Kinetics and effects of H₂ partial pressure on hydrotreating of heavy gas oil

Abstract

The impact of H₂ partial pressure (H₂ pp) during the hydrotreating of heavy gas oil, derived from Athabasca bitumen, over commercial NiMo/ɣ-Al₂O₃ catalyst was studied in a micro-trickle bed reactor. The experimental conditions were varied as follows: temperature: 360 to 400°C, pressure: 7 to 11 MPa, gas/oil ratio: 400 to 1270 mL/mL, H₂ purity range of 0 to 100 vol. % (with the rest either CH4 or He), and LHSV range of 0.65 to 2 h⁻¹. The two main objectives of the project were to study the nature of the dependence of H₂ pp on temperature, pressure, gas/oil ratio, LHSV (Liquid Hourly Space Velocity), and H₂ purity. The project was divided into three phases: in phase one the effect of H₂ purity on hydrotreating of heavy gas oil (HGO) was studied, in phase two the nature of H₂ pp dependency and the effect of H₂ pp on hydrotreating of HGO was investigated, and in phase three kinetic studies were carried out using different kinetic models. The objective of phase one was to study the effect of hydrogen purity on hydrotreating of HGO was studied in a trickle bed reactor over a commercial Ni−Mo/ɣ-alumina catalyst. Methane was used as a diluent for the hydrogen stream, and its effect on the catalyst performance was compared to that of helium, which is inert toward the catalyst. Furthermore, a deactivation study was conducted over a period of 66 days, during which the catalyst was subjected to H₂ purities ranging from 75 to 95% (with the rest methane); no significant deterioration in the hydroprocessing activities of the catalyst was observed. Therefore, it was concluded that methane was inert toward a commercial Ni−Mo/ɣ-alumina catalyst. However, its presence resulted in hydrogen partial pressure reduction, which in turn led to a decrease in hydrodesulphurization (HDS), hydrodenitrogenation (HDN), hydrodearomatization (HDA) conversions. This reduction can be offset by increasing the total pressure of the system. HDS, HDN, HDA, and mild hydrocracking (MHC) conversions were studied. Also determined were cetane index, density, aniline point, diesel index, and fractional distribution of the products. The main objective of phase two was to study the effects of H₂ pp on hydrotreating conversions, feed vaporization, H₂ dissolution, and H₂ consumption were studied. The results show that HDN and HDA are significantly more affected by H₂ partial pressure than HDS; with the HDN being the most affected. For instance as the inlet H₂ partial pressure was increased from 4.6 to 8.9 MPa HDS, HDN, and HDA conversions increased for 94.9%, 55.1%, and 46.0% to 96.7%, 83.9%, and 58.0% , respectively. Moreover, it was observed that H₂ dissolution and H₂ consumption increased with increasing H₂ pp. No clear trend was observed for the effect of H₂ pp on feed vaporization. In phase three the kinetics of HDS, HDN, and HDA were studied. The power law, multi-parameter, and Langmuir - Hinshelwood type models were used to fit the data. The prediction capacities of the resulting models were tested. It was determined that, while multi-parameter model yielded better prediction, L-H had an advantage in that it took a lesser number of experimental data to determine its parameters. Kinetic fitting of the data to a pseudo-first-order power law model suggested that conclusions on the effect of H₂ pp on hydrotreating activities could be equally drawn from either inlet or outlet hydrogen partial pressure. However, from the catalyst deactivation standpoint, it is recommended that such conclusions are drawn from the outlet H₂ partial pressure, since it is the reactor point with the lowest hydrogen partial pressure.