line 68 while stripping steam may be added through line 70 to strip combustion gases from the heated solids air is charged to the burner through line 69 combustion gases are removed from the burner through line 72 for passage to heat and energy recovery systems not shown while regenerated hot solids which are relatively free of coke are removed from the burner through line 32 for recycle to riser 28 in order to produce a cracked product containing ethylene and molecular hydrogen petroleum residual oil is passed through the catalytic hydrodesulfurization zone in the presence of hydrogen at a temperature between 650 u00b0 f and 900 u00b0 f with the hydrogen being chemically combined with the oil during the hydrocycling step the hydrosulfurization residual oil passes through the thermal cracking zone together with the entrained inert hot solids functioning as the heat source and a diluent gas at a temperature between about 1300 u00b0 f and 2500 u00b0 f for a residual time between about 0 05 to 2 seconds to produce the cracked product and ethylene and hydrogen for the production of ethylene by thermally cracking a hydrogen feed at least 90 volume percent of which comprises light gas oil fraction of a crude oil boiling between 400 u00b0 f and 650 u00b0 f the hydrocarbon feed along with diluent gas and entrained inert hot gases are passed through the cracking zone at a temperature between 1300.