PICT:0803104/III/1 Heavy residual petroleum stock is cracked by contacting it with a fluidized bed of hot solid particles to crack and to convert at least part of the stock to vapours, the vapours comprising cracked and vaporized stock, removing the vapours from the bed, through a disperse phase above the bed, to a recovery zone where a gas oil is recovered, thermally cracking a feed comprising the gas oil and passing the hot thermally-cracked products into the dispersed phase. In a preferred embodiment, a heavy residual petroleum stock is fed to a system through an inlet line 21, a surge drum 23 and a line 29 into a fluidized bed of solids such as finely divided petroleum coke preheated to 900-1150 DEG F. before coming to a reactor 11. As fast as the feed is converted, the vapours pass through the interface between the bed and the disperse zone above it as indicated at 31. The vapours pass into a scrubbing unit 39 and are fractionated in a tower 43 into coker naphtha and gases. A gas oil is taken off through line 47 and passes through a surge drum 91 and to a thermal cracking circuit 17 operating at 20-300 p.s.i.g. The outlet temperature from the cracking coil should be 950-1200 DEG F. The thermally-cracked gas oil emerges from the furnace through line 121 and passes through one or more lines 123 and 125 into the dispese phase at the top of the reactor. The thermally cracked vapours thus introduced into the disperse phase pass upwardly through the cyclone 33 and through its outlet 37 into the scrubber section 39 and fractionator 43 above it.
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