Main Reactions in FCC Catalysis

1
Main Reactions in FCC Catalysis
2
Key Developments in FCC Technology
3
Carbenium Ion Cracking Mechanism
4
Zeolite Structure

• Zeolites are a well-defined class of crystalline
  aluminosilicate minerals whose 3-dimensional
  structure is derived from a framework of SiO44-
  and AlO45- 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-jzH2O where z is about 250
  
• and j is between 48 and 76.

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 70C and
• 300C 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.

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 550C, water loss from Bronsted
• sites leads to unstable Lewis sites,
• leading to so-called true Lewis sites
• through expelling an Al species.

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

8
Synthesis of FCC Catalysts Utilizing Zeolites
9
Paraffin Cracking Catalyzed by ZSM-5
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

11
Modern FCC Complex
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 (500C) 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.

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.

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.

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.

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.