initial days of operation. The rate of increase will be reduced to very low levels
under normal processing conditions. A carbon level of 5 wt% may be tolerated
without a significant decrease in desulfurization. However, denitrogenation activity
would be reduced.
The sulfur and nitrogen found in the feed could be considered contaminants to
the extent that they produce hydrogen sulfide and ammonia which can react to form
ammonium bisulfide. The water injected into the reactor effluent dissolves the
ammonium bisulfide and prevents exchanger fouling. Organic nitrogen in the
feed, if present in amounts higher than expected, will require higher reactor
temperatures for processing and will lead to a reduction in catalyst life.
If the water injection should be stopped for any period of time, the H 2 S and NH 3
may accumulate in the recycle gas and result in a sudden loss in catalyst activity.
The activity will return to normal once wash water is reestablished. Catalyst bed
temperatures should not be increased to compensate for the temporary activity loss.
Small amounts of molecular nitrogen, CO, and CO 2 that enter the system with
the makeup gas are not harmful to the catalyst, but must be vented to prevent
accumulation in the recycle gas. Excessive amounts of CO and CO 2 may have an
adverse effect on catalyst performance, as they may be methanated by the catalyst
taking up active sites and liberating heat. This will raise the outlet temperature and
reduce the apparent catalyst activity. The unit should never be pressured up with
high CO + CO 2 containing makeup gas as a temperature runaway may result.
Permanent
Permanent loss of catalyst activity is usually caused by the gradual accumulation of
inorganic species picked up from the feed, makeup hydrogen, or effluent wash
water. Examples include arsenic, lead, calcium, sodium, silicon, and phosphorus.
Low concentrations of these elements (and other alkaline metals) can cause deactivation over time as they are deposited on the catalyst.
Organic metal compounds are decomposed and typically deposit in the upper
section of the catalyst bed as a metal sulfide. The graded catalyst bed, if used, may
contain demetallation catalysts that have a high metal retention capacity. Some of
these catalysts may retain as much as 100 wt% of the fresh catalyst weight as metals
from the feed. These demetallation catalysts typically have a lower activity for
desulfurization and denitrogenation.
Hydrotreating catalysts exhibit a moderate tolerance for metals such as arsenic
and lead. Total metal content of 2–3 wt% of the hydrotreating catalyst have been
observed. However, product analysis frequency should be increased to monitor
breakthrough when calculations show the metal level on the hydrotreating catalyst
exceeds 0.5 wt%. Metals cannot be removed by catalyst regeneration. Catalyst
replacement should be considered when metal loading of 1–2 wt% is exceeded on
the hydrotreating catalyst.
Apparent catalyst deactivation may be caused by the accumulation of deposits
on top of the catalyst bed. Solid material, such as corrosion products and coke, will
lead to rapid fouling of the catalyst bed if allowed to enter the reactor. This problem
is remedied by skimming a portion of the catalyst, screening, and reloading.
432
P. Kokayeff et al.
under normal processing conditions. A carbon level of 5 wt% may be tolerated
without a significant decrease in desulfurization. However, denitrogenation activity
would be reduced.
The sulfur and nitrogen found in the feed could be considered contaminants to
the extent that they produce hydrogen sulfide and ammonia which can react to form
ammonium bisulfide. The water injected into the reactor effluent dissolves the
ammonium bisulfide and prevents exchanger fouling. Organic nitrogen in the
feed, if present in amounts higher than expected, will require higher reactor
temperatures for processing and will lead to a reduction in catalyst life.
If the water injection should be stopped for any period of time, the H 2 S and NH 3
may accumulate in the recycle gas and result in a sudden loss in catalyst activity.
The activity will return to normal once wash water is reestablished. Catalyst bed
temperatures should not be increased to compensate for the temporary activity loss.
Small amounts of molecular nitrogen, CO, and CO 2 that enter the system with
the makeup gas are not harmful to the catalyst, but must be vented to prevent
accumulation in the recycle gas. Excessive amounts of CO and CO 2 may have an
adverse effect on catalyst performance, as they may be methanated by the catalyst
taking up active sites and liberating heat. This will raise the outlet temperature and
reduce the apparent catalyst activity. The unit should never be pressured up with
high CO + CO 2 containing makeup gas as a temperature runaway may result.
Permanent
Permanent loss of catalyst activity is usually caused by the gradual accumulation of
inorganic species picked up from the feed, makeup hydrogen, or effluent wash
water. Examples include arsenic, lead, calcium, sodium, silicon, and phosphorus.
Low concentrations of these elements (and other alkaline metals) can cause deactivation over time as they are deposited on the catalyst.
Organic metal compounds are decomposed and typically deposit in the upper
section of the catalyst bed as a metal sulfide. The graded catalyst bed, if used, may
contain demetallation catalysts that have a high metal retention capacity. Some of
these catalysts may retain as much as 100 wt% of the fresh catalyst weight as metals
from the feed. These demetallation catalysts typically have a lower activity for
desulfurization and denitrogenation.
Hydrotreating catalysts exhibit a moderate tolerance for metals such as arsenic
and lead. Total metal content of 2–3 wt% of the hydrotreating catalyst have been
observed. However, product analysis frequency should be increased to monitor
breakthrough when calculations show the metal level on the hydrotreating catalyst
exceeds 0.5 wt%. Metals cannot be removed by catalyst regeneration. Catalyst
replacement should be considered when metal loading of 1–2 wt% is exceeded on
the hydrotreating catalyst.
Apparent catalyst deactivation may be caused by the accumulation of deposits
on top of the catalyst bed. Solid material, such as corrosion products and coke, will
lead to rapid fouling of the catalyst bed if allowed to enter the reactor. This problem
is remedied by skimming a portion of the catalyst, screening, and reloading.
432
P. Kokayeff et al.