Fischer-Tropsch (FT) Synthesis
Liquid transportation hydrocarbon fuels can be produced from syngas via a well-known catalytic chemical reaction called Fischer-Tropsch (FT) synthesis, named after the original German inventors, Franz Fischer and Hans Tropsch in the 1920s. During World War II, FT synthesis provided the needed liquid hydrocarbon fuels for the German war effort. Later, facing isolation during the apartheid era, South Africa turned to FT synthesis from coal gasification to supply significant quantities of its hydrocarbon fuel needs. Since then, many refinements and adjustments to the technology have been made, including catalyst development and reactor design. Depending on the source of the syngas, the technology is often referred to as coal-to-liquids (CTL) and/or gas-to-liquids (GTL). Examples of current operating CTL plants include Sasol’s Sasolburg I and II plant, and an example of a GTL FT process is Shell’s plant in Bintulu, Malaysia. Several world-class GTL and CTL plants are currently at various stages of engineering and construction in Nigeria, Qatar and China. Recently, methanol synthesis combined with new methanol-to-gasoline processes have become a competing technology for the traditional FT approach.
The Fischer-Tropsch process is a catalytic chemical reaction in which carbon monoxide (CO) and hydrogen (H2) in the syngas are converted into hydrocarbons of various molecular weights according to the following equation:
(2n+1) H2 + n CO → Cn H(2n+2) + n H2O
Where n is an integer. Thus, for n=1, the reaction represents the formation of methane, which in most CTL or GTL applications is considered an undesirable byproduct. The Fischer-Tropsch process conditions are usually chosen to maximize the formation of higher molecular weight hydrocarbon liquid fuels which are higher value products. There are other side reactions taking place in the process, among which the water-gas-shift reaction
CO + H2O → H2 + CO2
is predominant. Depending on the catalyst, temperature, and type of process employed, hydrocarbons ranging from methane to higher molecular paraffins and olefins can be obtained. Small amounts of low molecular weight oxygenates (e.g., alcohol and organic acids) are also formed. The Fischer-Tropsch synthesis reaction, in theory, is a condensation polymerization reaction of CO. Its products obey a well-defined molecular weight distribution according to a relationship known as Shultz-Flory distribution.
A variety of catalysts can be used for Fischer-Tropsch synthesis, but the most common are transition metals of iron, cobalt, nickel and ruthenium. FT catalyst development has largely been focused on the preference for high molecular weight linear alkanes and diesel fuels production. Among these catalysts, it is generally known that:
- Nickel (Ni) tends to promote methane formation, as in a methanation process; thus generally it is not desirable
- Iron (Fe) is relatively low cost and has a higher water-gas-shift activity, and is therefore more suitable for a lower hydrogen/carbon monoxide ratio (H2/CO) syngas such as those derived from coal gasification
- Cobalt (Co) is more active, and generally preferred over ruthenium (Ru) because of the prohibitively high cost of Ru
- In comparison to iron, Co has much less water-gas-shift activity, and is much more costly.
In addition to the active metal, the catalysts typically contain a number of promoters, including potassium and copper, as well as high surface area binders/supports such as silica and/or alumina.
FT catalysts are sensitive to the presence of sulfur compounds in the syngas and can be poisoned by them. The sensitivity of the catalyst to sulfur is higher for Co-based catalysts than for their iron counterparts, which contributes to higher catalyst replacement costs for Co. For this reason, Co catalysts are preferred for FT synthesis with natural gas derived syngas, where the syngas has a higher H2:CO ratio and is relatively lower in sulfur content. Iron catalysts are preferred for lower quality feedstocks such as coal.
The Fischer-Tropsch reaction is highly exothermic; therefore heat removal is an important factor in the design of a commercial reactor. In general, three different types of reactor design might be used for FT synthesis:
- Fixed bed reactor
- Fluidized bed reactor
- Slurry bed reactor.
All three types of reactors are in use commercially. The multitubular fixed-bed reactors, known as Arge reactors, were developed jointly by Lurgi and Ruhrchemie and commissioned in the 1955. They were used by Sasol to produce heavy FT liquid hydrocarbons and waxes in Sasolburg, in what Sasol called it now their Low-Temperature FT Synthesis Process, aiming for liquid fuels production. Most, if not all, of these types of Arge reactors are now be replaced by slurry-bed reactors, which is considered the state-of-the-art technology for low temperature FT synthesis. Slurry-bed FT reactors offer better temperature control and higher conversion. Slurry-bed FT reactors are also being developed by other Fischer-Tropsch technology vendors, namely Exxon and Shell.
Fluidized-bed FT reactors were developed for high temperature FT synthesis to produce low molecular gaseous hydrocarbons and gasoline. It was originally developed in a circulating mode, e.g., Sasol’s Synthol reactors, and they have since been replaced by a fixed fluidized bed type of design called Advanced Synthol reactors. These types of reactors have high throughputs.