Optimization and control of CFB is very important and requires an accurate, real time, dynamic model to describe and quantify the process. The present work focuses on modeling the transient behavior of large CFB units, whose flow characteristics were shown to yield C-shaped voidage profiles using cork as the fluidizing material and air at ambient conditions. The riser is modeled in two ways: 1) as a set of well-mixed tanks connected in series; 2) as a 1-D axisymetric cluster flow. The tanks-in-series model visualizes the riser as consisting of a series of well-mixed vessels. When the gas flow rate is constant and solids flow rate changes as a sine wave function, the voidages along the riser indicate a family of sine waves with significant phase shifts. Using this method, the dynamic response time at different locations along the riser was estimated successfully. The cluster flow model assumes that gas and solids flows are unidirectional with no mixing in the axial direction, and the solids move upward in the riser as clusters. This model can be used to predict the smooth changes in voidage profiles for transient processes. (C) 2006 Elsevier B.V. All rights reserved.