Experimental and theoretical assessment of CO2 capture by adsorption on clinoptilolite

Abstract

Clinoptilolite is a low-cost and abundant natural zeolite with a high CO2 selectivity, which is abundant in many regions around the world, but with limited industrial use. By combining experimental and theoretical (molecular simulation) approaches, the present work aims at contributing with fundamental data to assist the design of adsorption processes based on this zeolite. A sample of naturally occurring (raw) clinoptilolite was characterized by TGA, XRD, XRF, N2 adsorption isotherms at −196 °C and CO2 at 0 °C. Single-component (CO2, N2, CH4 and O2) and binary (CO2/CH4 and CO2/N2) equilibrium isotherms were measured gravimetrically. Column dynamics experimental data were obtained in order to validate a fixed-bed model, which was then used to evaluate the performance of separation processes. Molecular simulation (MS) was used to build a predictive zeolite model validated with experimental data for CO2, N2, CH4 and O2 able to investigate adsorption of mixtures and gas diffusion. TGA results demonstrated structural stability up to 600 °C with the X-ray diffraction pattern revealing clinoptilolite as the main phase, together with cristobalite and other silicon phases as contaminants. The textural properties obtained from N2 adsorption isotherms at −196 °C of the studied sample were comparable to those reported in the literature, though inaccurate for the actual clinoptilolite structure, which poses significant diffusional resistance for N2 at such temperature. IAST combined with Sips model was used to predict the behavior of mixtures. The proposed clinoptilolite MS model was used to verify the suitability of IAST in predicting mixture equilibrium on clinoptilolite and to predict the selectivity of CO2 over O2. The MS model was able to assess the equilibrium effects in the adsorption of the CH4-N2 mixture and to confirm the relationship between the diffusivity of gases and the positioning and nature of cations in the structure. The fixed-bed model highlighted the different performances of the raw and exchanged clinoptilolite samples, including the theoretical Na-clinoptilolite crystal predicted by the MS model. Together with experimental data, the MS model can be used to aid the design of separation processes of such gas stream with multiple components using clinoptilolite.