Catalytic cracking of n-alkane naphtha: The impact of olefin addition and active sites differentiation

Abstract

An extended dual kinetic model allows to fit the n-heptane cracking results working in a wide range of reaction conditions. The duality of the model is provided by the contribution of monomolecular and bimolecular cracking mechanisms. It takes into account the role played by the olefins formed on the global cracking or added within the feed. Furthermore by means of this model and the kinetic parameters obtained when cracking n-heptane on ZSM-5, it has been observed that, while some characterization techniques show a homogeneous zeolite surface from the point of view of the active sites, rigorous kinetic experiments point to the possibility that the reactant sees a heterogeneous surface with, at least, two group of cracking active sites. Those differentiated active sites give different cracking rates and different activation energies for the process and, in the case of ZSM-5, could be assimilated to sites pointing to the 10R channels and sites pointing into the crossing of the 10R channels, mainly due to differences in acid site location and confinement effects.3

Theta-1 zeolite catalyst for increasing the yield of propene when cracking olefins and its potential integration with an olefin metathesis unit

“…Overall, the above kinetic results suggest that, at 500 °C and a feed molar fraction below 0.251, monomolecular cracking of hexene over ZSM-5 and Theta-1 is the predominant mechanism. It explains propene being the main product, P/B ratios in great excess over the thermodynamic value, and is consistent with the cracking modes available for hexene and the low surface coverage of the catalyst under these conditions (27).…”

“…where τ W = p A0 • WHSV −1 • M A is the modified contact time (27). Therefore, if the kinetic results are truly first order and ε is small, given τW a certain conversion will be observed, irrespective of the partial pressure of the reactant in the feed.…”

Mechanistic Insights into the Pore Confinement Effect on Bimolecular and Monomolecular Cracking Mechanisms of N-Octane over HY and HZSM-5 Zeolites: A DFT Study

“…ethylene and propylene) will increase 5 . On the contrary, if the bimolecular cracking is dominant, the primary product will contain a large number of iso-paraffins and iso-olefins 6 . N-octane is a typical alkane for two cracking mechanisms and has appropriate number of carbon atoms.…”

“…If the monomolecular mechanism is dominant, the yield of light olefins (e.g., ethylene and propylene) will increase . On the contrary, if the bimolecular cracking is dominant, the primary product will contain a large number of isoparaffins and isoolefins …”

“…5 On the contrary, if the bimolecular cracking is dominant, the primary product will contain a large number of isoparaffins and isoolefins. 6 N-Octane is a typical alkane for two cracking mechanisms and has an appropriate number of carbon atoms. Therefore, mechanistic investigation on n-octane catalytic cracking is of prime scientific and industrial importance.…”