simplification other convection heating coils steam drum 11 and the transfer line exchanger 9 are not included in fig3 the amount of hydrogen to be used in this invention is a variable dependent upon the overall economics of the olefins plants i e the cost increase of the external vaporization system vis a vis the extra cost of the associated equipment for hydrogen recovery and purification it has been found that with the use of a hydrogen hydrocarbon feed ratio of 0 01 to 0 15 wt the external vaporization system can be eliminated since the molecular weight of hydrogen is low and the molecular weight of the heavy hydrocarbon feedstock is extremely high addition of even small quantities of hydrogen leads to a high concentration of hydrogen in that section of the convection section where the vaporization of the hydrocarbon feedstock takes place specifically the addition of 0 05 wt of hydrogen to a hydrocarbon feed having a molecular weight of about 700 results in 15 vol in hydrogen hydrocarbon mixture assuming that fig4 and 5 are accurate representations with respect to a particular feedstock at room temperature this would correspond to a reduction in molecular weight of the polymer by a factor of 2 to 3 and to a reduction in the polymerization rate of 25 at the higher temperatures encountered in the convection section however it is anticipated that the inhibition effect exhibited by the hydrogen would be considerably greater utilization of a level 0 05 wt of hydrogen represents about 10 of the hydrogen yield achieved in the furnace effluent during pyrolysis this would not have any significant impact on the downstream equipment size and utilities consumption without wishing or desiring to be limited to any theoretical explanation for the salutary.