535
Catalytic Cracking
operating at a catalyst circulation limit. This can be achieved by increasing reactor heat load or
switching to a lower coke selective (i.e., lower delta coke) catalyst. Reactor heat load can be raised
by increased reactor temperature or lower feed preheat temperature. This, in turn, increases the cat/
oil ratio to maintain the unit in heat balance.
The lower the carbon on regenerated catalyst, the higher the availability of cracking sites,
since less coke is blocking acid cracking sites. The carbon on the regenerated catalyst is reduced
by increasing regeneration efficiency through the use of carbon monoxide oxidation promoters.
Carbon on the regenerated catalyst can also be reduced by more efficient air and spent catalyst contact. Increased regenerator bed levels also reduce the amount of carbon on the regenerated catalyst
through increased residence time but this must be traded off with reduced dilute phase disengager
residence time and the possibility for increased catalyst losses.
19.6.4 ProCess vArIABles
As already noted, there are primary variables available to the operation of fluid catalytic cracking
units for maximum unit conversion for a given feedstock quality can be divided into two groups:
catalytic variables and process variable. In addition to the catalyst variables (q.v.), there are also
process variables that include (1) pressure, (2) reaction time, and (3) reactor temperature. Higher
conversion and coke yield are thermodynamically favored by higher pressure. However, pressure
is usually varied over a very narrow range due to limited air blower horsepower. Conversion is not
significantly affected by unit pressure, since a substantial increase in pressure is required to significantly increase conversion.
An increase in reaction time available for cracking also increases conversion. Fresh feed rate,
riser steam rate, recycle rate, and pressure are the primary operating variables that affect reaction time for a given unit configuration. Conversion varies inversely with these stream rates due to
limited reactor size available for cracking. Conversion has been increased by a decrease in rate in
injection of fresh feedstock. Under these circumstances, overcracking of gasoline to LPG and to dry
gas may occur due to the increase in reactor residence time. One approach to offset any potential
gasoline overcracking is to add additional riser steam to lower hydrocarbon partial pressure for
more selective cracking. Alternatively, an operator may choose to lower reactor pressure or increase
the recycle rate to decrease residence time. Gasoline overcracking may be controlled by reducing
the availability of catalytic cracking sites by lowering cat/oil ratio.
Increased reactor temperature increases feedstock conversion, primarily through a higher rate
of reaction for the endothermic cracking reaction and also through increased cat/oil ratio. A 10°F
increase in reactor temperature can increase conversion by 1%–2% absolute, but, again, this is
feedstock-dependent. Higher reactor temperature also increases the amount of olefins in gasoline
and in the gases. This is due to the higher rate of primary cracking reactions relative to secondary hydrogen transfer reactions, which saturate olefins in the gasoline boiling range and lower
gasoline octane.
However, these variables are not always available for maximizing conversion, since most fluid
catalytic cracking units operate at an optimum conversion level corresponding to a given feed rate,
feed quality, set of processing objectives and catalyst at one or more unit constraints (e.g., wet gas
compressor capacity, fractionation capacity. air blower capacity, reactor temperature, regenerator
temperature, catalyst circulation). Once the optimum conversion level is found, there are very few
additional degrees of freedom for changing the operating variables.
19.6.5 AddItIves
In addition to what to cracking catalyst described previously, a series of additives has been
developed that catalyze or otherwise alter the primary catalyst’s activity/selectivity or act as pollution control agents. Additives are most often prepared in microspherical form to be compatible
Catalytic Cracking
operating at a catalyst circulation limit. This can be achieved by increasing reactor heat load or
switching to a lower coke selective (i.e., lower delta coke) catalyst. Reactor heat load can be raised
by increased reactor temperature or lower feed preheat temperature. This, in turn, increases the cat/
oil ratio to maintain the unit in heat balance.
The lower the carbon on regenerated catalyst, the higher the availability of cracking sites,
since less coke is blocking acid cracking sites. The carbon on the regenerated catalyst is reduced
by increasing regeneration efficiency through the use of carbon monoxide oxidation promoters.
Carbon on the regenerated catalyst can also be reduced by more efficient air and spent catalyst contact. Increased regenerator bed levels also reduce the amount of carbon on the regenerated catalyst
through increased residence time but this must be traded off with reduced dilute phase disengager
residence time and the possibility for increased catalyst losses.
19.6.4 ProCess vArIABles
As already noted, there are primary variables available to the operation of fluid catalytic cracking
units for maximum unit conversion for a given feedstock quality can be divided into two groups:
catalytic variables and process variable. In addition to the catalyst variables (q.v.), there are also
process variables that include (1) pressure, (2) reaction time, and (3) reactor temperature. Higher
conversion and coke yield are thermodynamically favored by higher pressure. However, pressure
is usually varied over a very narrow range due to limited air blower horsepower. Conversion is not
significantly affected by unit pressure, since a substantial increase in pressure is required to significantly increase conversion.
An increase in reaction time available for cracking also increases conversion. Fresh feed rate,
riser steam rate, recycle rate, and pressure are the primary operating variables that affect reaction time for a given unit configuration. Conversion varies inversely with these stream rates due to
limited reactor size available for cracking. Conversion has been increased by a decrease in rate in
injection of fresh feedstock. Under these circumstances, overcracking of gasoline to LPG and to dry
gas may occur due to the increase in reactor residence time. One approach to offset any potential
gasoline overcracking is to add additional riser steam to lower hydrocarbon partial pressure for
more selective cracking. Alternatively, an operator may choose to lower reactor pressure or increase
the recycle rate to decrease residence time. Gasoline overcracking may be controlled by reducing
the availability of catalytic cracking sites by lowering cat/oil ratio.
Increased reactor temperature increases feedstock conversion, primarily through a higher rate
of reaction for the endothermic cracking reaction and also through increased cat/oil ratio. A 10°F
increase in reactor temperature can increase conversion by 1%–2% absolute, but, again, this is
feedstock-dependent. Higher reactor temperature also increases the amount of olefins in gasoline
and in the gases. This is due to the higher rate of primary cracking reactions relative to secondary hydrogen transfer reactions, which saturate olefins in the gasoline boiling range and lower
gasoline octane.
However, these variables are not always available for maximizing conversion, since most fluid
catalytic cracking units operate at an optimum conversion level corresponding to a given feed rate,
feed quality, set of processing objectives and catalyst at one or more unit constraints (e.g., wet gas
compressor capacity, fractionation capacity. air blower capacity, reactor temperature, regenerator
temperature, catalyst circulation). Once the optimum conversion level is found, there are very few
additional degrees of freedom for changing the operating variables.
19.6.5 AddItIves
In addition to what to cracking catalyst described previously, a series of additives has been
developed that catalyze or otherwise alter the primary catalyst’s activity/selectivity or act as pollution control agents. Additives are most often prepared in microspherical form to be compatible