Environmental Engineering Reference
In-Depth Information
evidenced by the different deactivation patterns for different feeds and different catalysts
observed initially. In this case, the method used for catalyst presulfiding may be an important
factor for controlling the initial coke deposition.
Undoubtedly, during the hydroprocessing of heavy feeds containing metals, the structure of
coke on catalyst will be progressively influenced by the metals deposited from the feed. This
was indeed observed by Zeuthen et al.
[194,195]
. In this case, the coke formed in proximity of
the deposited V was more refractory, i.e., it had lower H/C ratio than the coke in the interior of
pores. This suggests that in the course of the experiment, the V enhanced dehydrogenation of
coke. Then, different forms of coke may be present on catalyst surface. The influence of metals
on properties of coke was reported by Galiasso Tailleur and Caprioli
[196]
. They observed
that, initially, coke filled pores before depositing on the exterior of the catalyst particles.
However, because of its permeability, the liquid phase could reach catalyst surface. The
permeability of the coke was gradually decreasing before catalyst was completely deactivated.
The permeability decrease was complemented by the increased deposition of metals on
catalyst surface. Therefore, it was suggested that metals contributed to the loss in permeability.
In this case, the vacuum residue derived from Venezuelan crude was studied at 23.6MPa and
between 683 and 703 K over the NiMo/Al
2
O
3
catalyst.
After deposition on catalyst surface, V and Ni are gradually converted to sulfides. The
overwhelming information showed that V tends to deposit on the external surface of catalyst
particles, whereas the radial distribution of Ni is more uniform
[10,191,79,197]
. This is
illustrated in
Fig. 4.11 [197]
. However, for a macroporous HDM catalyst (pore volume of
Figure 4.11: Effect of fractional radius on deposition of vanadium, nickel and iron [From ref.
197
.
Reprinted with permission].
