Making on Specification Jet Fuel Through Syncrude Refining

by Qiang Li (Visiting scholoar, PEI)

Figure 1

Background

Today, petroleum is still the dominant energy source in transportation sector. The transportation fuels are primarily produced from crude oil, while alternative sources such as renewables, coal and natural gas still having a small market share. However, this situation could change quickly, since there are deep concerns about current and prospective high oil prices and supply insecurity have stimulated the creation of synthetic fuel from coal (CTL), natural gas (GTL) and biomass (BTL).

The Department of Defense (DOD) of the United States is a major consumer of energy, representing 97% of the total federal government’s total energy use. Within the DOD, the Air Force is the major consumer of jet fuel, accounting for 64% of total DOD energy use. Both the DOD and the Air Force have established goals to reduce energy use, pursue alternative energy sources to reduce operating costs, reduce contribution to greenhouse gas (GHG) emissions, and reduce dependence on foreign energy resources. These energy strategies reflect a challenge to which synthetic fuels offer a solution.

The synthetic fuel can be produced from syngas (CO+H2) through the well-known catalytic reaction called Fischer-Tropsch (FT) synthesis, which is named after the original German inventors in the 1920s. The feed material for the FT synthesis can, in principle, be any carbon source. The feedstock must be gasified or reformed in order to produce syngas. Then the syngas is converted into hydrocarbons which range from light gases to long chain wax through FT reaction. After that, the products are cooled and separated. The formation and distribution of FT products is very sensitive to the types of catalysts and reactors. The operating parameters of pressure, temperature, H2/CO ratio, and space velocity also govern the products distribution. To maximize jet fuel production, a cobalt-based catalyst, tubular fixed bed reactor (TFBR) low temperature Fischer-Tropsch (LTFT, H2/CO=2, Temperature:220-250℃, 25-45bar, 500hr-1) process is usually recommended. Figure 1 is a simplified scheme of the FT process.

 

Syncrude Refining to Produce Jet Fuel

The primary products from FT synthesis have to be refined to produce final products. Therefore, to some extent, the FT products can be seen as syncrude oil.  The properties of jet fuel jet A-1 can be seen in ASTM D1655. Kerosene range distillates are typically in the carbon number range C9-C14, which are the main components of jet fuel. The upper carbon number limit of jet fuel is set by the final boiling point specification(300℃, about n-C17), while the lower limit is set indirectly by the density(775-840kg/m3) and viscosity(maximum 8 cst) specifications.

To maximize jet fuel yield, gaseous products, naphtha, distillate and residue need to be converted into kerosene. The key conversion technologies that have been identified for jet fuel production are oligomerization, hydrotreating, aromatization, catalytic reforming and hydrocracking.

  1. C3-C5 hydrocarbons. The LTFT derived C3-C4 hydrocarbons constitute about 5-8w% of the syncrude and contain 45-65w% olefins. Hence, solid phosphoric acid (SPA) based olefin oligomerization is an ideal technology. Alkylation of propylene with benzene is also an efficient way to produce an aromatic component that is well suited for jet fuel. The product of oligomerization and alkylation which contains oligomers and alkyl aromatics, is then hydrogenated to produce jet fuel. The unconverted C3-C4 paraffins can be sold as commercial product of LPG and C5 paraffins can be blended into motor-gasoline products.
  2. C6-C10 naphtha. The straight run C6-C10 hydrocarbons contain less than 40w% olefins. Mild hydroisomerization is beneficial for this fraction. Aromatization and catalytic reforming of C6-C8 may also be used to increase the yield of jet fuel.
  3. C11and heavier syncrude. Most of the components of heavier LTFT syncrude are chain paraffins, including straight run kerosene, distillate and wax. The best way to refine C11+syncrude is feed it to hydrocracker. The hydrocracker cracks the long chain hydrocarbons to kerosene range and hydroisomerize the linear hydrocarbons to isomeric hydrocarbons. The olefins are saturated and oxygenates are eliminated by hydrogenation. Branched paraffins are essential to meet the freezing point specification (maximum -47℃) of jet fuel. The product of hydrocracking is separated into kerosene, C5-C8 naphtha, and LPG. The LPG and C6-C8 naphtha can be fed to aromatization unit to get aromatic jet fuel components and C5-C6 fraction is hydroisomerized to produce motor-gasoline blending components.

 

Figure 2 illustrates a typical LTFT jet fuel refinery flowscheme. By this FT refinery design, the yield of jet fuel could reach as high as 71w%.

Figure 2

Prospect

Jet fuel is only one example of synthetic fuel production through Fischer-Tropsch synthesis. With increasing desire for alternative transportation fuel, several world-class GTL, CTL and BTL plants in Nigeria, Qatar, USA and China are currently at various stages of engineering and construction. Synthetic fuel is becoming a competitive technology compared to crude-oil-based liquid fuels.

 

Reference

  1. Arno de Klerk, Fischer-Tropsch Refining, WILEY-VCH Verlag GmbH&Co.KGaA, 2011
  2. Guangjian Liu, Eric D. Larson, Robert H. Williams etc., Making Fischer-Tropsch Fuels and Electricity from Coal and Biomass: Performance and Cost Analysis, energy&fuels,25,415-437(2011)
  3. J.Eulers, S.A. Postuma, S.T.Sie, The Shell Middle Distillate Synthesis Process(SMDS), Catalysis Letters, 7,253-270(1990)
  4. Arno de Klerk, Fischer-Tropsch Fuels Refinery Design, Energy&Environmental Science, 4, 11771205(2011)
  5. Arno de Klerk, Fischer-Tropsch Refining, Doctoral Dissertation, 2008, University of Pretoria, South Africa
  6. ASTM D1665-05, Standard Specification for Aviation Turbine Fuels, ASTM: West Conshohocken, PA, 2005

 

 


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