JPS5813599B

A process for controlling the coke balance as well as the size of coke particles within a fluidized bed in the range adapted for continuous operation of a heavy residual oil cracking apparatus, wherein thermal cracking of heavy residual oil is performed under fluidization of coke particles and steam, which comprises classifying one portion of the coke particles withdrawn from the fluidized bed into coarse particles and fine particles by means of a pneumatic classifier, and after the size of said coarse particles have been reduced by combustion, returning them to the fluidized bed along with said fine particles.