KINETICS OF HEXANE PYROLYSIS AT VERY HIGH-PRESSURES .3. APPLICATION TO GEOCHEMICAL MODELING

A simplified mechanism consisting of 31 free radical reactions is used to calculate the overall rate of pyrolysis of n-hexane over the 77-500-degrees-C temperature range, at 210, 2000, 6000 and 15,000 bar. The overall activation energy of hexane pyrolysis, E, is found to increase with increasing pressure and decreasing temperature. For example, at 210 bar, E remains almost constant at about 70 kcal/mol between 77 and 200-degrees-C, and then decreases to about 60 kcal/mol at 450-degrees-C. The results of this model are compared with the results obtained using the usual geochemical modeling approach, which uses first-order stoichiometric reactions with activation energies independent of temperature. The activation energies are usually derived from high temperature laboratory experiments and the rate constants are then extrapolated to low temperatures. In the present case, the data obtained between 427 and 500-degrees-C are used to derive the Arrhenius parameters for the overall rate constant of hexane pyrolysis. The use of these rate parameters at low temperatures yields hexane pyrolysis rates different by a factor as high as 350 from the rates obtained with the free radical mechanism. The implications of these results for geochemical modeling are discussed, and an experimental method is proposed to obtain rate parameters for the pyrolysis of crude oils that would be valid at the temperatures of sedimentary basins.