50
at the inlet of the reactor catalyst bed, while most of the carbon deposits at the outlet
of the reactor bed.
Properties of the catalyst such as surface area, pore volume, and mean pore diameter are also important factors that determine the metal tolerance of the hydrotreating catalysts. Metal poisoning or deactivation is irreversible and thus affects the
overall catalyst life. Catalyst regeneration also cannot reverse the effects of metal
deactivation. Incorporation of a guard bed containing a demetallization catalyst
ahead of the main HDT catalyst bed will protect the high activity hydrotreating
catalyst from metal poisoning.
5.3 Deactivation by Poisoning of Active Sites
Catalyst poisons are substances that are strongly chemisorbed on the active sites and
decrease the number of such sites available for the desired reaction resulting in
lower catalyst activity. Also, these poisons can compete with the reactant molecules
for adsorption in the same type of active site. In most of the cases, this kind of poisoning is reversible, and removal of the poisoning compound from the system will
gradually restore the catalyst activity by reversing the poisoning effect. Some of the
reversible poisons that hamper the activity of hydroprocessing catalysts are basic
nitrogen compounds, hydrogen sulfide, and carbon monoxide. The catalyst activity
loss due to the effect of temporary poisons is compensated in some cases by operating the unit at higher reactor temperatures for enhancing the rate of reaction.
However, this will result in a higher rate of catalyst deactivation through an enhanced
rate of coke formation.
5.4 Change in the Structure of the Catalyst (Metal Sintering/
Agglomeration)
High temperatures encountered during hydrotreating operations affect gradual
changes in the structure of the catalyst active phase resulting in slow and irreversible
loss in the catalyst activity and stability. Deactivation caused by high temperatures
is primarily due to the following reasons with respect to changes in the catalyst
active phase:
• Sintering and segregation of active phase through diffusion of active metals into
support.
• Recrystallization of the active material into a different form.
• Interaction of the metals present in the feed with catalyst metals resulting in the
changes in the active sites.
G. Valavarasu and B. Ramachandrarao
at the inlet of the reactor catalyst bed, while most of the carbon deposits at the outlet
of the reactor bed.
Properties of the catalyst such as surface area, pore volume, and mean pore diameter are also important factors that determine the metal tolerance of the hydrotreating catalysts. Metal poisoning or deactivation is irreversible and thus affects the
overall catalyst life. Catalyst regeneration also cannot reverse the effects of metal
deactivation. Incorporation of a guard bed containing a demetallization catalyst
ahead of the main HDT catalyst bed will protect the high activity hydrotreating
catalyst from metal poisoning.
5.3 Deactivation by Poisoning of Active Sites
Catalyst poisons are substances that are strongly chemisorbed on the active sites and
decrease the number of such sites available for the desired reaction resulting in
lower catalyst activity. Also, these poisons can compete with the reactant molecules
for adsorption in the same type of active site. In most of the cases, this kind of poisoning is reversible, and removal of the poisoning compound from the system will
gradually restore the catalyst activity by reversing the poisoning effect. Some of the
reversible poisons that hamper the activity of hydroprocessing catalysts are basic
nitrogen compounds, hydrogen sulfide, and carbon monoxide. The catalyst activity
loss due to the effect of temporary poisons is compensated in some cases by operating the unit at higher reactor temperatures for enhancing the rate of reaction.
However, this will result in a higher rate of catalyst deactivation through an enhanced
rate of coke formation.
5.4 Change in the Structure of the Catalyst (Metal Sintering/
Agglomeration)
High temperatures encountered during hydrotreating operations affect gradual
changes in the structure of the catalyst active phase resulting in slow and irreversible
loss in the catalyst activity and stability. Deactivation caused by high temperatures
is primarily due to the following reasons with respect to changes in the catalyst
active phase:
• Sintering and segregation of active phase through diffusion of active metals into
support.
• Recrystallization of the active material into a different form.
• Interaction of the metals present in the feed with catalyst metals resulting in the
changes in the active sites.
G. Valavarasu and B. Ramachandrarao