Nonlinear Frequency Response measurements of gas adsorption equilibrium and kinetics: New apparatus and experimental verification

• New apparatus for NFR measurements has been designed and constructed.
• Second-order FRF, F2(ω,ω), was obtained experimentally.
• F2(ω,ω) agrees with theoretical one for nonisothetmal macropore diffusion.
• Diffusion and heat transfer coefficients and heat of adsorption were estimated.

A new apparatus for experimental validation of the Nonlinear Frequency Response (NFR) method for investigation of gas adsorption is presented. The apparatus has been designed as a batch adsorber with sinusoidal modulation of the volume, with an amplitude that can be varied. Fast and accurate measurements of pressure, gas temperature and particle temperature responses are employed. The measurement routine and data analysis are demonstrated by CO2 adsorption on zeolite 5A particles of two different diameters. The responses for input amplitudes of 3% and 7% of the volume change are measured. The concept of blank FRFs is introduced as a new way to incorporate the results of blank experiments for elimination of spurious effects. The first- and second-order FRFs which relate the adsorbed quantity to the pressure (F1(ω) and F2(ω,ω)) are calculated based on the measured pressure responses. The first-order FRF which relates the particle temperature to the pressure (H1(ω)) is calculated from the particle temperature responses. The characteristics of F1(ω), F2(ω,ω) and H1(ω) for spherical beads of 1.7 mm agree with the corresponding theoretical ones for nonisothermal macropore diffusion model. The equilibrium and kinetic parameters are estimated from these functions. The results of this study confirm the applicability of NFR method for characterization of gas adsorption and show significance of the extension of analysis to the nonlinear range.