Cascading - Spent Hydroprocessing Catalysts - page 228

as part of the layered bed with fresh catalyst. In the first two hydrodemetallization (HDM)
reactors, the regenerated catalysts from the third and fourth reactors may be used as part of a
layered bed to provide HDS/hydrodenitrogenation (HDN) activity. In this case, the layer of the
regenerated catalyst would be placed at the end/bottom of fixed-bed. The HDS is usually the
main function of the last reactor in series. It has been established that the HDS activity of
catalyst is severely inhibited by the metals deposited from the feed, particularly V [197] . This
suggests that the cascading of the spent-regenerated catalyst from preceding reactors may not
be feasible. However, in one case, even a heavily metal-fouled catalyst after regeneration
exhibited an adequate activity during mild HDS of gas oil as the lowest activity user option
[518] . However, this gas oil was derived from sweet crude; therefore, this option may not be
representative of the current situation in refineries. Another example of the strategy used for
catalyst cascading in a multireactor system was presented by Rockwell [519] . In this case, a
portion of spent catalyst withdrawn from third reactor would be cascaded to the front of the
second reactor.
Apparently, it makes more sense to use the spent-regenerated catalyst in a preceding reactor
though it operates at a higher severity. In this regard, the level of catalyst deactivation may be a
decisive factor. For example, in the last reactor of the ARDS and/or HYVAHL process, the
catalyst deactivation is dominated by coke deposition. After decoking, such catalyst may still
pose an adequate activity to be used in one of the preceding reactors. As part of the mixed bed
or layered bed with an HDM catalyst possessing a high metal storage capacity, spent-decoked
catalysts may exhibit desirable hydrogenation and hydrocracking activities required for
asphaltenes conversion in the upstream reactors with dedicated HDM functions. One report
indicates on the use of spent-regenerated catalyst from the most severe operation to the next
most severe service in the multistage system [520] .
Fixed-bed guard reactors are sometimes placed upstream of the HDM and HDS/HDN reactors
to prolong the operation of the latter. For example, the advanced version of the HYVAHL
process incorporates two guard reactors operating in a perturbating mode. A composite bed
comprising a typical guard reactor material and spent-decoked catalyst may be another option
for the reversed cascading of spent catalysts. Primary function of guard chamber is the
removal of the solids dispersed in heavy feed, whereas in guard reactor removal of these solids
and a partial HDM is anticipated. It is believed that after decoking, heavily deactivated
non-regenerable catalysts can still be utilized either in guard chamber or added to the front of
the fixed-bed in the first reactor to prevent the crust formation.
Practical cases of cascading of spent hydroprocessing catalysts from low severity to high
severity service can be found in the literature. For example, Sakabe and Yagi [408] used a
tubular type reactor to evaluate the spent hydroprocessing catalyst for upgrading atmospheric
and vacuum residues. Rather high activity for hydrocracking of the residues was observed.
Refinery practice indicates the practical case of cascading from lower to higher severity