A demand-driven pressure swing adsorption biogas upgrading application is modelled using monolayer and multilayered (bilayer) beds, to gain insight on the impact of the adsorbent pellet size on the overall performance of such processes. Pellet radii in the range of 0.1–2.4 mm were studied, for fixed cycle settings and column dimensions. Varying the pellet size influences the sorption kinetics and flow resistance, resulting in the existence of an optimum pellet size for monolayered beds. For fixed cycle settings, small pellets may yield higher purities at low total productivities, yet show a more rapid decrease in product purity with increasing productivities due to the higher pressure drop. Furthermore, 18 configurations with beds containing a layer of larger pellets and a second layer of smaller pellets (bilayer) were investigated. Bilayered beds with 0.3 mm, 0.6 and 2.4 mm radius pellets were combined, with the first layer taking up 25, 50 or 75% of the bed. With respect to upward flow in the adsorption step, beds with the smallest pellet size in the top layer (LS beds) can offer higher product purity than beds with the smallest pellet in the bottom layer (SL beds).