The flow behavior in the riser of a circulating fluidized-bed reactor was studied experimentally and computationally. Laser Doppler anemometry was applied to measure the flow behavior of FCC catalysts. Particle diameter and mean and fluctuating velocity for different particle sizes were measured at different flow conditions. A typical core-annulus flow with a relative velocity between particles of different sizes was obtained. A significant radial segregation of the mean diameter was measured in the riser. A multifluid computational fluid dynamics model was developed and verified against the experimental results. The flow model is based on a Eulerian description of the phases where the kinetic theory for granular flow forms the basis for the turbulence modeling in the solid phases. The model is generalized for one gas phase and N number of solids phases to enable a realistic description of the particle-size distribution in gas/solid flow systems. Each solid phase is characterized by a diameter, form factor, density, and restitution coefficient. Simulations with one gas and two solid phases were conducted, and the computational results agreed generally well with the measurements.