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The Chemistry and Technology of Petroleum
fire when the unit is opened up to the atmosphere. Proper shutdown procedures, which ensure that
the internals have sufficiently cooled before air enters the reactor will eliminate this problem. In
fact, the only defence against having coke plugging problems during start-up is to thoroughly clean
the unit during the turnaround and remove all the coke. If strainers on the line(s), they will have
to be cleaned frequently.
The two basic principles to minimize coking are to avoid dead spots and prevent heat losses. An
example of minimizing dead spots is using purge steam to sweep out stagnant areas in the disengager
system. The steam prevents collection of high-boiling condensable products in the cooler regions.
Steam also provides a reduced partial pressure or steam distillation effect on the high-boiling
constituents and cause enhanced vaporization at lower temperatures. Steam for purging should preferably be superheated since medium-pressure low-velocity steam in small pipes with high heat losses
is likely to be very wet at the point of injection and will cause more problems. Cold spots are often
caused by heat loss through the walls in which case increased thermal resistance might help reduce
coking. The transfer line, being a common source of coke deposits, should be as heavily insulated as
possible, provided that stress-related problems have been taken into consideration.
In some cases, changing catalyst type or the use of an additive (q.v.) can alleviate coking problems. The catalyst types that appear to result in the least coke formation (not delta coke or catalytic
coke) contain low or zero earth zeolites with moderate matrix activities. Eliminating heavy recycle
streams can lead to reduced coke formation. Since clarified oil is a desirable feedstock to make
needle coke in a coker, then it must also be a potential coke maker in the disengager.
One of the trends in recent years has been to improve product yields by means of better feed
atomization. The ultimate objective is to produce an oil droplet small enough so that a single particle of catalyst will have sufficient energy to vaporize it. This has the double benefit of improving
cracking selectivity and reducing the number of liquid droplets, which can collect to form coke
nucleation sites.
19.6.3 CAtAlyst vArIABles
The primary variables available to the operation of fluid catalytic cracking units for maximum unit
conversion for a given feedstock quality include catalytic variables such as (1) catalyst activity and
(2) catalyst design, which includes availability of cracking sites and the presence of carbon on the
regenerated catalyst.
The equilibrium catalyst activity, as measured by a microactivity test (MAT), is a measure of
the availability of zeolite and active matrix cracking sites for conversion. Therefore, an increase
in the unit activity can effect an increase in conversion and activity is increased by one, or a combination of: (1) increased fresh catalyst addition rate, (2) increased fresh catalyst zeolite activity,
(3) increased fresh catalyst matrix activity, (4) addition of catalyst additives to trap or passivate the
deleterious effects of feed nitrogen, alkalis (i.e., calcium and sodium), vanadium and other feed
metal contaminants, and (5) increased fresh catalyst matrix surface area to trap or remove feedstock
contaminants.
In general, a two-digit increase in the activity as determined by the MAT activity appears to
coincide with a 1% absolute increase in conversion. The increased matrix surface area improves
conversion by providing more amorphous sites for cracking high boiling range compounds in the
feedstock that cannot be cracked by the zeolite. Increased zeolite, on the other hand, provides the
necessary acid-cracking sites for selectively cracking the amorphous cracked high-boiling compounds and lighter boiling compounds.
In addition to zeolite and matrix activity, many of the catalyst’s physical and chemical properties
(catalyst design) contribute to increased conversion through selectivity differences. These include
zeolite type, pore size distribution, relative matrix to total surface area, and chemical composition.
Increasing the concentration of catalyst in the reactor, often referred to as cat/oil ratio, will
increase the availability of cracking for maximum conversion, assuming the unit is not already
The Chemistry and Technology of Petroleum
fire when the unit is opened up to the atmosphere. Proper shutdown procedures, which ensure that
the internals have sufficiently cooled before air enters the reactor will eliminate this problem. In
fact, the only defence against having coke plugging problems during start-up is to thoroughly clean
the unit during the turnaround and remove all the coke. If strainers on the line(s), they will have
to be cleaned frequently.
The two basic principles to minimize coking are to avoid dead spots and prevent heat losses. An
example of minimizing dead spots is using purge steam to sweep out stagnant areas in the disengager
system. The steam prevents collection of high-boiling condensable products in the cooler regions.
Steam also provides a reduced partial pressure or steam distillation effect on the high-boiling
constituents and cause enhanced vaporization at lower temperatures. Steam for purging should preferably be superheated since medium-pressure low-velocity steam in small pipes with high heat losses
is likely to be very wet at the point of injection and will cause more problems. Cold spots are often
caused by heat loss through the walls in which case increased thermal resistance might help reduce
coking. The transfer line, being a common source of coke deposits, should be as heavily insulated as
possible, provided that stress-related problems have been taken into consideration.
In some cases, changing catalyst type or the use of an additive (q.v.) can alleviate coking problems. The catalyst types that appear to result in the least coke formation (not delta coke or catalytic
coke) contain low or zero earth zeolites with moderate matrix activities. Eliminating heavy recycle
streams can lead to reduced coke formation. Since clarified oil is a desirable feedstock to make
needle coke in a coker, then it must also be a potential coke maker in the disengager.
One of the trends in recent years has been to improve product yields by means of better feed
atomization. The ultimate objective is to produce an oil droplet small enough so that a single particle of catalyst will have sufficient energy to vaporize it. This has the double benefit of improving
cracking selectivity and reducing the number of liquid droplets, which can collect to form coke
nucleation sites.
19.6.3 CAtAlyst vArIABles
The primary variables available to the operation of fluid catalytic cracking units for maximum unit
conversion for a given feedstock quality include catalytic variables such as (1) catalyst activity and
(2) catalyst design, which includes availability of cracking sites and the presence of carbon on the
regenerated catalyst.
The equilibrium catalyst activity, as measured by a microactivity test (MAT), is a measure of
the availability of zeolite and active matrix cracking sites for conversion. Therefore, an increase
in the unit activity can effect an increase in conversion and activity is increased by one, or a combination of: (1) increased fresh catalyst addition rate, (2) increased fresh catalyst zeolite activity,
(3) increased fresh catalyst matrix activity, (4) addition of catalyst additives to trap or passivate the
deleterious effects of feed nitrogen, alkalis (i.e., calcium and sodium), vanadium and other feed
metal contaminants, and (5) increased fresh catalyst matrix surface area to trap or remove feedstock
contaminants.
In general, a two-digit increase in the activity as determined by the MAT activity appears to
coincide with a 1% absolute increase in conversion. The increased matrix surface area improves
conversion by providing more amorphous sites for cracking high boiling range compounds in the
feedstock that cannot be cracked by the zeolite. Increased zeolite, on the other hand, provides the
necessary acid-cracking sites for selectively cracking the amorphous cracked high-boiling compounds and lighter boiling compounds.
In addition to zeolite and matrix activity, many of the catalyst’s physical and chemical properties
(catalyst design) contribute to increased conversion through selectivity differences. These include
zeolite type, pore size distribution, relative matrix to total surface area, and chemical composition.
Increasing the concentration of catalyst in the reactor, often referred to as cat/oil ratio, will
increase the availability of cracking for maximum conversion, assuming the unit is not already