The removal of residual volatile components such as monomers, oligomers, moisture, solvents or other impurities from polymers has gained growing attention as an important processing step. To meet the high quality requirements of the final product and to comply with the tight environmental regulations these undesired volatile components are usually removed by devolatilization during the extrusion process. The devolatilization process is carried out in partially filled screw sections and involves the diffusion of the volatiles to the polymer-vapor interface, evaporation of the volatiles at the interface and their subsequent removal by a vacuum venting system with condensation. To extend the understanding of the transport phenomena in single-screw or multi-screw extrusion units during devolatilization, a numerical model based on finite volume method is developed. The model investigates the two-dimensional flow situation in a partially filled venting section and analyzes the mass transport processes in the screw channel during devolatilization. Therefore, the velocity field describing the circulatory flow in the melt region of the screw channel due to drag flow is calculated and connected to Fick’s diffusion equations. The residual volatile concentration is analyzed depending on various input parameters including residence time, material properties and processing conditions. Taking the simulation results into account, an analytical equation describing the course of concentration in the devolatilization zone is derived using a heuristic approach. The new analytical equation can be used to detect the influence of important testing parameters on the residual volatile concentration in the devolatilization zone. As a result, it offers a useful tool to design efficient devolatilization screw sections.