ABSTRACT
In modern FCC units, the cracking reaction takes place in a vertical pipe, or riser, where hot catalyst introduced at the bottom is carried upward at high velocity by lift gas plus the hydrocarbon vapors formed from the oil feed. The riser may be 1-2 m in diameter and up to 40 m tall, and the exit vapor velocity is generally 15-20 m/sec [1], giving a gas residence time of a few seconds. At the top of the riser, catalyst is recovered by cyclones, stripped with steam, and sent to the regenerator. A sketch of an FCC unit was shown in Figure 9.18. The mass flow rate of catalyst in the riser is usually about six times that of the gas, but the volume fraction of solids in the reactor is quite small. Over much of the length, the average suspension density is about 50 kg/m3, corresponding to a solids fraction (1 Þ ffi 0:04, but near the inlet, where the particles are accelerated, the suspension density may be three to four times greater [2].
