Octane enhancement in fluid catalytic cracking: I. Role of ZSM-5 addition and reactor temperature

Abstract

Addition of ZSM-5 to an ultra-stabilized fluid catalytic cracking (FCC) catalyst leads to significant losses in gasoline yield, the gasoline being cracked preferentially to propylene, iso-butane and butylene. The octane boost resulting from ZSM-5 addition is due to an increase in gasoline aromatics concentration as well as a decrease in molecular weight. Rather than being due to a particular reaction mechanism, the increase in aromatics concentration is in essence a concentration effect resulting from preferential removal of the non-cyclic gasoline components through cracking. No increase in gasoline component branching is observed on ZSM-5 addition. Of particular importance is the effect of ZSM-5 addition on the octane potential of the low-octane gasoline heart-cut (b.p. 70–130°C) from the FCC unit. A boost in motor octane number (MON) is observed due to cracking of low-MON components, e.g. olefins, to gas. However, no effect on heart-cut research octane number (RON) is seen, since these olefins have a significant RON contribution. A comparative study has been made of the effects of ZSM-5 addition and operation at higher reactor temperatures on gasoline octane enhancement in fluid catalytic cracking. Higher FCC reactor temperatures give rise to higher research octane levels than ZSM-5 addition does due to the catalytic formation of gasoline aromatics and preservation of olefins; the latter are preferentially cracked out in the presence of ZSM-5. Hence, a higher gasoline yield is realized, even at high motor octane levels. One detrimental factor, however, is the increased production of dry gas.