Rejuvenation - Spent Hydroprocessing Catalysts - page 224

using the supercritical and sub-critical solution of ammonia [508] . However, with respect to
catalyst rejuvenation, the removal of Co is an unwanted reaction.
After a solvent wash to remove process oils, a substantial removal of coke was achieved by
treating the spent catalyst with an organic solvent, such as N-methyl-2-pyrrolidone, at a
temperature of 100 to about 250 ◦ C for a period of about 1 to about 12 h [495] . After this
treatment the catalyst was ready for reuse providing that the content of the contaminant metals
in spent catalyst was low. Optionally, the solvent-treated catalyst can be oxidatively
regenerated. In another version, the solvent-treated catalyst can be acid leached using diluted
H 2 SO 4 and/or HCl to remove contaminant metals and then decoked.
The spent hydroprocessing catalyst used for bitumen upgrading was extracted with hydrogen
donor solvents such as tetralin and tetralin-pyrene mixture [439] . These solvents were
compared with gas oil. The following order in the effectiveness of coke removal was
established: gas oil < tetralin < tetralin-pyrene. The highest effectiveness of tetralin-pyrene
mixture was attributed to the ability of pyrene to form very active H-donors such as
hydropyrenes. The HDS activity of the regenerated catalyst was significantly enhanced.
Because this extraction was conducted under H 2 pressure, this mode of spent catalyst
treatment was discussed in the section dealing with reductive regeneration.
Strong solvents such as pyridine and quinoline were used for the extraction of coke from the
spent CoMo/Al 2 O 3 catalyst [509] . In this case, the amount of coke was decreased from
27 wt.% to 14 and 12 wt.%, respectively. The isolated extract contained V, Ni, and Fe, whereas
Co and Mo were not present. The drawback of this method was that a special treatment was
necessary to remove the solvent adsorbed on the catalyst surface, unless catalyst was subjected
to decoking.
7.4 Biorejuvenation
From time to time, biorejuvenation as the subject of discussion has been noted. In this case, a
microorganism-aided removal of V and Ni from spent catalyst while leaving active metals
intact would be an ideal method for rejuvenation. Bioleaching of metals from various solids
including spent catalysts has been attracting attention and is the subject of the later chapter. It
will be shown that potential of this method for metal recovery is limited. It is believed that
biorejuvenation of spent hydroprocessing catalysts is still in its infancy. Thus, no experimental
studies dealing with this issue could be found in literature. This sub-chapter was added only
with the aim to clarify the issue of biorejuvenation, which so far only attracted very limited
interest. However, there is good scope for research in this area, particularly in the development
of microorganisms, which can selectively remove the contaminant metals (e.g., V) leaving
active metals (Co and Mo) in the catalyst.