Fischer-Tropsch process is the process of converting Carbon Monoxide into light hydrocarbons by reacting with hydrogen. This process was originally developed by Franz Fischer and Hans Tropsch in the early 1920s. The feedstock is typically natural gas, coal and biomass. The feedstock is first converted into syn-gas by gasification process. The catalyst used for this process generally consists of iron, cobalt or ruthenium.
The syngas is passed through the metal catalyst and undergoes polymerization into hydrocarbon chains via reaction-
n CO + (2n+1) H2 → Cn H (2n+2) + n H2O
where ‘n’ is an integer.
Since the FT reaction is exothermic, excess heat needs to be removed to avoid catalyst deactivation. The FT process also yields small quantities of organic acids and alcohol.
The transition metals like ruthenium, nickel, cobalt and iron are used as catalysts. The choice of catalysts depend upon the end product.
Nickel promotes methane formation. Iron has high water-gas-shift activity. Iron catalyst is more suitable for production of low hydrogen to carbon monoxide ratio gases. Iron catalyst is used for deriving synthetic gas from coal. Cobalt catalyst is used for production of synthetic gas derived from natural gas. Synthetic gas from natural gas have low sulphur content and high hydrogen to carbon monoxide ratio. Ruthenium is the most expensive catalyst. Ruthenium is capable for producing highest molecular weight hydrocarbons without the addition of any promoters.
The catalyst also has promoters such as copper, potassium and high surface area binders such as alumina or silica. The presence of sulphur is undesirable and can act as poison to FT catalysts. Cobalt based catalyst is more sensitive to sulphur than iron based catalyst. Therefore, cobalt catalyst is not used for high sulphur content gas.
The primary process for FT is the Synthol process. The syn gas goes to the synthol reactor at 315 -330 degree Celsius temperature and 2.2 MPa pressure. The product is then sent to catalyst recovery unit where the catalyst is recovered and recycled back to the reactor. The product then moves to hydrocarbon scrubber where heavy oils are dissolved and removed. The remaining product is then sent to tail gas recovery unit where gaseous product is recovered and recycled back to the feed stream. The rest of the product is then distilled into gasoline, jet fuel and diesel.
The resulting products are very clean. The products have very low sulfur content and low aromatic compound content. The FT synthesis gives very high quality diesel fuel because of high straight chain paraffin and low aromatic content but gives low quality low octane number gasoline fuel because of low branch chain paraffin and low aromatic content.
Three types of reactors are used in Fischer-Tropsch process. They are-
- Slurry Bed Reactor
- Fluidized bed reactor
- Fixed bed reactor
Heavy FT hydrocarbon liquids are produced in multi-tubular fixed bed reactors. Most of these reactors are now replaced by slurry bed reactors which have higher conversion rate and better temperature control.
Fluidized bed FT reactors are used to produce low molecular weight gaseous hydrocarbons and gasoline through high temperature FT synthesis. They have been replaced by advanced fixed fluidized bed reactors which have high troughputs.
Several Companies use FT process for production of fuels. This process is not much popular because of high maintenance and operation costs and high capital costs.
The companies that use FT process are:-
1. Sasol, South Africa
The series of plants operated by Sasol uses FT technology for the production of diesel fuel and other synthetic petroleum products. Sasol uses coal and natural gas as feedstocks.
2. Ras Laffan, Qatar
Pearl GTL plant at Ras Laffan, Qatar is the largest FT plant. This plant uses cobalt catalyst at 230 degree Celsius for converting natural gas to petroleum liquids. This plant uses Shell Middle Distillate Synthesis process.
3. Shell, Bintulu, Malaysia
Shell MDS based in Bintulu, Malaysia uses Shell Middle Distillate Synthesis process, to convert natural gas into hydrocarbon liquids.