Main Reactions in FCC Catalysis

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Key Developments in FCC Technology

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Zeolite StructureZeolites 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 thestructure has a radius of approx.1.2 nm.

A range of compositions exist, with a unitcell formula typically beingNaj[(AlO2)j(SiO2)192-j]zH2O where z is about 250 and j is between 48 and 76.

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Zeolite StructureZeolite structure depends on the Si/Al ratio, template agents and preparation, but allhave unique micropore structures.

Virtually all acid catalyzed reactions can be conducted with acidic form of zeolites, provided the reactant is small enough to enter the pores.

The acidic sites in HZSM-5 are strong enough to protonate paraffins, leading to widespread use as an industrial cat. cracking catalyst.

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Structure of ZSM-5The 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 crystallizedfrom alkaline aqueous gels attemperatures between 70C and300C to produce a sodium salt.

In addition to structure, key propertiesare the Si/Al ratio, the particle sizeand the nature of the (exchanged) cation.

These primary structure/composition factors influence acidity, thermal stability and overall catalytic activity.

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Acidity of ZeolitesThe 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 exchangefollowed by calcination

Lewis Acid Sites:

At 550C, water loss from Bronstedsites leads to unstable Lewis sites,leading to so-called true Lewis sitesthrough expelling an Al species.

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Cracking Catalyst FormulationsIn addition to acidity, physical characteristics must be considered, including:1. Mechanical stabilityfunction of zeolite, matrix and binder composition, degree of zeolite dispersion and bulk density.

2. Pore volume, pore size distribution and surface areadetermined by matrix and zeolite composition, effects activity and yield through introduction of diffusion effects.

3. Thermal and hydrothermal stabilityRecrystallization and structure collapse

4. Particle size distributionFluidization and entrainment specifications require 60-80m particle diameter

5. Bulk Density

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Synthesis of FCC Catalysts Utilizing Zeolites

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Paraffin Cracking Catalyzed by ZSM-5

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Shape Selectivity Imposed by Zeolite StructureVariable 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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Modern FCC Complex

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Schematic View: Short Contact Time FCC UnitA 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.

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Schematic View: Catalyst RegeneratorCoke 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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Really, Really Big ReactorsThis 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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Bifunctional CatalysisHydrocrackingCatalytic 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 ReformingThe 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.

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