MECHANISTIC MODELING OF N-HEPTANE CRACKING ON HZSM-5

A mechanistic model for the catalytic cracking of n-heptane was develo ped using a novel mechanism-based lumping scheme that exploits the che mical similarities within reaction families. The formal application of 13 reaction family matrices, which correspond to the 11 reaction fami lies in the model, to the matrix representations of the reactants and derived products generated 70 species, 235 elementary steps and 70 ord inary differential equations. The reaction family concept was further exploited to constrain the kinetics within each reaction family to fol low a quantitative structure/reactivity Polanyi relationship. Ultimate ly, four Polanyi relationship parameters and one catalyst specific par ameter were optimized using experimental data obtained from the cracki ng of n-heptane at 500 degrees C over HZSM-5 with a Si/Al ratio of 21. 25. The model correlations were excellent, as were the a priori predic tions of experimental results at 450 and 550 degrees C with an HZSM-5 Si/Al ratio of 21.25 and at 500 degrees C with HZSM-5 Si/Al ratios of 35.25 and 63.5. The thus validated model was then used to probe the co ntrolling elementary steps of n-heptane cracking. Carbonium ion cracki ng, beta-scission, and hydride transfer were the kinetically significa nt reactions.