Process for controlling the size of coke particles within a fluidized bed

In a process for the thermal cracking of heavy residual oil or crude oil with a fluidized bed of a particulate coke heat carrier in a system comprising a fluidized bed reaction zone for cracking said heavy residual oil or crude oil and a heating zone for heating said particulate coke heat carrier and wherein heavy residual oil or crude oil is thermally cracked at a high temperature by means of a fluidized bed consisting of coke particles and steam under conditions which maintain a positive coke balance in the system such that an amount of coke is formed that is greater than an amount of coke lost during operation of said process;

• particulate coke from the fluidized bed reaction zone is circulated to said heating zone wherein it is heated and said heated particulate coke heater carrier is returned to said fluidized bed reaction zone;
• the improvement for controlling the coke balance and the size of the particles of said particulate coke heat carrier in a range adapted for continuous operation comprising:
• withdrawing a portion of the particulate coke heated in the heating zone from a position intermediate said heating zone and said reaction zone;
• providing a storage zone for receiving said withdrawn particulate coke, the particles of the particulate coke heat carrier being maintained in a fluidized state in the storage zone by means of a gas introduced thereinto;
• passing a controlled amount of the withdrawn particulate coke to a pneumatic classifier wherein the particles of the particulate coke are classified into relatively coarse particles and relatively fine particles;
• contacting the relatively coarse particles with an oxygen-containing gas to cause partial combustion thereof thereby reducing the particle size and, thereafter, returning said sizereduced particles with said relatively fine particles and the flue gas resulting from the combustion of the coarse particles to the heating zone.