To design cyclic separation processes by adsorption, such as pressure swing adsorption (PSA) processes, one has to simulate the start-up time in order to determine the cyclic steady state (CSS), which can be very time-consuming. A fast way to estimate the CSS reached by a PSA process is proposed by Suzuki (1985)-assuming equivalency between the PSA system and a continuous countercurrent flow system (CCF). The CCF model is improved in the present paper, taking into account the gaseous phase located in a column when the system switches from a constant pressure step to another step. By performing a parametric study, the limits of the CCF model are tested. It is shown that this model can quantitatively predict the purities and recoveries obtained at CSS for kinetically controlled separation by comparison with a detailed model of PSA. It is also shown that the CCF model can also give a good approximation of the dynamic behavior of the PSA process. Finally, the simulation of dry air separation is proposed. (c) 2005 Elsevier Ltd. All rights reserved.