Cascading - Spent Hydroprocessing Catalysts - page 229

service during the operation of LC-Fining process employing three ebullated-bed reactors in
the series. Thus, the least deactivated catalyst from the third reactor is transferred to the second
reactor before it must be replaced. In this way, the consumption of the fresh catalyst in the
second reactor can be decreased. Perhaps, the simplest case of cascading of spent-regenerated
catalyst was reported by Duddy et al. [521] . These authors used the regenerated catalysts from
the end of the fixed-bed and placed it on the front of the same bed before the subsequent
utilization cycle.
8.3 Cascading of Rejuvenated Catalysts
According to the established refinery practice, the desirable recovery of activity via oxidative
regeneration cannot be achieved in the case that spent catalysts contain more than 5 wt.% of
contaminant metals [517,518] . Such catalysts can be successfully reactivated by combining the
removal of contaminant metals with decoking as applied during rejuvenation. The activity of
catalyst obtained by oxidative regeneration of the spent catalyst lightly deactivated by metals
may approach that of a catalyst obtained by rejuvenation of the spent catalyst severely
deactivated by metals. This suggests that cascading options for rejuvenated catalysts may be
similar as those applied for regenerated catalysts. For example, for a process employing
several ebullated-bed reactors in a series, rejuvenated catalyst could be cascaded to the most
upstream reactor.