Mar 15 2013
The Fischer-Tropsch (FT) process, originally developed by Franz Fischer and Hans Tropsch in early 1920s, is a series of chemical reactions that involve the conversion of hydrogen and carbon monoxide into liquid hydrocarbons by using a catalyst.
This process is a key component of gas to liquid technology. It produces synthetic lubrication oil and synthetic fuel including natural gas, biomass or coal. Generally, these products are of higher quality than those derived through conventional means, having no sulphur or aromatics.
In the FT process, typical cobalt catalysts are produced using Oxford Catalysts' patented technology with superior stability, selectivity and activity to conventional catalysts, and without any loss of performance.
The FT process has gained importance as a source of low-sulfur diesel fuel and in addressing the cost and supply of petroleum-derived hydrocarbons.
Process Chemistry
The FT process involves a catalytic chemical reaction in which carbon monoxide (CO) and hydrogen (H2) present in the synthetic gas are converted into hydrocarbons of different molecular weights based on the following equation:
(2n+1) H2 + n CO → Cn H(2n+2) + n H2O,
Where 'n' is an integer.
For n=1, the reaction represents production of methane, which is considered as an undesirable by-product in most gas-to-liquids and/or coal-to-liquid applications.
The FT process conditions are chosen such that the formation of hydrocarbon liquid fuels having higher molecular weight is maximized. The process also involves some other side reactions of which the water-gas-shift reaction is predominant.
CO + H2O → H2 + CO2
Hydrocarbons ranging from methane to higher molecular olefins and paraffin can be obtained based on the temperature, process employed and catalyst.
The FT process also yields small quantities of low molecular weight oxygenates such as organic acids and alcohol. The FT synthesis reaction is a condensation polymerization reaction of CO.
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The Fischer-Tropsch Process in three steps. Image Credits: Stanford.edu
Fischer-Tropsch Catalysts
Although several catalysts can be used for Fischer-Tropsch synthesis, the transition metals of ruthenium, nickel, cobalt and iron are some of the most common catalysts. Selection of FT process catalysts is based on the diesel fuels production and high molecular weight linear alkanes.
Nickel can be also be used as the catalyst, but it tends to promote methane formation. Cobalt is more active and usually preferred over ruthenium owing to the high cost of ruthenium.
On the other hand, iron is relatively inexpensive and has high water-gas-shift activity, so it is more suitable for obtaining synthetic gas with low hydrogen/carbon monoxide ratio like those derived through coal gasification.
In addition to the active metal, the catalysts include various promoters such as copper, potassium and high surface area binders such as alumina or silica. The presence of sulfur compounds in the synthetic gas can poison the FT catalysts.
The cobalt-based catalysts have higher sensitivity to sulfur than its iron counterparts, which in turn contributes to higher catalyst replacement costs for Co.
Therefore, cobalt catalysts are preferred for FT synthesis of synthetic gas derived from natural gas, where the synthetic gas has relatively low sulphur content and high hydrogen to carbon monoxide ratio. Iron catalysts are preferred deriving synthetic gas from low quality feedstocks such as coal.
Fischer-Tropsch Reactors
The reactions involved in the FT process are highly exothermic therefore, the elimination of heat is important when designing a commercial reactor. Three types of reactors are generally used for FT synthesis:
- Slurry bed reactor
- Fluidized bed reactor
- Fixed bed reactor
Commercially, all three types of reactors are in use. Heavy FT liquid hydrocarbons are produced in Arge reactors, the multitubular fixed-bed reactors developed by Ruhrchemie and Lurgi and used by Sasol.
Most of these Arge reactors are now replaced by slurry-bed reactors, which are regarded as the state- of-the-art technology for low temperature FT synthesis. Slurry-bed FT reactors have higher conversion rate and better temperature control.
Fluidized-bed FT reactors were developed for producing low molecular gaseous hydrocarbons and gasoline through high temperature FT synthesis.
They were originally developed in a circulating mode and have been replaced by Advanced Synthol reactors of fixed fluidized bed type design. These reactors have high throughputs.
Fischer-Tropsch Process - Commercial Aspects
The FT process has been used for large-scale applications in certain industrial sectors, eventhough its popularity is hampered by high maintenance and operation costs, high capital costs and uncertain price of crude oil.
Several companies are developing the process to enable practical exploitation of stranded gas reserves. Some of them include the following:
- Sasol - Series of plants operated by Sasol in South Africa were implemented with FT technology. The company now uses coal and natural gas as feedstocks and produces a wide range of synthetic petroleum products.
- PetroSA - It is another South African company having the world's largest Gas to Liquids complexes. It uses this technology to convert coal, biomass and natural gas into synthetic fuels.
- Linc Energy - The Australian company constructed the world's first gas-to-liquid plant operating on synthesis gas in 1999, using underground coal gasification. The plant first produced liquids using FT technology in 2008.
Some coal-producing states have invested in FT plants in the US. Waste Management and Processors, Inc.
in Pennsylvania was funded by the state to implement FT technology licensed from Sasol and Shell to convert waste coal into low-sulfur diesel fuel.
Future Development
Currently, the conversion of natural gas to hydrocarbons is one of the most promising technologies in the energy industry due to economic utilization of natural gas to environmentally clean fuels, waxes and other chemicals.
Alternatively, coal or heavy residues can be used on sites where these are available at low costs. With a view to have the greatest impact on the economics of the process, future breakthroughs should focus on decreasing the capital costs of generating synthetic gas and/or improving the thermal efficiency of the FT plant.
An obvious way of enhancing the thermal efficiency of the process is to combine it with a power generation plant. The combination will in turn promote a more efficient utilization of the low pressure steam produced by the FT process.
In addition, the optimization of gas-to-liquids plants can significantly reduce the capital and operating costs of the plants.
Sources and Further Reading