CATALYTIC CRACKING
Catalytic Cracking
Feedstock From Process Typical products . . . . To
Gas oils . . . . Towers, Decomposition, Gasoline . . . . . . . . . Treater or blend
coker alteration Gases . . . . . . . . . . Gas plant
visbreaker
Deasphalted Middle distillates . . .Hydrotreat,
oils . . . . . . . Deasphalter blend, recycle
Petrochem feedstock . . Petrochem or
other
Residue . . . . . . . . . . . Residual fuel
blend
Catalytic cracking
What it does?
Catalyticcracking breaks complex hydrocarbons into
simpler molecules
Why?
Inorder to increase the quality and quantity of lighter,
more desirable products
decrease the amount of residuals.
Catalytic cracking
This process rearranges the molecular structure of
hydrocarbon compounds to convert heavy
hydrocarbon feedstock into lighter fractions such
as kerosene, gasoline, LPG, heating oil, and
petrochemical feedstock.
Conditions
FCC is a low pressure, intermediate to high
temperature process.
Reactor Temperature, 0C 470-540
Regenerator Temperature, 0C 590-610
Reactor Pressure, kPa 232
Regenerator Pressure, kPa 274
Fractionator Pressure, kPa 225
Basic functions in the catalytic cracking
process
Reaction: Feedstock reacts with catalyst and
cracks into different hydrocarbons
Regeneration: Catalyst is reactivated by burning
off coke
Fractionation: Cracked hydrocarbon stream is
separated into various products.
Types of catalytic cracking processes
Fluid catalytic cracking (FCC)
Moving-bed catalytic cracking
Thermofor catalytic cracking (TCC)
Fluid Catalytic Cracking
Working of FCC
Fluid cracker consists of
Catalyst section
Reactor
catalyst circulation unit
regenerator,standpipe and riser
Fractionating section
FCC Process
The fluid catalyst is continuously circulated
between the reactor and the regenerator using air,
oil vapors, and steam as the conveying media
FCC Process
mixing a preheated hydrocarbon charge with hot,
regenerated catalyst as it enters the riser leading
to the reactor
The charge is combined with a recycle stream
within the riser, vaporized, and raised to reactor
temperature (900°-1,000° F) by the hot catalyst
FCC Process
As the mixture travels up the riser, the charge is
cracked at 10-30 psi
This cracking continues until the oil vapors are
separated from the catalyst in the reactor cyclones.
The resultant product stream (cracked product) is
then charged to a fractionating column where it is
separated into fractions, and some of the heavy oil is
recycled to the riser.
FCC Process
Spent catalyst flows through the catalyst stripper
to the regenerator, where most of the coke
deposits burn off at the bottom where preheated
air and spent catalyst are mixed.
Catalysts
A catalyst consists of an active substance applied
on a carrier substance having large extended
surface area
Particles have enormous porosity and very large
internal surface area
Promotors: Added to improve the performance of
the catalysts
Catalysts
Activity: Estimated in terms of yield of the end
product relative to the use of the starting material
Selectivity: The capability of the catalyst to
accelerate a desirable reaction and retain the rate
of the undesirable one
Activity and selectivity is gradually lost due to
ageing
Regeneration
By burning off coke deposited on the catalyst
particles
By raising the temperature within the reactor
By adding fresh circulating catalysts to compensate for
the loss of the original one
Catalyst
Catalysts used Form
zeolite powders
aluminum beads
hydrosilicate pellets or
treated bentonite clay shaped materials
fuller's earth called extrudites
bauxite
silica-alumina
Process Variables: Reactor Section
Fresh feed rate Catalyst to oil ratio
Fresh feed Recycle rate
temperature Recycle temperature
Reactor Temperature Catalyst activity
Reactor level
Reactor pressure
Stripping steam rate
Fresh Feed rate
Increasing the feed rate will increase the space
velocity and lower the conversion
Space velocity (Weight Hourly Space Velocity-
WHSV) = kg/h of feed divided by kg of catalyst hold
up in the reaction zone
Fresh Feed temperature
Combined feed temperature increases. This leads
to
Decrease in conversion
Increase in regenerator temperature
Reduction in coke yield
Reduction in oil/catalyst ratio
Reactor temperature
Increase in reactor temperature leads to
Increase in C1, C2, C3 and C4 yield
Decrease in gasoline yield
Increase in olefin content
Increase in RON
Reactor level
Increasing the reactor level (Weight of catalyst)
leads to
Increase in conversion
Decrease gasoline yield
Increase regenerator temperature
Increase coke yield
Decrease in olefin content
Reactor Pressure
Increase in pressure will lead to
Increase in conversion
Increase in regeneration temperature
Decrease in reactor velocity and cyclone inlet velocity
Stripping steam rate
Determined by catalyst circulation rate
Generally 1-2.5 kg per ton of catalyst circulated
Conditions that demand for increase in the rate
Increase in feed rate and recycle rate
Increase in reactor pressure
Decrease in combined feed temperature and reactor
temperature
A deterioration in catalyst particle size distribution
Catalyst/Oil ratio
Increase in this ratio leads to increase in
conversion
It increases with increase in reactor temperature
and decrease in regenerator and combined feed
temperatures.
Recycle rate
Cycle oils recycled to increase gasoline yield
Recycle temperature
Increases combined feed temperature
Catalyst activity
Results in higher conversion
Average specific surface of about 80-100 m2/g is
desirable for maximizing middle distillate
production
Process Variables: Regeneration Section
Dense Phase temperature
Regenerator pressure
Catalyst circulation rate
Regeneration air rate
Air distribution
Torch oil
Spray water
Catalyst condition
Dense phase temperature
Regenerator temperature increases with increase
in:
Sp.gravity or Avg. B.P of the feed
Carbon residue
combined feed temperature
reactor level
Reactor temperature
Reactor pressure
Regenerator pressure
Lowering of pressure will
Decrease blower power consumption
Improve air distribution
Increase catalyst entrainment to cyclones
Increase cyclone efficiency
Catalyst circulation rate
Increase in catalyst circulation rate reduces catalyst
residence time and has an adverse effect on
regeneration
Regeneration air rate
Most important variable.. Can be controlled by the
operator
Insufficient air leads to coke build up
Too much air leads to excess oxygen in flue gas and
after burning occurs
Regeneration air rate
Most important variable.. Can be controlled by the
operator
Insufficient air leads to coke build up
Too much air leads to excess oxygen in flue gas and
after burning occurs
Oxidation of CO to CO2
Increases temperature
Air Distribution
Unconsumed oxygen passing through the section
of the bed will cause abnormal afterburning
Torch oil
Distillate sprayed through nozzles to give
additional coke to satisfy heating requirements of
the unit
Spray water
At the inlet of first stage cyclone
To protect the cyclone from excessive temperature
During upset conditions
Catalyst condition
A substantial loss of fines will result in poor
fluidization and will increase the carbon content.
This will lead to after burning
Moving-bed catalytic cracking process
It is similar to the FCC process.
The catalyst is in the form of pellets that are
moved continuously to the top of the unit by
conveyor or pneumatic lift tubes to a storage
hopper, then flow downward by gravity through
the reactor, and finally to a regenerator.
The cracked product is separated into recycle gas,
oil, clarified oil, distillate, naphtha, and wet gas.
Thermofor Catalytic Cracking
The preheated feedstock flows by gravity
through the catalytic reactor bed.
The vapors are separated from the catalyst and
sent to a fractionating tower.
The spent catalyst is regenerated, cooled, and
recycled.
The flue gas from regeneration is sent to a
carbon-monoxide boiler for heat recovery.
