Page 150 - Fluid Catalytic Cracking Handbook

CHAPTER 4

Chemistry of FCC

Reactions

A complex series of reactions (Table 4-1) take place when a large
gas-oil molecule comes in contact with a 1,200°F to 1,400°F (650°C
to 760°C) FCC catalyst. The distribution of products depends on many
factors, including the nature and strength of the catalyst acid sites.
Although most cracking in the FCC is catalytic, thermal cracking
reactions also occur. Thermal cracking is caused by factors such as
non-ideal mixing in the riser and poor separation of cracked products
in the reactor.
The purpose of this chapter is to:

• Provide a general discussion of the chemistry of cracking (both
thermal and catalytic).
• Highlight the role of the catalyst, and in particular, the influence
of zeolites.
• Explain how cracking reactions affect the unit's heat balance.

Whether thermal or catalytic, cracking of a hydrocarbon means the
breaking of a carbon to carbon bond. But catalytic and thermal crack-
ing proceed via different routes. A clear understanding of the different
mechanisms involved is beneficial in areas such as:

• Selecting the "right" catalyst for a given operation
• Troubleshooting unit operation
• Developing a new catalyst formulation
Topics discussed in this chapter are:

• Thermal cracking
• Catalytic cracking
• Thermodynamic aspects

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