Catalyst Regeneration
The activity decline due to coke laydown can be recovered by burning the coke off
in a controlled atmosphere. The regeneration can be accomplished in any of three
ways: in situ with steam/air, in situ with nitrogen/air, or ex situ. The majority of
commercial catalyst regenerations, at least in the industrialized world, are
performed ex situ, by specialized contractors, because of environmental considerations as well as because it results in a superior regenerated catalyst. Prior to
regeneration, the spent catalyst must be de-oiled; this activity can be done in situ
at the refinery or ex situ at the service provider’s facility. De-oiling is needed to
prevent the formation of sulfates during the regeneration. The formation of sulfates
will lead to disappointing catalyst performance and shorter cycle length. During
regeneration/combustion, coke is converted to CO 2 and H 2 O. In the absence of
excess oxygen, CO may also form. Hydrotreating catalysts contain sulfur, as metal
sulfides. In the regeneration process, the metal sulfides are converted into the
corresponding metal oxides and the sulfur is emitted as SO 2 . In general, sulfur
dioxide emission starts at lower temperature than CO 2 emission. Regeneration
service providers rely on several forms of equipment. One vendor uses a continuous
rotolouver, which is a cylindrical drum rotating slowly on a horizontal axis and
enclosing a series of overlapping louvers. The spent catalyst passes slowly through
the rotolouver, where it encounters a countercurrent of hot air. Another uses a belt
furnace. The catalyst is conveyed with the stainless steel belt through a stationary
furnace. A third vendor makes use of an ebullated bed furnace/reactor to perform
the catalyst regeneration. Regardless of the process, the spent catalyst is submitted
to de-oiling prior to regeneration, which eliminates as much hydrocarbon as
possible and which removes as much sulfur as possible, preventing the formation
of sulfates which could deposit on the catalyst and not be removed during regeneration. Sulfates are deleterious to catalyst performance. While the in situ regeneration results in about 90 % catalyst activity recovery, ex situ catalyst regeneration
can result in 95–97 % catalyst activity recovery. Following regeneration, Type II
hydrotreating catalysts require a rejuvenation step, wherein the active phase is
re-dispersed as it was when fresh.
Design and Operation of Hydrotreating Reactors
Design and Construction of Hydrotreating Reactors
Hydrotreating reactors are downflow, fixed-bed catalytic reactors, generally operating in trickle-flow regime. Because hydrotreating occurs at moderately high pressure
and relatively high temperature and in the presence of hydrogen and hydrogen
sulfide, the reactors are vessels with relatively thick wall. The reactors are usually
cylindrical vessels, and while those used for naphtha hydrotreating as well as many of
the older vintage reactors are made from lower alloys, most of those designed in
the last 10 years are typically constructed of 1¼ Cr–½ Mo or 2¼ Cr–1 Mo
424
P. Kokayeff et al.
The activity decline due to coke laydown can be recovered by burning the coke off
in a controlled atmosphere. The regeneration can be accomplished in any of three
ways: in situ with steam/air, in situ with nitrogen/air, or ex situ. The majority of
commercial catalyst regenerations, at least in the industrialized world, are
performed ex situ, by specialized contractors, because of environmental considerations as well as because it results in a superior regenerated catalyst. Prior to
regeneration, the spent catalyst must be de-oiled; this activity can be done in situ
at the refinery or ex situ at the service provider’s facility. De-oiling is needed to
prevent the formation of sulfates during the regeneration. The formation of sulfates
will lead to disappointing catalyst performance and shorter cycle length. During
regeneration/combustion, coke is converted to CO 2 and H 2 O. In the absence of
excess oxygen, CO may also form. Hydrotreating catalysts contain sulfur, as metal
sulfides. In the regeneration process, the metal sulfides are converted into the
corresponding metal oxides and the sulfur is emitted as SO 2 . In general, sulfur
dioxide emission starts at lower temperature than CO 2 emission. Regeneration
service providers rely on several forms of equipment. One vendor uses a continuous
rotolouver, which is a cylindrical drum rotating slowly on a horizontal axis and
enclosing a series of overlapping louvers. The spent catalyst passes slowly through
the rotolouver, where it encounters a countercurrent of hot air. Another uses a belt
furnace. The catalyst is conveyed with the stainless steel belt through a stationary
furnace. A third vendor makes use of an ebullated bed furnace/reactor to perform
the catalyst regeneration. Regardless of the process, the spent catalyst is submitted
to de-oiling prior to regeneration, which eliminates as much hydrocarbon as
possible and which removes as much sulfur as possible, preventing the formation
of sulfates which could deposit on the catalyst and not be removed during regeneration. Sulfates are deleterious to catalyst performance. While the in situ regeneration results in about 90 % catalyst activity recovery, ex situ catalyst regeneration
can result in 95–97 % catalyst activity recovery. Following regeneration, Type II
hydrotreating catalysts require a rejuvenation step, wherein the active phase is
re-dispersed as it was when fresh.
Design and Operation of Hydrotreating Reactors
Design and Construction of Hydrotreating Reactors
Hydrotreating reactors are downflow, fixed-bed catalytic reactors, generally operating in trickle-flow regime. Because hydrotreating occurs at moderately high pressure
and relatively high temperature and in the presence of hydrogen and hydrogen
sulfide, the reactors are vessels with relatively thick wall. The reactors are usually
cylindrical vessels, and while those used for naphtha hydrotreating as well as many of
the older vintage reactors are made from lower alloys, most of those designed in
the last 10 years are typically constructed of 1¼ Cr–½ Mo or 2¼ Cr–1 Mo
424
P. Kokayeff et al.