A process for vaporizing and pyrolyzing a portion of a hydrocarbon feedstock to olefins, and separating an unvaporized portion of said feedstock containing undesirable coke precursors and/or high boiling pitch fractions that cannot be completely vaporized under convection section conditions of a typical pyrolysis furnace, said process comprising: a) feeding the hydrocarbon feedstock to a first stage preheater provided in a convection zone of a pyrolysis furnace, and heating said feedstock within the first stage preheater to produce a heated gas-liquid mixture, b) withdrawing the heated gas-liquid mixture from the first stage preheater, combining it with high temperature steam and feeding the combined stream to a first vapor-liquid separator, c) separating and removing the gas from the liquid in the first vapor-liquid separator, heating the gas in a vapor phase superheater provided in said convection zone to a temperature of about 450 to 700°C, feeding all or the majority portion of the heated gas to a second vapor-liquid separator, and the remaining portion into a radiant zone of the pyrolysis furnace and pyrolyzing the gas to produce olefins and other pyrolysis products, d) withdrawing the liquid from the first vapor-liquid separator, and heating the removed liquid to a temperature of about 425 to about 510°C by combining it with the majority portion of the vapor from the first vapor-liquid separator after it is further heated in said superheater in the convection zone and feeding the stream to a second vapor-liquid separator, e) separating and removing the gas from the liquid fraction in the second vapor-liquid separator, feeding the removed gas into a radiant zone of the pyrolysis furnace and pyrolyzing the gas to produce olefins and other pyrolysis products, and f) removing the remaining liquid fraction from the second vapor-liquid separator, wherein the portion of the heated gas from said preheater in step c) routed to the second vapor-liquid separator is in the range of 60 and 100 volume percent of the heated gas and the portion of the heated gas routed to the radiant zone of the pyrolysis furnace is in the range of 0 and 40 volume percent, and wherein pyrolytic cracking conditions include a pyrolytic cracking temperature of from about 700°C to about 900°C, a pyrolytic cracking pressure of from about 1 Bar to about 3 Bar, and wherein the gaseous fractions are exposed to the pyrolytic cracking conditions within the radiant zone for a pyrolytic cracking time period upwardly to a maximum of about 10 seconds. a) feeding the hydrocarbon feedstock to a first stage preheater provided in a convection zone of a pyrolysis furnace, and heating said feedstock within the first stage preheater to produce a heated gas-liquid mixture, b) withdrawing the heated gas-liquid mixture from the first stage preheater, combining it with high temperature steam and feeding the combined stream to a first vapor-liquid separator, c) separating and removing the gas from the liquid in the first vapor-liquid separator, heating the gas in a vapor phase superheater provided in said convection zone to a temperature of about 450 to 700°C, feeding all or the majority portion of the heated gas to a second vapor-liquid separator, and the remaining portion into a radiant zone of the pyrolysis furnace and pyrolyzing the gas to produce olefins and other pyrolysis products, d) withdrawing the liquid from the first vapor-liquid separator, and heating the removed liquid to a temperature of about 425 to about 510°C by combining it with the majority portion of the vapor from the first vapor-liquid separator after it is further heated in said superheater in the convection zone and feeding the stream to a second vapor-liquid separator, e) separating and removing the gas from the liquid fraction in the second vapor-liquid separator, feeding the removed gas into a radiant zone of the pyrolysis furnace and pyrolyzing the gas to produce olefins and other pyrolysis products, and f) removing the remaining liquid fraction from the second vapor-liquid separator,