Operation of a CFB riser as versus (adapted from Mahmoudi et al. [5, 6]). Zone : Zone I: transition zone and/or inaccuracy in prediction; Zone : zone of choking; Zone II: dilute riser flow (DRF); Zone III: core-annulus flow (CAF) only; Zone IV: CAF with turbulent fluidized bed at the bottom (TFBB) of the riser; Zone V: dense riser upflow (DRU). —: transition DRF-CAF: , - - - - -: transition CAF-CAF with TFBB: ,   ........: transition CAF with TFBB-DRU: , and -.-.-.-: 84 μm sand: (6) predicted.