(684f) Temperature-Dependent Global Kinetic Rate Parameters for the Production of Polycyclic Aromatic Hydrocarbons from the Supercritical Pyrolysis of Ethylcyclohexane

In its pre-combustion role as coolant, fuel in future high-speed aircraft will be expected to sustain temperatures and pressures as high as 600 °C and 100 atm. These conditions—supercritical for most hydrocarbons—will cause the fuel to undergo pyrolytic reactions that may lead to polycyclic aromatic hydrocarbons (PAH)—precursors to fuel-line solids that can hinder the aircraft’s safe operation. Since temperature plays a major role in determining the nature of pyrolysis products, it is imperative to understand the temperature-dependent supercritical fuel pyrolysis behavior as well as determine the temperature-dependent global kinetic rate parameters—essential for predicting PAH product yields—under supercritical pyrolysis conditions.

To this end, we have conducted supercritical pyrolysis experiments with the model fuel ethylcyclohexane—a cyclic-alkane component of jet fuels. The experiments are carried out in an isothermal, silica-lined stainless steel flow reactor at 94.6 atm, 133 sec, and at seven temperatures—held constant during each experiment—in the range of 500 ˚C to 570 ˚C. The resulting complex PAH product mixture is separated using two-dimensional high-pressure liquid chromatography and isomer-specifically identified and quantified using ultraviolet-visible diode-array and mass-spectrometric detection.

The yields of the pyrolysis products are reported as functions of temperature, and the temperature-dependent experimental data is used to determine the temperature-dependent global kinetic rate parameters, apparent activation energy E_app (in kcal/mol) and apparent pre-exponential factor A_app (in s^-1), which appear in the expression for the kinetic rate constant, k=A_appexp[-E_app/RT] . For these determinations, it is assumed that the rate of production of each aromatic product is first-order in the concentration of ethylcyclohexane fuel. The derived global kinetic rate parameters are reported for each aromatic product and for PAH grouped by ring number. The predicted yields, calculated from these parameters, are then compared to those obtained from the experiments.