The industrial-scale adsorptive separation of p-xylene from a mixture of C-8 aromatics in a four-section simulated moving bed (SMB) unit is analyzed through simulation, In the order to describe the behavior of the SMB unit by means of a mathematical model, two approaches were used: the true moving bed (TMB) approach and the SMB approach. Both approaches assume a constant selectivity nonstoichiometric Langmuir isotherm, an axial dispersion for the fluid flow, and a linear driving force for the intraparticle mass transfer. The TMB and SMB model predictions of steady-state performance of the SMB unit are very close. Therefore, the TMB model was selected to study the effect of the switching time period, adsorbent deactivation, and mass-transfer resistances on the process performance. The adsorbent deactivation which occurs during its lifetime will influence the SMB performance; to keep the p-xylene purity and acceptable recovery, the switching time should be decreased. The TMB package is a useful tool for fast visualization of the SMB process behavior under particular operating conditions, The novel "separation volume" methodology is used to find the operating conditions of the SMB unit in the presence of mass-transfer resistances.