Non-catalyst Metals Deposition
Some metals may come into the system via additives, such as silicon compounds used in delayed coking unit coke drums to reduce foaming, or feed
contaminants such as Pb, Fe, As, P, Na, Ca, and Mg, or as organometallic
compounds in the feed primarily Ni and V. Ni and V deposition occurs at the
pore entrances and near the outer surface of the catalyst, creating a rind layer
which effectively chokes off the access to the interior part of the catalyst, where
most of the surface area resides. Metal deposition can damage the acid sites, the
metal sites, or both. Deposition of metals is not reversible even with catalyst
regeneration.
Catalyst Support Sintering
Catalyst support sintering is another reason for loss of catalyst activity and it
also is irreversible. Sintering is also a result of high temperatures and particularly
in connection with high water partial pressures. In this case the catalyst
support material can lose surface area from a collapse of pores or from an
increase in the diameter of pores, with the pore volume remaining approximately
constant.
Catalyst Regeneration
A coked catalyst is usually regenerated by combustion in a stream of diluted
oxygen or air, although steam or steam-air mixtures have also been used in the
past. Upon combustion, coke is converted to CO 2 and H 2 O. In the absence
of excess oxygen, CO may also form. Except for the noble metal catalysts,
hydrocracking catalysts contain sulfur, as the metals are in a sulfide form.
In the regeneration process, the sulfur will be emitted as SO 2 . In general, sulfur
oxide emission starts at lower temperature than CO 2 emission. Regeneration
of commercial catalysts can be done in situ or ex situ. The majority of commercial catalyst regeneration is performed ex situ because of environmental
considerations. Several companies operate ex situ regeneration by using different
equipment for burning off the coke. One company 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 company
uses a porous moving belt as a regenerator. The catalyst is moved with
the stainless steel belt through a stationary tunnel furnace vessel where the
regeneration takes place. Yet a third regeneration company uses ebullated bed
technology to perform the catalyst regeneration. Regardless of the regeneration
process, the spent catalyst is submitted to de-oiling prior to regeneration.
This is to eliminate as much hydrocarbon as possible as well as to remove as
much sulfur as possible to prevent formation of sulfates which could deposit on
the catalyst and not be removed during regeneration. Sulfates are deleterious to
catalyst performance.
348
M. Bricker et al.
Some metals may come into the system via additives, such as silicon compounds used in delayed coking unit coke drums to reduce foaming, or feed
contaminants such as Pb, Fe, As, P, Na, Ca, and Mg, or as organometallic
compounds in the feed primarily Ni and V. Ni and V deposition occurs at the
pore entrances and near the outer surface of the catalyst, creating a rind layer
which effectively chokes off the access to the interior part of the catalyst, where
most of the surface area resides. Metal deposition can damage the acid sites, the
metal sites, or both. Deposition of metals is not reversible even with catalyst
regeneration.
Catalyst Support Sintering
Catalyst support sintering is another reason for loss of catalyst activity and it
also is irreversible. Sintering is also a result of high temperatures and particularly
in connection with high water partial pressures. In this case the catalyst
support material can lose surface area from a collapse of pores or from an
increase in the diameter of pores, with the pore volume remaining approximately
constant.
Catalyst Regeneration
A coked catalyst is usually regenerated by combustion in a stream of diluted
oxygen or air, although steam or steam-air mixtures have also been used in the
past. Upon combustion, coke is converted to CO 2 and H 2 O. In the absence
of excess oxygen, CO may also form. Except for the noble metal catalysts,
hydrocracking catalysts contain sulfur, as the metals are in a sulfide form.
In the regeneration process, the sulfur will be emitted as SO 2 . In general, sulfur
oxide emission starts at lower temperature than CO 2 emission. Regeneration
of commercial catalysts can be done in situ or ex situ. The majority of commercial catalyst regeneration is performed ex situ because of environmental
considerations. Several companies operate ex situ regeneration by using different
equipment for burning off the coke. One company 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 company
uses a porous moving belt as a regenerator. The catalyst is moved with
the stainless steel belt through a stationary tunnel furnace vessel where the
regeneration takes place. Yet a third regeneration company uses ebullated bed
technology to perform the catalyst regeneration. Regardless of the regeneration
process, the spent catalyst is submitted to de-oiling prior to regeneration.
This is to eliminate as much hydrocarbon as possible as well as to remove as
much sulfur as possible to prevent formation of sulfates which could deposit on
the catalyst and not be removed during regeneration. Sulfates are deleterious to
catalyst performance.
348
M. Bricker et al.