Computer simulation of turbine oil oxidation. 1: Consumption of a hindered phenol antioxidant in model hydrocarbon systems at 115°C

Computer simulation of the inhibited oxidation of model hydrocarbon systems representing turbine oils of different composition, hexadecane/tetralin mixtures containing di-tert-butyl pcresol (DBPC), has been achieved using the kinetic and mechanistic information obtained from fundamental hydrocarbon autoxidation and inhibition studies. The oxidation life, the time to complete consumption of DBPC and its reactive intermediates, was calculated using previously reported and newly measured absolute rate constants for reaction involved. The results of this calculation of oxidation life were in good agreement with values experimentally determined. The oxidation life has been found to decrease with increasing amount of tetralin in the mixtures. This can be explained by the increased rate of consumption of antioxidants, caused by an increase in the rate of free radical formation, via the direct reaction of hydrocarbons with oxygen, and via the homolytic decomposition of hydroperoxides. The observed decrease in oxidation life, however, is not significant, despite the fact that the addition of tetralin considerably increases the rate of free-radical formation. The relatively low sensitivity of oxidation life to the rate of free-radical formation caused by the addition of an easily oxidisable substrate can be attributed to the occurrence of a direct oxidation of antioxidants. That is, kinetic analysis for the consumption of antioxidants disclosed that antioxidant consumption mostly occurred not through the inhibition of oxidation, but through direct oxidation. The direct oxidation of antioxidants obeys the rate equation -d[AH]/dt = k₆[AH]^1.0[O₂]^1.5 where k₆ can be calculated, at various temperatures, from Arrhenius parameters obtained in this study (log (A/M⁻¹s⁻¹) = 9.6 and Ea = 22.6 kcall mol).