Environmental Engineering Reference
In-Depth Information
CHAPTER 8
Cascading
While the objective of regeneration and rejuvenation is the reuse of spent catalysts at the point
of their origin as many times as possible, cascading offers an additional option for the reuse of
spent catalysts in less and more severe operations. Generally, severity of hydroprocessing
increases with increasing temperature and H
2
pressure is employed during the operations. This
is usually accompanied by the increased hydrogen consumption. The cases in which
spent-regenerated catalysts are used either in more or less severe operation can be encountered
in the refinery practice. Thus, with increasing size of refinery, the number hydroprocessing
reactor increases. Consequently, the number of applications for regenerated/rejuvenated
catalysts increases as well. According to the literature, the following options for catalyst
cascading may be identified
[511-513]
:
(1) Cascading to less severe operation
After regeneration of catalyst used in a severe service (e.g., gas oil) the catalyst is
returned to a less severe operation (e.g., naphtha or straight run distillate).
(2) Cascading to high metal service
After regeneration, catalyst is used to treat high contaminant metals content feed. The
catalyst can be used in a guard bed as metal trap or a guard reactor to protect catalysts in
the downstream reactors of a multistage process.
(3) Cascading to a high silicon service
In this case, the catalyst contamination due to silicon from the anti-foaming agent such
as used during delayed coking may be more serious problem than deactivation by coke.
For this application, using a relatively cheap catalyst than the fresh catalyst may be more
advantageous.
(4) Make-up catalyst after reactor skim
In some cases, catalytic reactors, i.e., front of the fixed-bed require skim to alleviate
pressure drop build-up at the front of the bed. Such service can be adequately provided by
spent regenerated/rejuvenated catalysts.
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