A circulating fluid bed reactor such as that used in fluid coking processes has a circular dense bed reaction section above the reactor base where the fluidizing gas is injected and a plurality of frusto-conical baffles in the dense bed reaction section, each of which depends downwardly and radially inwards from the reactor wall to a lower, inner edge defining a central aperture. The baffles are preferably provided with downcomers which permit downward flow of solids and upward flow of gas through the baffles.