OCTANE ENHANCEMENT IN FLUID CATALYTIC CRACKING .2. OPERATION IN THE OVERCRACKING REGIME

The chemistry of gasoline Research and Motor Octane Number enhancement was studied for FCCU operation at high severities (cat/oil ratio) i.e. in the overcracking regime (greater than 75% conversion), where substantial amounts of gasoline are secondary cracked (with loss in selectivity) to C3, C4 olefins and coke. Negligible increases in dry gas are observed during operation at these severities. Gasoline aromatics, the source of Research and Motor octane, are mainly due to: - Rapid dealkylation of higher feed mono-aromatics. - Slow cyclization of olefins. - Conversion of cyclics to aromatics by hydrogen transfer. The reduction in gasoline yield is also a factor in causing the sharp octane boost, i.e. by concentrating the aromatics in the gasoline fraction. The aromatics formed are in the C8C10 range, which potentially reduces the impact of government gasoline legislation limiting benzene content to 5 vol-%. The overcracking regime is also characterized by significant losses in branched olefins and saturated components from the gasoline and increased production of iso-butane in the gas fraction. However, operating in the overcracking regime gives negligible improvement to the octane potential of typical FCCU gasoline heart-cut (70-130°C fraction). © 1990.