The linear driving force (LDF) approximation has been extensively used to represent the intraparticle diffusion in adsorptive processes regardless of its real nature. This work introduces a bilinear driving force (bi-LDF) approximation to account for intraparticle diffusion and to obtain concentration profiles in the macropores and microparticles of the adsorbent. The bi-LDF approximation is applied to the simulation of glucose/fructose breakthrough curves in a fixed bed and to their separation in a simulated moving bed (SMB). Equivalence with the simple LDF model is well-established for both fixed-bed operation and the SMB mode. The flow rate constraints that define a region of separation (both product purities higher than 99.5%) for a SMB configuration are shown to be greatly affected by relatively small deviations in the macro- and microparticle time constants, which illustrate the relevance of the proposed approximation.