-In this study, a lab-scale continuous packed-bed bioreactor with multi-sized distributed particles has been simulated in a structured layout using computational fluid dynamics. The goal was to achieve a well-estimated simple model which can describe the hydrodynamic regime of flow as well as mass transfer of species within the bioreactor. A MATLAB program was coded to create the packed-bed geometry by generating non-overlapping spherical particles. The flow hydrodynamics was assumed to be single-phase creeping flow regime (Stokes flow). The kinetic data of batch experiments at similar operating conditions were used to formulate the rate of anaerobic reactions. The anaerobic methano-genesis reactions were assumed to occur at the particle surfaces (on the biofllm). The bioreactor temperature was fixed at 37 degrees C. The commercial software COMSOL Multiphysics 5.0 was employed to solve the governing equations. Velocity profiles and concentration distribution of volatile fatty acids (VFAs) are shown inside the packed-bed reactor. Simulation results were found to be in good agreement with the experimental data at various influent flow rates (23.8 - 43.5 mL/h) and various influent concentrations of VFAs (0.001 - 0.042 M). In all cases, the relative error was less than 15% which is acceptable. This proposed model is applicable for scale-up and optimization purposes as well as evaluation of other kinetic models. (C) 2021 Elsevier Ltd. All rights reserved.