An experimental investigation of the effects of <i>n</i>-decane on the supercritical pyrolysis of toluene

Abstract

Future and current high-speed jet aircraft will require their fuels to act as the primary coolants as well as propellants. Fuels will be exposed to severe temperatures and pressures in hypersonic aircraft, up to 700°C and 130 atm, respectively, conditions that are supercritical for most pure hydrocarbons. Under supercritical conditions, hydrocarbon fuels undergo pyrolytic reactions, which may lead to the formation of polycyclic aromatic hydrocarbons (PAH), known precursors to carbonaceous solid deposits. Such deposits may clog fuel lines and injection nozzles, hindering safe engine performance. Hence, it is important to understand the reactions that lead to the formation of PAH. While jet fuels are composed primarily of alkanes, a significant portion of their composition is comprised of aromatics. In our effort to understand PAH formation, we must first understand the interactions of aliphatic and aromatic fuel components. Therefore, the aromatic model fuel toluene (critical temperature, 319°C; critical pressure, 41 atm) has been pyrolyzed both alone and in the presence of the aliphatic model fuel n-decane (critical temperature, 345°C; critical pressure, 20.8 atm) in an isothermal flow reactor at 570°C and 600°C, 94.6 atm, and 133 sec. Analyses of 12 gas-phase and 53 condensed-phase hydrocarbon products were performed with gas chromatography (GC) with flame-ionization detection (FID) and GC/FID coupled with mass spectrometry (MS), respectively. Results indicate that n-decane addition increases toluene conversion and product yields. In n-decane-doped toluene pyrolysis at 570°C, n-decane conversion is inhibited by the smaller radical pool relative to n-decane-only pyrolysis at the same temperature. 1-Alkene formation is generally enhanced compared to n-alkanes at 570°C, a result that contrasts with results obtained from n-decane-only pyrolysis at the same temperature. Additionally, results suggest that interactions of alkenes and benzylic-type radicals are important to the formation of high-ring number aromatics. Increasing the temperature to 600°C increases the conversions of both toluene and n-decane and causes a general rise in product yields. Yields of aliphatics and n-alkylbenzenes with long-carbon chains decrease at 600°C due to decomposition of the alkyl chain. Product yields as functions of n-decane concentration at 570°C and 600°C are presented, and possible product formation pathways are discussed.