The three-dimensional simulation of an industrial-scale fluid catalytic cracking riser reactor is performed using a novel density-based solution algorithm. The particle-level fluctuations are modeled in the framework of the kinetic theory of granular flow. The reactor model includes separate continuity equations for the components in the bulk gas and inside the solid phase. The results show a core-annular flow pattern in the major part of the riser with a significant densification of the core region halfway up the riser. The higher solid fraction and a lower solid velocity result in a higher conversion in the annulus region than in the core. However, the concentration profiles of the gaseous components are relatively flat due to the excellent radial mixing in the riser. The radially averaged axial solid velocity shows a considerable slip from the gas phase, amounting to a typical slip factor (ratio of gas/solid axial velocities) of about 2. The high slip factor results from the radial segregation of particles. The simulated product yields using the 3D model are comparable with the plant data. A comparison of the results obtained using the 3D and a 1D simulation model shows that the latter is unable to predict the high slip factor adequately unless the solid particle diameter is significantly altered.