Evaluation of the Effectiveness of a Metal Deactivator and Other Additives in Reducing Insolubles in Aviation Fuels

Abstract

Surface fouling in aircraft fuel lines resulting from autoxidation of aviation fuel leads to reduced efficiency as deposits collect on heat exchangers, nozzles, and servocontrols and may ultimately lead to system failure. Metal surfaces and trace quantities of metals dissolved in the fuel exacerbate the surface-fouling problem because they can catalyze free-radical initiation, thereby accelerating autoxidation. Additives and additive packages containing antioxidants, dispersants, and metal deactivators (MDA) have been shown to reduce insolubles in some fuels. Because of metal chelation and possible metal-surface passivation, MDA has been proposed as an additive component to be included in all fuels, even those without dissolved metals. The goal of the present study was to obtain fundamental information on the behavior of MDA under conditions where surface-passivation effects are minimal. Experiments have been conducted to (1) study the effects of adding MDA to fuels containing a significant concentration of dissolved metals (i.e., chelation) and to those containing minor concentrations of dissolved metals and (2) investigate interactions when MDA is used in conjunction with an antioxidant and a dispersant. Simple fuel-line-fouling simulations with a single-pass tubular heat exchanger operated under near-isothermal conditions have been conducted to study the thermal behavior at 185°C of several neat and MDA-treated jet fuels as well as fuels treated with MDA plus other additives. Comparison of neat and treated fuels is based on several criteria: (1) dependence of autoxidation on stress duration, (2) dependence of surface deposition on stress duration, and (3) quantity of total insolubles (bulk filterables and surface deposits). Potential advantages and disadvantages of using MDA alone and in combination are discussed.