1. Main Reactions in FCC Catalysis
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2. Key Developments in FCC Technology
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3. Carbenium Ion Cracking Mechanism
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4. Zeolite Structure
Zeolites are a well-defined class of crystalline aluminosilicate minerals whose 3-dimensional structure is derived from a framework of [SiO4]4- and [AlO4]5- coordination polyhedra.
Usually zeolites are classified according to common structural units (secondary building units, sbus)
Tetrahedra are arranged to yield an open framework structure, as shown below in the most important FCC catalyst, zeolite Y or faujasite.
This class of zeolite has 0.74nm
apertures, and the supercage of the
structure has a radius of approx.
1.2 nm.
A range of compositions exist, with a unit
cell formula typically being
Naj[(AlO2)j(SiO2)192-j]•zH2O where z is about 250
and j is between 48 and 76.
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5. Structure of ZSM-5
The zeolite ZSM-5 is finding greater application in FCC as an octane enhancing catalyst, as it cracks/isomerizes low octane components in the gas boiling range to higher octane value while generating propylene and butylene for subsequent alkylation.
Zeolites are usually crystallized
from alkaline aqueous gels at
temperatures between 70°C and
300°C to produce a sodium salt.
In addition to structure, key properties
are the Si/Al ratio, the particle size
and the nature of the (exchanged) cation.
These primary structure/composition factors influence acidity, thermal stability and overall catalytic activity.
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6. Acidity of Zeolites
The acidic properties of zeolites are dependent on the method of preparation, form, temperature of dehydration and the Si/Al ratio.
Bronsted Acid Sites:
generated by ion exchange
followed by calcination
Lewis Acid Sites:
At 550°C, water loss from Bronsted
sites leads to unstable Lewis sites,
leading to so-called ‘true’ Lewis sites
through expelling an Al species.
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7. Cracking Catalyst Formulations
In addition to acidity, physical characteristics must be considered, including:
1. Mechanical stability
function of zeolite, matrix and binder composition, degree of zeolite dispersion and bulk density.
2. Pore volume, pore size distribution and surface area
determined by matrix and zeolite composition, effects activity and yield through introduction of diffusion effects.
3. Thermal and hydrothermal stability
Recrystallization and structure collapse
4. Particle size distribution
Fluidization and entrainment specifications require 60-80m particle diameter
5. Bulk Density
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8. Synthesis of FCC Catalysts Utilizing Zeolites
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9. Paraffin Cracking Catalyzed by ZSM-5
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10. Shape Selectivity Imposed by Zeolite Structure
Variable channel and pore sizes of zeolites can create unique selectivity effects.
Reactant Selectivity:
Products Selectivity:
Restricted transition state selectivity
Transalkylation of a
dialkylbenzene
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11. Modern FCC Complex
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12. Schematic View: Short Contact Time FCC Unit
A modern FCC unit is a short-residence time, adiabatic process where atomized feed is contacted with hot catalyst (500°C) in a relatively narrow riser.
The reaction riser is a fluidized bed, with mixing promoted by differential particle-gas velocity and large scale turbulence.
Upon exiting the riser, the fluid velocity drops, and entrained catalyst settles. The overhead product stream is isolated through a cyclone to remove smaller particles.
Large-scale coke formation deactivates the catalyst, limiting the single-pass activity.
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13. Schematic View: Catalyst Regenerator
Coke formation during FCC blocks access to acidic sites within the active zeolite.
While limiting the lifetime of the catalyst, regeneration by coke combustion is very efficient.
The heat of combustion drives the endothermic cracking process by heating the catalyst prior to reintroduction to the riser.
As much as 30 tons per minute of catalyst is regenerated in a full-scale FCC unit.
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14. FCC Heat Balance
The predominate heat effects in FCC operations are the endothermic cracking process QRX and the exothermic coke combustion process QRG.
Feed preheating is done to facilitate atomization and improve vapourization upon contact with catalyst.
Flue gas heat is recovered.
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15. Really, Really Big Reactors
This FFC installation has the regeneration unit constructed above the cracking riser. Side-by-side configurations are also used.
A typical plant can run continuously for several hundred days, processing millions of barrels of oil.
In the foreground of the photo is a heating furnace.
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16. Bifunctional Catalysis
Hydrocracking
Catalytic cracking and olefin hydrogenation are combined processes in hydrocracking units.
FCC Zeolites, combined with dispersed metals (Ni, Pt, Pd) on a standard matrix generates a bifunctional catalyst capable of utilizing reforming by-product hydrogen.
Naptha Reforming
The low octane number of small paraffins (naptha) can be improved by isomerization without concurrent cracking or alkylation.
Pt/SiO2-Al2O3 is a bifunctional
catalyst preparation, wherein the
metal catalyzes dehydrogenation
and /hydrogenation and the
acidic support catalyzes skeletal
isomerization.
CHEE 323
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