Abstract
Ultra deep hydrodesulfurization (HDS) of several types of gas oil were performed over NiMo catalyst under various conditions and the carbonaceous deposits on the spent catalysts were characterized by temperature programmed oxidation (TPO). CH3CN was formed in addition to H2O, CO, CO2 and SO2 by TPO, indicating that the carbonaceous compounds deposited on the spent catalysts contain nitrogen atoms. Curve fitting analyses showed that both COx and CH3CN profiles could be deconvolved into two or three Gauss-Lorentz type peaks. Two peaks always appeared below 680 K whereas another peak was observed in the range from 680-690 K if ultra deep HDS was performed above 580 K. These results indicated that at least two or three types of carbonaceous compounds with different combustion properties are deposited on the spent catalysts depending on the reaction conditions. No clear correlation was found between the amounts of carbon and nitrogen atoms contained in the carbonaceous compounds combusted below 680 K and the reaction conditions or the feed compositions such as a total nitrogen content and 90% distillation temperature, T90. On the other hand, the amounts of carbon and nitrogen atoms included in the refractory carbonaceous compounds increased with both higher T90 and reaction temperature. Therefore, the carbon and nitrogen atoms included in the refractory carbonaceous compounds originate from the adsorption of heavy molecule(s) containing nitrogen atom(s) in the feed. At higher reaction temperatures, polymerization of the adsorbed species was facilitated, resulting in larger amounts of the refractory carbonaceous compounds. Larger amounts of refractory carbonaceous compounds were associated with lower residual HDS activity, suggesting that the deposition of such compounds is one of the reasons for the catalyst deactivation.
