In this contribution, the fluid flow and mixing dynamics inside planetary roller extruders are simulated using the finite element method (FEM) and the mesh superposition technique (MST). Three-dimensional configurations with planetary spindles of varying number and geometry of planetary spindles were created to analyse the influence of the spindle configuration and the rotational speed on the process behavior. Therefore, pressure gradients, flow velocities and directions, shear rates, the mixing index and residence time distributions were evaluated. The distributive and dispersive mixing efficiencies varied depending on the planetary spindle configuration, and these configurations thus suit different processing tasks. In comparison to the standard planetary spindles, the TT3 spindles, with their incomplete toothing, and the knob spindles, with their double transversal helical toothing, showed intense axial and radial mixing. In general, the mixing performance of the planetary roller extruder is explained by a high rate of extensional flow and frequent changes in flow type. The reported numerical approach allows, for the first time, a comprehensive observation of the process behavior of planetary roller extruders.