The thermal stability of a high-quality hydrotreated Jet-A fuel, an average-quality straight-run Jet-A fuel, and several blends of each fuel has been studied during flow through a single-pass 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.

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Recently it has been reported that autoxidation of a Jet-A fuel which occurs during passage through heated stainless-steel tubing is accelerated by the tubing walls and that this effect is mitigated as the surface becomes fouled. To investigate the generality of this finding, we have studied the depletion of dissolved oxygen in 16 aviation fuels in a single-pass tubular heat exchanger. Experimental conditions of temperature and tube dimension were held constant, but the chemical composition of the inner wall surfaces was varied. Reaction was compared in commercial stainless-steel (304) tubes and in passivated or surface-treated (Silcosteel) tubes which are noted for their inert inner walls. The aviation fuels selected for study ranged from the highly thermally stable JPTS to an unstable Cu-doped Jet-A. The fuels contained a fixed amount of oxygen (air-saturated at room temperature) and were stressed under identical conditions at 185 °C during passage through 0.216-cm-i.d. tubing. Results reported herein show that autoxidation occurring as fuel flows through stainless-steel tubes is accelerated as compared to that occurring in treated tubes. The magnitude of this effect is fuel dependent, ranging from a low of 10-20% (barely detectable) to ∼75%. The role of surfaces in catalyzing aviation-fuel autoxidation in narrow-bore tubing and possible ramifications with regard to surface fouling in aircraft fuel lines are discussed.

The thermooxidative stability of blends of a straight-run Jet-A fuel (POSF-2827) and a paraffinic/cycloparaffinic solvent (Exxsol D-80) has been studied in a single-pass tubular heat exchanger operated isothermally at 185 °C. Autoxidation of most blends is found to be significantly slower than that of either fuel or solvent. Surface fouling relative to the solvent is increased by addition of jet fuel; this is attributed to reactions of natural antioxidants present in the fuel. Surface fouling relative to the jet fuel is reduced following paraffin addition under conditions of partial O2 conversion; this is attributed to a lower initiation rate as the concentration of aromatics is reduced. The impact of these findings for mitigation of surface fouling in aircraft fuel lines is discussed.

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