56
consumption due to the difference in the reaction mechanism at which both these
catalysts operate.
The Ni-Mo catalyst is best suited for hydrodenitrogenation and hydrogenation of
unsaturated hydrocarbon molecules such as olefins and aromatics due to its high
hydrogenation activity compared to Co-Mo catalysts. Although Ni-Mo-type catalysts are well suited for nitrogen removal, Co-Mo catalysts also exhibit reasonably
good hydrodenitrogenation activity for lighter feedstock. However, Co-Mo catalysts are not a good choice when the feed nitrogen content is substantially high even
for light feeds due to catalyst poisoning. For high nitrogen-containing feeds, Ni-Mo
is the best choice. Also, Ni-Mo catalysts are frequently chosen when processing
feedstocks containing more than 20% of cracked stock due to their high hydrogenation activity. In some applications, Ni-W based catalysts are used where there is a
requirement of high activity for the saturation of aromatics. In two-stage hydrotreating processes, noble metal (Pt or Pd)-based hydrogenation catalysts are used for
aromatic saturation applications in sulfur-free, pure second-stage environment,
especially to improve cetane number of diesel fuel. Since noble metal catalysts possess high activity for hydrogenation of aromatics, they can be operated at lower
reactor temperatures compared to Co-Mo- or Ni-Mo-based catalysts in a clean reaction environment.
Ni-Mo hydrotreating catalysts are usually preferred for vacuum gas-oil HDT/
HDS, FCC feed pretreatment, and hydrocracker pretreat applications, which mainly
use light and heavy vacuum gas-oil feedstocks. VGO hydrotreating application
requires catalysts with high hydrogenation activity such as Ni-Mo/Al 2 O 3 due to the
following reasons:
• Deep sulfur removal from predominantly refractory sulfur species such as alkylated dibenzothiophenes.
• Deep HDN for higher FCC and hydrocracker conversions.
• Aromatic saturation for improved FCC conversions.
Klimov et al. [55] studied the use of trimetallic Co-Ni-Mo/Al 2 O 3 catalysts with
varying Co and Ni contents for deep HDT of VGO and showed that the catalyst
containing 1.8% Co and 1.2% Ni being the highly active ones compared to other
compositions. The higher activity of these trimetallic catalysts was ascribed to the
presence of mixed Ni-Co-Mo-S active phases. They proposed the formation of
Table 4 Selection of common catalysts for hydrotreating applications
Catalyst type
Applications
Co-Mo/Al 2 O 3
HDS of straight-run feeds
Ni-Mo/Al 2 O 3
Hydrodenitrogenation
Hydrogenation of aromatics/olefins
HDS of cracked feeds
Ni-W
Aromatic saturation
Pt or Pd
Aromatic saturation in a pure sulfur-free environment
G. Valavarasu and B. Ramachandrarao
consumption due to the difference in the reaction mechanism at which both these
catalysts operate.
The Ni-Mo catalyst is best suited for hydrodenitrogenation and hydrogenation of
unsaturated hydrocarbon molecules such as olefins and aromatics due to its high
hydrogenation activity compared to Co-Mo catalysts. Although Ni-Mo-type catalysts are well suited for nitrogen removal, Co-Mo catalysts also exhibit reasonably
good hydrodenitrogenation activity for lighter feedstock. However, Co-Mo catalysts are not a good choice when the feed nitrogen content is substantially high even
for light feeds due to catalyst poisoning. For high nitrogen-containing feeds, Ni-Mo
is the best choice. Also, Ni-Mo catalysts are frequently chosen when processing
feedstocks containing more than 20% of cracked stock due to their high hydrogenation activity. In some applications, Ni-W based catalysts are used where there is a
requirement of high activity for the saturation of aromatics. In two-stage hydrotreating processes, noble metal (Pt or Pd)-based hydrogenation catalysts are used for
aromatic saturation applications in sulfur-free, pure second-stage environment,
especially to improve cetane number of diesel fuel. Since noble metal catalysts possess high activity for hydrogenation of aromatics, they can be operated at lower
reactor temperatures compared to Co-Mo- or Ni-Mo-based catalysts in a clean reaction environment.
Ni-Mo hydrotreating catalysts are usually preferred for vacuum gas-oil HDT/
HDS, FCC feed pretreatment, and hydrocracker pretreat applications, which mainly
use light and heavy vacuum gas-oil feedstocks. VGO hydrotreating application
requires catalysts with high hydrogenation activity such as Ni-Mo/Al 2 O 3 due to the
following reasons:
• Deep sulfur removal from predominantly refractory sulfur species such as alkylated dibenzothiophenes.
• Deep HDN for higher FCC and hydrocracker conversions.
• Aromatic saturation for improved FCC conversions.
Klimov et al. [55] studied the use of trimetallic Co-Ni-Mo/Al 2 O 3 catalysts with
varying Co and Ni contents for deep HDT of VGO and showed that the catalyst
containing 1.8% Co and 1.2% Ni being the highly active ones compared to other
compositions. The higher activity of these trimetallic catalysts was ascribed to the
presence of mixed Ni-Co-Mo-S active phases. They proposed the formation of
Table 4 Selection of common catalysts for hydrotreating applications
Catalyst type
Applications
Co-Mo/Al 2 O 3
HDS of straight-run feeds
Ni-Mo/Al 2 O 3
Hydrodenitrogenation
Hydrogenation of aromatics/olefins
HDS of cracked feeds
Ni-W
Aromatic saturation
Pt or Pd
Aromatic saturation in a pure sulfur-free environment
G. Valavarasu and B. Ramachandrarao