In the present work a simulation module is developed for the description of a bubbling fluidized bed reactor and for the riser reactor of a circulating fluidized bed which is suitable for use in the framework of a flowsheet simulation system. A population balance approach is used to determine the particle size distribution within the reactor in steady-state. The model allows to describe the individual fate of particles in terms of attrition (abrasion and shrinkage) and transport effects. The fluidized bed reactor model is one-dimensional. Models of fluid mechanics are combined with kinetic approaches for the description of the local reaction. The implementation within the framework of a simulation package for solids processes allows for the flexible coupling of process models, e.g. of the fluidized bed reactor with one or more cyclone stages for gas cleaning or with other fluidized beds in fluidized bed systems. (C) 2011 Elsevier B.V. All rights reserved.