High temperature cracking and deposition behavior of an n-alkane mixture

Advanced jet engine designs and the need for jet fuel in aircraft to handle increasing heat loads has generated much interest in investigating the thermal stability of jet fuels at temperatures greater than 500{degrees}C. A mixture of C12 to C15 normal paraffins, was used to model the high temperature deposition and cracking behavior of jet fuels. The model hydrocarbon mixture was pumped through a single tube heat exchanger under supercritical conditions and heated to a final temperature of 550{degrees}C. Gas and liquid products were analyzed by gas chromatography/mass spectrometry, GC/MS, and gas chromatography with a flame ionization detector, GC FID. Amounts of carbon deposit through the tube were also determined by carbon burnoff analysis. Results showed the long chain normal paraffins to be stable in the oxidative deposition region, 150 to 300{degrees}C, while creating large amounts of pyrolytic deposits at temperatures greater than 500{degrees}C. The normal paraffins were found to crack to form smaller chain alkanes and alkenes with highly stressed samples then forming higher numbered olefins and cyclohexanes. This model mixture was also highly useful in observing the effects of fuel additives and tube surfaces on chemistry and deposit formation. Both high temperature hydrogen donors and an inert surfacemore » were found to increase the thermal stability of the paraffin mixture.« less