Oxidative Stability of Middle Distillate Fuels. Part 1: Exploring the Soluble Macromolecular Oxidatively Reactive Species (SMORS) Mechanism with Jet Fuels

Abstract

The soluble macromolecular oxidatively reactive species (SMORS) mechanism has recently been applied to jet fuel thermal oxidative degradation at high temperatures (250–550 °C). The primary purpose of this work is to further test the extant SMORS mechanism with carefully designed experiments at lower temperatures. First, synthetic SMORS precursors, 2-methylindole and 1,4-benzoquinone, were doped into a stable Jet A-1 in both mono- and oligomeric forms. Flask oxidative stress of these solutions at 90 °C for 60 min with an oxygen sparge significantly increases jet fuel thermal oxidative degradation. Second, a model compound experiment suggests SMORS precursors, phenol and 1,4-benzoquinone, are generated in situ from flask oxidation of a natural jet fuel component cumene (isopropylbezene) at 160 °C with an air sparge for 300 min. This observation is particularly significant for the thermal oxidative degradation of ultra-low sulfur diesel (ULSD) because it suggests that fuels with low heteroatom content may oxidatively degrade by the SMORS mechanism. Third, doping parts per million (ppm) levels of 2,4-dimethylpyrrole into oxidatively stable jet fuels followed by flask oxidation at 95 °C with an air sparge for 30 min results in significant oxidation of the jet fuels. This observation is particularly significant for the storage and thermal oxidative stability of Athabasca-tar-sands-derived middle distillates, which have previously been shown to contain alkylpyrroles; these middle distillates are predominate across the northern tier of the United States.

Exploring the Molecular Origin of Jet Fuel Thermal Oxidative Instability through Statistical Analysis of Mass Spectral Data

“…It is unclear whether the compounds with the general elemental compositions of C n H y OS, C n H y NO, and C n H y NOS are related to the poor thermal oxidative stability of J2971. The general elemental composition of C n H y OS could be related to hydroxybenzenethiols, which have been discussed as possible reactants with quinones, yet none of the detected elemental compositions fit this compound class. This general elemental composition could also fit a sulfur containing analog of quinone.…”

“…Synthesis of SMORS intermediates has been explored previously and could offer a way to determine the retention behavior of these compounds by this LC/MS method and add them to the library. 12 Increasing the number of passing and failing samples could help to improve the understanding of whether or not C n H y OS, C n H y NO, and C n H y NOS truly have a statistically significant correlation to thermal oxidative stability. With the introduction of more samples, there could be some samples that pass ASTM D3241 and have high levels of one or more of these groups of compounds, whereas some samples that fail ASTM D3241 do not.…”

“…This is a critical property of jet fuel because fuel is exposed to high temperature and pressure conditions when in service, and deposit formations during flight may lead to catastrophic failure. Trace amounts of polar molecules containing heteroatoms have been shown to lead to ASTM D3241 failures. − However, a simple correlation based on the concentrations of these molecules in a fuel have failed to predict ASTM D3241 failures. It has also been shown that trace amounts of dissolved metals can lead to thermal oxidative instability in jet fuel …”

“…The soluble macromolecular oxidatively reactive species (SMORS) model for distillate fuel instability was first proposed to explain storage stability issues in diesel fuel and has been applied to help understand thermal oxidative instability in jet fuels (Figure ). , The first step is the oxidation of phenol or an alkyl phenol which acts as an antioxidant in the autoxidation pathway (Figure ). , This oxidation leads to the formation of hydroquinone and quinone .…”

The Effects of Storage on Turbine Engine Fuels

“…The initial step of the SMORS mechanism is the reaction of phenol with a peroxy radical to abstract a hydrogen atom. A similar reaction would also be possible with anilines and thiophenols that may Flight Physics -Models, Techniques and Technologiesalso be present [17]. The importance of the phenol has been demonstrated by conversion of active hydrogen containing species to silyl ethers which resulted in a dramatic decrease in deposit formation [18].…”