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Thermal Stability of Jet-A Fuel Blends E. Grant Jones*,† Innovative Scientific Solutions, Inc., 3845 Woodhurst Ct., Beavercreek, Ohio 45430
Lori M. Balster and Walter J. Balster Systems Research Laboratories, An Operation of Calspan SRL Corporation, 2800 Indian Ripple Road, Dayton, Ohio 45440-3696 Received September 28, 1995. Revised Manuscript Received December 22, 1995X
The thermal stability of a high-quality hydrotreated Jet-A fuel, an average-quality straightrun Jet-A fuel, and several blends of each fuel has been studied during flow through a singlepass tubular heat exchanger at 185 °C. The goal was to obtain fundamental information concerning the thermal stability of fuel blends using dynamic isothermal methods. Autoxidation was tracked as a function of stress duration by measurements of dissolved oxygen and hydroperoxides; insoluble formation was determined by measurements of the average surface deposition rate and the total quantity of bulk insolubles collected on in-line filters. On the basis of several thermooxidative stability criteria, some of the blends were found to be less stable than either neat fuel. Furthermore, benefits of blending were not realized until the lesser-quality fuel had been diluted more than 8-fold. The implications of blending high- and lesser-quality fuels are discussed in the context of autoxidation and insoluble formation.
Introduction Thermooxidative stability is a very important factor in the selection of aviation fuels. Storage at low temperature can cause discoloration, buildup of hydroperoxides, and the formation of insoluble gums and solids. Stressing at higher temperature that occurs during passage over hot surfaces of heat exchangers, nozzles, and flow-divider valves of aircraft fuel lines can cause fouling, which has serious consequences in terms of engine downtime and long-term aircraft operation. Since advanced aircraft will generate increased heat loads, the search for methods of mitigating the effects of surface fouling has taken on increased importance. Two such approaches based on enhancing fuel thermal stability are special refining techniques and the incorporation of additives. Fuels with certain S- and N-containing components tend to have reduced stability or increased tendency toward surface fouling as compared to similar fuels which have been subjected to hydrotreatment at the refinery and, therefore, contain reduced levels of polar components.1 Previous studies2,3 of fuel stability within our laboratory were conducted on the former (the lesserquality fuels), with the primary goal of creating measurable quantities of insolubles and addressing potential methods for their reduction. Such straight-run fuels are subject to slower autoxidation because they contain natural inhibitors.4 However, frequently the basis of the inhibition (namely, reactions of S- and N-containing
components with peroxy radicals and hydroperoxides) is also the source of the insolubles.5 Fuels in the latter category are frequently costly prime fuels that are of less interest for study because they produce negligible surface fouling in laboratory simulations. Hydrotreated fuels, like pure hydrocarbons, contain few natural inhibitors and, thus, at elevated temperatures undergo more rapid autoxidation, hydroperoxide buildup, and autocatalysis, forming gums but few surface insolubles. Chain-breaking antioxidants such as butylated hydroxytoluene (BHT) are usually added to these fuels to reduce gum formation and improve long-term storage stability.6 Our laboratory has been interested in fundamental methods for quantitatively assessing fuel thermal stability, with the goal of identifying lesser-quality fuels and improving them by introducing additives; such an approach may be more practical and less costly than refining techniques. An obvious extension of this approach is to consider the potential impact on thermal stability of adding or blending a second fuel. For example, if straight-run and hydrotreated fuels represent the two extremes with respect to surface fouling, it would be of interest to determine the fouling tendency of different blends of each fuel. Blending may be viewed alternatively as (1) the contamination of a high-quality fuel with its lesser-quality counterpart or (2) the dilution of a lesser-quality fuel with a prime fuel. Fuel blending has frequently led to thermal-stability problems.7-11 For example, the process of introducing
†
Tel.: (513) 252-4264. Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Hazlett, R. N. Thermal Oxidation Stability of Aviation Turbine Fuels, ASTM Monograph 1; American Society for Testing and Materials: Philadelphia, 1991. (2) Jones, E. G.; Balster, W. J. Energy Fuels 1993, 7, 968-977. (3) Jones, E. G.; Balster, L. M.; Balster, W. J. Energy Fuels 1995, 5, 906-912. (4) Bolshakov, G. F. Sulfur Rep. 1987, 7, Part 5, 379-392. X
0887-0624/96/2510-0509$12.00/0
(5) Kendall, D. R.; Mills, J. S. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 360-366. (6) Turner, L. M.; Kamin, R. A.; Nowack, C. J.; Speck, G. E. Effect of Peroxide Content on the Thermal Stability of Hydrocracked Aviation Fuel Proceedings of 3rd International Conference on Stability and Handling of Liquid Fuels; Institute of Petroleum: London, Nov 1988. (7) Gomes, H. O.; Pereira, R. C. L. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39 (3), 917-922.
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light cycle oil (LCO) into diesel formulations which has been used at the refinery to meet the increasing need for diesel fuels has been reported to reduce thermal stability.7 Datschefski9 and Speck10 have also observed decreased stability with the introduction of heavier components at a level of 1-10%. Upon storage, certain blends fail to pass thermal-stability tests such as the Jet Fuel Thermal Oxidation Test, JFTOT; this problem has been traced to fuel incompatibility.11 In contrast, thermal stability has been reported to improve following dilution of a Jet-A fuel with a paraffinic solvent blend, Exxsol D-80.12 Although blending can be accomplished by design at the refinery, it occurs routinely during storage-tank and aircraft refilling operations. Such blending is rarely documented, and its consequences remain unknown. The goal of the current study was to investigate the impact on thermal stability of blending two distinct types of fuels; namely, a hydrotreated Jet-A-1 (POSF2747) and a straight-run Jet-A (POSF-2827). Both fuels were obtained from a local airport and meet all commercial and USAF specifications; recent results indicate that the hydrotreated fuel is of excellent quality and that the straight-run fuel is a very typical Jet-A aviation fuel having average thermal stability. These fuels have served as references in our laboratory and have been the subject of USAF-sponsored research.13 The neat fuels and selected blends of each were studied in order to sample the continuum between the two fuels and to determine whether the stability of blends could be predicted based simply upon the thermal behavior of the neat fuels. If the addition of small quantities of a prime fuel to an average fuel were found to improve thermal stability, the result would be an inexpensive method of upgrading fuel. The approach was to conduct a simulation of the complicated stressing that occurs in aircraft fuel lines using simplified laboratory tests under well-defined chemical-reaction conditions. The neat fuels and several blends were stressed under high-pressure, isothermal conditions at 185 °C during flow through the heated tubes of a single-pass heat exchanger, and the results of each experiment were compared with respect to established thermal-stability criteria.3 The stress conditions were selected to allow the complete conversion of initial oxygen (air-saturated at room temperature) and the completion of the major deposition processes. For each blend the following measurements were made as a function of stress duration: (1) the concentration of the primary reactant, oxygen, (2) the concentration of one of the first autoxidation products, hydroperoxide, (3) the surface-deposition rate, and (4) the quantity of surface insolubles. In addition, the total quantity of (8) Hazlett, R. N., Fuel Sci. Technol. Int. 1988, 6, 185-208. (9) Datschefski, G. Fuel Sci. Technol. Int. 1988, 6, 609-631. (10) Speck, G. E. Feasibility of Marine Diesel Fuel as an Emergency Aircraft Fuel, Report NAPC-PE 44; Naval Air Propulsion Center: Trenton, NJ, Feb 1981. (11) Goetzinger, J. W.; Ripley, D. L.; French, C. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1989, 39 (1), 804-808. (12) Zabarnick, S.; Zelesnik, P.; Grinstead, R. R. Jet Fuel Deposition and Oxidation: Dilution, Materials, Oxygen, and Temperature Effects. ASME Paper No. 95-GT-050, Presented at the International Gas Turbine and Aeroengine Congress and Exposition, Houston, TX, 5-8 June, 1995. (13) Edwards, T.; Anderson, S. D.; Pearce, J. A.; Harrison, W. E. High Temperature Thermally Stable JP FuelssAn Overview. AIAA Paper No. 92-0683, Presented at the 30th Aerospace Sciences Meeting and Exhibit, Reno, NV, 6-9 January 1992.
Jones et al. Table 1. Fuel Properties type treatment JFTOT breakpoint (°C) total sulfur (%) mercaptan sulfur (%) freezing point (°C) viscosity at -20 °C (cSt) aromatics (vol%) total acid no. (mg of KOH/g) Cu (ppb) Fe (ppb) Zn (ppb)
POSF-2747
POSF-2827
Jet-A-1 hydrotreated 332 0.004 0.000 -60 4 19 0.0
