Environmental Engineering Reference
In-Depth Information
The rejuvenation process described by Duddy et al.
[497]
used the same acids but
experimental system was different. Thus, it comprised a pressurizable vertically oriented
vessel with inlet and outlet openings for the catalyst and washing liquids. With this
arrangement, solvent liquid washing, water washings, and acid treatment could be carried out
in successive steps. After rejuvenation, catalyst is withdrawn from the vessel conical-shaped
lower portion downwardly for further processing. The water-soluble solvent and acid
treatment liquids can be usually recovered by distillation for reuse in the catalyst rejuvenation
process. The solvent-washed and acid-treated catalyst is dried and decoked to obtain
completely rejuvenated catalyst. A similar rejuvenation process was disclosed by Tesker and
Milligen
[498]
. In this process, the vessel assembly is arranged to permit solvent washing,
vacuum drying and acid treatment, and gas drying of the used catalyst in a bed supported
above the conical grid, by upward flow and recycle of the washing liquids and fluidization of
the catalyst. Following rejuvenation step, the catalyst is withdrawn from the vessel
downwardly through the conical shaped grid for further processing, i.e., decoking.
Several studies were selected to illustrate the use of inorganic agents for selectively leaching
the contaminant metals from spent hydroprocessing catalysts
[499-501]
. For example, the
spent CoMo/Al
2
O
3
catalyst used for hydroprocessing of a vacuum residue was decoked by the
oxidative burn-off before being extracted with either Fe
3+
HCl or Fe
3+
H
2
SO
4
solutions
[499]
. The spent catalyst contained about 15 wt.% of coke, 14 wt.% of V, and 3.5 wt.% of Ni.
The solutions removed contaminant metals as well as the active metals. However, after
presulfiding of the decoked catalyst, selectivity for the removal of the former metals
significantly improved. The activity of the rejuvenated catalyst was measured in an autoclave
using a residual feed. In this case, the ratios of rate constants for the rejuvenated catalyst to
that of the fresh catalyst were determined. The values of 0.7, 0.7, 3.9, and 2.5 were obtained
for HDS, hydrodenitrogenation (HDN), hydrodevanadization (HDV), and
hydrodenickelization (HDNi), respectively. Although only 70% of the original HDS and HDN
activity was recovered, the HDM activity of the rejuvenated catalyst was superior to that of the
fresh catalyst, suggesting that the former may be suitable for cascading, e.g., in a guard
reactor. However, this would have to be confirmed during long-run experiments. In another
treatment, the spent catalyst was presulfided at 813 K in 5 vol.% H
2
S in He and then extracted
with the boiling solution of Fe
3+
+
+
H
2
SO
4
for 20min before being decoked
[500]
. This
treatment resulted in the significant removal of both V and Ni. This is confirmed in
Fig. 7.19
[500]
comparing the spent and rejuvenated catalysts. Significantly greater amount of V than
that of Ni in spent catalysts should be noted. In spite of small amount of Mo and Co removed,
the HDM activity of the rejuvenated catalysts was not affected. Physical properties of the
fresh, spent, and rejuvenated catalysts are shown in
Table 7.10 [500]
. Most likely, the
improvement in HDM activity resulted from the increase in the mean pore diameter.
+
The efficient leaching of the metals (V
Ni) from spent catalyst with H
2
SO
4
could be
achieved by optimizing the concentration of the latter
[501]
. For example, for 15% H
2
SO
4
, the
+
