Main Reactions in FCC Catalysis

1. Main Reactions in FCC Catalysis

2. Key Developments in FCC Technology

3. 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.

4. Zeolite Structure • Zeolite structure depends on the • Si/Al ratio, template agents • and preparation, but all • have 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.

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.

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.

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 (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.

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. 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.

15. 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.