Thermal hydrocracking of heavy hydrocarbon oils with heavy oil recycle

In a process for hydrocracking a heavy hydrocarbon oil feed stock, a substantial proportion of which boils above 524.degree. C., wherein an intimate mixture of the heavy hydrocarbon oil and hydrogen is passed under upflow liquid conditions through a tubular hydrocracking zone having an L/D ratio of at least 2, said hydrocracking zone being maintained at a temperature between about 400.degree. and 490.degree. C. and a pressure between about 500 and 3,500 psig, a mixed effluent containing a gaseous phase comprising hydrogen and vaporous hydrocarbons and a liquid phase comprising heavy hydrocarbons is removed from the top of the hydrocracking zone and passed into a separate hot separator vessel also having an L/D ratio of at least 2, a gaseous stream comprising hydrogen and vaporous hydrocarbons is withdrawn from the top of the separator and a liquid stream comprising heavy hydrocarbons is withdrawn from the bottom of the separator,

• the improvement which comprises discharging the mixed effluent into the hot separator vessel in a lower region thereof below the liquid level in the separator to provide vigorous mixing action in the bottom of the separator and thereby substantially prevent coke deposits in the separator, said separator being maintained at a temperature between about 350.degree. and 490.degree. C., and recycling at least part of the liquid stream from the bottom of the separator without further treatment other than temperature adjustment to the bottom of the hydrocracking zone at a volume ratio of recycle liquid to feed stock of at least 2:1 to provide a liquid hourly space velocity in the hydrocracking zone of about 0.5 to 4.0 and a superficial liquid upflow velocity in the hydrocracking zone of at least 0.25 cm/sec such that deposition of coke in the hydrocracking zone is also substantially eliminated.