The Petrochemical FCC Unit

Propylene is a key building block in the petrochemical industry. The vast majority of propylene supply comes from steam crackers (approximately 60%). Fluid Catalytic Cracking (FCC) units provide approximately 35% and the balance is made up by propane dehydrogenation units (PDH). Feedstock diet of steam crackers is changing with the advent of shale oils to lighter feeds. These lighter feeds, and especially natural gas liquids change the yield pattern of steam cracker products towards more ethylene and less propylene. Typically FCC yields 5 wt% of propylene from a heavy feed, but this can be easily increased by the use of ZSM-5 additives. Petrochemical FCC units can achieve yields of typically 9 to 12 wt% propylene on feed using ZSM-5 additives. Where addition of small amounts of ZSM-5 additive to a base catalyst do not change FCC chemistry dramatically, adding substantial amounts of ZSM-5 additive (10 to 25 wt% to inventory) have a profound effect on the chemistry of gasoline and LPG as they can now react over two very different shape selective zeolites. Hydrogen transfer reactions over ulstrastiblized Y (main zeolite in FCC base catalyst), which are very dominant in FCC, are now ‘diluted’ by alternative cracking routes over ZSM-5. A new network of reactions appears in which FCC olefins are now no longer only a desired final product but an intermediate as well. The effect of ZSM-5 on light and heavy fractions of the gasoline is different. In the light gasoline the effects of de-alkylation and recombination of LPG olefins is more pronounced than in the heavy gasoline. In both fractions the change in hydrogen transfer over the catalytic system are very apparent. Changes in the gasoline and LPG composition show that these are a function of the process and catalyst both.