48
In commercial hydrotreating units, reactor temperature needs to be gradually
increased to maintain constant catalytic activity in order to obtain required desulfurization levels. The degree of deactivation is usually measured in terms of the profile
of temperature rise as a function of time during the operations. Normally, catalyst
deactivation rates are dependent on the feed source and process conditions. Heavier
feedstock such as residue will deactivate at a faster rate compared to lighter feeds.
Generally, heavier feeds contain higher amounts of contaminants, and thus they
need to be operated at higher process severity (high temperature, hydrogen partial
pressure, etc.), resulting in shorter catalyst life. Also, rates of deactivation are not
the same for different hydroprocessing reactions such as hydrodesulfurization,
hydrodenitrogenation, hydrodearomatization, etc. for different feeds and process
conditions.
Deactivation rates commonly encountered during hydroprocessing of petroleum
fractions vary from <1 to 4–5 °C/month depending upon the type and nature of
feedstock. Lighter hydrocarbon fractions such as straight-run naphtha deactivate at
a slower rate of <1 °C/month, while cracked naphtha deactivates at the rate of
2-4 °C/month during hydrotreating. Straight-run and light hydrocarbon feeds deactivate at a slower rate compared to cracked stock and heavy hydrocarbons. Furimsky
and Massoth [45] presented a detailed review of the deactivation of hydroprocessing
catalysts covering all aspects of deactivation.
5.1 Deactivation Due to Coke
Coke is usually a mixture of carbon (90 wt%) and hydrogen (10 wt%). It is deposited gradually on the surface of the hydrotreating catalyst due to polymerization and
condensation reactions of polycyclic aromatic species and other unsaturated intermediates formed by the hydrotreating and hydrocracking reactions. Coke reduces
the activity of the catalyst, and thus reactor temperature needs to be increased over
time to obtain the same HDS conversions. The higher temperatures will further
aggravate the coke formation through enhanced coking reactions.
Deactivation due to coke formation occurs at a rapid rate during the initial stages
of operation, resulting in a rapid build-up of coke on the catalyst surface. After this
initial stage of rapid coke deposition, the rate of coke laydown becomes gradual and
attains a steady state. Again, the coke builds up faster with the mostly deactivated
catalyst, especially due to severe processing conditions such as higher reactor temperatures employed near the end-of-run conditions. During this period, the catalyst
needs to be unloaded and regenerated for reuse.
Aromatic compounds, especially polycyclic aromatics and asphaltenes, are the
primary precursors responsible for coke formation during hydroprocessing through
polycondensation/polymerization reactions. As the amount of coke increases on the
catalyst, the pore diameter and pore volume both decrease, resulting in the shift of
G. Valavarasu and B. Ramachandrarao
In commercial hydrotreating units, reactor temperature needs to be gradually
increased to maintain constant catalytic activity in order to obtain required desulfurization levels. The degree of deactivation is usually measured in terms of the profile
of temperature rise as a function of time during the operations. Normally, catalyst
deactivation rates are dependent on the feed source and process conditions. Heavier
feedstock such as residue will deactivate at a faster rate compared to lighter feeds.
Generally, heavier feeds contain higher amounts of contaminants, and thus they
need to be operated at higher process severity (high temperature, hydrogen partial
pressure, etc.), resulting in shorter catalyst life. Also, rates of deactivation are not
the same for different hydroprocessing reactions such as hydrodesulfurization,
hydrodenitrogenation, hydrodearomatization, etc. for different feeds and process
conditions.
Deactivation rates commonly encountered during hydroprocessing of petroleum
fractions vary from <1 to 4–5 °C/month depending upon the type and nature of
feedstock. Lighter hydrocarbon fractions such as straight-run naphtha deactivate at
a slower rate of <1 °C/month, while cracked naphtha deactivates at the rate of
2-4 °C/month during hydrotreating. Straight-run and light hydrocarbon feeds deactivate at a slower rate compared to cracked stock and heavy hydrocarbons. Furimsky
and Massoth [45] presented a detailed review of the deactivation of hydroprocessing
catalysts covering all aspects of deactivation.
5.1 Deactivation Due to Coke
Coke is usually a mixture of carbon (90 wt%) and hydrogen (10 wt%). It is deposited gradually on the surface of the hydrotreating catalyst due to polymerization and
condensation reactions of polycyclic aromatic species and other unsaturated intermediates formed by the hydrotreating and hydrocracking reactions. Coke reduces
the activity of the catalyst, and thus reactor temperature needs to be increased over
time to obtain the same HDS conversions. The higher temperatures will further
aggravate the coke formation through enhanced coking reactions.
Deactivation due to coke formation occurs at a rapid rate during the initial stages
of operation, resulting in a rapid build-up of coke on the catalyst surface. After this
initial stage of rapid coke deposition, the rate of coke laydown becomes gradual and
attains a steady state. Again, the coke builds up faster with the mostly deactivated
catalyst, especially due to severe processing conditions such as higher reactor temperatures employed near the end-of-run conditions. During this period, the catalyst
needs to be unloaded and regenerated for reuse.
Aromatic compounds, especially polycyclic aromatics and asphaltenes, are the
primary precursors responsible for coke formation during hydroprocessing through
polycondensation/polymerization reactions. As the amount of coke increases on the
catalyst, the pore diameter and pore volume both decrease, resulting in the shift of
G. Valavarasu and B. Ramachandrarao