Separation efficiency in the cyclic steady state for periodic countercurrent adsorption

Abstract

Using an equilibrium dynamics model and one taking into account internal diffusion resistance, the separation efficiency is calculated for systems obeying the Langmuir isotherm in periodic countercurrent adsorption. The equilibrium and kinetic parameters, the cycle time and the number of columns are varied over ranges considered to be reasonable for technical applications. If other parameters are constant, there is an optimum cycle time yielding maximum separation efficiency in all cases. Maximum separation efficiency depends more strongly on the curvature of the isotherm than on the kinetic parameter or the number of columns in series. All results were evaluated in the cyclic steady state, taking into account the desorption levels achieved within the finite cycle times available to ensure continuous flow. In general, the benefits of periodic counter-current operation are less than calculated by previous authors, who assumed that a column of completely regenerated adsorbent is available whenever needed. Single-column reverse-flow desorption appears to be an adequate alternative to periodic counter-current operation under most conditions. Rough approximations indicate the advantages of thermal-swing or displacing-agent desorption and demonstrate that the desorption step is the decisive phase in modelling adsorber performance.