Catalyst regenerators and methods of their use are provided. A catalyst regenerator includes a combustion chamber with a combustion chamber diameter and a combustion chamber bottom. A mixing chamber is fluidly coupled to the combustion chamber at the combustion chamber bottom, where the mixing chamber has an exterior wall and a mixing chamber diameter less than the combustion chamber diameter. A first and second catalyst inlet are fluidly coupled to the mixing chamber, and a mixing cylinder is within the mixing chamber. The mixing cylinder and the exterior wall define an annular space there-between, and the mixing cylinder includes a cylinder opening.