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 . <EOS>