98
greater influence on catalyst activity than additive coke. There are five main types
of coke identified in catalytic cracking [86]:
• Catalytic coke—from condensation and dehydrogenation.
• Catalyst-to-oil coke—hydrocarbons entrained in the small pores and not removed
by the stripper.
• Thermal coke—formed by a free radical mechanism; it is important at high reaction temperatures and also yields hydrogen. It is less important than catalytic
coke due to the low extent of thermal cracking at typical FCC conditions.
• Additive coke (or Conradson coke)—from heavy molecules already present in
the feed. Its amount correlates directly with the Conradson carbon residue (residue remaining after the fuel has been pyrolyzed by raising the temperature to
800 °C).
• Contaminant coke—from dehydrogenation catalyzed by Ni, Fe, and V (Fig. 21).
It is generally accepted that the unsaturated molecules formed by carbenium ion
cracking as well as aromatics formed by the isomerization and cyclization are the
main precursors to coke. These precursors polymerize to form complex polycyclic
hydrocarbons that deposit in the pores of the catalyst due to diffusional restrictions.
Coke-selective catalysts focus on these precursors and crack them so that they can
escape the lattice cage structure before getting trapped as coke. Coke yield is directly
linked to the riser operating temperature. At higher temperature, a greater amount of
Fig. 21 Catalytic coke formation [37]
A. R. Khande et al.
greater influence on catalyst activity than additive coke. There are five main types
of coke identified in catalytic cracking [86]:
• Catalytic coke—from condensation and dehydrogenation.
• Catalyst-to-oil coke—hydrocarbons entrained in the small pores and not removed
by the stripper.
• Thermal coke—formed by a free radical mechanism; it is important at high reaction temperatures and also yields hydrogen. It is less important than catalytic
coke due to the low extent of thermal cracking at typical FCC conditions.
• Additive coke (or Conradson coke)—from heavy molecules already present in
the feed. Its amount correlates directly with the Conradson carbon residue (residue remaining after the fuel has been pyrolyzed by raising the temperature to
800 °C).
• Contaminant coke—from dehydrogenation catalyzed by Ni, Fe, and V (Fig. 21).
It is generally accepted that the unsaturated molecules formed by carbenium ion
cracking as well as aromatics formed by the isomerization and cyclization are the
main precursors to coke. These precursors polymerize to form complex polycyclic
hydrocarbons that deposit in the pores of the catalyst due to diffusional restrictions.
Coke-selective catalysts focus on these precursors and crack them so that they can
escape the lattice cage structure before getting trapped as coke. Coke yield is directly
linked to the riser operating temperature. At higher temperature, a greater amount of
Fig. 21 Catalytic coke formation [37]
A. R. Khande et al.