A process for combusting coke from catalyst comprising: a. contacting hydrocarbon feed with catalyst to produce cracked products and coked catalyst; b. separating said cracked products from said coked catalyst; c. delivering coked catalyst to a lower chamber of a regeneration vessel; d. contacting coked catalyst with oxygen to combust coke from the coked catalyst in a partial burn mode; e. transporting catalyst and flue gas from the lower chamber to an upper chamber of the regeneration vessel through a catalyst distributor into a dense catalyst bed in the upper chamber, wherein the top of the outlet of the catalyst distributor is disposed in a lower third of the upper chamber so that the catalyst distributor is submerged in the dense catalyst bed, below a top surface of the dense catalyst bed, and wherein the catalyst distributor radially discharges partially regenerated catalyst into the dense catalyst bed from under the top surface of the dense catalyst bed; f. contacting coked catalyst with oxygen to combust coke from the coked catalyst in the dense catalyst bed of the upper chamber, wherein combustion gas containing the oxygen is delivered to a combustion gas distributor in the upper chamber and distributed through outlets to the upper chamber of the regenerator vessel, and wherein the top of the outlet of the combustion gas distributor is disposed in a lower third of the upper chamber, and wherein the combustion gas distributor is submerged in the dense catalyst bed; g. separating flue gas from regenerated catalyst in the upper chamber; h. discharging regenerated catalyst from said upper chamber; and i. discharging flue gas from said upper chamber. a. contacting hydrocarbon feed with catalyst to produce cracked products and coked catalyst; b. separating said cracked products from said coked catalyst; c. delivering coked catalyst to a lower chamber of a regeneration vessel; d. contacting coked catalyst with oxygen to combust coke from the coked catalyst in a partial burn mode; e. transporting catalyst and flue gas from the lower chamber to an upper chamber of the regeneration vessel through a catalyst distributor into a dense catalyst bed in the upper chamber, wherein the top of the outlet of the catalyst distributor is disposed in a lower third of the upper chamber so that the catalyst distributor is submerged in the dense catalyst bed, below a top surface of the dense catalyst bed, and wherein the catalyst distributor radially discharges partially regenerated catalyst into the dense catalyst bed from under the top surface of the dense catalyst bed; f. contacting coked catalyst with oxygen to combust coke from the coked catalyst in the dense catalyst bed of the upper chamber, wherein combustion gas containing the oxygen is delivered to a combustion gas distributor in the upper chamber and distributed through outlets to the upper chamber of the regenerator vessel, and wherein the top of the outlet of the combustion gas distributor is disposed in a lower third of the upper chamber, and wherein the combustion gas distributor is submerged in the dense catalyst bed; g. separating flue gas from regenerated catalyst in the upper chamber; h. discharging regenerated catalyst from said upper chamber; and i. discharging flue gas from said upper chamber.