Probing the Supercritical Pyrolysis Regime by Mass Spectrometry: The Effects of Aliphatic versus Aromatic Content on the Composition of an Endothermic Fuel

Abstract

To more effectively utilize jet fuel as a thermal management fluid on board an aircraft, it is necessary to understand the changing chemical composition of the fuel under high temperature (350–800 °C) and pressure (300–1000 psi) pyrolytic conditions. Toward this aim, we have performed in situ characterization of neat supercritical fuel surrogates and a fuel (Jet A) primarily comprised of saturated hydrocarbons using quadrupole mass spectrometry. We directly probe the pyrolytic fluid via supersonic expansion into a vacuum, which rapidly cools the reaction mixture. We ionize the resulting molecular beam using electron impact ionization (10 eV electron kinetic energy) and identify reactants, intermediates, and products with a quadrupole mass filter. Although different fuels exhibit distinct pathways by which they crack into lighter intermediates, the aromatic product distributions become indistinguishable as the temperature is increased. We attribute these similarities to the fast cracking rates at extreme temperatures, which funnel into relatively few small hydrocarbon intermediates (C2 and C3) from which benzene and larger aromatics are synthesized. The smaller aromatics and their methyl-substituted derivatives evolve into larger polycyclic aromatic hydrocarbons at the highest temperatures, providing insight into the elementary steps of coke formation. Such a mechanism, which is consistent with that of soot formation in flames, is sensible in a high-temperature regime, in which cracking rates increase and residence times lengthen as coke is deposited on the reactor nozzle.

In Situ Diagnostic of Supercritical Fuel Surrogates: Probing Heterogeneous Catalysis by Collision-Induced Dissociation in a Molecular Beam Tandem Mass Spectrometer

“…Previously, we observed similar product distributions of mostly aromatic compounds (benzene, toluene, naphthalene, xylene, anthracene, etc.) for the pyrolysis of neat fluids of aliphatic and alicyclic compounds under supercritical conditions at the limit of high temperatures and long residence times in the absence of a catalyst . These previously shown product distributions are displayed in Figure b,d.…”

“…When probing the neat fluid in situ using mass spectrometry, we observed similar aromatic product distributions (benzene, toluene, xylene, naphthalene, etc.) at the limit of high temperatures (700–800 °C) and long residence times (>5 min) . This distribution did not depend on the identity of the reactant fluid for a variety of aliphatic fuel surrogates ( n -hexane, cyclohexane, n -dodecane, and decalin) and for Jet A aviation fuel, which is mostly aliphatic.…”

“…at the limit of high temperatures (700−800 °C) and long residence times (>5 min). 16 This distribution did not depend on the identity of the reactant fluid for a variety of aliphatic fuel surrogates (nhexane, cyclohexane, n-dodecane, and decalin) and for Jet A aviation fuel, which is mostly aliphatic. The results suggested that all aliphatic fuels break down to a common set of C2 and C3 intermediates from which the polyaromatic hydrocarbons (PAHs) are synthesized.…”

“…In Figure , a diagram of the instrument is given, which shows the supercritical pyrolysis source and the two mass spectrometers. In our previous studies, , we demonstrated the single quadrupole mass analyzer combined with 10 eV EI ionization to interrogate the molecular beam formed from the pyrolysis source. In this setup, the ions that are generated by EI are turned 90° into a quadrupole mass filter, which is scanned at a rate of 0.53 amu/ms.…”

“…Mass spectrometry is an alternative analytical method to optical spectrometry and is well suited for detailed chemical analysis of in situ pyrolysis reactions because species can be distinguished by their mass-to-charge ratios (m/z) with unit mass resolution using conventional quadrupole mass analyzers. Previously, we have shown that pyrolysis intermediates and products of neat fluids (n-hexane, cyclohexane, n-dodecane, and decalin), 16 binary mixtures (hexane and benzene), 17 and real fuels (Jet A) 16 can be interrogated by forming a supersonic expansion of the reacting fluid into the gas phase. The advantage of our approach when compared to standard analytical methods (e.g., LC-MS, GC-MS, etc.)…”