A predictive model is developed in order to define the hydrodynamics of the acceleration zone in the riser of a circulating fluidized bed (CFB). The riser is divided into three distinct zones, i.e., acceleration, fully developed and deceleration zones. The acceleration zone is the region between the top of the dense bed and the fully developed zone. This study has employed both fundamental principles and empirical correlations in order to predict the hydrodynamic parameters in the acceleration zone. It is assumed that the solids move up in the riser as clusters (cluster-based approach, CBA) rather than single particles (particle-based approach, PBA). The model is thus based on the force balance on a single cluster. The cluster properties were estimated from the existing correlations from the literature. The model predictions were compared with the data obtained from the literature in term of pressure drop against riser height. Unlike the particle-based approach, there is a good agreement between the proposed model predictions and the experimental data. Therefore, it has been concluded that clusters are the key controller of the acceleration zone hydrodynamics. (C) 2004 Elsevier B.V. All rights reserved.