In the last few years, adsorption technology has become increasingly important in market and industry. This increased commercialization of the technology stimulates further research into adsorption process. One of the most important applications of adsorption process is in the Pressure Swing Adsorption (PSA) systems. In such systems, the adsorption process occurs in a packed bed of adsorbents and usually under high pressure. To have a good comprehension about the adsorption phenomenon, especially in PSA systems, it is crucial to simulate this process and recognize important parameters involved in it. Present study is focused on the effect of adsorbent size on the adsorption process using CFD method. Unlike previous studies, which simulate the adsorption process using porous media approach, in this research, the details of the packed bed geometry have been taken into account. This study can facilitate more accurate simulation of all the steps of the PSA cycle in near future. The adsorption phenomenon is assumed to occur on spherical adsorbents placed in a cylindrical bed. In order to investigate the grain size effect on the adsorption phenomenon, five packed beds with unisized grains have been considered which are similar in height and radius but are filled with various sizes of adsorbents. The geometry has been generated based on the Discrete Element Method (DEM) algorithm. The meshing process has been done using ICEM CFD commercial software. Due to the complexity of the geometry, tetrahedral elements have been utilized and in order to reduce the computations, inner parts of the adsorbents have not been meshed. In this work, the air (which consists of 79% nitrogen and 21% oxygen) is passed through the packed beds of adsorbents and the adsorption process occurs on the surfaces of the adsorbents. Nitrogen is considered as the only sorbate of the flow. The process of nitrogen adsorption is assumed to be isothermal and the flow regime is set to be laminar. Equations to be solved are continuity, Navier-Stokes and species traort which the latter is only solved for the adsorbed component (nitrogen). The Linear Driving Force (LDF) model, along with Langmuir isotherm, has been used to simulate the adsorption phenomenon on the adsorbents. The problem has been solved using CFD method by means of ANSYS FLUENT 13.0 commercial CFD software. In order to apply boundary conditions and source terms to the governing equations, parallel User Defined Functions (UDF) programming has been employed. The objective of present study is to examine the effect of adsorbent size on the adsorption process and this objective leads us to simulate air separation on five different packed beds. The results have gone along with the Langmuir model and shown the importance of porosity and total mass of adsorbents. They have shown that with respect to the LDF model, for two different packed beds with similar porosity and different mass of adsorbents, the one which has more adsorbents mass can give us flow with more oxygen purity. Also, for two different packed beds with similar mass of adsorbents and different porosity, the one with lower porosity can give us flow with more oxygen purity albeit with lower oxygen mass flow rate. Key Words: adsorption, packed bed, LDF model, CFD simulation, PSA.