Environmental Engineering Reference
In-Depth Information
tends to lose Ni because of the enhanced formation of NiAl
2
O
4
. Similarly, Mahadjev et al.
[386]
observed the gradual loss of Co during the repeated regeneration-utilization cycles
involving the CoMo/Al
2
O
3
catalyst. In the study of Madeley and Wanke
[417]
, severe
sintering of
-Al
2
O
3
was observed after regeneration of the spent NiMo/Al
2
O
3
in pure O
2
,
while under moderate regeneration conditions a highly dispersed metal oxide phases, such as
present in fresh catalyst, were found.
In the study of Yoshimura et al.
[390]
on the spent NiW/Al
2
O
3
catalyst, the EXAFS data
revealed that WS
2
-like structures, which were laterally grown during hydroprocessing run,
were redispersed to nearly the same level as that of the fresh catalysts when carefully
controlled oxidizing conditions were used (e.g., 1.5% O
2
gas). The XPS data showed that
surface compositions of Ni and W were recovered to almost the level of fresh catalysts, but the
Ni/W ratio was slightly less than that of the fresh ones. Catalytic activities and selectivities
were successfully recovered by low-temperature oxidation. On the contrary, for the CoMo
catalyst on which MoS
2
-like sulfides were laterally grown, some of the Co aggregated to
Co
9
S
8
. In this case, it was not possible to recover the same level of structural properties as
those of the fresh catalysts because of small amounts of metals such as Ni, Fe, and V being
present. While the catalytic activities and selectivities were almost recovered by
low-temperature oxidation, at higher regeneration temperatures, a slight loss of hydrogenation
activity and a large increase in the hydrocracking activity were noted.
The study on regeneration and resulfidation of the NiMo/Al
2
O
3
catalyst conducted by
Bogdanor and Rase
[392]
confirmed a significant intraparticle migration of the Ni and Mo
metals. This is evident in
Fig. 6.24
from the radial variation in the Mo/Al, Ni/Al, and Mo/Ni
ratios determined by SEM. Interestingly enough, for spent catalyst, the net result of
regeneration followed by resulfidation was the initial distribution as observed for the fresh
catalyst. Several qualitative observations can be made from the results in
Fig. 6.24
.For
example, the fresh catalyst exhibited a modest maldistribution of Ni and Mo, which then
changed on sulfidation followed by reaction, i.e., the Mo profile became flat, whereas the Ni
profile increased in concentration towards the center. This caused the Ni/Mo ratio to increase
and/or Mo/Ni ratio to decrease towards the center. In the spent catalyst, Mo was concentrated
towards the pellet center and Ni towards the pellet exterior. Therefore, a high Mo/Ni ratio at
the center of pellet was observed. Increasing the temperature of regeneration resulted in the
migration of both Ni and Mo towards the pellet exterior in agreement with the results
published by Gellerman et al.
[418]
. The spent catalyst and the same spent catalyst that was
regenerated under different conditions and subsequently resulfided had the same Mo/Ni ratio.
Generally, Ni appeared to be more mobile than Mo. During the reaction (thiophene HDS) over
the fresh-sulfided catalyst, Ni migrated towards the center, whereas for the
spent-regenerated-sulfided catalyst, towards the exterior. The contaminant metals, such as V
and Fe, exhibited little change during the regeneration followed by resulfidation.
