By: Chuck Sorensen, Ph.D., P.E.
Introduction
Conventional jet fuel is made in oil refineries by various processes that convert crude oil into intermediate streams that boil within the allowable range of jet fuel. These intermediates, called kerosene, are then blended together so that the final product meets jet fuel specifications. The amount of refining needed to produce finished fuel is a strong function of the composition of the crude oil, including the amount of kerosene-range boiling compounds inherently present, their hydrocarbon makeup (e.g., n-paraffin, iso-paraffin, naphthene/cyclo-paraffin, olefin, and aromatic content, i.e. PIONA), and the amount of heteroatom contaminants such as organo-sulfur and nitrogen compounds.[1]
Production Methods
The principal production method used to make the largest quantity of petro-jet components is distilling crude oil into the kerosene boiling range fraction. This boiling range is a variable that can overlap with naphtha at lower boiling points and diesel fuel at the high end. Typically, the kerosene distillation cut, called “straight run kero”, is between 200 and 305C – the same boiling range as jet fuel. Depending on sulfur content, the kero is hydrotreated (HDT) in a desulfurization reactor to remove sulfur atoms by hydrogenating the heteroatom to produce hydrogen sulfide by-product (e.g., H2S) and a low-sulfur hydrocarbon product after separation. Other heteroatoms present such as nitrogen or oxygen are also reduced to comparably low levels, in parallel with sulfur removal during hydrotreatment. If sulfur is low enough in the kero fraction for certain advantaged crude oils, alternative treatment options (such as Merox sweetening) can be used instead of hydrotreating to address sulfur odor and corrosion concerns.
This low-sulfur kerosene thus contains hundreds of different hydrocarbon molecules that differ by carbon number and structure and which boil across a range of temperatures. The principal components are normal paraffins, iso-paraffins, naphthenes, and relatively lower amounts of aromatics, typically less than 25%[2]. See Figure 1.
Figure 1 – Molecular Type and Carbon Number Distribution of Typical Jet Fuels
The aromatic content is directly correlated to an important jet fuel product specification called “smoke point”, which is limited in order to minimize the emissions of soot particles in the jet engine exhaust. However, aromatics are energy dense which helps benefit the airline range. Aromatics have historically been required for material compatibility with the jet fuel distribution system, both on the ground and in the aircraft.
Jet Fuel Specifications
Some important jet fuel specifications that related directly to the distillation step include flash point, freeze point, and end point. For fire safety, the flash point specification is a minimum of 38C. The flash point is governed by the quantity and vapor pressure of light hydrocarbons in the front end of the cut. The freeze point specification limits the molecular weight and type of hydrocarbons boiling at the back end of the range. Linear n-paraffins above 16C are generally responsible for fuel cold flow property limitations.
Also on the back end, there is a boiling point overlap between the end of kerosene and the front end of gasoil which is the refinery precursor to diesel fuel. Refiners often adjust the degree of distillation range overlap between these products to increase or decrease the production of one product over another, subject to constraints imposed by each product’s specifications. Thus, there is some overlap between naphtha and kero, and kero and diesel fractions. Bio-fuel production of sustainable aviation fuel blendstocks by Fisher Tropsch (FT)) or hydrogenation of esters, fats, and oils (HEFA) routes can take advantage of this flexibility to swing the yields between products by choice of distillation cut points when needed.
Increasing Yields by Hydrocracking
To increase kerosene yield and/or to improve the properties of the final product, other conventional refining steps are used. One major upgrading technology is hydrocracking (HDC). In this process, the heavy fraction of crude oil is hydrogenated and catalytically cracked to produce more valuable naphtha, kero, and gasoil range products. These are then added to their respective total refinery production amounts by blending them with a straight run and other comparable intermediate streams. Thus, in many refineries, jet fuels are made by blending the hydrotreated kerosene fraction from crude oil with the hydrocracked kerosene product made from the heavier parts of the crude. Care is taken so that the blended product meets the intended specifications, even though there can be cases where the individual streams alone would not.
Jet Fuel and SAF
According to industry convention, this precursor blend of the differently sourced kerosenes is officially designated as jet fuel only after testing confirms that it meets specifications and that special additives have been blended in. These additives are chemical formulations present at low levels and used to control corrosion, thermal stability, gum formation, fuel icing, and static electricity dissipation. After blending, testing, and additizing, the finished jet fuel is then ready for distribution to airports.
With the airline industry’s drive to decarbonize air travel, current jet fuel formulations are turning toward Sustainable Aviation Fuel (SAF) which is a blend of conventional, petroleum-sourced jet fuel with some renewable materials. CO2 emissions for air travel are directly proportional to the amount of renewable carbon in the fuel and the renewable fuel’s carbon intensity value.
Renewable blended hydrocarbons are made from renewable feedstocks such as corn ethanol, fermentation iso-butanol, or seed oils (like canola and soybean). After the conversion of these feeds by chemical processing, the resulting kerosene-range products contain sub-sets of identically similar molecules found in petroleum jet fuel. When blended with petroleum-sourced jet fuel, the resulting product is Sustainable Aviation Fuel (SAF) which produces less non-renewable CO2 emissions when combusted in a jet engine. SAF is required to meet all of the physical and chemical specifications of conventional petro-jet. The proportion of renewable content in SAF is currently limited by the industry to a maximum of 50 volume percent depending on the process and feedstock used for the renewable blend stock.
The implementation of this technology is in its infancy, and the industry is expected to grow significantly in the coming decade.
About the Author. Chuck Sorensen, Ph.D., P.E. has 37 years of experience with commercializing new chemical & fuel process and product technologies, bio-fuels & bio-chemicals, conventional oil refineries, and chemical plant operations, process engineering & design, and capital project analysis. and oversees matters involving Refinery Co-Processing, Biofuel Blending and Distribution, Renewable Diesel, SAF, FT Upgrading, Marine Biofuels, TEA & Business Case Analyses.
Footnotes:
[1] Alkyl-sulfides and alkyl-thiophenes are examples of the types of organo-sulfur compounds present in kerosene which is part of crude oil boiling in the jet fuel range.
[2] Edwards, Tim and Research, Air Force. Conventional & Renewable Jet Fuel Testing Review. https://www.caafi.org/news/pdf/Edwards_AIAA-2017-0146_Reference_Jet_Fuels.pdf.
