ABSTRACT
ABSTRACT Two series of bimodal CoMo/A120 3 catalyst samples have been characterized using temperature-programmed oxidation, NMR spectroscopy and nuclear microprobe analysis. The samples are withdrawn from the first-and third-stage reactors of an ebullating-bed resid hydrotreating pilot unit after varying times on stream of up to 120 days. The results show that coke and metals are deposited rather rapidly. The coke deposited on the third-stage catalyst is shown to be more aromatic than the coke on first-stage samples. Temperature-programmed oxidation studies show that vanadium catalyzes the oxidation of coke and that the coke contains significant amounts of sulfur and nitrogen. With increasing time on stream, the coke in both the first-and third-stage samples becomes more hydrogen and sulfur deficient as evidenced by increasing concentration of aromatic carbon (from NMR), falling H/C ratio (from nuclear microprobe) and a lowering in concentration of S associated with the coke (from TPO). There is evidence to the effect that the sulfur in the coke is associated with the upper layers of coke and that nitrogen may adsorb preferentially during the initial coke laydown. Model compound activity measurements carried out on the samples showed that the HDS activity is less sensitive to the effects of coke deposition than the HDN and hydrogenation activities. It was also shown that the initial metal deposits have a stronger effect on the loss of activity for HDS than for HDN and hydrogenation, indicating that different sizes are involved in these hydrotreating reactions.
