Mechanism of Deposit Formation in Wing Tanks

Deposits form in the “empty” wing tank environment as a result of a complex autoxidative degradation of the hydrocarbon fuel. For the past two years, Esso Research and Engineering Co. has been carrying out a fundamental study for the Air Force of the variables which control such deposit formation. Extensive studies have been carried out in a specially designed apparatus which simulates the kinetic environment found in a wing tank. Independent Advanced Aircraft Fuel System Simulator studies have shown that this apparatus is the only small test apparatus currently available which accurately rates the wing tank deposit formation tendency of jet fuels.

Rigorous exclusion of oxygen has been demonstrated to eliminate deposit formation at temperatures up to 500 F. The rate of deposit formation from typical jet fuels exhibits a 10 kcal/mole apparent activation energy and a 0.2 order in oxygen partial pressure at moderate oxygen partial pressure levels. Both homogeneous and heterogeneous metals affect the kinetics of deposit formation, presumably by catalyzing the free radical initiation reactions. Vanadium containing titanium alloys are much more reactive toward deposit formation than pure titanium, aluminum, or stainless steel. Various soluble metal acetylacetonates increased the rate of deposit formation up to a factor of 120 at the 50 ppm level. Trace levels of sulfur and nitrogen compounds markedly accelerate the deposit formation process. Sulfur compounds increase deposit formation by initiating the complex autoxidative process. Pure compound studies showed that the structure of the individual sulfur compound determines its effect on deposit formation. Highly stable sulfur compounds such as diphenyl sulfide and dibenzothiophene did not increase the rate even at the 1000 ppm S level; less stable compounds increased the rate by factors ranging from 5-30.