A process and apparatus described is for distributing fluidizing gas to a riser. Fluidizing gas is delivered to a plenum below the riser. A first stream of fluidizing gas is distributed from the plenum into a chamber in a riser and a second stream of fluidizing gas is distributed from the plenum into the riser outside of the chamber. First nozzles in the plenum have a first outlet in the chamber and second nozzles in the plenum have a second outlet outside of the chamber. Streams of regenerated catalyst and carbonized catalyst may be passed to the riser and mixed around the chamber in a lower section of a riser.