Abstract: Guard catalysts of the diesel hydrocracking unit in a refinery were collected and characterized by X-ray diffraction (XRD), X-ray fluorescence spectrum (XRF), carbon-sulfur analysis, thermogravimetry-mass spectrometry analysis (TG-MS), N2 adsorption-desorption and scanning electron microscopy with energy dispersive X-ray (SEM-EDX) after toluene extraction treatment. The results showed that along the cross section of the guard catalyst, the non-metallic impurities Si and P were evenly distributed in the interior and exterior of the guard catalyst, and the impurities Ca and Fe were mainly deposited on the outer surface of the guard catalyst. A large amount of Si, P, Fe, Ca, C and other impurities deposited and formed a “shell” over the guard catalyst, which blocked the pore channel and covered the outer surface of the catalyst, resulting in a significant reduction in the ability of containing impurities. The falling off of the “shell” eventually leads to the increasing pressure drop in the reactor.

Key words: diesel, hydrotreating, hydrocracking, guard catalyst, impurity, deposit