Abstract: As to the significant increasing of by-products such as dry gas and coke in residue catalytic cracking process, an experimental investigation was conducted. The study revealed that catalyst particles solids hold up (volume fraction of catalyst particles in the reactor) play a critical role in modulating the thermal cracking reactions across the reactor, there is a hidden thermal cracking reaction zone in the lower part of the riser. Through expanding the diameter of the first reaction zone of the variable-diameter fluidized bed reactor, the solids hold-up of catalyst particles is increased, and a novel variable-diameter fluidized bed reactor is designed. On this basis, a high-selectivity catalytic cracking (MSC) technology for low-cost production of high-octane gasoline is developed. Commercial application results demonstrate that the MSC technology exhibits significant advantages in converting low-value feedstocks into high-value products and in heavy oil conversion, with improved utilization efficiency of hydrocarbon resources. For the gasoline+LPG production scheme, despite the deterioration of feedstock properties, the product distribution is notably improved: the combined yield of gasoline and LPG increases, while the yields of dry gas and coke decrease significantly. The research octane number of FCC gasoline is raised from 90.4 to 93.9, and the entire refinery's gasoline is blended and sold as No. 95 gasoline, resulting in a substantial enhancement of the enterprise's economic benefits.

Key words: catalytic cracking reaction, thermal cracking reaction, variable-diameter fluidized bed, MSC technology, high-octane gasoline, LPG