|dc.description.abstract||Two separate mechanistic kinetic models have been developed for the hydrocracking of
complex 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 a
more fundamental level as compared to the first one. Both models are based on an
exhaustive computer generated reaction network of elementary steps.
In the first model, the dehydrogenation/hydrogenation steps occurring on the metal sites
to generate/consume the reactive olefinic intermediates are assumed to be very fast so
that the acid site steps are considered as the rate determining steps. The frequency
factors for acid site steps are modeled using the single-event concept and the activation
energies based on the nature of the reactant and product carbenium ions.
This model utilizes a detailed composition of the vacuum gas oil characterized by 16
different molecular classes up to carbon number 40. These classes are divided into 45
subclasses by distinguishing the isomers of a class according to the number of methyl
branches. The kinetic model is plugged into an adiabatic multi-bed trickle flow reactor
model. The model contains 33 feedstock and temperature independent parameters which
have been estimated from the experimental data.
The model has been used to study the effect of the operating conditions on the yield and
composition of various products. A sensitivity analysis of the distribution of isomers of a class among its different subclasses has been performed showing that the total
conversion increases when the content of isomers with a higher degree of branching is
increased in the feed.
In the second model, the dehydrogenation/hydrogenation steps on the metal sites are also
assumed to be rate determining. The rate coefficients for the dehydrogenation steps are
modeled depending on the nature of the carbon atoms forming the double bond. The
frequency factors for the acid site steps are modeled using the single-event concept. A
more rigorous approach has been selected to model the activation energies of the acid
site steps by implementing the Evans-Polanyi relationship. The 14 model parameters,
which are independent of the temperature and feedstock composition, have been
estimated from the experimental data. The model elucidates the effect of the relative
metal/acid activity of the catalyst on the isomerization/cracking selectivities and on the
carbon number distribution of the products.||en