46
activity for thiophene and toluene due to the large fraction of corner sites, whereas
Mo/ASA exhibited low intrinsic HDS activities, which was compensated by very
high hydrogenation activity.
The patent literature also describes the making of high-activity HDS catalysts
using various methods and applications [39–41].
4 Catalyst Activation/Sulfidation
Hydrotreating catalysts need to be activated before starting the reactions either
through sulfiding or reduction step depending on the type of catalyst to make them
catalytically active. HDT catalysts are usually supplied in the oxide form of metals
(Ni, Co, Mo, W oxides, or Pt/Pd oxides). The oxide form is not the active form to
carry out HDT/HDS reactions; this has to be converted into the sulfide form of metals before carrying out HDT/HDS reactions in the case of nonnoble metal or base
metal catalysts. Thus, fresh hydrotreating catalysts or regenerated catalysts need to
be activated through catalyst sulfiding or presulfiding before processing with actual
feed. The fresh or regenerated catalyst should not be exposed to hydrogen while in
an oxidized state above 200 °C because hydrogen could reduce the catalyst metal
oxides into metals and render the catalyst useless for the HDT reactions. Also, the
reduction reaction is highly exothermic and occurs at a faster rate due to which there
are chances of catalyst damage. There are two approaches employed by refiners for
sulfiding/activation of the HDT catalysts: in situ sulfiding and ex situ sulfiding.
In situ sulfiding is the most commonly used sulfiding method in refineries and
performed in either vapor or liquid phase. Liquid phase sulfiding is carried out with
or without the aid of a sulfiding agent. Sulfiding is accomplished with the help of a
sulfiding feed, which may be a straight-run distillate such as diesel or kerosene with
at least 1.0–2.0 wt% sulfur or straight-run distillate spiked with a sulfiding agent
such as dimethyldisulfide (CH 3 -S-S-CH 3 ) (DMDS), dimethyl sulfide (CH 3 -S-CH 3 ),
etc. Many refiners prefer liquid phase sulfiding using spiking agents such as DMDS
along with straight-run distillates compared to either vapor phase or liquid phase
sulfiding without spiking agents due to the advantages in terms of less sulfidation
and start-up time. Sulfiding is normally carried out in the presence of hydrogen gas.
Cracked feeds with the content of unsaturated olefinic hydrocarbons are not preferable for sulfidation of HDT/HDS catalysts since they are prone to coking and
polymerization and interfere with proper sulfiding of the catalysts. DMDS is the
widely used sulfiding agent for the presulfiding of hydrotreating catalysts due to
some of their advantages compared to other sulfiding agents in terms of its higher
sulfur content and lower sulfidation/start-up period. Another advantage of using
DMDS is the better control of exothermic sulfiding reactions due to two-step
decomposition and sulfiding, which reduces the chance of reactor exotherms. The
sulfiding agent such as DMDS initially breaks down into hydrogen sulfide and then
initiates the sulfiding process. Complete sulfiding of Co-Mo- and Ni-Mo-type HDS
catalysts require about 10 wt% sulfur on the catalyst. Vapor phase sulfiding is
G. Valavarasu and B. Ramachandrarao
activity for thiophene and toluene due to the large fraction of corner sites, whereas
Mo/ASA exhibited low intrinsic HDS activities, which was compensated by very
high hydrogenation activity.
The patent literature also describes the making of high-activity HDS catalysts
using various methods and applications [39–41].
4 Catalyst Activation/Sulfidation
Hydrotreating catalysts need to be activated before starting the reactions either
through sulfiding or reduction step depending on the type of catalyst to make them
catalytically active. HDT catalysts are usually supplied in the oxide form of metals
(Ni, Co, Mo, W oxides, or Pt/Pd oxides). The oxide form is not the active form to
carry out HDT/HDS reactions; this has to be converted into the sulfide form of metals before carrying out HDT/HDS reactions in the case of nonnoble metal or base
metal catalysts. Thus, fresh hydrotreating catalysts or regenerated catalysts need to
be activated through catalyst sulfiding or presulfiding before processing with actual
feed. The fresh or regenerated catalyst should not be exposed to hydrogen while in
an oxidized state above 200 °C because hydrogen could reduce the catalyst metal
oxides into metals and render the catalyst useless for the HDT reactions. Also, the
reduction reaction is highly exothermic and occurs at a faster rate due to which there
are chances of catalyst damage. There are two approaches employed by refiners for
sulfiding/activation of the HDT catalysts: in situ sulfiding and ex situ sulfiding.
In situ sulfiding is the most commonly used sulfiding method in refineries and
performed in either vapor or liquid phase. Liquid phase sulfiding is carried out with
or without the aid of a sulfiding agent. Sulfiding is accomplished with the help of a
sulfiding feed, which may be a straight-run distillate such as diesel or kerosene with
at least 1.0–2.0 wt% sulfur or straight-run distillate spiked with a sulfiding agent
such as dimethyldisulfide (CH 3 -S-S-CH 3 ) (DMDS), dimethyl sulfide (CH 3 -S-CH 3 ),
etc. Many refiners prefer liquid phase sulfiding using spiking agents such as DMDS
along with straight-run distillates compared to either vapor phase or liquid phase
sulfiding without spiking agents due to the advantages in terms of less sulfidation
and start-up time. Sulfiding is normally carried out in the presence of hydrogen gas.
Cracked feeds with the content of unsaturated olefinic hydrocarbons are not preferable for sulfidation of HDT/HDS catalysts since they are prone to coking and
polymerization and interfere with proper sulfiding of the catalysts. DMDS is the
widely used sulfiding agent for the presulfiding of hydrotreating catalysts due to
some of their advantages compared to other sulfiding agents in terms of its higher
sulfur content and lower sulfidation/start-up period. Another advantage of using
DMDS is the better control of exothermic sulfiding reactions due to two-step
decomposition and sulfiding, which reduces the chance of reactor exotherms. The
sulfiding agent such as DMDS initially breaks down into hydrogen sulfide and then
initiates the sulfiding process. Complete sulfiding of Co-Mo- and Ni-Mo-type HDS
catalysts require about 10 wt% sulfur on the catalyst. Vapor phase sulfiding is
G. Valavarasu and B. Ramachandrarao