Understanding of the relation between physico-chemical structure and activity of aged hydrodesulfurization catalysts
Résumé
Hydrodesulfurization (HDS) is a major process of petroleum refining destined to the removal of sulphur compounds from fuels, in order to satisfy environmental concerns. At the present day, the legislation for gasoline and diesel admits 10 ppm of sulphur. HDS catalysts are composed by Ni or Co promoted molybdenum sulfide phases dispersed in an alumina support with high specific surface, preferentially in the form of the so-called CoMoS phase. In the industrial process, these catalysts can perform during an average of 2 years, depending on the type of feed used. During the cycle of a HDS process, deactivation of the catalyst is attributed to three main causes: coke deposition (inducing both pore plugging and catalytic site blocking), metals deposition and segregation of the active phase. In the industrial units, to keep necessary performance, the catalyst deactivation is compensated by an increase in temperature. The aim of our work is to study the effect of coking and active phase changes on the deactivation of the HDS catalyst (of CoMo/Al2O3 type) applied to the HDS of a diesel feed. In order to understand these phenomena, the aging of the catalyst must be accelerated; in order to reproduce the characteristics observed in a 6 months to 2 years run on a much shorter time scale. The typical operating conditions of a HDS process on a straight-run gasoil (SRGO) are 340°C, 40 bar and a ratio H2/GO of 250Nl/l; the accelerated aging tests must be performed under more severe but still realistic conditions. Using a commercial CoMo/Al2O3 catalyst, different series of aging tests have been performed by increasing the temperature (up to 390°C), limiting the hydrogen supply (from 250 to 25 Nl/l), introducing nitrogen and aromatic inhibitors in the feedstock or using mixed feedstocks (SRGO and SRGO/LCO). After every aging test, the hydrotreated diesel samples and the aged catalysts have been recovered for further analysis of catalyst texture, active phase and coke structure. Aging tests were performed with durations between 90 and 250 hours, reaching a coke amount of 8%wt and a total volume of pores loss up to 30%. The ensemble of processes occurring during the deactivation as a function of accelerated aging conditions is discussed in this work.