Catalyst Deactivation - Spent Hydroprocessing Catalysts - page 70

Figure 4.18: Effect of H 2 pressure (1 /P H 2 ) on carbon on catalyst [From ref. 212 . Reprinted
with permission].
Rather complex deactivation patterns were observed in the study of Kumata et al. [214] on the
combined effect of temperature and H 2 pressure on the coke build-up. As the feed, they used
the partially demetallized atmospheric residue derived from the heavy Arabian crude. The
partial HDM of the feed was conducted over the typical HDM catalyst (4.2 wt.% of MoO 3 on
-Al 2 O 3 ). The experimental system comprised two trickle-bed reactors connected in a series.
The adjacent reactor was loaded with the typical HDS catalyst of the Ni/CoMo/Al 2 O 3
formulation. It was observed that at 653 K, coke was rather evenly distributed between the top
of the first HDS reactor and bottom of the second HDS reactor. However, at 683 K, the coke
build-up progressively increased in the same direction. When H 2 pressure was increased from
8.0 to 14.0MPa at 653 K, the amount of deposited coke decreased from about 20 to 14 wt.%.
This indicates that an optimal combination of temperature and H 2 pressure, for which coke
deposition can be minimized, may be established.
4.5 Effect of Mechanical Properties of Catalyst
The desirable performance of the bed of catalyst cannot be achieved without the catalyst
having an adequate mechanical strength. In the fixed-bed, the fine particles formed by attrition
may be carried out with liquid streams, thus depleting the original load of catalyst. Moreover,
in the fixed-bed, the fines may decrease the void space between the catalyst particles. This
would affect the flow patterns of the liquid and gaseous streams leading to the development of
pressure drops across the bed. Malfunctioning of the fixed-bed (e.g., development of
channels), ending with the discontinuation of the operation, could be an ultimate result of
these changes. Although this may not be catalyst deactivation in a true sense, the operating
problems caused by fines of catalyst require attention. It was indicated earlier that if not
removed from the feed, finely divided mineral matter might cause similar difficulties.