Abstract
Poisoning by very small amounts of nitrogen compounds was investigated for the catalysts for two types of hydrocracking processes. Nitrogen-free vacuum gas oil was used as feedstock with carbazole and tributyl amine as the model nitrogen compounds. Carbazole is abundant in hydrotreated oil as carbazole is difficult to remove. Tributyl amine is easily converted to ammonia in the reactor.
Vacuum gas oils containing various amounts of carbazole were passed over the main hydrocracking catalyst of the two-stage hydrocracking process. Strong effects on both hydrocracking activity and middle distillate selectivity were observed, especially if nitrogen content was less than 2 wtppm. The nitrogen poisoning effect was stronger on the catalyst with higher activity than on the catalyst with lower activity.
Poisoning of the main hydrocracking catalyst of the single-stage hydrocracking process, which is affected by both organic nitrogen compounds and ammonia, was evaluated with several ratios of combinations of carbazole and tributyl amine, maintaining total nitrogen concentration at 300 wtppm. Increased carbazole ratio lowered the hydrocracking activity and increased the middle distillate selectivity especially if the concentration of nitrogen as carbazole was less than 20 wtppm. These experimental results indicate that there is an optimum range of nitrogen concentration in the effluent to the main hydrocracking reactor to maximize middle distillate selectivity without severely affecting the hydrocracking activity.
