With an aim to optimize propylene production, petroleum oil such as gas oil is catalytically cracked by contacting the oil with catalyst mixture consisting of a base cracking catalyst containing an stable Y-type zeolite and small amounts of rare-earth metal oxide, and an additive containing a shape-selective zeolite, in a down-flow type fluid catalytic cracking apparatus having a regeneration zone, a separation zone, and a stripping zone, wherein conversion of hydrocarbon occurs under relatively severe conditions. According to this fluid catalytic cracking process, the production of light-fraction olefins such as propylene is maximized by applying appropriate process control, monitoring, and optimizing systems. Process model and historical data to test a predictive system can provide early warning of potential performance degradation and equipment failure in the FCC unit.
