A fluid coking unit for converting a heavy oil feed to lower boiling products by thermal has a centrally-apertured annular baffle at the top of the stripping zone below the coking zone to inhibit recirculation of solid particles from the stripping zone to the coking zone. By inhibiting recirculation of the particles from the stripping zone to the coking zone, the temperatures of the two zones are effectively decoupled, enabling the coking zone to be run at a lower temperature than the stripping zone to increase the yield of liquid products.