Environmental Engineering Reference
In-Depth Information
Figure 4.12: Effect of the type of porosity on radial distribution of vanadium [From ref. 199 .
Reprinted with permission].
0.95 cm 3 /g), the difference between the distribution patterns of V and Ni was less pronounced
[198] . An example of the effect of porosity on metal distribution is shown in Fig. 4.12
[143,199] . It was indicated that the presence of V in the vanadyl form is one of the reasons for
the enhanced reactivity of V-containing porphyrins compared with Ni-containing porphyrins
during the deposit formation on the catalyst surface. As the result of this, vanadium is
deposited on the surface before it could diffuse into the catalyst interior. It has been observed
that small amounts of V may deactivate catalyst because of the blocking active sites, whereas
large amount of deposited V deactivates catalyst due to pore mouth plugging [145,200] . The
pore mouth plugging by Ni deposits is much less evident. In fact, information suggests that the
Ni deposited on the catalyst from the feed may improve catalyst performance, its HYD activity
in particular [201,202] . Koyama et al. [203] proposed two regions of deactivation by metals,
i.e., the initial one involving the poisoning of active sites and the other causing the decrease in
effective diffusivity due to pore mouth plugging. It is believed that in both regions, the
deactivating effect of the V deposits was more pronounced than that of the Ni deposits.
Arsenic in spent catalysts has been attracting attention mainly for environmental and safety
reasons. Arsenic is naturally present in many crudes and usually concentrates in light fractions.
Deactivation due to the presence of arsenic deposited on catalysts has been reported [204] .
 
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