Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 69

temperature. This may be attributed to the enhanced conversion of asphaltenes to light
products. Consequently, the HDM rate should be increased as well.
At the same temperature, the H 2 pressure may be a critical parameter for controlling coke
formation. It is however believed that the decreased coke formation caused by an increase in
the H 2 pressure would favor the deposition of metals relative to that of coke. Richardson et al.
[212] used Athabasca bitumen to study the H 2 pressure effect on the initial coke formation
(between 1.5 and 5 h on stream) in the continuous stir tank reactor (CSTR) system and in an
autoclave reactor using the commercial NiMo/Al 2 O 3 catalyst at 703 K. After a rapid coke
build-up during the first hour on stream, the coke formation did not change with the increasing
ratio of the feed to catalyst. At the same time, increasing H 2 pressure from 7MPa to more than
15MPa decreased the amount of coke from about 17 wt.% to about 11 wt.%. In the study of
Gualda and Kasztelan [145] on hydroprocessing an atmospheric residue, the amount of coke
decreased from about 10 wt.% to about 4 wt.% by increasing the H 2 pressure from 2 to 15MPa
( Fig. 4.17 ). Moreover, the H 2 pressure had a pronounced effect on the H/C ration of coke on
the catalyst. Figure 4.18 [212] shows the effect of H 2 pressure on the steady-state level of
coke. It is believed that in the case of Athabasca bitumen, large asphaltenic molecules had the
predominant role during the initial stages of coke formation. Thus, there was a sufficient
amount of asphaltenes to form the same amount of coke even for the low feed/catalyst ratios.
Higashi et al. [213] studied the coke deposition on catalyst surface during the very early stages
on stream at a low H 2 pressure using an atmospheric residue as the feed. The study was
conducted in a pilot plant. They observed that the coke could not be removed and/or catalyst
activity could not be recovered by increasing the H 2 pressure at the same temperature, during
the later stage on stream. This indicated the permanent deactivation by coke. It is therefore
essential that the coke deposition control by H 2 pressure begins at the start of the run. In this
case, the loss of the HDS activity was noticed in particular. It was observed that the catalyst
presulfiding was an important factor in controlling the initial coke deposition.
Figure 4.17: Effect of H 2 pressure on H/C ratio and amount of carbon on catalyst (NiMo/Al 2 O 3 ,
atmospheric residue [AR], 663 K) [From ref. 145 . Reprinted with permission].