Abstract
Based on the analysis of the results obtained with the used catalysts used in a previous paper, the deactivation mechanism of HDN catalyst was studied. The analytical results of the catalysts, before and after reaction, are shown in Table 1.
The quantities of carbon deposited on the catalyst at various positions in the reactor are shown in Fig. 1. The quantity of carbon is found to be uniform at 400°C but if increases at 430°C. The quantity of carbon deposited at reactor inlet differs with the catalyst used, but Fig. 2 shows that the quantity deposited per unit surface area is well related to the pore diameter of the catalyst. As shown in Fig. 3 the average amount of carbon deposited per unit surface area for the whole reactor is in a good linear relationship with deactivation of the catalyst. These results suggest that a catalyst with a larger pore diameter can capture carbon precursors more easily and, it is therefore poisened by more extensive coking and its activity is declined more rapidly.
The N/C (atomic ratio) values shown in Table 1 suggest that there are two kinds of cokes which break out in different mechanisms from each other. One of them is formed directly from the feed (probably through dehydrogenation) and the other is formed from the cracked fragments (probably through polymerization). A model of carbon deposition is illustrated in Fig. 4.
By analysis of metal compounds deposited on the catalyst, V and Ni are probably in the forms of V3S4 and Ni3S2. From the densities of these sulfides and from the pore volume of the catalyst used, the specific gravity of coke was estimated.
By use of these values and effectiveness factors calculated from the deposition curves of V and Ni shown in Fig. 6, the degree of pore mouth plugging was obtained. The results are shown in Table 2.
The activities of the used catalyst sampled from the various parts of the reactor were examined, and they are shown in Table 3. The influence of carbon deposition on degradation of the catalyst can be recognized clearly but not that of vanadium. The AP catalyst taken out from the reactor inlet shows very low activity despite the deposition of only small quantities of carbon and metals on the catalyst, because the degree of pore mouth plugging of the catalyst is about 1.0.
