A Generalized Mechanistic Kinetic Model for the Hydroisomerization and Hydrocracking of Long-Chain Paraffins

Abstract

A mechanistic kinetic model has been developed for the three-phase hydroisomerization and hydrocracking of long-chain paraffins in which rate-determining steps are assumed to occur on both acid and metal sites of the bifunctional catalyst. The frequency factors for the acid site elementary steps are modeled using the single-event concept and the corresponding activation energies using the Evans−Polanyi relationship. The rate coefficients for the dehydrogenation reactions on the metal sites of the catalyst are classified into five different classes depending on the nature (i.e., primary, secondary, or tertiary) of the carbon atoms forming the double bond. The model contains 14 independent parameters for the hydroisomerization and hydrocracking of heavy paraffins. These parameters are estimated from experimental data on the three-phase isothermal hydrocracking of n-hexadecane in a tubular reactor. A number of reactor simulations have been performed for various operating conditions and hydrocarbons of different chain lengths. The effect of the relative metal to acid activity of the catalyst on the isomerization and cracking selectivities and on the carbon number distribution of the products is discussed in detail.