conditions and a temperature of from 800 1 000 u00b0 f generally the hydrogen consumption at a given resid conversion decreases with increasing reactor temperature however the tendency of the donor diluent and the cracked material to deposit coke in the cracking unit also increases with increasing reactor temperature such that the optimum temperature for a given situation is often a compromise between resid conversion level and coke deposition the process of this invention and particularly the use of a heavy premium coker gas oil as the hydrogen donor diluent enables the process to be carried out at a higher temperature than could be utilized using prior art diluents this is due to the improved thermal stability of premium coker gas oil compared to the prior art diluent materials the diluent in accordance with the invention is produced in a premium coker unit which is operated at conditions such that the coke produced has a coefficient of thermal expansion of about five times 10 7 u00b0 c or less these conditions include a transfer line temperature from the coker furnace of about 900 u00b0 960 u00b0 f preferably from 920 945 u00b0 f a coke drum pressure of about 10 to 150 psig preferably about 30 to 70 psig a recycle ratio furnace charge divided by fresh feed of 1 2 to 2 5 and a premium coke feedstock such as thermal tar decant oil from a fluid catalytic cracker or other similar stream having comparable aromaticity and boiling range a coke drum temperature of about 820 u00b0 900 u00b0 f preferably 840 u00b0 880 u00b0 f is normally obtained in a premium coker operation coke drum temperature is determined by measuring the temperature of.