Ind. Eng. Chem. Res. 1999, 38, 3557-3563
3557
Inhibition of Jet Fuel Oxidation by Addition of Hydroperoxide-Decomposing Species S. Zabarnick* and M. S. Mick University of Dayton Research Institute, Aerospace Mechanics Division, 300 College Park, Dayton, Ohio 45469-0140
We have explored the use of hydroperoxide decomposing species for inhibiting oxidation in jet fuel. We find that hydroperoxide decomposing species, such as alkyl sulfides, do not slow or delay oxidation in hydrocarbon solvents at 140 °C. However, when phenolic species are also present, such as those naturally occurring in fuel or by addition of hindered phenols, substantial delays in oxidation are observed. We used a pseudo-detailed chemical kinetic mechanism to provide insight into the oxidation process. The combination of hydroperoxide decomposer and hindered phenol can substantially inhibit oxidation of fuel under the conditions studied here. Introduction The introduction of additives into petroleum liquids to improve properties or characteristics has been used successfully for many decades. Currently, additives which may be used in the U. S. Air Force primary jet fuel, JP-8, include icing inhibitors, corrosion inhibitors, antioxidants, metal deactivators, and static dissipators. Recently, the U. S. Air Force JP-8+100 program has developed an additive package which significantly increases the thermal stability (tendency to resist deposit formation) of the fuel, preventing the formation of deposits which result from fuel oxidation within aircraft fuel systems.1,2 This additive package presently includes a dispersant, an antioxidant, and a metal deactivator. While a wide variety of additive types were investigated in the JP-8+100 program, the use of hydroperoxide decomposers to slow or delay oxidation has not received significant attention. Hydroperoxide-decomposing additives have been used in lubricants and polymers to decompose alkyl hydroperoxides formed during oxidation into nonradical products.3 In the present study, we explore the role that hydroperoxide decomposers play in slowing or delaying autoxidation of jet fuel. We also study the synergism of these species with other additive types. In addition, pseudo-detailed chemical kinetic modeling was performed to provide insight into the chemical mechanism of additive behavior. We have also used the insights provided by these studies to help in understanding the various oxidation characteristics of fuels. Experimental Section Fuel oxidation and deposition characteristics were evaluated in the quartz crystal microbalance (QCM) Parr bomb system that has been described in detail previously.4,5 Most fuel oxidation tests were run at 140 °C and 1 atm of air initial pressure. The reactor is heated with a clamp-on band heater, and its temperature is controlled by a PID controller via a thermocouple immersed in the fuel. The device is equipped with a * Telephone: (937) 255-3549. Fax: (937) 252-9917. E-mail:
[email protected].
pressure transducer (Sensotec) to measure the absolute headspace pressure and a polarographic oxygen sensor (Ingold) to measure the headspace oxygen concentration. The oxygen sensor and pressure gauge allow us to follow the oxidation process. As oxygen is consumed in the liquid, oxygen in the headspace diffuses into the liquid, resulting in a decrease in the headspace oxygen concentration. The reactor also contains a radio frequency feedthrough, through which the connection for the quartz crystal resonator is attached. The crystals are 2.54 cm in diameter and 0.33 mm thick and have a nominal resonant frequency of 5 MHz. The crystals were acquired from Maxtek Inc. and are available in crystal electrode surfaces of gold, silver, platinum, and aluminum. For the studies reported here gold crystal electrodes were used. The QCM measures deposition (i.e., an increase in mass) which occurs on overlapping sections of the two-sided electrodes. Thus, the device responds to deposition which occurs on the metal surface and does not respond to deposition on the exposed quartz. Deposition measurements for the additized fuels (Exxsol D110 and F-2747) will not be shown in this study because the fuels, and fuels with additives, produce very low levels of deposition (
