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Fischer Tropsch

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The economy of the Fisher-Tropsch process is critically dependent on the effectiveness of the cobalt catalyst used. Therefore the design of FT catalysts tends to focus on methods to improve the cobalt utilization by optimizing metal dispersion, improving reductability and or increasing stability. 

Fischer-Tropsch synthesis rate increases linearly with cobalt metal dispersion irrespective of the chemical identity of the underlying support. As the crystal structure depends only weakly on crystalline diameter over this dispersion range, it is not certain that above conclusions can be extrapolated to higher dispersions.

Process

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The Fischer-Tropsch Process is a key part of the technology that is needed to convert one type of carbon fuel into another.This in turn allows industry to choose which feedstock and which technique is most suitable for a given purpose. A number of composite technologies known as XTL have been developed.

A number of composite technologies known as XTL have beel developed; CTL (coal-to-liquids), GTL (gas-to-liquids), BTL (biomass-to-liquids) and WTL (waste-to-liquids). Thus for example, GTL reforms natural gas (mainly methane) by partial oxidation into syngas.

CH4 + H2O $\rightleftharpoons$ CO + 3H2

CH4 + CO2 $\rightleftharpoons$ 2CO + 2H2

Alternatively CTL for example, makes syngas from coal.

2C + H2O + O2 $\rightarrow$ CO + CO2 + H2

Synthesis

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The Fischer-Tropsch synthesis (FTS) is a rather old catalytic process to convert syngas into liquid fuels. In the last decade the interest in the FTS has been considerably revived in view of exploiting both natural gas from remote sources and associate gas from oil wells. A key target is to achieve high selectivities to heavy paraffins, which can be cracked subsequently to hydrocarbons in the fuel range, and specific to diesel components with high cetane numbers. The process is exothermal and the chain growth to heavy hydrocarbons proceeds via a polymerization like scheme based on sequential additions leading to the well known Anderson-Schultz-Flory distribution.

Fischer-Tropsch process are still not completely understood, even though the reaction was discovered 90 years ago. In the Fischer Tropsch synthesis hydrocarbons are synthesized from carbon monoxide (CO) and hydrogen (H2) (in other words, syngas). The Fischer-Tropsch process is an established technology and already applied on a large scale. Synfuels produced by the Fischer-Tropsch process are nowadays more expensive than natural oil based hydrocarbon fuels. However, under certain conditions and in the next century, the process economics may become favorable.
The process deserves special attention where
  1. Coal reserves are significant and available at a cheaper rate and
  2. Natural gases are abundant. In the long run, production of hydrocarbon synthesis based on coal will exceed that of oil.

Reaction

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The Fischer-Tropsch reaction named after the original inventors, is essentially the conversion of synthesis gas (CO + H2) to a mixture of hydrocarbons, and to a lesser extent oxygenated hydrocarbons. The commercial Fischer-Tropsch reaction such as the one practiced at SASOL in South Africa uses potassium and copper promoted heterogeneous iron catalysts. A wide range of hydrocarbons, containing one or more than 100 carbon atoms, is produced. Paraffins and to a lesser extent alkenes are the main products. These are thought to arise through carbide intermediates that are converted to CH2 groups. There is no homogeneous Fischer-Tropsch that gives paraffin or alkene in good yield.

The CO used for the carbonylation reaction always contains some hydrogen. The side products in the BASF carbonylation process arise due to Fischer-Tropsch reaction catalyzed by the cobalt catalyst. The high temperatures and pressures used in the BASF process are conditions under which the Fischer Tropsch reaction with soluble cobalt catalyst can take place. In the Monsanto process the reaction conditions are much milder, and the side product forming Fischer-Tropsch reaction is avoided. 

The proposed catalytic cycle for the Fischer-Tropsch reaction, giving oxygenated hydrocarbons is shown below. 

Fischer Tropsch Reaction

However under high pressure and temperature this reaction may be kinetically favorable or may take place by an intermolecular mechanism.

Fischer Tropsch Reactor

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The Fischer-Tropsch reactors are operate at pressures ranging from 10 to 40 bar. Upgrading usually means a combination of hydro treating, hydro cracking and hydro isomerization in addition to product separation. In the cobalt-catalyzed Fischer-Tropsch reaction, oxygen is mainly rejected as water and this will generate high partial pressures of water at the reactor exit for fixed-bed reactors. As a consequence of extensive back mixing in slurry reactors, high water concentrations and low reactant concentrations will exit throughout the entire reactor. Water will therefore always be present in varying quantities during the reaction.

The general Fischer-Tropsch process flow diagram is shown below. 

Fischer Tropsch Reactor

There are four main steps to producing Fischer-Tropsch products: syngas generation, gas purification, Fischer-Tropsch synthesis and product upgrading.