Abstract: It was found that the surface of hydrofining catalyst on the top of hydrofining reactor was covered with scale during the turnaround of a diesel hydrocracking unit and replacement of catalysts. After toluene extraction and regeneration, the hydrofining catalysts were analyzed using specific surface area and pore structure analyzer, carbon & sulfur analyzer, and scanning electron microscope (SEM) techniques. The results showed that the main cause of the catalyst deactivation is the impurities deposited on hydrofining catalyst. The deposition was inorganic components containing P, Si and Fe, showing eggshell type distribution on cross section of the catalyst. It is suggested that the refinery should strictly control the impurities content, such as P, Si and Fe in the feed, and load some silicon trap catalyst and optimize the grading of guard catalyst, so as to ensure the activity of the main catalyst and realize the smooth operation of the unit.

Key words: diesel, hydrocracking, hydrofining, catalyst, deactivation