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

Abstract

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 (F1(ω) and F2(ω,ω)), 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 F1(ω), F2(ω,ω) exhibits features which unambiguously distinguish the pore-surface diffusion model from pure pore diffusion and micropore diffusion. Based on the characteristic features of F1(ω) and F2(ω,ω), a new methodology for direct estimation of the separate values of the pore and surface diffusion coefficients has been established.

Identification of mass transfer resistances in microporous materials using frequency response methods

“…In their later theoretical study of adsorption governed by parallel macropore and micropore diffusion, they found the second-order FRFs exhibit bimodal characteristics that reflect the dynamics of the parallel macropore and micropore diffusion processes. In the combination of second-order FRFs, NFR offers the potential to estimate the values of the macropore and micropore diffusion coefficients, both of practical importance [102]. This suggests the potential of NFR to tackle some of the difficulties of interpreting FR data, while allowing for the simultaneous determination of multiple dynamic rates with shorter experimental times, although the complexity of this analysis is significantly increased.…”

Determination of Pore and Surface Diffusivities from Single Decay Curve in CSBR Based on Parallel Diffusion Model

“…Danica et al applied the high-order frequency response function obtained by nonlinear frequency response analysis to pore diffusion analysis [26]. They successfully applied the pioneering research of the frequency response analysis performed by Petkovska et al [27] to separately estimate the pore and surface diffusion coefficients.…”