ABSTRACT
Catalyst deactivation is mainly due to carbon and metal deposition, which enhances with time-on-stream (TOS) leaving unutilized catalytic active sites. During hydrotreating, polynuclear aromatic (PNA) compounds and asphaltenes are the main element responsible for coke formation (Abotsi and Scaroni, 1989). Adsorption of PNAs and asphaltenes is mainly due to the presence of nitrogen compounds (Choi and Gray, 1988; Deng et al., 1997). Various authors have reported the presence of nitrogen (N) in coke (Furimsky, 1978; Sattereld et al., 1978; Speight, 1987; Choi and Gray, 1988). Furimsky concluded that adsorption of nitrogen compounds is stronger compared to other hydrocarbons, and adsorption is carried out by interaction of the unpaired electrons of N with Lewis sites. According to Speight (1987), nitrogen remains in coke as a nonvolatile compound after thermal reaction of asphaltenes, and Reynolds (1998) has pointed out that the majority of sulfur, nitrogen, and metals, which are the most important asphaltene constituents, are deposited on the coke when thermal decomposition of asphaltene is carried out.
