Packed bed reactors with moderate tube-to-particle diameter ratios are studied numerically using the computational fluid dynamics approach. Irregular particle arrangements within the reactors are generated by a modified ballistic deposition method. The study is focused on mass transfer phenomena in the reactors, while two different case studies are selected for numerical analysis, basically imitating the relevant experimental procedures. The first one is the dissolution of benzoic acid particles in water, the second one is the sublimation of naphthalene particles in air. To validate the simulation results, experimental mass transfer correlations from literature are used and a good agreement between numerical and experimental results is found. The numerical analysis provides useful data that can be applied for the improvement of the packed bed performance as well as for a deeper understanding of the fundamental transport phenomena in such reactors. (C) 2012 Elsevier Ltd. All rights reserved.