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Energy & Fuels 1999, 13, 796-802
Autoxidation of Dilute Jet-Fuel Blends E. Grant Jones* and Lori M. Balster Innovative Scientific Solutions, Inc., 2766 Indian Ripple Road, Dayton, Ohio 45440-3638 Received September 24, 1998. Revised Manuscript Received May 6, 1999
The depletion of dissolved O2 has been measured at 185 °C for a series of 12 jet fuels diluted 10-fold in a paraffinic solvent. Reaction is limited by the fixed amount of O2 present in air-saturated fuel. Because of dilution, aromatics, olefins, and also species such as dissolved metals and natural secondary antioxidants that influence oxidation by collision with hydroperoxides are less important. Under such dilute conditions, autoxidation is simplified, being governed mainly by the residual natural primary antioxidants acting as retarders or inhibitors to slow oxidation of the diluent. Oxidation of diluted fuels is, therefore, characterized by a time delay, followed by reaction acceleration. The time required to achieve 50% conversion of O2 has been used as a measure of the efficiency and concentration of primary antioxidants present in the diluted fuel. In turn, it is proposed that this time is an indirect measure of primary antioxidants originally present in the neat fuel. Similarity in the observed oxidation behavior of diluted fuels and hydrotreated fuels and the parallel of removing polar species either by dilution or by hydrotreatment lead to classification of such diluted fuels as surrogate hydrotreated fuels. Improved thermal stability following dilution, differences in the response of the neat and the diluted fuel to several additives, the effect of increased inital dissolved O2, and the measured concentration of hydroperoxides further support the analogy between dilution and hydrotreatment.
Introduction In recent studies concerning the liquid-phase oxidation of neat and blended aviation fuels,1,2 many blends have been observed to oxidize more slowly than either of their component fuels. In particular, blends of lowthermal-stability jet fuels with paraffins or other severely hydrotreated fuels tend to undergo unusually slow oxidation. Since hydrotreatment removes many polar species that are natural antioxidants, refined fuels usually oxidize more rapidly and require the addition of synthetic antioxidants to improve stability during storage.3 Thus, species associated with the lower thermal stability fuel have been postulated to play important roles in determining the oxidation behavior of such blends.1,2,4 Among the components to be considered are primary and secondary antioxidants, aromatics, and dissolved metals; however, no single factor has been found to be responsible for the observed behavior. Variability of aviation-fuel composition and the complex dependence of oxidation on the concentration of trace components make it difficult to assess the relative contributions of antioxidants, aromatics, and dissolved metals in determining oxidation behavior.2 * Corresponding author telephone, (937) 252-4264; e-mail, joneseg@ innssi.com. (1) Balster, L. M.; Balster, W. J.; Jones, E. G. Energy Fuels 1996, 10, 1176-1180. (2) Jones, E. G.; Balster, L. M.; Balster, W. J. Energy Fuels 1998, 12, 990-995. Balster, L. M.; Jones, E. G. Prepr.sAm. Chem. Soc., Div. Fuel Chem. 1998, 43 (1), 49-52. (3) Turner, L. M.; Kamin, R. A.; Nowack, C. J.; Speck, G. E. Proceedings of the 3rd International Conference on Stability and Handling of Liquid Fuels; Institute of Petroleum: London, Nov 1988; pp 338-349. (4) Zabarnick, S.; Zelesnik, P.; Grinstead, R. R. ASME J. Eng. Gas Turb. Power 1996, 118, 509-515.
By considering only dilute blends with paraffins, autoxidation becomes less complicated for several reasons. First, the role of aromatics is greatly reduced because of their lowered concentration (∼ 2 vol %). Second, reactive olefins are reduced to < 1%. Third, dilution discriminates against collisions between trace fuel components and hydroperoxides (ROOH), thereby significantly reducing the role of both secondary antioxidants and dissolved metals. Secondary antioxidants slow oxidation by destroying ROOH without radical initiation, thereby competing with radical initiation by homolysis. Dissolved metals accelerate oxidation by catalytically decomposing ROOH with the formation of additional free radicals. Primary antioxidants such as phenols, amines, and thiols interfere with radical chain propagation through the formation of stable or hindered radicals, the result being an induction or a delay time before rapid (or the autocatalytic phase of) oxidation. These species are known to function most efficiently at an optimum concentration.5 An excess of natural antioxidants is associated with fuels of lower thermal stability; therefore, reducing the concentration of these natural antioxidants by dilution may lead to a relative enhancement of primary-antioxidant efficiency. Some compositional changes caused by dilution are analogous to those occurring as a result of hydrotreatment conducted at the refinery to improve fuel thermal stability. For example, olefins are saturated and trace metals eliminated by refining. Depending on the severity of the hydrotreatment, aromatic concentration can also be lowered. Naturally occurring N-, S-, and O(5) Scott, G. Atmospheric Oxidation and Antioxidants; Elsevier: New York, 1965; Chapter 4, pp 115-169.
10.1021/ef980198x CCC: $18.00 © 1999 American Chemical Society Published on Web 06/12/1999
Autoxidation of Dilute Jet-Fuel Blends
Energy & Fuels, Vol. 13, No. 4, 1999 797 Table 1. Fuels Studied
fuel
type
treatment
aromatics %,vol
total surface insolubles formed at 185 °C, µg/mL
total sulfur ppm
Exxsol D-110 POSF-3084 POSF-3119 POSF-3166 POSF-2985 POSF-2827 POSF-2959 POSF-2963 POSF-2962 POSF-2980 POSF-2926 POSF-2747 POSF-2976
paraffin/c-paraffin Jet-A Jet-A Jet-A JP-5 Jet-A Jet-A JP-5 JP-5 Jet-A Jet-A Jet-A-1 JPTS
hydrotreated straight-run straight-run straight-run straight-run straight-run Merox-treated straight-run straight-run Merox-treated hydrotreated hydrotreated hydrotreated
