Cracking apparatus for cracking a hydrocarbon feedstock containing resid, comprising (i) a heating zone for heating said hydrocarbon feedstock to provide heated hydrocarbon feedstock; (ii) 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; (iii) a vapor/liquid separation apparatus for treating the mixture stream, comprising (a) a cylindrical vertical drum having an upper cap section, middle section comprising a circular wall, and a lower cap section; (b) an overhead vapor outlet attached to said upper cap section; (c) at least one tangentially positioned inlet in the circular wall of said middle section for introducing said flow along said wall; (d) an annular structure located in the middle section, comprising an annular ceiling section extending from the circular wall and an internal vertical side wall to which said ceiling section extends, said side wall being positioned concentrically to, but away from, said circular wall, for blocking the upward passage of said vapor/liquid mixtures along the circular wall beyond said ceiling section, said annular structure surrounding an open core having sufficient cross-sectional area to permit vapor velocity low enough to avoid significant entrainment of liquid for permitting vapor flow to the overhead vapor outlet; and (e) a concentrically positioned, cylindrical boot of less diameter than said middle section, said boot communicating with said lower cap section and comprising a liquid outlet at its lower end, (iv) 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; (v) a means for quenching the effluent; and (vi) a recovery train for recovering cracked product from the quenched effluent, characterized in that at least one baffle is located in said vapor/liquid separation apparatus at a lower part of said middle section providing a surface slanting downwardly from the center of said drum toward the circular wall and providing a gap between said baffle and said circular wall for directing liquid along or near the circular wall to said lower cap section and the boot has an inlet for quench oil. (i) a heating zone for heating said hydrocarbon feedstock to provide heated hydrocarbon feedstock; (ii) 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; (iii) a vapor/liquid separation apparatus for treating the mixture stream, comprising (a) a cylindrical vertical drum having an upper cap section, middle section comprising a circular wall, and a lower cap section; (b) an overhead vapor outlet attached to said upper cap section; (c) at least one tangentially positioned inlet in the circular wall of said middle section for introducing said flow along said wall; (d) an annular structure located in the middle section, comprising an annular ceiling section extending from the circular wall and an internal vertical side wall to which said ceiling section extends, said side wall being positioned concentrically to, but away from, said circular wall, for blocking the upward passage of said vapor/liquid mixtures along the circular wall beyond said ceiling section, said annular structure surrounding an open core having sufficient cross-sectional area to permit vapor velocity low enough to avoid significant entrainment of liquid for permitting vapor flow to the overhead vapor outlet; and (e) a concentrically positioned, cylindrical boot of less diameter than said middle section, said boot communicating with said lower cap section and comprising a liquid outlet at its lower end, (a) a cylindrical vertical drum having an upper cap section, middle section comprising a circular wall, and a lower cap section; (b) an overhead vapor outlet attached to said upper cap section; (c) at least one tangentially positioned inlet in the circular wall of said middle section for introducing said flow along said wall; (d) an annular structure located in the middle section, comprising an annular ceiling section extending from the circular wall and an internal vertical side wall to which said ceiling section extends, said side wall being positioned concentrically to, but away from, said circular wall, for blocking the upward passage of said vapor/liquid mixtures along the circular wall beyond said ceiling section, said annular structure surrounding an open core having sufficient cross-sectional area to permit vapor velocity low enough to avoid significant entrainment of liquid for permitting vapor flow to the overhead vapor outlet; and (e) a concentrically positioned, cylindrical boot of less diameter than said middle section, said boot communicating with said lower cap section and comprising a liquid outlet at its lower end, (iv) 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; (v) a means for quenching the effluent; and (vi) a recovery train for recovering cracked product from the quenched effluent, characterized in that at least one baffle is located in said vapor/liquid separation apparatus at a lower part of said middle section providing a surface slanting downwardly from the center of said drum toward the circular wall and providing a gap between said baffle and said circular wall for directing liquid along or near the circular wall to said lower cap section and the boot has an inlet for quench oil.