Historically, the refining of liquid products derived from the conversion of gas and coal by the Fischer-Tropsch (FT) synthesis has focused on fuels production. Nevertheless, the FT primary products are rich in chemicals that can be extracted. Olefins, especially linear alphaolefins, oxygenates, linear paraffins and waxes are the most abundant chemicals that can be considered for extraction, although the nature and concentration of the chemicals depend on the FT catalyst and operating conditions.
Some processes for FT chemicals extraction have been commercialised, but to date no FT refinery has been built with chemical production as primary aim. Since chemicals generally are more valuable than fuels, a chemical refinery could conceivably be more profitable than a fuels refinery.
The traditional approach is to consider the FT product as fuel, with add-on units siphoning off the valuable chemicals. In the present analysis the FT synthetic crude has been considered as chemical feedstock, with fuels being a low value chemical product. It will be shown that this not only simplifies chemical recovery, but also offers refining advantages based on the nature of the FT synthetic crude.
The product from a Fischer-Tropsch (FT) process1 is rich in functionalised molecules. Olefins, especially linear alpha-olefins, oxygenates, linear paraffins and waxes are the most abundant chemicals that can be considered for extraction. Such molecules are reactive for conversion to high value products and some are commodity chemicals in their own right. Historically FT synthesis has been used mostly for fuels production. Although some processes for FT chemical extraction have been commercialised, no FT refinery has been built with chemical production as primary aim. Even the most recent FT based gas-to-liquids (GTL) plant, which is currently being constructed in Qatar, has been designed for fuels production. In a fuels refinery the total FT product is considered a syncrude, which is refined to produce transportation fuel. Chemicals extraction is done by add-on units in a way that does not negatively affect the fuel refinery viability. The design philosophy of an FT based chemicals refinery is fundamentally different. In a chemicals refinery the design aims to convert most of the material to chemicals by exploiting the unique characteristics of the FT product. The remainder is considered syncrude, which can be converted to fuel.
The scope of this paper is limited to FT product refining. Obvious FT production synergies, like ammonia synthesis and syngas based technologies like hydroformylation and methanol synthesis have not been explored here. Extensive downstream integration has not been considered either. The design philosophy is illustrated by a design based on a High Temperature Fischer-Tropsch (HTFT) product slate, similar to that being produced at the Sasol refinery in Secunda, South Africa.
The FT refinery at Sasolburg, South Africa, was originally designed to produce fuels, but was later converted to produce only chemicals. Considering that this is a Low Temperature Fischer-Tropsch (LTFT) facility, one may ask why HTFT is the preferred platform for an FT based chemicals refinery.
