Mechanistic kinetic modeling of the hydrocracking of complex feedstocks

摘要

Two separate mechanistic kinetic models have been developed for the hydrocracking ofcomplex feedstocks. The first model is targeted for the hydrocracking of vacuum gas oil.The second one addresses specifically the hydrocracking of long-chain paraffins, but at amore fundamental level as compared to the first one. Both models are based on anexhaustive computer generated reaction network of elementary steps.In the first model, the dehydrogenation/hydrogenation steps occurring on the metal sitesto generate/consume the reactive olefinic intermediates are assumed to be very fast sothat the acid site steps are considered as the rate determining steps. The frequencyfactors for acid site steps are modeled using the single-event concept and the activationenergies based on the nature of the reactant and product carbenium ions.This model utilizes a detailed composition of the vacuum gas oil characterized by 16different molecular classes up to carbon number 40. These classes are divided into 45subclasses by distinguishing the isomers of a class according to the number of methylbranches. The kinetic model is plugged into an adiabatic multi-bed trickle flow reactormodel. The model contains 33 feedstock and temperature independent parameters whichhave been estimated from the experimental data.The model has been used to study the effect of the operating conditions on the yield andcomposition of various products. A sensitivity analysis of the distribution of isomers of a class among its different subclasses has been performed showing that the totalconversion increases when the content of isomers with a higher degree of branching isincreased in the feed.In the second model, the dehydrogenation/hydrogenation steps on the metal sites are alsoassumed to be rate determining. The rate coefficients for the dehydrogenation steps aremodeled depending on the nature of the carbon atoms forming the double bond. Thefrequency factors for the acid site steps are modeled using the single-event concept. Amore rigorous approach has been selected to model the activation energies of the acidsite steps by implementing the Evans-Polanyi relationship. The 14 model parameters,which are independent of the temperature and feedstock composition, have beenestimated from the experimental data. The model elucidates the effect of the relativemetal/acid activity of the catalyst on the isomerization/cracking selectivities and on thecarbon number distribution of the products.