The Supercritical Pyrolysis of 1-Octene

Abstract

In the pre-combustion environment, fuels for future high-speed aircrafts are predicted to take on increasing heat loads in their role as the primary coolant in order to remove excess heat from engine subsystems. While acting in this role, these fuels are expected to experience temperatures and pressures up to 700 °C and 130 atm, conditions which are supercritical for jet fuels and most hydrocarbons. Such extreme conditions can cause fuel decomposition and subsequent pyrolytic reactions, which can lead to the formation of polycyclic aromatic hydrocarbons (PAH). PAH are precursors to solids deposits that clog fuel-delivery lines, causing reduced engine performance and eventual failure. Therefore, it is vital to understand the reaction pathways which govern PAH formation and growth in the supercritical fuel pyrolysis environment. Previous work has shown that n-alkanes, a major class of jet fuel components, are prone to solids formation, and 1-alkenes are abundant in the supercritical n-alkane pyrolysis environment. In order to better understand the role 1-alkenes have in PAH formation and growth, 1-octene (critical temperature, 294 °C; critical pressure, 24.6 atm), a representative product of supercritical n-alkane pyrolysis, has been pyrolyzed in an isothermal, isobaric reactor at 94.6 atm, 133 sec, and five temperatures between 450 to 535 °C. Analyses of 18 C1-C4 aliphatic and one-ring aromatic gas-phase products and 54 C5-C14 aliphatic and one- and two-ring aromatic liquid-phase products were performed by gas chromatography coupled to flame-ionization and mass spectrometry. A two-dimensional high-pressure liquid chromatographic technique was employed to separate the PAH products. Identification and quantification of 116 PAH products of three to nine rings was performed by diode-array ultraviolet-visible and mass spectrometry, an isomer-specific technique for PAH analysis. The facile scission of the weak allylic C–C bond of 1-octene translates to its rapid conversion. Results indicate that the interactions of alkenes with resonantly stabilized allyl, methylallyl, arylmethyl, and phenalenyl-type radicals are important to the growth and formation of high-ring number aromatics. PAH formation and growth are significantly enhanced in the supercritical 1-octene pyrolysis environment at 535 °C compared to the supercritical n-decane pyrolysis environment at 530 °C and 540 °C.