Thermodynamic analysis of diesel hydrotreating reactions

Thermodynamic analysis of diesel hydrotreating reactions-Hydrodesulfurization (HDS), Hydrodenitrogenation (HDN) and Hydrodearomatization (HDA)-were studied by Gibbs free energy minimization to understand the effect of process variables such as temperature (200-600 °C), pressure (10-80 bar(a)) and H2/oil ratio (stoichiometric ratio - 5 times of stoichiometric ratio) on the product distribution. Dibenzothiophene (DBT), naphthalene and pyridine were considered as representative compounds to study the HDS, HDA and HDN reactions respectively and the reactions were studied in both gas phase and two phase (gas-liquid). It was found that the HDN reaction has no thermodynamic limitation while HDA reaction has significant thermodynamic limitation at low pressures and high temperatures. Higher H2 concentration favors the HDA equilibria but not very significantly. Overall HDS reaction has no thermodynamic limitation but for refractory sulfur compounds like DBT and alkyl substituted DBTs, the preference for individual reaction pathways-Direct Desulphurization (DDS) and Hydrogenation (HYD)-changes with change in process variables. It was shown that while the selection of operating temperature and H2 concentration for a commercial diesel hydrotreating reactor are governed by the reaction kinetics, the reactor pressure is mostly governed by the HDA reaction equilibria.