PURPOSE: To supply a feedstock which enables a uniform addn., by a process wherein a fluid with two phases of gas and liq. is separated into a main stream and a branch, the latter is separated into liq. and gas, the gas is mixed with powder, and the mixture is then joined in the main stream.

CONSTITUTION: Heavy residual oil O as a fluid with two phases of gas and liq. taken out of a heating furnace is separated into main stream A and branch B to supply to a coke drum 1. In branch B, residual oil O is separated into gas G and liq. L by a gas-liq. separation cyclone 2, and liq. L is fed to the main stream A while gas G is fed to a Venturi mixer 3. Into the Venturi mixer 3, an alkaline earth metal compd. powder P is supplied from a pressurized powder storage tank 4, and gas G is mixed with powder P. Fluid mixture M of gas G with powder P is joined with heavy residual oil O in the main stream A at a secondary mixing bend 7, mixed, and supplied to the coke drum 1.