52
diluent. Catalyst regeneration is an exothermic reaction. It usually restores the catalyst activity lost due to the gradual accumulation of coke on the catalyst, but it does
not recover the loss in activity due to poisoning by metals such as nickel, vanadium,
iron, etc.
There are two modes of catalyst regeneration: in situ and ex situ regeneration. Ex
situ regeneration is also called off-site regeneration. In the in situ mode, regeneration is carried out within the reactor itself and the coke is burned off using air,
steam/air, or nitrogen/air mixtures. In the ex situ regeneration, the catalyst is
removed from the reactor and the regeneration is carried out off-site. In the past, in
situ regeneration was commonly practiced in refineries using a lengthy procedure
involving unit downtime and production loss. Also, better control of regeneration
could not be achieved during in situ regeneration resulting in fines generation and
increased pressure drop. Poor in situ regeneration usually results in poor catalyst
performance with reduced unit throughputs and/or lower severity operation and ultimately ends up in premature or prescheduled replacement of the catalyst.
Ex situ regeneration involves regeneration by any third-party on a chargeable
basis. Since the refiner is ready with a fresh catalyst charge, the fresh catalyst is
loaded in the reactor and the spent catalyst is sent to any of the service providers. At
present, most of the refiners prefer ex situ regeneration in place of in situ regeneration due to the following advantages:
• Better control of regeneration operation such as exothermicity, oxygen diffusion,
formation of water and SO 2 , etc.
• Better recovery of catalyst activity.
• Less unit downtime and better economics.
• Minimization of environmental and safety hazards.
Regeneration of hydroprocessing catalysts burns off the coke, but this will not
ensure proper re-dispersion of the active metals on the catalyst surface. For redispersion of active metals, a wet process called “rejuvenation” is performed immediately after the regeneration step. More than 90% of the original catalyst activity is
usually restored in the ex situ regeneration process utilizing the combination of both
regeneration and rejuvenation processes. Kallinikos et al. [50] studied HDS catalyst
deactivation in a laboratory reactor through a hybrid neural network model and
compared the results with the deactivation of industrial hydrotreater catalysts. They
found that the deactivation of the industrial reactor is more or less uniform without
significant variation at different locations. Also, faster deactivation was observed for
hydrogen consuming reactions than the HDS reactions, which indicates lower
hydrogen consumption with time on stream for specified product sulfur content.
Pimerzin et al. [51] studied reactivation of the spent industrial CoMo/Al 2 O 3 HDT
catalyst using a combination of oxidative regeneration and rejuvenation methods.
The spent catalyst was obtained from an industrial, low-pressure gas-oil HDT unit
(4.5 MPa) after 2 years and 2 months of operation in ULSD service. The MoO 3 and
CoO content of the industrial catalyst was about 16.2% and 4.6%, respectively.
Rejuvenation of the catalyst was performed using different organic acids such as
citric and thioglycolic acids, glycols such as ethylene and triethylene glycols, and
G. Valavarasu and B. Ramachandrarao
diluent. Catalyst regeneration is an exothermic reaction. It usually restores the catalyst activity lost due to the gradual accumulation of coke on the catalyst, but it does
not recover the loss in activity due to poisoning by metals such as nickel, vanadium,
iron, etc.
There are two modes of catalyst regeneration: in situ and ex situ regeneration. Ex
situ regeneration is also called off-site regeneration. In the in situ mode, regeneration is carried out within the reactor itself and the coke is burned off using air,
steam/air, or nitrogen/air mixtures. In the ex situ regeneration, the catalyst is
removed from the reactor and the regeneration is carried out off-site. In the past, in
situ regeneration was commonly practiced in refineries using a lengthy procedure
involving unit downtime and production loss. Also, better control of regeneration
could not be achieved during in situ regeneration resulting in fines generation and
increased pressure drop. Poor in situ regeneration usually results in poor catalyst
performance with reduced unit throughputs and/or lower severity operation and ultimately ends up in premature or prescheduled replacement of the catalyst.
Ex situ regeneration involves regeneration by any third-party on a chargeable
basis. Since the refiner is ready with a fresh catalyst charge, the fresh catalyst is
loaded in the reactor and the spent catalyst is sent to any of the service providers. At
present, most of the refiners prefer ex situ regeneration in place of in situ regeneration due to the following advantages:
• Better control of regeneration operation such as exothermicity, oxygen diffusion,
formation of water and SO 2 , etc.
• Better recovery of catalyst activity.
• Less unit downtime and better economics.
• Minimization of environmental and safety hazards.
Regeneration of hydroprocessing catalysts burns off the coke, but this will not
ensure proper re-dispersion of the active metals on the catalyst surface. For redispersion of active metals, a wet process called “rejuvenation” is performed immediately after the regeneration step. More than 90% of the original catalyst activity is
usually restored in the ex situ regeneration process utilizing the combination of both
regeneration and rejuvenation processes. Kallinikos et al. [50] studied HDS catalyst
deactivation in a laboratory reactor through a hybrid neural network model and
compared the results with the deactivation of industrial hydrotreater catalysts. They
found that the deactivation of the industrial reactor is more or less uniform without
significant variation at different locations. Also, faster deactivation was observed for
hydrogen consuming reactions than the HDS reactions, which indicates lower
hydrogen consumption with time on stream for specified product sulfur content.
Pimerzin et al. [51] studied reactivation of the spent industrial CoMo/Al 2 O 3 HDT
catalyst using a combination of oxidative regeneration and rejuvenation methods.
The spent catalyst was obtained from an industrial, low-pressure gas-oil HDT unit
(4.5 MPa) after 2 years and 2 months of operation in ULSD service. The MoO 3 and
CoO content of the industrial catalyst was about 16.2% and 4.6%, respectively.
Rejuvenation of the catalyst was performed using different organic acids such as
citric and thioglycolic acids, glycols such as ethylene and triethylene glycols, and
G. Valavarasu and B. Ramachandrarao