Catalyst Deactivation
Catalyst deactivation is the gradual loss of the catalyst’s activity to convert the feed
conversion. In practical terms, it is the temperature required to obtain a fixed
conversion. As the run progresses, the catalyst loses activity. Catalyst will lose
activity in several ways described below.
Catalyst Coking
Coke deposition is a by-product of the cracking reactions. The deposition of coke
on a catalyst is a function of time and temperature. The longer the catalyst is in
service and/or the higher the temperature of the process, the deactivating effect will
be more severe. The coking of the catalyst begins with the adsorption of highmolecular-weight, low hydrogen/carbon ratio ring compounds; it proceeds with
further loss of hydrogen content and ends with varying degrees of hardness of coke.
This coke can cover active sites and/or prevent access to these sites by physical
blockage of the entrance to the pores leading to the sites. Coke is not a permanent
poison. Catalyst, which has been deactivated by coke deposition, can be, relatively
easily, restored to near original condition by regeneration.
Reversible Poisoning/Catalyst Inhibition
Catalyst poisoning is primarily the result of strong chemisorption of impurities on
active sites. Hydrocracking catalysts in a first-stage reaction environment such as
once-through or single-stage configuration have to operate with high concentrations
of both NH 3 and H 2 S in the gas phase. This type of poisoning is reversible, i.e.,
when the deactivating agent is removed, the deactivating effect is gradually
reversed. It is possible that if the catalyst temperature is raised, it can compensate
for the deactivating effects. However, raising temperatures has to be done cautiously since the rate of coke deposition will accelerate. For example, NH 3 will
adsorb strongly on the acidic sites of the catalysts. If there is a temporary exposure
to high ammonia, the catalyst will require higher temperature to maintain activity.
However, if the NH 3 concentration lowers to the more typical value, the catalyst
should return to the normal activity as the ammonia desorbs from the surface.
Another example of a reversible poison is carbon monoxide, which can impair the
hydrogenation reactions by preferential adsorption on active metal sites. Another
example is H 2 S, which in moderate to high concentrations can reduce the desulfurization rate constant. In this case, the removal of H 2 S from the recycle gas
system solves the problem.
Catalytic Metal Agglomeration
Another reversible form of catalyst deactivation is the agglomeration of the hydrogenation component of the catalyst. It can be caused by poor catalyst activation
conditions in which a combination of high water partial pressure and high temperature may exist for a prolonged period. Regeneration can restore the catalyst to near
original condition.
Hydrocracking in Petroleum Processing
347
Catalyst deactivation is the gradual loss of the catalyst’s activity to convert the feed
conversion. In practical terms, it is the temperature required to obtain a fixed
conversion. As the run progresses, the catalyst loses activity. Catalyst will lose
activity in several ways described below.
Catalyst Coking
Coke deposition is a by-product of the cracking reactions. The deposition of coke
on a catalyst is a function of time and temperature. The longer the catalyst is in
service and/or the higher the temperature of the process, the deactivating effect will
be more severe. The coking of the catalyst begins with the adsorption of highmolecular-weight, low hydrogen/carbon ratio ring compounds; it proceeds with
further loss of hydrogen content and ends with varying degrees of hardness of coke.
This coke can cover active sites and/or prevent access to these sites by physical
blockage of the entrance to the pores leading to the sites. Coke is not a permanent
poison. Catalyst, which has been deactivated by coke deposition, can be, relatively
easily, restored to near original condition by regeneration.
Reversible Poisoning/Catalyst Inhibition
Catalyst poisoning is primarily the result of strong chemisorption of impurities on
active sites. Hydrocracking catalysts in a first-stage reaction environment such as
once-through or single-stage configuration have to operate with high concentrations
of both NH 3 and H 2 S in the gas phase. This type of poisoning is reversible, i.e.,
when the deactivating agent is removed, the deactivating effect is gradually
reversed. It is possible that if the catalyst temperature is raised, it can compensate
for the deactivating effects. However, raising temperatures has to be done cautiously since the rate of coke deposition will accelerate. For example, NH 3 will
adsorb strongly on the acidic sites of the catalysts. If there is a temporary exposure
to high ammonia, the catalyst will require higher temperature to maintain activity.
However, if the NH 3 concentration lowers to the more typical value, the catalyst
should return to the normal activity as the ammonia desorbs from the surface.
Another example of a reversible poison is carbon monoxide, which can impair the
hydrogenation reactions by preferential adsorption on active metal sites. Another
example is H 2 S, which in moderate to high concentrations can reduce the desulfurization rate constant. In this case, the removal of H 2 S from the recycle gas
system solves the problem.
Catalytic Metal Agglomeration
Another reversible form of catalyst deactivation is the agglomeration of the hydrogenation component of the catalyst. It can be caused by poor catalyst activation
conditions in which a combination of high water partial pressure and high temperature may exist for a prolonged period. Regeneration can restore the catalyst to near
original condition.
Hydrocracking in Petroleum Processing
347