Nonlinear Frequency Response Analysis as a Tool for Identification of Adsorption Kinetics: Case Study-Pore-Surface Diffusion Control

In the present paper, the Nonlinear Frequency Response (NFR) analysis is applied for theoretical study of kinetics of adsorption governed by pore-surface diffusion. The concept of higher-order frequency response functions (FRFs) is used. Based on a nonlinear mathematical model for adsorption of pure gas and spherical adsorbent particles, the theoretical first-and second-order FRFs, which relate the adsorbate concentration in the particle to the surrounding pressure (F-1 (omega) and F-2 (omega,omega)), have been derived. The obtained FRFs have been simulated for different steady-state pressures and different ratios (between zero and one) of surface to pore diffusion coefficients. The results show that, unlike F-1 (omega), F-2 (omega,omega) exhibits features which unambiguously distinguish the pore-surface diffusion model from pure pore diffusion and micropore diffusion. Based on the characteristic features of F-1 (omega) and F-2 (omega,omega), a new methodology for direct estimation of the separate values of the pore and surface diffusion coefficients has been established.