Cascading - Spent Hydroprocessing Catalysts - page 227

8.1 Cascading of Spent Catalyst
A variable level of deactivation of spent catalyst removed from reactor has been noted. If
separated from the mixture, the least deactivated portion of the spent catalyst may be returned
either directly to the same operation or cascaded to the less severe service. In the case of
ebullated-bed reactors, the spent catalyst may comprise a portion of the fresh and/or very
lightly deactivated catalyst which was admixed with spent catalyst during ebullation. The cost
of catalyst inventory could be decreased if this portion of the catalyst can be separated from
spent catalyst mixture and reused.
It was indicated earlier that density grading of spent catalyst after de-oiling might separate the
least deactivated portion of spent catalyst. Sherwood et al. [514] disclosed the process in
which spent catalyst is subjected to elutriation by air. The flow of air directed upwardly was
maintained at the velocity sufficient to expand the bed at least 25% over its settled height. At a
sufficient time, a substantial segregation of catalyst particles occurred. The least deactivated
potion withdrawn from the top of fluidized bed was returned directly to the operation. In the
case of several ebullated-bed reactors operating in series, the catalyst is used in a preceding
reactor, whereas for fixed-bed systems, the elutraited catalyst may be placed on the front of the
bed in the same reactor. In large refineries, additional options for the reuse of the least
deactivated catalyst separated from the spent catalyst mixtures may be identified.
8.2 Cascading of Regenerated Catalysts
Several examples from literature are presented to illustrate practical cases of cascading. In the
study of Mittal et al. [515] , the spent-regenerated catalyst after more than four years of
operation during the hydrodesulfurization (HDS) of naphtha was used for mild hydrocracking
of a vacuum residue. The catalyst exhibited a good activity and selectivity to middle distillates.
A marginal cost of catalyst, low hydrogen consumption and a high yield of the desirable
product made this option economically attractive. In another study, the spent catalyst from
residue upgrading was used for the upgrading of naphtha [516] . In this case, the purpose was
to selectively remove sulfur, while avoiding the extensive hydrogenation of olefins with the
aim of minimizing the loss of octane number of the gasoline product.
In the case of the multistage processes employing either fixed-bed reactors (e.g., atmospheric
residue desulfurization [ARDS] and HYVAHL processes) or ebullated-bed reactors (e.g.,
LC-Fining and H-Oil processes) in a series used for upgrading heavy petroleum feeds, the
severity decreases from the first reactor towards the last reactor. The extent of catalyst
deactivation by metals decreases in the same order. This suggests that cascading of the
regenerated catalyst from fourth reactor to third reactor makes more sense than vice versa. In
fact, according to Suchanek [517] , regenerated catalyst from the downstream reactors could be
successfully used in the first reactor. In the same reactor, regenerated catalyst can also be used