The separation of a multicomponent mixture in a countercurrent adsorptive separation unit is analyzed. A procedure for the optimal and robust design of the operating conditions of the unit is developed in the frame of equilibrium theory, where the adsorption equilibria are described through the constant selectivity nonstoichiometric Langmuir model, while mass-transfer resistances and axial dispersion are neglected. This allows for extending the results previously obtained for systems characterized by either the linear or stoichiometric Langmuir isotherm to cases where the adsorbates exhibit rather different loading capacities. Analogies and differences among these three equilibrium models with respect to the criteria for the choice of operating conditions of countercurrent separation units are discussed. A comparison between model predictions and experimental data assesses the reliability and accuracy of the theoretical results achieved in this work.