Regeneration - Spent Hydroprocessing Catalysts - page 120

CHAPTER 6
Regeneration
For the purpose of this review, regeneration is considered to be the process for only removal of
the coke deposited on catalyst surface with the aim to restore as much as possible of the
original catalyst activity. This also requires the recovery of surface area and porosity. It is
generally believed that, to be suitable for regeneration, the content of contaminant metals (e.g.,
V and Ni) in spent catalyst should not exceed 5 wt.%. Otherwise, at least 80% recovery of the
original activity to make regeneration attractive may not be achieved. Such level of catalyst
activity recovery can be readily achieved for the spent catalysts after hydroprocessing
atmospherics distillates and lighter feeds. It is difficult to reach this level of the activity
recovery when both coke and metals, which were deposited from the feed, are present on the
catalyst surface. The removal of metals with the aim to restore activity is discussed as part of
the Chapter 7 dealing with catalyst rejuvenation.
The most widely used method for activity recovery involves the coke removal via oxidative
regeneration using diluted air and/or air and steam mixture. Potentially, other oxidative agents
can also be used. The oxidative regeneration technique has been used on the commercial scale
for several decades. Attempts have also been made to use steam and CO 2 as oxidative agents.
The coke removal may be achieved via reductive regeneration using H 2 . However, this method
has not yet reached a commercial stage. Attempts to remove the deposits from catalyst surface
by extraction using various solvents have also been noted. However, the potential of this option
seems to be rather limited. It was indicated earlier that after calcining to obtain necessary
mechanical strength, an enhancement of active metals dispersion may be achieved via
postcalcining treatment with chelating agents [329] . The same approach may also be applied
to regenerated catalysts.
6.1 Regenerability of Spent Catalysts
In petroleum refinery, a decision has to be made on the fate of spent catalyst after unloading
from reactor. This may require testing on a laboratory scale. A regeneration test should reveal
the potential level of recovery of catalyst activity. Generally, at least 80% recovery of
hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activity of the fresh catalyst is
the acceptable minimum [364] , although some refineries prefer 90% activity recovery [365] .
Otherwise, the spent catalyst is classified as non-regenerable. Compared with the pore volume
and size distribution, the surface area recovery may not be a precise indication of the activity
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