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Catalytic Cracking
The recognition that significant post riser cracking occurs in commercial catalytic cracking units
resulting in substantial production of dry gas and other low valued products (Avidan and Krambeck,
1990). There are two mechanisms by which this post-riser cracking occurs, thermal and dilute phase
catalytic cracking.
Thermal cracking results from extended residence times of hydrocarbon vapors in the reactor
disengaging area and leads to high dry gas yields via nonselective free-radical cracking mechanisms. On the other hand, dilute phase catalytic cracking results from extended contact between
catalyst and hydrocarbon vapors downstream of the riser. While much of this undesirable cracking
was eliminated in the transition from bed to riser cracking, there is still a substantial amount of
nonselective cracking occurring in the dilute phase due to the significant catalyst holdup.
Many catalytic cracking units are equipped with advanced riser termination systems to minimize
post-riser cracking (Long et al., 1993). However, due to the complexity and diversity of catalytic
cracking units, there are many variations of these systems and many such as closed cyclones and
many designs are specific to the unit configuration but all serve the same fundamental purpose of
reducing the undesirable post-riser reactions. Furthermore, there are many options for taking advantage of reduced post-riser cracking to improve yields. A combination of higher reactor temperature,
higher cat/oil ratio, higher feed rate, and/or poorer quality feed is typically employed. Catalyst modification is also appropriate and typical catalyst objectives such as low coke and dry gas selectivity
are reduced in importance due to the process changes, while other features such as activity stability
and bottoms cracking selectivity become more important for the new unit constraints.
Certain catalyst types seem to increase coke deposit formation. For example, these catalysts
(some rare earth zeolites) that tend to form aromatics from naphthenes as a result of secondary
hydrogen transfer reactions and the catalysts contribute to coke formation indirectly because the
products that they produce have a greater tendency to be coke precursors. In addition, high zeolite
content, low surface areas cracking catalysts are less efficient at heavy oil cracking than many
amorphous catalysts because the non-zeolite catalysts contained a matrix that was better able to
crack heavy oils and convert the coke precursors. The active matrix of some modern catalysts serves
the same function.
Once coke is formed, it is matter of where it will appear. Coke deposits are most often found in
the reactor (or disengager), transfer line, and slurry circuit and cause major problems in some units
such as increased pressure drops, when a layer of coke reduces the flow through a pipe, or plugging,
when chunks of coke spall off and block the flow completely. Deposited coke is commonly observed
in the reactor as a black deposit on the surface of the cyclone barrels, reactor dome, and walls. Coke
is also often deposited on the cyclone barrels 180° away from the inlet. Coking within the cyclones
can be potentially very troublesome, since any coke spalls going down into the dipleg could restrict
catalyst flow or jam the flapper valve. Either situation reduces cyclone efficiency and can increase
catalyst losses from the reactor. Coke formation also occurs at nozzles, which can increase the
nozzle pressure drop. It is possible for steam or instrument nozzles to be plugged completely, a serious problem in the case of unit instrumentation.
Coking in the transfer line between the reactor and main fractionator is also common, especially
at the elbow where it enters the fractionator. Transfer line coking causes pressure drop and spalling
and can lead to reduced throughput. Furthermore, any coke in the transfer line that spalls off can pass
through the fractionator into the circulating slurry system where it is likely to plug up exchangers,
resulting in lower slurry circulation rates and reduced heat removal. Pressure balance is obviously
affected, if the reactor has to be run at higher pressures to compensate for transfer line coking. On
units where circulation is limited by low slide valve differentials, coke laydown may then indirectly
reduce catalyst circulation. The risk of a flow reversal is also increased. In units with reactor grids,
coking increases grid pressure drop, which can directly affect the catalyst circulation rate.
Shutdowns and startups can aggravate problems due to coking. The thermal cycling leads to
differential expansion and contraction between the coke and the metal wall that will often cause
the coke to spall in large pieces. Another hazard during shutdowns is the possibility of an internal
Catalytic Cracking
The recognition that significant post riser cracking occurs in commercial catalytic cracking units
resulting in substantial production of dry gas and other low valued products (Avidan and Krambeck,
1990). There are two mechanisms by which this post-riser cracking occurs, thermal and dilute phase
catalytic cracking.
Thermal cracking results from extended residence times of hydrocarbon vapors in the reactor
disengaging area and leads to high dry gas yields via nonselective free-radical cracking mechanisms. On the other hand, dilute phase catalytic cracking results from extended contact between
catalyst and hydrocarbon vapors downstream of the riser. While much of this undesirable cracking
was eliminated in the transition from bed to riser cracking, there is still a substantial amount of
nonselective cracking occurring in the dilute phase due to the significant catalyst holdup.
Many catalytic cracking units are equipped with advanced riser termination systems to minimize
post-riser cracking (Long et al., 1993). However, due to the complexity and diversity of catalytic
cracking units, there are many variations of these systems and many such as closed cyclones and
many designs are specific to the unit configuration but all serve the same fundamental purpose of
reducing the undesirable post-riser reactions. Furthermore, there are many options for taking advantage of reduced post-riser cracking to improve yields. A combination of higher reactor temperature,
higher cat/oil ratio, higher feed rate, and/or poorer quality feed is typically employed. Catalyst modification is also appropriate and typical catalyst objectives such as low coke and dry gas selectivity
are reduced in importance due to the process changes, while other features such as activity stability
and bottoms cracking selectivity become more important for the new unit constraints.
Certain catalyst types seem to increase coke deposit formation. For example, these catalysts
(some rare earth zeolites) that tend to form aromatics from naphthenes as a result of secondary
hydrogen transfer reactions and the catalysts contribute to coke formation indirectly because the
products that they produce have a greater tendency to be coke precursors. In addition, high zeolite
content, low surface areas cracking catalysts are less efficient at heavy oil cracking than many
amorphous catalysts because the non-zeolite catalysts contained a matrix that was better able to
crack heavy oils and convert the coke precursors. The active matrix of some modern catalysts serves
the same function.
Once coke is formed, it is matter of where it will appear. Coke deposits are most often found in
the reactor (or disengager), transfer line, and slurry circuit and cause major problems in some units
such as increased pressure drops, when a layer of coke reduces the flow through a pipe, or plugging,
when chunks of coke spall off and block the flow completely. Deposited coke is commonly observed
in the reactor as a black deposit on the surface of the cyclone barrels, reactor dome, and walls. Coke
is also often deposited on the cyclone barrels 180° away from the inlet. Coking within the cyclones
can be potentially very troublesome, since any coke spalls going down into the dipleg could restrict
catalyst flow or jam the flapper valve. Either situation reduces cyclone efficiency and can increase
catalyst losses from the reactor. Coke formation also occurs at nozzles, which can increase the
nozzle pressure drop. It is possible for steam or instrument nozzles to be plugged completely, a serious problem in the case of unit instrumentation.
Coking in the transfer line between the reactor and main fractionator is also common, especially
at the elbow where it enters the fractionator. Transfer line coking causes pressure drop and spalling
and can lead to reduced throughput. Furthermore, any coke in the transfer line that spalls off can pass
through the fractionator into the circulating slurry system where it is likely to plug up exchangers,
resulting in lower slurry circulation rates and reduced heat removal. Pressure balance is obviously
affected, if the reactor has to be run at higher pressures to compensate for transfer line coking. On
units where circulation is limited by low slide valve differentials, coke laydown may then indirectly
reduce catalyst circulation. The risk of a flow reversal is also increased. In units with reactor grids,
coking increases grid pressure drop, which can directly affect the catalyst circulation rate.
Shutdowns and startups can aggravate problems due to coking. The thermal cycling leads to
differential expansion and contraction between the coke and the metal wall that will often cause
the coke to spall in large pieces. Another hazard during shutdowns is the possibility of an internal