The Effect of Solvent on the Kinetics and Mass Transfer Resistances for 4,6 DMDBT HDS

Abstract

The effect of different solvents for model hydrodesulfurization (HDS) reactions on both the kinetics and the mass transfer resistances on a wide range of hydrogen partial pressures has been investigated. Special attention was directed to study the effect of low hydrogen partial pressures. While some researchers associate solvent effects to competitive adsorption of the solvent molecules on the catalytic surface, there is evidence that other effects may also play an important role in the modification of the kinetics. In this work, HDS experiments were performed in a slurry stirred tank reactor using the model molecule 4,6-dimethyldibenzothiophene (4,6-DMDBT) over a commercial NiMoP/alumina catalyst. The effect of hydrogen pressure on the transformation of 4,6 DMDBT in different solvents was studied. Thermodynamic calculations for hydrogen solubility under the whole set of conditions was considered. From our experiments, modification of the catalyst activity was observed to occur only at low hydrogen pressures (less than 34.7 bar) when n-dodecane was the used solvent. On the other hand, when using decaline there seems to be practically no effect in the catalytic activity of the hydrogen pressure over the entire range of pressures that were studied (18.0-55.5 bar). Additionally, the selectivity to direct desulfurization (DDS) products increased, while simultaneously decreasing the selectivity to the hydrogenation (HYD) pathway products, as the hydrogen partial pressure diminished below 34.7 bar when both solvents were used. It was concluded that hydrogen solubility effects become important and modify the activity at pressures below 34.7 bar.