(a) a heating zone for heating said hydrocarbon feedstock to provide heated hydrocarbon feedstock;

(b) a mixing zone for mixing a primary dilution steam stream with said heated hydrocarbon feedstock to provide a heated two-phase stratified open channel flow mixture stream;

(c) a vapor/liquid separation zone for treating vapor/liquid mixtures of hydrocarbons and steam, said zone comprising:

i) a substantially cylindrical vertical drum having an upper cap section, a middle section comprising a circular wall, and a lower cap section;

ii) an overhead vapor outlet extending upwardly from said upper cap section;

iii) at least one inlet in the circular wall of said middle section for introducing said flow;

iv) a substantially concentrically positioned, substantially cylindrical boot extending downwardly from said lower cap section for receiving separated liquid, said boot being of less diameter than said middle section and communicating with said lower cap section, and further comprising a liquid outlet at its lower end; and further comprising at least one of

v) a means for introducing heat directly to said lower cap section and/or said boot using at least one heating coil to provide conditions to effect at least partial visbreaking of the separated liquid; and

vi) a means to regulate residence time of liquid present in said lower cap and/or boot to provide conditions to effect at least partial visbreaking of the separated liquid;

(d) a pyrolysis furnace comprising a convection section, and a radiant section for cracking the vapor phase from the overhead vapor outlet to produce an effluent comprising olefins;

(e) a means for quenching the effluent; and

(f) a recovery train for recovering cracked product from the quenched effluent.