Environmental Engineering Reference
In-Depth Information
Table 6.10: Effect of reactivation on catalyst activity [From ref.
430
. Reprinted with permission].
Catalyst
S in products (ppm)
N in products (ppm)
RVA-HDS
RVA-HDN
Fresh
165
83
100
100
Regenerated
290
220
68
67
Reactivated
201
105
95
93
RVA-HDS: relative volume activity-hydrodesulfurization; RVA-HYD: relative volume activity-hydrogenation.
numerous chelating agents, which may be used, e.g., citric acid, tartaric acid, oxalic acid,
malonic acid, butanediolglycolic aldehyde, acetaldol, various glycols, etc. The published
database confirmed that several spent hydroprocessing catalysts have been successfully
reactivated using this method.
Table 6.10 [430]
compares the HDS and HDN activities of the
fresh, regenerated, and reactivated catalyst via REACT method. The activities were measured
under typical hydroprocessing conditions using VGO as the feed. A significant improvement
in both HDS and HDN activities of the reactivated catalyst compared with the regenerated
catalyst was quite evident. A similar revitalization process was developed by Criterion
Catalyst Company for their CENTINEL catalyst
[328]
. Haldor Topsoe is offering a similar
refresh process for reactivating regenerated catalysts by chemical treatment
[433]
.
Although beneficial effects of reactivation with the aid of chelating agents have been
confirmed experimentally, there is little information on mechanistic aspects of reactivation. In
recent study, Mazoyer et al.
[434]
investigated the interaction of chelating agent such as
diammonium salt of ethylene diaminetetraacetic acid with the calcined CoMo/Al
2
O
3
catalyst.
The study confirmed that chelating agent dissolved undesirable CoMoO
4
crystalline phase and
redispersed Co
2+
cation on the surface of the catalyst in a better way to facilitate the Co-Mo-S
phase formation. The chelating agent was however not able to solubilize Co
2+
from CoAl
2
O
3
spinel structure. In a related study, Costa et al.
[435]
investigated the role glycol-based
additives in enhancing the activity of CoMo and CoMoP catalysts. For all dried as well as
calcined CoMo and CoMoP catalysts (P/Mo molar ratio less than 0.4), a redissolution
phenomenon was evidenced after the additive impregnation step leading to the formation of
the Anderson heteropolyanion such as AlMo
6
O
24O
. This redissolution phenomenon was
however affected by the low solubility of this Anderson heteropolyanion. In the case of the
CoMoP dried catalyst (P/Mo ratio greater than 0.4), characterization of the additive containing
catalyst evidenced the PCoMo
11
O
407-
formation. Redissolution and redispersion of the active
components of the catalyst by chelating agents, thus, appeared to be responsible for the
activity enhancement.
6.3.4 Regeneration Aided by Radiation Treatment
The curves 1 and 3 in
Fig. 6.28 [422]
compare the coke removal using the oxidative burn-off
with that employing a radiation thermal treatment. A significant enhancement in the rate of