Modeling of a three-phase reactor for bitumen-derived gas oil hydrotreating

CATEGORY: HYDROTREATING
Braz. J. Chem. Eng. vol.29 no.1 São Paulo Jan./Mar. 2012
Modeling of a three-phase reactor for bitumen-derived gas oil hydrotreating
R. Chacón I,*; A. Canale I; A. Bouza I; Y. Sánchez II
I Departamento de Termodinámica y Fenómenos de Transporte, Universidad Simón Bolívar, Caracas Venezuela. E-mail: ronaldchj1@gmail.com
II Departamento de Procesos y Sistemas, Universidad Simón Bolívar, Sartenejas 1081, Venezuela
ABSTRACT
A three-phase reactor model for describing the hydrotreating reactions of bitumen-derived gas oil was developed. The model incorporates the mass-transfer resistance at the gas-liquid and liquid-solid interfaces and a kinetic rate expression based on a Langmuir-Hinshelwood-type model.  Authors derived three correlations for determining the solubility of hydrogen (H2), hydrogen sulfide (H2S) and ammonia (NH3) in hydrocarbon mixtures and the calculation of the catalyst effectiveness factor was included. Experimental data taken from the literature were used to determine the kinetic parameters (stoichiometric coefficients, reaction orders, reaction rate and adsorption constants for hydrodesulfuration (HDS) and hydrodenitrogenation (HDN)) and to validate the model under various operating conditions. Finally, they studied the effect of operating conditions such as pressure, temperature, LHSV, H2/feed ratio and the inhibiting effect of H2S on HDS and NH3 on HDN.
INTRODUCTION
Hydrotreating (HDT) is a catalytic conversion process where the content of some crude oil contaminants such as sulfur, nitrogen, oxygen and heavy metals is reduced using hydrogen over a catalyst of NiMo or CoMo supported on Al2O3. It also causes the conversion of some heavy molecules into lighter ones. HDT includes reactions of: hydrodesulfuration (HDS), hydrodenitrogenation (HDN), hydrodeoxygenation (HDO), hydrodemetallization (HDM), hydrodearomatization (HDA) and others such as olefin saturation, decyclization, etc. This is one of the most important processes in crude oil refining, because it allows reducing the emission of SOx and NOx, which are synthesized by fuel combustion. These emissions are strong environmental contaminants; in addition, they can inhibit the performance of the catalysts used in refining processes, as well as the catalysts used in catalytic converters of vehicles. (Harding et al., 2001).

In order to maximize the sulfur and nitrogen conversion it is necessary to study the optimal operating conditions used in HDT, such as pressure, temperature, liquid hourly space velocity (LHSV) and H2/feed ratio. It is also very important to select the most appropriate catalyst (such as NiMo/Al2O3 or CoMo/Al2O3), reactor configuration (simple or series), reactor type (fixed bed, slurry or ebullated bed). These operating conditions vary according to the feed type to be hydrotreated; the severity degree can vary depending on the sulfur and nitrogen content and the molecular weight of the feed. (Yang et al., 2004).

The aim of this work was to develop and validate a three-phase reactor model for describing HDT reactions, which incorporates the mass-transfer resistance at the gas-liquid and liquid-solid interfaces. It is also an objective to develop correlations for predicting the gas solubility of H2, H2S and NH3 in hydrocarbon mixtures and including the calculation of the catalyst effectiveness factor. The reactor model was proposed by Hofmann (1997), with the assumptions, kinetic expressions and properties estimation made by Korsten and Hoffmann (1996), López and Dassori (2001) and Yang et al. (2004). The experimental data was taken from the work of Botchwey et al. (2003) for a pilot reactor, packed bed and concurrent descending flow.
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