Changes in the liquid-phase oxidation of a cycloparaffinic/paraffinic solvent resulting from the introduction of natural and synthetic fuel antioxidants are studied by tracking the depletion of dissolved O 2 in a closed system at 185 °C and 3.2 MPa. A Jet-A fuel of reduced thermal stability is the source of natural fuel antioxidants that are introduced by making dilute (<20%) blends with the solvent. The synthetic hindered-phenol antioxidant BHT is added at concentrations of 3-50 mg/L. The individual and combined effects of natural and synthetic antioxidants on paraffin autoxidation are determined on the basis of increased reaction time required to deplete initial dissolved O 2 by 50%. Synergistic effects of ∼40% are observed for certain antioxidant concentrations. The roles of natural and synthetic antioxidants in the liquid-phase oxidation of jet fuel are discussed.
Autoxidation of a thermally stable JPTS aviation fuel has been studied at 185 °C in a single-pass heat exchanger by monitoring the disappearance of dissolved O2 as a function of reaction time. All measurements were made in a single liquid phase at elevated pressure. We report empirical changes in autoxidation caused by (1) introducing a hindered phenol antioxidant (BHT), (2) adding a phenylenediamine antioxidant mixture (A0−24), (3) introducing natural fuel inhibitors in the form of a straight-run fuel, (4) adding selected combinations of the above antioxidants, (5) varying initial O2 concentration, (6) diluting (1/1) with a paraffinic solvent, and, finally, (7) introducing dispersants.
The depletion of dissolved O2 has been measured at 185 °C for a series of 12 jet fuels diluted 10-fold in a paraffinic solvent. Reaction is limited by the fixed amount of O2 present in air-saturated fuel. Because of dilution, aromatics, olefins, and also species such as dissolved metals and natural secondary antioxidants that influence oxidation by collision with hydroperoxides are less important. Under such dilute conditions, autoxidation is simplified, being governed mainly by the residual natural primary antioxidants acting as retarders or inhibitors to slow oxidation of the diluent. Oxidation of diluted fuels is, therefore, characterized by a time delay, followed by reaction acceleration. The time required to achieve 50% conversion of O2 has been used as a measure of the efficiency and concentration of primary antioxidants present in the diluted fuel. In turn, it is proposed that this time is an indirect measure of primary antioxidants originally present in the neat fuel. Similarity in the observed oxidation behavior of diluted fuels and hydrotreated fuels and the parallel of removing polar species either by dilution or by hydrotreatment lead to classification of such diluted fuels as surrogate hydrotreated fuels. Improved thermal stability following dilution, differences in the response of the neat and the diluted fuel to several additives, the effect of increased inital dissolved O2, and the measured concentration of hydroperoxides further support the analogy between dilution and hydrotreatment.
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