Fluid catalytic cracking of vacuum gas oil with a refractory fluid quench

Vacuum gas oil is catalytically cracked in the presence of a fluid zeolite catalyst in a riser reactor and then quenched before discharging into the disengaging section of a stripper vessel. The quench fluid is injected into the last 10 vol % of the riser reactor to prevent over cracking. In this manner, the amount of cracking which takes place in the disengaging vessel is reduced. The quench injection nozzles should be located at a point near the riser outlet but far enough from the outlet that the mixture in the riser and the quench fluid have come to temperature equilibrium by the time the mixture reaches the riser outlet. The quench fluid should be essentially inert to cracking, e.g. water or selected hydrocarbon fractions such as an intermediate cycle gas oil or heavy recycle naphtha.