A practical countercurrent fluid catalytic cracking (FCC) regenerator model with improved descriptions of gas and solid flow patterns is proposed. A three-zone and two-phase gas model was utilized to describe the gas flow through the regenerator, addressing the different phase mass-transfer properties in the different zones. A new two-continuously stirred-tank reactor-with-interchange model was used to describe the solid flow and to address the effect of freeboard on catalyst regeneration. Otherwise, this model also considered the usually adopted expanding section for reducing solid carryover. The model was programmed in Matlab language with coupled hydrodynamics and reaction kinetics models and tested and validated by the data from an industrial FCC regenerator operated under both partial and full CO combustion modes. After fitting a single model parameter, the interchange solid flux between the dense bed and freeboard, the model predictions were in reasonable agreement with the commercial data for both modes. (c) 2011 American Institute of Chemical Engineers AIChE J, 2012