(300c) Modeling of Hydrocracking: Methodology
Hydrocracking process has become a major operation unit in today's oil refining industry. Such an interest is explained by the increasing demand of middle distillates with excellent product quality. Hydrocracking is a catalytic cracking process converting high-boiling fractions like Vacuum Gas Oil (VGO) into lower-boiling and more valuable fractions like middle distillates. It is carried out on bifunctional catalyst combining both a metal and an acid phase, for example a NiMoS/Y catalyst, in the presence of a high hydrogen partial pressure. On the metal phase, hydrogenation/dehydrogenation reactions take place while on the acid phase, protonation/deprotonation, isomerization and cracking reactions occur. To optimize the yield of the desired products and because of the complexity of the feedstock and the reactions, hydrocracking modeling is essential. The developed model considers an hydrotreated feedstock composed of aromatic, naphthenic and paraffinic hydrocarbons. Its purposes are both to realize a relevant molecular reconstruction of the effluents and a kinetic model taking into account the industrial context. Analytical techniques are not yet powerful enough to detect and quantify in detail all the components of the effluents. Thus a molecular reconstruction is required to get a better and more precise representation of the effluent compositions. The proposed method is to use analytical results provided by high-temperature two-dimensional gas chromatography (HT-2D-GC) to create an initial set of molecules. Then, the molar fractions of these molecules are optimized by the maximization entropy method. Finally a detailed and relevant distribution of molecules is obtained. For the kinetic model, goals are both to introduce the aromatic and naphthenic hydrocarbons in the model and to consider the reactions on the metal phase and on the acid phase. So first, the structures of the molecules introduced in the model have to be defined, using analytical results (The model considers paraffin, naphthenes and aromatic compounds with zero to three condensed rings and maximum one alkyl chain. The maximum number of bonds allowed for each molecule is three and only methyl bonds are taken into account.). Secondly, the reactions on the metal phase have to be accounting for as well as those on the acid phase. So for the metal phase, a reaction mechanism of hydrogenation/dehydrogenation has to be defined and implemented into the kinetic model[3,4,5]. For the acid phase, the kinetic model is based on the Single-Event approach initiated by Froment et al.. Its biggest interest is to use a limited number of kinetic parameters independent of feedstock composition. Finally, the global model allows to simulate the hydrocracking process in industrial conditions considering complex feedstock.
References:  T. Dutriez T et al., High-temperature two-dimensional gas chromatography of hydrocarbons up to nC(60) for analysis of vacuum gas oils, Journal of Chromatography A, 1216, 2905-2912 (2009)  D. Hudebine and J. Verstraete, Molecular reconstruction of LCO gasoils from overall petroleum analyses, Chemical Engineering Science, 59, 4755-4763 (2004)  H. Kumar et G. F. Froment, Mechanistic Kinetic Modeling of the Hydrocracking of Complex Feedstocks, such as Vacuum Gas Oils, Ind. Eng. Chem. Res., 46, 5881-5897 (2007)  H. Kumar et G. F. Froment, A Generalized Mechanistic Kinetic Model for the Hydroisomerization and Hydrocracking of Long-Chain Paraffins, Ind. Eng. Chem. Res., 46, 4075-4090 (2007)  J.C. Chavarría-Hernández , J. Ramírez, M.A. Baltanás, Single-event-lumped-parameter hybrid (SELPH) model for non-ideal hydrocracking of n-octane, Catalysis Today, 130, 455-461 (2008)  M. A. Baltanas et al., Fundamental Kinetic Modeling of Hydroisomerization and Hydrocracking on Noble-Metal-Loaded Faujasites. 1. Rate Parameters for Hydroisomerisation, Ind. Eng. Chem. Res., 28, 899-910 (1989)