47
carried out using a mixture of about 2–5 vol.% of H 2 S in hydrogen, and this is not
being commonly practiced in refineries.
Although DMDS is the most common sulfiding agent used for catalyst sulfiding
application, use of tertiary-butyl polysulfide (TBPS) is also gaining significance in
recent times [42]. However, sulfur content of TBPS (54%) is lower than that of
DMDS (68%). TBPS is preferred in certain sulfiding applications where a lower
decomposition temperature and lower methane make is desirable. Some of the other
sulfiding agents are dimethyl sulfide (DMS) and dimethyl sulfoxide (DMSO).
van Haandel et al. [43] studied the activation of CoMo/Al 2 O 3 catalysts prepared
with phosphoric acid (PA) and citric acid as additives in both gas phase (using H 2 /
H 2 S at 1 or 20 bar pressure) and liquid phase activation (using a mixture of DMDS/
n-hexadecane at 20 bar H 2 pressure) methods. Gas-phase sulfiding with H 2 /H 2 S was
found to occur gradually from room temperature to 350 °C. Activation at elevated
pressure (20 bar) was beneficial in terms of increased sulfidation rate and higher
degree of Mo sulfidation without affecting the particle size. Citric acid increased the
gas-phase sulfidation rate at higher pressure due to reduced metal–support interaction (MSI) and enhanced Mo reducibility. Sulfidation in the liquid phase predominantly formed single layers of MoS 2 in contrast to the multilayer stacks formed after
gas phase activation, emphasizing the importance of activation conditions on the
formation of MoS 2 stacks. Catalysts prepared with PA were reported to be most
stable and active for gas-oil HDS compared to those prepared using CA. During ex
situ presulfiding process, sulfiding conditions play an important role in the morphology and performance of CoMoS/γ-Al 2 O 3 catalysts [44]. Some companies such
as Eurocat and Porocel offer patented ex situ sulfiding and activation services for
hydroprocessing catalysts.
5 Catalyst Deactivation
Hydrotreating catalysts gradually lose their catalytic activity during the course of
operation, and this deactivation phenomenon hampers the catalyst’s ability with
respect to the extent of sulfur removal in the case of HDT/HDS operations. Catalyst
activity usually depends on the number of active sites available on the catalyst for
the desirable reactions. The main cause of deactivation is due to the loss of active
sites of the catalyst. Catalyst activity is measured in terms of relative rates of HDS
reaction for HDT/HDS type of operation. In Industrial applications, catalyst activity
is measured in terms of the temperature required for obtaining specific HDS levels.
Hydrotreating catalysts undergo gradual deactivation over the cycle life due to
the following reasons:
• Metal deposition on the catalyst surface.
• Coke laydown on the surface.
• Poisoning of the active sites by strongly adsorbed species.
• Change in the catalyst structure (metal sintering/agglomeration).
• Pore mouth plugging by metals or coke.
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part II—Catalyst…
carried out using a mixture of about 2–5 vol.% of H 2 S in hydrogen, and this is not
being commonly practiced in refineries.
Although DMDS is the most common sulfiding agent used for catalyst sulfiding
application, use of tertiary-butyl polysulfide (TBPS) is also gaining significance in
recent times [42]. However, sulfur content of TBPS (54%) is lower than that of
DMDS (68%). TBPS is preferred in certain sulfiding applications where a lower
decomposition temperature and lower methane make is desirable. Some of the other
sulfiding agents are dimethyl sulfide (DMS) and dimethyl sulfoxide (DMSO).
van Haandel et al. [43] studied the activation of CoMo/Al 2 O 3 catalysts prepared
with phosphoric acid (PA) and citric acid as additives in both gas phase (using H 2 /
H 2 S at 1 or 20 bar pressure) and liquid phase activation (using a mixture of DMDS/
n-hexadecane at 20 bar H 2 pressure) methods. Gas-phase sulfiding with H 2 /H 2 S was
found to occur gradually from room temperature to 350 °C. Activation at elevated
pressure (20 bar) was beneficial in terms of increased sulfidation rate and higher
degree of Mo sulfidation without affecting the particle size. Citric acid increased the
gas-phase sulfidation rate at higher pressure due to reduced metal–support interaction (MSI) and enhanced Mo reducibility. Sulfidation in the liquid phase predominantly formed single layers of MoS 2 in contrast to the multilayer stacks formed after
gas phase activation, emphasizing the importance of activation conditions on the
formation of MoS 2 stacks. Catalysts prepared with PA were reported to be most
stable and active for gas-oil HDS compared to those prepared using CA. During ex
situ presulfiding process, sulfiding conditions play an important role in the morphology and performance of CoMoS/γ-Al 2 O 3 catalysts [44]. Some companies such
as Eurocat and Porocel offer patented ex situ sulfiding and activation services for
hydroprocessing catalysts.
5 Catalyst Deactivation
Hydrotreating catalysts gradually lose their catalytic activity during the course of
operation, and this deactivation phenomenon hampers the catalyst’s ability with
respect to the extent of sulfur removal in the case of HDT/HDS operations. Catalyst
activity usually depends on the number of active sites available on the catalyst for
the desirable reactions. The main cause of deactivation is due to the loss of active
sites of the catalyst. Catalyst activity is measured in terms of relative rates of HDS
reaction for HDT/HDS type of operation. In Industrial applications, catalyst activity
is measured in terms of the temperature required for obtaining specific HDS levels.
Hydrotreating catalysts undergo gradual deactivation over the cycle life due to
the following reasons:
• Metal deposition on the catalyst surface.
• Coke laydown on the surface.
• Poisoning of the active sites by strongly adsorbed species.
• Change in the catalyst structure (metal sintering/agglomeration).
• Pore mouth plugging by metals or coke.
Recent Advances in Hydrotreating/Hydrodesulfurization Catalysts: Part II—Catalyst…