The Fischer-Tropsch (FT) synthesis has been used for long time for the production of synthetic hydrocarbons from synthesis gas (syngas), a mixture of gases comprising mostly hydrogen (H2) and carbon monoxide (CO).
The FT synthesis is a chemical reaction conducted over a metal oxide catalyst where the active metal comprises iron (Fe), cobalt (Co), ruthenium (Ru) and nickel (Ni). These catalysts can be produced by precipitation or can be supported in a metal oxide like alumina, titania, zirconia, magnesia and the like.
The FT synthesis primary reaction can be described as follows:2H2+CO→—[CH2]—+H2O
In this reaction —[CH2]— represents the primary building block of a paraffinic hydrocarbon, sometimes also referred as alkanes.
The process is carried out at elevated temperatures normally in the 200-400° C. range and high pressures of up to 40 bar-g. It can be conducted in many reactor designs like (i) tubular fixed beds, (ii) 3-phase slurry beds (a.k.a. bubble columns), (iii) high temperature circulating beds, and (iv) 2-phase fluidised beds. These had been extensively described in the literature in works like the Fischer-Tropsch Technology book edited by A P Steynberg and M E Dry (Elsevier, 2004).
This reaction proceeds by a mechanism known to those in the art as chain propagation. The reaction above described repeats many times resulting in the production of long chain species with up to 100 carbon atoms. The products from this synthesis are often described in the art as primary FT products, covering a very broad distillation range including FT hydrocarbon gasses (C2 to C4), FT liquid products (C5 to C21) and solid FT products (C22 and heavier) at ambient conditions. This blend has also been described as a synthetic crude or syncrude in many publications like “Processing of Fischer-Tropsch Syncrude and Benefits of Integrating its Products with Conventional Fuels” (NPRA Paper AM-00-51, 2000).
The primary products or syncrude from the FT synthesis can be separated into various liquid streams at processing conditions for example a C3 to C5 range, a naphtha C6 to C8 range and a C9 and heavier range.
Fuels refineries, irrespective of whether they refine crude oil, Fischer-Tropsch syncrude, coal liquids, oil shales or tar sands, generally produce a product slate that may include a kerosene cut.
The most common refining pathway for producing jet fuel (Jet A-1) from the kerosene range material is by hydroprocessing. In some instances it is desirable to produce more kerosene range material to ultimately boost jet fuel production and suggestions for doing this have been made, for example U.S. Pat. No. 4,409,092.
More recently, the United States Department of Defense expressed interest in a universal fuel for military use and preferably from a synthetic process to improve energy security (Forest, et al. 2005). This so-called Battlefield Use Fuel of the Future (BUFF), is very similar to jet fuel in specifications, but has a more stringent flash point specification (60° C. like JP-5). Ideally such a synthetic fuels refinery should produce only kerosene range material. Although a “jet fuel only” refinery may be conceptually devised, there are limits to the yield of kerosene range material that can be obtained in practise. Even conversion processes known in the art to be very kerosene selective, such as the conversion of propylene over solid phosphoric acid (Jones 1954), do not exclusively yield kerosene range material.
Straight run Fischer-Tropsch products have some inherent drawbacks in meeting Jet A-1 and/or BUFF specifications, namely a high linearity that results in a high freezing point and low temperature viscosity and a tow aromatics content. Furthermore, the Anderson-Schultz-Flory distribution often used to describe the carbon number distribution of Fischer-Tropsch products, show that the volume of straight run syncrude material in the kerosene range is limited, irrespective of the Fischer-Tropsch process. Using the Anderson-Schultz-Flory description it may be shown that the straight run kerosene production from a Fischer-Tropsch process can be optimised by tailoring the Fischer-Tropsch catalyst to have a chain growth probability factor (α-value) preferably In the range 0.76-0.86. Most current commercial Fischer-Tropsch technologies operate outside of this range, with high temperature Fischer-Tropsch (HTFT) technology operating with an α-value of less than 0.7, while low temperature Fischer-Tropsch (LTFT) technology generally operates with an α-value higher than 0.9. However, even with a Fischer-Tropsch conversion process optimised for the production of kerosene range material, the straight run yield of kerosene is less than 30%. Nevertheless, the synthesis of jet fuel components from Fischer-Tropsch products and the blending thereof to produce semi-synthetic jet fuel and fully synthetic jet fuel are known in the art.
In this context, kerosene may be understood as a fraction having a carbon number range of C9 to C16, typically having a boiling point range of from 149° C. to 288° C., although variations to this definition may exist.
Preparation of Jet A-1 from low temperature Fischer-Tropsch (LIFT) hydrocracker products by distillation results in a small C8-C13 fraction that meets most of the specifications, but contains less that the required 8% aromatics of Jet A-1
The initial boiling point of the material is limited by the flash point specification of jet fuel, while the final boiling point is restricted by the Jet A-1 specification (maximum 300° C.). However, in practise a lower final boiling point is often required to meet the freezing point specification (maximum −47° C.). It has also been pointed out that the low temperature viscosity of kerosene range material from Low Temperature Fischer-Tropsch (LTFT) syncrude may present a problem by being too viscous (Lamprecht 2006).
The LTFT hydrocracker design of Shell, as commercially used in their gas-to-liquids facility in Bintulu, Malaysia, has a maximum kerosene mode of operation which is reportedly capable of refining 50% of the LTFT syncrude to kerosene range material while the naphtha and distillate products from such an operation are only blending components and not transportation fuels meeting motor-gasoline and diesel fuel specifications.
Preparation of Jet A-1 from high temperature Fischer-Tropsch (HTFT) hydrogenated straight run kerosene and iso-paraffinic kerosene from short chain olefin oligomerisation over Solid Phosphoric Acid (SPA), meets all the specifications, including density, however, the volumes that can be produced are unfortunately limited.
What is needed is a synthetic fuels facility that can maximise kerosene yield in such a way that the kerosene meets Jet A-1/JP-8 and/or JP-5/BUFF specifications.
A need has been identified for a kerosene dominant refinery in which the naphtha and distillate that are co-produced are easily refinable to meet final fuel specifications, rather than having to be sold as naphtha or distillate blend stock. This is especially pertinent in instances where the refinery is located far from markets for such intermediate products, or in instances where the refinery is used as strategic asset for the production of fuel for military use.