Fischer–Tropsch process

The Fischer–Tropsch process is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen, known as syngas, into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, typically at temperatures of and pressures of one to several tens of atmospheres. The 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 FT, are produced from coal, natural gas, or biomass in a process known as gasification. The process then converts these gases into synthetic lubrication oil and synthetic fuel. This process has received intermittent attention as a source of low-sulfur diesel fuel and to address the supply or cost of petroleum-derived hydrocarbons. Fischer–Tropsch process is discussed as a step of producing carbon-neutral liquid hydrocarbon fuels from CO₂ and hydrogen.
The process was first developed by Franz Fischer and Hans Tropsch at the Kaiser Wilhelm Institute for Coal Research in Mülheim an der Ruhr, Germany, in 1925.

Reaction mechanism

The Fischer–Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbons, ideally having the formula. The more useful reactions produce alkanes as follows:
where n is typically 10–20, resulting mostly in the formation of higher alkanes. The formation of methane 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.
The reaction is a highly exothermic reaction due to a standard reaction enthalpy of −165 kJ/mol CO combined.

Fischer–Tropsch intermediates and elemental reactions

Converting a mixture of H₂ and CO into aliphatic products is a multi-step reaction with several intermediate compounds. 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 → + 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 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, hydroxycarbene, hydroxymethyl, methyl, methylene, methylidyne, and hydroxymethylidyne. 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 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'.

Feedstocks: Carbon Dioxide

has emerged as an important carbon source for replacement of chemicals and fuels derived from fossil fuels. Based on the pioneering work of Sasol in South Africa using gasification to produce Syngas, an entire slate of downstream chemicals and fuels can manufactured. Fischer-Tropsch using high-temperature iron-based catalysts yields a wide array of short-chain paraffins, olefins, and aromatics. Low-temperature cobalt-based catalysts produce a majority of longer-chain n-paraffin species, mainly as liquids and waxes. These can be processed into a multitude of products such as zero-sulfur sustainable aviation fuel, diesel, base oils, and naphtha feedstock that can be catalytically reformed for BTX production of polymer precursors. By substituting carbon dioxide as the carbon source, entire fossil fuel supply chains can be replaced with Fischer-Tropsch products. Production of a Syngas feedstock made from carbon dioxide can use the following catalyzed chemical reactions:

Reverse Water Gas Shift reaction: CO2 + H2 -> CO + H2O
Dry Methane Reforming: CO2 + CH4 -> 2 CO + 2 H2

The reverse water gas shift reaction uses carbon dioxide and an excess of hydrogen such as from water electrolysis to yield a Syngas mixture of carbon monoxide and hydrogen with the desired H2:CO Syngas ratio for a downstream Fischer-Tropsch catalyst; a commercial catalytic process that has been pioneered by .Dry methane reforming utilizes CO and methane to yield a 1:1 H2-to-CO Syngas mixture that can be augmented with an external source of hydrogen or by the water-gas shift reaction to achieve the desired Syngas ratio. Commercial dry methane Reforming catalysts are available from and .
Carbon dioxide is not a typical direct feedstock for FT 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.

Feedstocks: gasification

Fischer–Tropsch plants associated with biomass or coal or related solid feedstocks must first convert the solid fuel into gases. These gases include CO, H₂, and alkanes. This conversion is called gasification. Synthesis gas obtained from biomass/coal gasification is a mixture of hydrogen and carbon monoxide. The H₂:CO ratio is adjusted using the water-gas shift reaction. Coal-based FT 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 process.

Feedstocks: GTL

Carbon monoxide for FT catalysis is derived from hydrocarbons. In gas to liquids technology, the hydrocarbons are low molecular weight materials that often would be discarded or flared. Stranded gas provides relatively cheap gas. For GTL to be commercially viable, gas must remain relatively cheaper than oil.
Several reactions are required to obtain the gaseous reactants required for FT catalysis. First, reactant gases entering a reactor must be desulfurized. Otherwise, sulfur-containing impurities deactivate the catalysts required for FT 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:
For FT plants that use methane as the feedstock, another important reaction is dry reforming, which converts the methane into CO and H₂:

Process conditions

Generally, the Fischer–Tropsch process is operated in the temperature range of. 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 the 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 deactivate the catalyst 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 their intrinsic water-gas shift reaction activity. This reactivity can be important for synthesis gas derived from coal or biomass, which tend to have relatively low H₂:CO ratios.

Design of the Fischer–Tropsch process reactor

Efficient removal of heat from the reactor is the basic need of FT reactors since these reactions are characterized by high exothermicity. Four types of reactors are discussed:

Multi tubular fixed-bed reactor

Entrained flow reactor

Slurry reactors

Fluid-bed and circulating catalyst (riser) reactors

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:
where W_n 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 FT 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. This way they can drive the reaction so as to minimize methane formation without producing many long-chained hydrocarbons. Such efforts have had only limited success.