51
Sintering and segregation of active metals could result in the growth in the size
of MoS 2 slabs and decreased metal dispersion, which can cause reduced catalytic
activity due to less number of active metallic sites. Sintering also depletes MoS 2
slabs of promoters (Co or Ni) affecting the catalyst activity due to reduced promotional effect since the active phase for the HDT reactions is the CoMoS or NiMoS
phase. Metals (arsenic, sodium, iron, etc.) present in the feedstock can interact with
the catalyst metals (Co, Mo, W) and form compounds such as AsMo 4 , Co 3 As 2 O 8 ,
Na 2 MoO 4 , Na 2 WO 4 , FeMoO 4 , CoFe 2 O 4 , etc. that will alter the nature of catalytic
active sites and cause deactivation of the catalyst.
Zhang et al. [46] studied the deactivation of HDS catalysts and mechanism with
unsupported nano MoS 2 and supported catalysts using LCO as feedstock under
extreme process conditions. Spent nano MoS 2 was found to exhibit lower deactivation through coke deposition compared to the supported catalyst and also resulted in
the formation of soft coke in the absence of acidic supports. The decomposition of
the active phase played an insignificant role in the deactivation of nano MoS 2 unlike
supported catalysts. Marafi et al. [47] studied the deactivation of Mo/Al 2 O 3 , Ni-Mo/
Al 2 O 3 , and Ni-MoP/Al 2 O 3 catalysts in atmospheric residue HDS and found that coking tendency of the reactions was of the following order: HDS ← HDM ← HDS/
HDN. Initial coke build-up was very rapid during hydrotreating of residue with
carbon deposition up to 20 wt% reached within 120 h in their study. There are two
types of coke reported such as soft coke and refractory surface coke on the spent
catalyst with the deposition mainly through adsorption at the catalyst support without affecting the metal sulfide phase. The nature of coke deposition depends on the
type of catalyst as the HDM and HDS catalysts contained more aromatic and condensed type of coke with less alkyl substituents, while the HDS/HDN catalyst contained coke with low aromaticity due to its high hydrogenation activity. The study
showed the strong influence of physicochemical properties (SA, PV, metal content)
on catalyst deactivation by both coke and metals.
6 Catalyst Regeneration
Hydrotreating catalysts gradually get deactivated during the operation and need to
be regenerated to restore the lost activity and reuse it for another cycle. Regeneration
gains significance due to high cost of fresh HDT catalysts and the stringent environmental regulations for spent catalyst disposal. It is not possible to restore the catalyst activity to the levels of a fresh catalyst after regeneration, and in general, the
HDT catalyst loses 5–15% of original activity with each regeneration. Usually, the
catalyst can be used with 2–3 regenerations beyond which it has to be dumped and
the fresh catalyst has to be replaced in the reactors. The extent of recovery of catalytic activity after regeneration depends on the amount of metals deposited on the
catalyst and the severity of the process. Catalyst regeneration is performed to burn
off the coke deposited on the catalyst in a controlled manner by oxidation with air
or diluted oxygen. Along with air or oxygen, steam or nitrogen is also used as a
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part II—Catalyst…
Sintering and segregation of active metals could result in the growth in the size
of MoS 2 slabs and decreased metal dispersion, which can cause reduced catalytic
activity due to less number of active metallic sites. Sintering also depletes MoS 2
slabs of promoters (Co or Ni) affecting the catalyst activity due to reduced promotional effect since the active phase for the HDT reactions is the CoMoS or NiMoS
phase. Metals (arsenic, sodium, iron, etc.) present in the feedstock can interact with
the catalyst metals (Co, Mo, W) and form compounds such as AsMo 4 , Co 3 As 2 O 8 ,
Na 2 MoO 4 , Na 2 WO 4 , FeMoO 4 , CoFe 2 O 4 , etc. that will alter the nature of catalytic
active sites and cause deactivation of the catalyst.
Zhang et al. [46] studied the deactivation of HDS catalysts and mechanism with
unsupported nano MoS 2 and supported catalysts using LCO as feedstock under
extreme process conditions. Spent nano MoS 2 was found to exhibit lower deactivation through coke deposition compared to the supported catalyst and also resulted in
the formation of soft coke in the absence of acidic supports. The decomposition of
the active phase played an insignificant role in the deactivation of nano MoS 2 unlike
supported catalysts. Marafi et al. [47] studied the deactivation of Mo/Al 2 O 3 , Ni-Mo/
Al 2 O 3 , and Ni-MoP/Al 2 O 3 catalysts in atmospheric residue HDS and found that coking tendency of the reactions was of the following order: HDS ← HDM ← HDS/
HDN. Initial coke build-up was very rapid during hydrotreating of residue with
carbon deposition up to 20 wt% reached within 120 h in their study. There are two
types of coke reported such as soft coke and refractory surface coke on the spent
catalyst with the deposition mainly through adsorption at the catalyst support without affecting the metal sulfide phase. The nature of coke deposition depends on the
type of catalyst as the HDM and HDS catalysts contained more aromatic and condensed type of coke with less alkyl substituents, while the HDS/HDN catalyst contained coke with low aromaticity due to its high hydrogenation activity. The study
showed the strong influence of physicochemical properties (SA, PV, metal content)
on catalyst deactivation by both coke and metals.
6 Catalyst Regeneration
Hydrotreating catalysts gradually get deactivated during the operation and need to
be regenerated to restore the lost activity and reuse it for another cycle. Regeneration
gains significance due to high cost of fresh HDT catalysts and the stringent environmental regulations for spent catalyst disposal. It is not possible to restore the catalyst activity to the levels of a fresh catalyst after regeneration, and in general, the
HDT catalyst loses 5–15% of original activity with each regeneration. Usually, the
catalyst can be used with 2–3 regenerations beyond which it has to be dumped and
the fresh catalyst has to be replaced in the reactors. The extent of recovery of catalytic activity after regeneration depends on the amount of metals deposited on the
catalyst and the severity of the process. Catalyst regeneration is performed to burn
off the coke deposited on the catalyst in a controlled manner by oxidation with air
or diluted oxygen. Along with air or oxygen, steam or nitrogen is also used as a
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part II—Catalyst…