The axial density profile is an important characteristic of the CFB riser and a key parameter in the CFB design. A simple, but reliable model is needed to predict the density profiles. Elutriation-based models treat the dense phase at the bottom of the riser as a dense bubbling bed whereas the dilute phase higher up can be looked upon as the entrainment zone above the dense bed. The elutriation model, as originally presented by Rhodes et al. (1986) and based on Wen et al. (1982) is extensively studied and modified. In spite of the modifications, the use of entrainment models has certain clear limitations due to a wide range of predictions as evident from Table 1. Elutriation rates are calculated based on the hydrodynamic phenomena in the dense bed and a fitting procedure for the entrainment decay constant (a) was performed.