Rejuvenation - Spent Hydroprocessing Catalysts - page 191

Figure 7.1: Options and process for spent catalyst handling and utilization [From ref. 462 .
Reprinted with permission].
It has been indicated above that the primary objective of rejuvenation is the removal of metals
deposited on the catalyst surface during the operation, while leaving the active metals intact.
Ideally, if contaminant metals could be removed selectively by chemical treatments without
significantly altering the chemical and physical characteristics of the original catalyst, then
spent catalysts could be rejuvenated and reused in the operation. It is also more advantageous
to conduct rejuvenation using spent catalyst particles in their original size and shape, rather
than using an altered form (e.g., a pulverized form). In the latter case, a reprocessing of the
solid material would be necessary to obtain an operating size and form of catalyst. It was
indicated earlier that in the conventional hydroprocessing catalysts the active metals exist in
the Co(Ni)-S-Mo(W) phase, although under hydroprocessing conditions, the presence of other
phase cannot be excluded. At the early stages of operation, the active phase accounts for most
of the metals on the support. With time on stream, inorganic solids in the feed as well as the V
and Ni produced during hydrodemetallization (HDM) reactions are deposited on catalyst
surface. The active phase metals together with these deposits represent rather complex
mixture, particularly in the later stages of the operation. This may include sulfides and mixed
sulfides of V, Ni, Fe, and other metals. To certain extent, Co and Ni promoters in the active
phase may be replaced by V and Fe and, as such, decrease the catalyst activity. The objective
of rejuvenation is to liberate the active phase from the unwanted deposits in the mixture.