Use of higher-order frequency response functions for identification of nonlinear adsorption kinetics: Single mechanisms under isothermal conditions

Abstract

The concept of higher-order frequency response functions (FRFs), which is based on Volterra series expansion of nonlinear functions, is used for analysis of kinetics of nonlinear adsorption systems. Four different kinetic mechanisms: Langmuir kinetics, film resistance control, micropore diffusion control and pore-surface diffusion control were analyzed and the results were compared. It was shown that, contrary to the linear frequency response characteristic functions, the higher-order FRFs corresponding to different mechanisms differ in shape. This result offers great potential for the identification of the adsorption-diffusion mechanism governing the process. It is shown that the second order FRFs give sufficient information for distinguishing different mechanisms.