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Energy & Fuels 1999, 13, 756-760
Studies of Jet Fuel Thermal Stability, Oxidation, and Additives Using an Isothermal Oxidation Apparatus Equipped with an Oxygen Sensor Becky Grinstead* and Steven Zabarnick Aerospace Mechanics Division, University of Dayton Research Institute, 300 College Park, Dayton, Ohio 45469-0140 Received November 13, 1998. Revised Manuscript Received January 26, 1999
An isothermal oxidation apparatus (IOA) has been modified to include an oxygen sensor so that both oxidation and deposition data can be measured for jet fuels that are thermally stressed. This modification is useful for comparing the relative oxidation rates of jet fuels and fuels blended with additives. Antioxidants can be evaluated in this apparatus by comparing the delay in the onset of oxidation at a given temperature in a fast oxidizing fuel. Other additive types, such as dispersants, can be evaluated by their ability to reduce deposition.
Introduction In military aircraft, jet fuel is used as both a coolant and a propellant. Future high-performance aircraft engines will produce more excess heat than current engines. Consequently, the jet fuel will be exposed to higher temperatures. When fuel is heated, it reacts with small amounts of dissolved oxygen to form soluble and insoluble oxidation products. Insoluble products (both bulk and surface deposits) pose a problem because they can clog fuel lines and foul close tolerance valves. Ultimately, the fouling caused by deposits could result in engine failure. To address this issue, the U.S. Air Force developed a program called JP-8+100.1 The primary goal of the JP-8+100 program is to increase the heat-sink capacity of JP-8 fuel by 100 °F (55 °C) using a thermal stability additive package. This corresponds to an increase in bulk fuel temperature at the nozzle from 163 to 218 °C. Thermal stability refers to the deposit-forming tendency of a fuel. It is generally accepted that dissolved oxygen initiates the deposition process in freshly refined fuels. Usually, over a range of fuels, an inverse relationship exists between thermal stability and oxidative stabilitysa fuel’s tendency to oxidize. In other words, a fuel that oxidizes slowly is likely to form more deposits than a fuel that oxidizes rapidly.2,3 An exception would be a highly refined hydrotreated fuel, which is required to have an antioxidant added at the refinery to prevent oxidation. This fuel would demonstrate low oxidation and low deposition until the antioxidant was consumed. To this end, many laboratory tests have been developed to investigate the thermal and oxidative stability of fuels. Because fuel in an aircraft experiences various temperatures, flow rates, and oxygen levels, laboratory tests (1) Heneghan, S. P.; Zabarnick, S.; Ballal, D. R.; Harrison, W. E., III J. Energy Resour. Technol. 1996, 118, 170-179. (2) Heneghan, S. P.; Zabarnick, S. Fuel 1994, 73, 35-43. (3) Hardy, D. R.; Beal, E. J.; Burnett, J. C. Proc. 4th Int. Conf. Stability Handling Liq. Fuels 1992, 260-271.
must cover a wide range of conditions. In general, there are two types of laboratory tests: static and flowing. In flowing tests, air-saturated fuel passes through a single-tube heat exchanger. The oxygen content is limited to that which is dissolved in the fuel, which is realistic; however, accelerated temperatures are usually needed to produce measurable deposits. Frequently, deposits are measured by carbon burnoff of the tube at the completion of the test. Due to the large amounts of fuel (many gallons) and relatively long test times (days) that may be required, many flowing tests are not practical for screening a large number of candidate additives. Rather, the flowing rigs are useful for testing a few promising additives and for studying jet fuel behavior at various flow rates, temperatures, and oxygen availabilities. The Phoenix rig is an example of a flowing test that has been used for additive testing and fundamental studies of jet fuel.4 In a static test, fuel is heated in the presence of air or oxygen in a flask or reactor. Generally, deposits are separated from the stressed fuel by filtration after suitable cooling and then measured gravimetrically. Static tests can be accelerated in time, temperature, and oxygen availability. For example, Kendall and Mills used a flask oxidation test to study the thermal stability of kerosines.5 Static tests are useful for screening a large number of additives because test times are short (hours) and sample volumes are small (
