Two-stage aromatics hydrogenation of bitumen-derived light gas oil

In this research, two-stage hydrotreating of bitumen-derived light gas oil (LGO) from Athabasca oil sands was studied. The objective was to catalytically upgrade the LGO by reducing the aromatics content and enhancing the cetane content via inter-stage removal of hydrogen sulfide. The impact of hydrogen sulfide inhibition on aromatics hydrogenation (HDA), hydrodenitrogenation (HDN) and hydrodesulfirization (HDS) activities was investigated. Experiments for this study were carried out in a trickle-bed reactor loaded with commercial NiMo/Al2O3 and lab-prepared NiW/Al2O3 in the stage I and stage II reactors, respectively. Temperature was varied from 350 to 390 oC at the optimum LHSV and pressure conditions of 0.6 h-1 and 11.0 MPa, respectively. The results from two-stage process showed significant improvement in HDA, cetane rating and HDS activities compared to the single-stage process after the inter-stage removal of hydrogen sulfide. Hence, the presence of hydrogen sulfide in the reaction retarded both the HDA and HDS processes in the single-stage operation. Negligible hydrogen sulfide inhibition was however, observed in the HDN process. Prior to the two-stage hydrotreating study, single-stage hydrotreating reactions were carried out over commercial NiMo/Al2O3 catalyst to determine the optimum operating conditions for maximizing hydrogenation of aromatics. A statistical approach via the Analysis of Variance (ANOVA) technique was used to develop regression models for predicting the conversion of aromatics, sulfur and nitrogen in the LGO feed. Experiments were performed at the following operating conditions: temperature (340-390 oC); pressure (6.9-12.4 MPa) and liquid hourly space velocity, LHSV (0.5-2.0 h-1). Hydrogen-to-oil ratio was maintained constant at 550 ml/ml. The results showed that the two-level interaction between temperature and pressure was the only significant interaction parameter affecting HDA while interaction between temperature and LHSV was the most important parameter affecting both HDS and HDN activities. A maximum 63 % HDA was obtained at 379 oC, 11.0 MPa and 0.6 h-1. Experiments with NiW/Al2O3 were also performed in a single-stage reactor with LGO blend feedstock by varying temperature from 340-390 oC at the optimum pressure and space velocity of 11.0 MPa and 0.6 h-1, respectively. The following order of ease of hydrogenation was observed: poly- > di- >> monoaromatics. The order of ease of hydrogenation in other LGO feedstocks (atmospheric light gas oil, ALGO; hydrocrack light gas oil, HLGO; and vacuum light gas oil, VLGO) was studied and found to follow the order: VLGO > ALHO > HLGO. Studies on mild hydrocracking (MHC) in the gas oil feedstocks showed a net increase in gasoline with a corresponding decrease in diesel with increasing temperature. Both the single and two-stage HDA and HDS kinetics were modeled using Langmuir-Hinshelwood rate equations. These models predicted the experimental data with reasonable accuracy. The degree of conversion of the gas oil fractions in ALGO, HLGO and VLGO via mild hydrocracking was best described by a pseudo-first order kinetic model based on a parallel conversion scheme