Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis (FT) is a technical process for converting synthesis gas into liquid and solid paraffinic hydrocarbons. It was developed by German chemists Franz Fischer and Hans Tropsch and patented in 1925. The liquid hydrocarbons are used as fuels, the solid hydrocarbons are highly pure waxes that are used as raw materials for the chemical or pharmaceutical industry. The background of the development was the constant increase in motorisation at the time and the desire for a self-sufficient supply of fuel and raw materials for the petrochemical industry. Local hard coal was used as the raw material back then. A total of nine FT facilities were put into operation by 1945 in the former German Reich with an annual capacity of 600,000 tonnes.

In the post-war period, Fischer-Tropsch synthesis  soon became less  interesting because of the generally cheap supply of crude oil and natural gas. The Republic of South Africa was the only country to continue industrial development due to its political and economic situation. Coal was also used as a raw material there. These facilities are still running today and are supplemented by plants that use natural gas as a raw material. In 1993, the Shell oil company set up a facility in Bintulu, Malaysia, with an annual capacity of 520,000 tonnes. It converts natural gas into highly pure waxes and fuels using FT synthesis.

CO + 2H2 –> (-CH2-) + H2O

The reaction occurs x-times. A straight chain of x -CH2- components is formed. If the chain, for example, consists of 16 CH2 components, for example, this hydrocarbon is called n-hexadecane or also commonly cetane. It is a very important component of diesel fuel.

The optimum ratio of H2 to CO  for the Fischer-Tropsch is two to one. The reaction is exothermal. This means that the heat transfer represents a decisive engineering challenge. A certain temperature needs to be kept constant for the synthesis. A considerable rise would lead to fast coking of the catalyst and would thus stop the synthesis. Basically only iron and cobalt are used as catalysts. Nickel is also active in terms of the hydrogenation of carbon monoxide, but it mainly forms methane. In practice, alloys (additives: alkali metals, copper, ammoniac, manganin, vanadium, titanium) with which specific product compositions can be obtained are used.

The task of carrier materials in the heterogeneous catalysis (i.e. catalyst and reactant are in different aggregate states: e.g. gaseous reactants and solid catalyst) is the formation of a large catalyst surface with a fine distribution of catalyst metals as well as the prevention of Sinter processes in the metal phase. Carrier materials are metal oxides that are difficult to reduce, activated carbon, polymeres and zeolites (crystalline aluminosilicates with a skeleton structure).

To ensure a stable process, the synthesis gas needs to be cleaned completely of catalyst poison before it is injected into the FT synthesis reactor. All materials that react with the catalyst and form an oxide layer or stick to the catalyst are considered to be catalyst poison. Both reduce or block the catalyst effect. The synthesis gas therefore needs to be free of oxygen, sulphur and tar. The sulphur content needs to be less than 5 ppb (parts per billion), for example – that is smaller than 0.000000005.

Advantage: The product is a highly pure fuel, which is free of sulphur and aromatics, can be reproduced with a high level of precision and can be produced with any raw material. This high-quality fuel allows a drastic reduction in emissions, as engine tests at Volkswagen have proven. Furthermore this manufacturing process provides the basis for future engine concepts like CCS, an engine that combines the low emissions of a modern petrol engine with the low fuel consumption of a TDI.