catalyst components such as alkali metals or copperincreased. Furthermore, the pore size distribution of the support, the calcination and reduction conditions and the resulting particle sizes of the active catalyst metal affect the catalytic activity. Substances such as alkali metals, which are good promoters for iron catalysts, act as catalyst poison, for example in the case of cobalt catalysts. Cobalt, nickel and ruthenium remain in the metallic state during the reaction, while iron forms a series of oxides and carbides. However, it is believed that cobalt oxides, which are left behind by 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. Fischer’s and Tropsch’s original catalyst was prepared by co-precipitation of cobalt, thorium, and magnesium nitrate, with diatomaceous earth added to the freshly precipitated catalyst. Further steps such as shaping, drying and reduction of the cobalt salt significantly influence the activity of the catalyst. Cobalt catalysts show only low activity in the water-gas shift reaction, whereas iron catalysts catalyze them.
The process is dictated by the need to remove the large heat of reaction of about 3000 kilojoules per cubic meter of converted synthesis gas. The temperature is dissipated by water whose temperature is controlled by adjusting the pressure. Excessively high temperatures lead to methane formation and rapid coking of the catalyst.
The typical Fischer-Tropsch product contains about 10-15% liquefied gases (propane and butanes), 50% gasoline, 28% kerosene (diesel oil), 6% soft paraffin (paraffin gossip) 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:
where W n is the weight fraction of 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 to various products, such as olefins as raw materials for the chemical industry, can be controlled.
The process 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 known as high-load or argon synthesis, was commercialized by a consortium of Ruhrchemie and Lurgi . In this case, the conversion of the coal gasification products to copper and potassium carbonate doped iron contacts 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. The products obtained are paraffin / olefin mixtures, so-called Gatsch.
The reaction is exothermic with 158 kilojoules per mol of CH 2 group formed at 250 ° C:
One problem is the removal of the high heat of hydrogenation to ensure the most isothermal reaction possible. The Arge reactor originally had a diameter of three meters and was equipped with 2052 catalyst tubes, which hold about 35 tons or 40 cubic meters of catalyst. The catalyst is arranged in narrow, lapped by water pipes. The heat of reaction is removed by boiling water under pressure. Insufficient heat removal leads to a temperature gradient across the catalyst bed and can lead to increased methane production or coking of the contacts. A decreasing catalytic activity of the contacts is compensated by an increase in the reaction temperature.
The catalyst volume in modern reactors is about 200 m3 . A Fischer-Tropsch plant with several reactors requires about 1,500,000 m 3 per hour under standard conditions of synthesis gas and produces about 2,000,000 t of hydrocarbons per year. The synthesis is carried out in three stages with a total conversion of about 94%. In addition to the implementation in fixed-bed reactor there are process variants with fluidized bed process (Hydrocol process), as a flue gas synthesis in which the catalyst is present as fluidized fly ash, or in an oil suspension ( Rheinpreußen – Koppers method).
A reaction variant is the Synthol synthesis developed by the companies Sasol and Kellogg. It is not to be confused with the method of the same name developed by Fischer and Tropsch. The process is a flue-gas synthesis; in him the catalyst is metered in as a powder with the reaction gas. The process works at 25 bar and temperatures above 300 ° C. As a result, preferably form low molecular weight hydrocarbons. The ratio of carbon monoxide to hydrogen is about 1: 2.
Cobalt-based catalysts are highly active, although iron may be more suitable for certain applications. 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. Synthesis gases derived from these hydrogen-poor feedstocks has a low-hydrogen-content and require the water–gas shift reaction. 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.
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. 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. C≥5 and CO2 selectivity increase while methane and C2–C4 selectivity decrease. In addition, the alkene to alkane ratio increases.
Fischer–Tropsch catalysts are sensitive to poisoning by sulfur-containing compounds. Cobalt-based catalysts are more sensitive than their iron counterparts.
Fischer–Tropsch iron catalysts need alkali promotion to attain high activity and stability (e.g. 0.5 wt% K2O). Addition of Cu for reduction promotion, addition of SiO2, Al2O3 for structural promotion and maybe some manganese can be applied for selectivity control (e.g. high olefinicity). The working catalyst is only obtained when—after reduction with hydrogen—in the initial period of synthesis several iron carbide phases and elemental carbon are formed whereas iron oxides are still present in addition to some metallic iron. With iron catalysts two directions of selectivity have been pursued. One direction has aimed at a low-molecular-weight olefinic hydrocarbon mixture to be produced in an entrained phase or fluid bed process (Sasol–Synthol process). Due to the relatively high reaction temperature (approx. 340 °C), the average molecular weight of the product is so low that no liquid product phase occurs under reaction conditions. The catalyst particles moving around in the reactor are small (particle diameter 100 µm) and carbon deposition on the catalyst does not disturb reactor operation. Thus a low catalyst porosity with small pore diameters as obtained from fused magnetite (plus promoters) after reduction with hydrogen is appropriate. For maximising the overall gasoline yield, C3 and C4 alkenes have been oligomerized at Sasol. However, recovering the olefins for use as chemicals in, e.g., polymerization processes is advantageous today. The second direction of iron catalyst development has aimed at highest catalyst activity to be used at low reaction temperature where most of the hydrocarbon product is in the liquid phase under reaction conditions. Typically, such catalysts are obtained through precipitation from nitrate solutions. A high content of a carrier provides mechanical strength and wide pores for easy mass transfer of the reactants in the liquid product filling the pores. The main product fraction then is a paraffin wax, which is refined to marketable wax materials at Sasol; however, it also can be very selectively hydrocracked to a high quality diesel fuel. Thus, iron catalysts are very flexible.
Ruthenium is the most active of the F-T catalysts. It works at the lowest reaction temperatures, and it produces the highest molecular weight hydrocarbons. It acts as a Fischer Tropsch catalyst as the pure metal, without any promotors, thus providing the simplest catalytic system of Fischer Tropsch synthesis, where mechanistic conclusions should be the easiest—e.g., much easier than with iron as the catalyst. Like with nickel, the selectivity changes to mainly methane at elevated temperature. Its high price and limited world resources exclude industrial application. Systematic Fischer Tropsch studies with ruthenium catalysts should contribute substantially to the further exploration of the fundamentals of Fischer Tropsch synthesis. There is an interesting question to consider: what features have the metals nickel, iron, cobalt, and ruthenium in common to let them—and only them—be Fischer–Tropsch catalysts, converting the CO/H2 mixture to aliphatic (long chain) hydrocarbons in a ‘one step reaction’. The term ‘one step reaction’ means that reaction intermediates are not desorbed from the catalyst surface. In particular, it is amazing that the much carbided alkalized iron catalyst gives a similar reaction as the just metallic ruthenium catalyst.
HTFT and LTFT
High-temperature Fischer–Tropsch (or HTFT) is operated at temperatures of 330–350 °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 an iron or cobalt-based catalyst. This process is best known for being used in the first integrated GTL-plant operated and built by Shell in Bintulu, Malaysia.
Choren Industries has built a plant in Germany that converts biomass to syngas and fuels using the Shell Fischer–Tropsch process structure. The company went bankrupt in 2011 due to impracticalities in the process.
Biomass gasification (BG) and Fischer–Tropsch (FT) synthesis can in principle be combined to produce renewable transportation fuels (biofuels).
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).
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 unsaturated short-chain hydrocarbons are also produced. Upon introduction to the catalyst’s support, ceria functions as a reverse water gas shift catalyst, further increasing the yield of the reaction. The short-chain hydrocarbons were upgraded to liquid fuels over solid acid catalysts, such as zeolites.
Using conventional FT technology the process ranges in carbon efficiency from 25 to 50 percent and a thermal efficiency of about 50% for CTL facilities idealised at 60% with GTL facilities at about 60% efficiency idealised to 80% efficiency.
Fischer-Tropsch in Nature
A Fischer–Tropsch-type process has also been suggested to have produced a few of the building blocks of DNA and RNA within asteroids. Similarly, naturally occurring FT processes have also been described as important for the formation of abiogenic petroleum.
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