Rejuvenation - Spent Hydroprocessing Catalysts - page 192

Moreover, because of the deposition in pores, the metals have to be removed to approach, as
much as possible, the original porosity of catalyst. This appears to be rather challenging task.
It should be noted that several methods, employing a wide range of leaching agents, have been
tested for the removal of contaminant metals with a varying degree of success.

Besides removal of the unwanted metals, catalyst has to be decoked by oxidative burn-off. The
latter may be performed either prior to decoking or on the decoked catalyst. General
observations confirm that the removal of metals by various solvents from spent catalysts can
be accomplished more readily on decoked catalyst. However, the selectivity of leaching out
contaminant metals may be more favorable in the presence of coke. Therefore, for
rejuvenation purposes, the spent catalyst still deposited by coke has been usually employed.
Nevertheless, it is desirable that during decoking the temperature is carefully controlled to
prevent sintering, which would affect the availability of active metals on catalyst surface
during subsequent utilization cycle. Some studies indicate that the extraction affected
mechanical properties of the rejuvenated catalyst [414] . Contrary with this observation,
numerous studies confirmed that desirable mechanical properties of rejuvenated catalysts
could be established on rejuvenation [475-478] .

A wide range of solvents has been used for the metal extraction from spent hydroprocessing
catalysts. Both organic and inorganic solvents have been evaluated. The former ensure the
non-corrosive environment, requiring much less safety precautions than that which has been
present during the leaching using inorganic agents. Moreover, their leaching efficiency can be
increased and/or modified by various additives with the aim to enhance the selectivity for
removal contaminant metals.

At KISR, the world leader in research on various aspects of rejuvenation, considerable effort
has been made as part of the research program on handling and utilization of spent
hydroprocessing catalysts [462] . Factors influencing the selectivity for removal of major
foulant metals (e.g., V and Ni) from spent catalysts were investigated [475-478] .
Improvements in the essential surface properties such as surface area and pore volume as well
as the activity for hydrodesulfurization (HDS) and HDM of atmospheric residues after metal
leaching were achieved. Extensive database established in the bench scale unit was used for a
preliminary design and economic assessment of the commercial rejuvenation process.

7.1 Organic Agents

This method of metal extraction is based on the pioneering work of Beuther and Flynn [479]
who recognized that some organic agents were capable of forming complexes with the
transition metals, which are part of the hydroprocessing catalysts. Rather small concentration
of acids, i.e., 1 wt.%, was sufficient for removing substantial amount of the metals from

spent catalysts. The tested organic agents included acids such as oxalic, lactic, citric, glycolic,

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