In this paper, Direct Numerical Simulation (DNS) was performed to understand the effects of confining walls on mass transfer through a packed bed for laminar regime (Re <= 100). The X, Y and Z coordinates of the center of the spheres in a randomly packed bed with varying ratios D/d (D is the diameter of the column and d is the diameter of the particle) were generated using a Discrete Element Method Computational Fluid Dynamics (DEM-CFD) code. Naphthalene-air system (Sc similar to 2.52) was considered for all the cases. The grid resolution, method and boundary conditions were validated by comparing the computed (overall) Sherwood number with the published experimental data. Local Sherwood number was computed around each particle for all ratios, Did, and spatial and probability distributions throughout the packed bed column as a function of D/d are reported. It was observed that for D/d's <= 8.6, the effects of wall on particle's Sherwood number was evident, while for D/d's <= 10.8, the Sherwood number was predominantly uniform all through the column. (C) 2017 Elsevier Ltd. All rights reserved.