Identification of adsorption kinetics with dual mechanisms by nonlinear frequency response method

Abstract

A characteristic example of the inability of the classical linear frequency response (FR) method to identify the correct kinetic mechanism is adsorption of n-hexane on silicalite-1. The linear FR has resulted in bimodal FR characteristic functions which fit equally well three different mechanisms, namely, nonisothermal micropore diffusion, two independent isothermal diffusion processes, and isothermal diffusion-rearrangement process. In this study, it is shown that the highest order FR functions obtained from the nonlinear FR can be used to discriminate among these three mechanisms. Starting from the nonlinear models, theoretical expressions are derived for the first and second order FR functions corresponding to these three mechanisms and it is shown that different shapes of the second order FR functions are obtained for each mechanisms. This will enable identification of the actual mechanisms form nonlinear FR data.