Environmental Engineering Reference
In-Depth Information
Figure 4.4: Effect of Mo loading on coke selectivity (vacuum gas oil [VGO], 3MPa) [From
ref.
174
. Reprinted with permission].
resulted in the formation of N-intermediates possessing a higher basicity than the
corresponding-containing reactants in the feed
[144]
. An ultimate result of this was the
diminished availability of the active surface hydrogen
[55]
.
In the studies of de Jong et al.
[173-175]
on hydroprocessing of a VGO, the coke formation
was observed to be dependent on the catalyst structure. Thus, according to
Fig. 4.4
, the
addition of a small amount of Mo to
-Al
2
O
3
resulted in the significant decrease in coke
formation
[173,174]
. The coke build-up increased with the increasing amount of Mo, i.e., with
the increasing catalyst activity. Therefore, the formation of this coke, termed as the “chemical
reaction coke”, was associated with hydroprocessing reactions. The influence of catalyst
structure on the coke formation was further demonstrated in the study on aging of the
CoMo/Al
2
O
3
and Mo/Al
2
O
3
catalysts
[175]
. The aging was conducted at 723 K and 3MPa in
the fixed-bed reactor using VGO. For both catalysts, the amount of deposited coke was similar.
However, the former catalyst was more deactivated because of the much greater coverage by
coke, i.e., about 90 and 50% for the CoMo/Al
2
O
3
and Mo/Al
2
O
3
catalysts, respectively. For
the latter catalyst, the islands of coke were present, whereas for CoMo/Al
2
O
3
the coke was
more evenly distributed. The detailed spectroscopic evaluations of the spent catalysts from the
hydroprocessing of a VGO conducted by van Dorn et al.
[176-178]
provided the information
