This study focuses on continuum model validation of the flow of air and small catalyst particles in a circulating fluidized bed. Comparison with available experimental data of pressure drop and solids circulation rate in the riser clearly demonstrates the need to modify the homogeneous drag model to accurately predict the formation of clusters of particles, which are typically observed in the fluidization of small particles. The need to correct the drag law is also demonstrated in simulations of polydisperse powder flows wherein three solids species are used to represent a typical catalyst size distribution. Finally, particle-wall friction is found to have the most significant effect on the vertical gas pressure gradient while particle-particle friction has only a minor effect. Published 2011 American Institute of Chemical Engineers AIChE J, 58: 427-439, 2012