A particular arrangement of an FCC unit creates an immediate and sustained gravity separation of catalyst and hydrocarbon vapors in short contact time cracking operation. A transverse feed contactor ejects the mixture of catalyst and hydrocarbon vapors transversely into a central portion of a separation vessel for vertical disengagement of catalyst from vapors. The vapors travel upwardly in the vessel into an inertial separator that quickly segregates entrained catalyst from the hydrocarbon vapors and collects separated catalyst at a higher elevation for stripping of adsorbed hydrocarbons from the catalyst.