CS Knowledge Base

The Fischer–Tropsch process (FT) 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 150–300 °C (302–572 °F) 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.^[1]

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.^[2] 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.^[3][4][5]

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.^[6]

The Fischer–Tropsch process involves a series of chemical reactions that produce a variety of hydrocarbons, ideally having the formula (C_nH_2n+2). The more useful reactions produce alkanes as follows:^[7]

(2n + 1) H₂ + n CO → C_nH_2n+2 + n H₂O

where n is typically 10–20, resulting mostly in the formation of higher alkanes.^[8] 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.^[9]

The reaction is a highly exothermic reaction due to a standard reaction enthalpy (Δ_rH) of −165 kJ/mol CO combined.^[10]

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 be manufactured.^[13] Fischer-Tropsch using high-temperature iron-based catalysts yields a wide array of short-chain paraffins, olefins, and aromatics.^[14][15] Low-temperature cobalt-based catalysts produce a majority of longer-chain n-paraffin species, mainly as liquids and waxes.^[16] 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.^[17][18] 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:

1. Reverse Water Gas Shift reaction:
2. Dry Methane Reforming:

The reverse water gas shift reaction uses carbon dioxide and an excess of hydrogen such as from water electrolysis (green hydrogen) to yield a Syngas mixture of carbon monoxide (CO) and hydrogen (H2) with the desired H2:CO Syngas ratio for a downstream Fischer-Tropsch catalyst; a commercial catalytic process that has been pioneered by Dimensional Energy.^[19][20] Dry methane reforming utilizes CO and methane (CH4) 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 HYCO1 and BASF.

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.^[21] Upon introduction to the catalyst's support, ceria functions as a reverse water-gas shift catalyst, further increasing the yield of the reaction.^[22] The short-chain hydrocarbons were upgraded to liquid fuels over solid acid catalysts, such as zeolites.

Fischer–Tropsch plants associated with biomass or coal or related solid feedstocks (sources of carbon) must first convert the solid fuel into gases. These gases include CO, H₂, and alkanes. This conversion is called gasification.^[23] Synthesis gas ("syngas") 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.

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 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 (< 1).

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

This type of reactor contains several tubes with small diameters. These tubes contain catalysts and are surrounded by cooling water which removes the heat of the reaction. A fixed-bed reactor is suitable for operation at low temperatures and has an upper-temperature limit of 257 °C (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

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 297 °C (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 (> 297 °C, 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 FT 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 impractical economics due to increasing petroleum availability, this development was discontinued. Fluid-bed FT synthesis has been reinvestigated by Sasol. One reactor with a capacity of 500,000 tons per annum is in operation. The process has been used for C₂ and C₇ alkene production. 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 Sasolburg, South Africa, 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. The circulating catalyst process can be replaced by 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. Since 1990, low-temperature FT 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 isomerized to produce diesel fuel, by Exxon and Sasol. Slurry-phase (bubble column) low-temperature FT synthesis is efficient. This 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.^[24]