Commercial residue hydrodesulfurization catalyst deactivation

The Mizushima Oil Refinery of Japan Energy Corporation first implemented a high conversion operation of vacuum residue, versus a constant desulfurization operation, in the commercial residue hydrodesulfurization unit equipped with fixed-bed reactors, to produce more middle distillates as well as fuel oil with lower viscosity. The catalysts will be replaced when the sulfur content in the product oil reaches the allowable limit. Since we have believed that an increase in the residue conversion decreases the catalyst activity by coke deposition, we have been interested in controlling the coke deactivation to maximize the residue conversion during a scheduled operating period. 

Those deactivation models accounting for both coke and metal sulfides are rather simple. Coke and metals foul residue hydrodesulfurization catalysts simultaneously via different processes, and decrease both intrinsic reaction rate and effective diffusivity. They never uniformly distribute in the commercial reactors. We have examined the activity and diffusivity of the aged and regenerated catalysts which were used at the different conditions as well as during the different periods. This paper describes the effects of vacuum residue conversion, reactor position, and time on-stream on the catalyst deactivation. Two mechanisms of the catalyst deactivation, depending on residue conversion level and reactor position, are also proposed. 

A study on the residue hydrodesulfurization catalysts used in the commercial reactors has suggested that there exists two deactivation mechanism such as metal-controlled deactivation and coke-controlled deactivation, depending on a residue conversion level. In the second and third bed, the deactivation is controlled by metal deposition. However, in the fourth bed, a coke-controlled deactivation appears at a high residue conversion. We also have proposed that there exist two stages in the metal-controlled deactivation. During the first stage, metal sulfides partially poison the active sites and... 

Feed properties and operation conditions determine catalyst life in the residue hydrodesulfurization. In a high conversion operation of vacuum residue, catalyst deactivation due to coke is as important as the one due to metals. Though many researchers have worked on understanding and modelling deactivation of residue hydrodesulfurization catalysts, there has still been a controversy in a coke deactivation mechanism [2, 3]. Very few publications are available discussing an effect of a bed temperature profile on catalyst deactivation in large scale adiabatic commercial reactors. Most of the studies on deactivation of residue hydrodesulfiirization catalysts have been done with small-scale isothermal reactors [2,3,4,5]. The activity tests of the used catalysts were conducted to study the catalyst deactivation in the commercial reactors. This paper also describes an effect of a bed temperature profile on coke deactivation, which was tested in the commercial reactors. 

The activity tests of the catalysts used in the commercial reactors were conducted in the bench-scale reactor. The aged catalyst samples were taken from the second bed through the fourth, where the hydrodesulfurization catalyst was packed. The aged catalysts were Soxhlet-extracted with toluene followed by drying. The activity tests were conducted for the fresh and aged catalysts with Arabian Heavy atmospheric residue at a temperature of 360 °C and pressure of 12 MPa. A detail of the study on the catalyst deactivation in the commercial reactors will be discussed elsewhere [9]. 

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