Process modeling hydrotreating

Reaction studies with model compounds representative of the metal species in petroleum are discussed to provide insight into the fundamental rate processes which occur. It will be demonstrated that this information can be used to successfully interpret the behavior of real feedstocks in commercial hydrotreating reactors. 

Metal deposition in hydrotreating of heavy oils is one of the most important phenomenon causing catalyst deactivation. Present work focuses on the modeling of hydrodemetallisation catalyst deactivation by model compound vanadyl-tetraphenylporphyrin. Intrinsic reaction kinetics, restrictive diffusion and the changing catalyst porous texture are the relevant phenomena to describe this deactivation process. The changing catalyst porous texture during metal depositon can be described successfully by percolation concepts. Comparison of simulated and experimental metal deposition profiles in catalyst pellets show qualitative agreement. 

Vanadium deposition profiles in catalyst pellets have been determined by various researchers (14-30). For this purpose porphyrinic model compounds and industrial feedstocks are used. The used catalyst are mainly conventional hydrotreating catalysts with narrow pores. Therefore, metal deposition profiles show mainly deposition in the outer shells of the catalyst pellets (M- or U-shaped profiles), indicating that the metal deposition process is diffusion limited. 

The usefulness of the of artificial neural networks as a modelling tool is apparent. A more general H-Oil product slate model can be developed by including the feed and catalyst properties. It can also easily be adapted to model the other aspects of the H-Oil process such the hydrotreating and hydrocracking reaction kinetics or coke lay down tendency in the separation units with the appropriate input and output patterns. 

If bitumens are regared as model substances for heavy residues, then their Arrhenius coefficients can serve as the basis for the calculation of the kinetics of pyrolysis reactions in thermal conversion processes (thermal cracking, visbreaking, hydrotreating etc.). Integration of the peak areas gives a value for the energy required for pyrolysis reactions. 

Ostrovskii and Bukhavtsova published several experimental and theoretical works on capillary condensation in catalytic reactions. Capillary condensation was found to accompany some gas-phase catalytic processes, in particular hydrotreating of jet fuel fractions [7]. The effects of gas-liquid interfacial surface, intra-particle diffusion, and of the ratio of gas to liquid reaction rates under conditions of capillary condensation were estimated [8]. The experimental study of /j-xylene hydrogenation on Pt/Si02 (as a model reaction) was carried out in order to demonstrate the influence of capillary condensation on reaction kinetics and process dynamics, and corresponding model was proposed [9]. Finally, the poisoning of the catalyst under capillary condensation was also considered [10]. 

Similar kinetic experiments were carried out by Fatemi et al. [54] for the reaction of thiophene hydrogenolysis at the elevated temperature (170-190°C) and pressure (9.5 atm) that are close to conditions of industrial hydrotreating processes. They used n-heptane as a feed containing 3.24% of thiophene. Owing to the preferential condensation of n-heptane rather than thiophene, they did not observe a relationship between reaction rate and thiophene conversion, like the trend shown in Figure 23.9. So, the dependency of the reaction rate vs. temperamre was used (Figure 23.11). To represent their experimental data, Fatemi et al. [54] have used a model similar to (Equation (23.23) and Equation (23.24)). 

But in literature still pseudo-homogeneous models are in use for scale-up in trickle-bed reactors, especially if hydrotreating processes of the petroleum industry are concerned, where the whole reaction dynamics is lumped totally either into the gas or the liquid phase,... 

E.g. in the so-called "pseudo-equilibrium model, developed by Sylvester [53-56], the same design procedure is used as in a single phase catalytic gas phase reaction, where the mass transfer resistance is replaced by a suitable overall term. Bulk flow and dispersion of the liquid phase are neglected and the whole transport mechanisms are lumped into the equilibrium of the reactant concentrations between gas-, liquid- and particle phase. It is an application of the same principle used successfully in fluid/fluid reactions [57], But the necessary precondition is that the rate of reaction is slow compared to the transfer rate across the phase boundaries, so that equilibrium can really by assured. This might be justified in some of the hydrotreating processes, but certainly not in case of an aqueous liquid phase, existing in waste water treating. Earlier models used in petroleum industry have taken in-... 

Verstraete JJ, Le Lannic K, Guibard I. Modeling fixed-bed residue hydrotreating processes. Chem. Eng ScL 2007 62 5402-5408. 

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