CO2 adsorption on diatomaceous earth modified with cetyltrimethylammonium bromide and functionalized with tetraethylenepentamine: Optimization and kinetics

Abstract

The carbon dioxide (CO2) adsorbent diatomaceous earth (DE) was modified with cetyltrimethylammonium bromide (CTAB) and functionalized with varying levels of tetraethylenepentamine (TEPA). The CO2 absorption at atmospheric pressure was optimized by varying the TEPA-loading level (0–40% (w/w)), operating temperature (40–80 °C) and water vapor concentration (0–16% (v/v)) in a 10% (v/v) CO2 feed stream in helium balance using a full 23 factorial design. The TEPA/CTAB-DE adsorbents were characterized by X-ray diffractometry, Fourier transform infrared spectrometry and thermogravimetric analyses. The CO2 adsorption capacity increased as each of these three factors increased. The TEPA loading level-water concentration interaction had a positive influence on the CO2 adsorption while the operating temperature–water concentration interaction was antagonistic. The optimal condition for CO2 adsorption on 40%TEPA/CTAB-DE, evaluated via a factorial design response surface method (RSM), was a temperature of 58–68 °C and a water vapor concentration of 9.5–14% (v/v), with a maximum CO2 adsorption capacity of 149.4 mg g−1 at 63.5 °C and 12% (v/v) water vapor concentration in the feed. Validation and sensitivity tests revealed that the estimated CO2 adsorption capacity was within ±4% of the experimental values, suggesting that the RSM model was satisfied and acceptable. From three kinetic models (pseudo-first-order, pseudo-second-order model and Avrami's equation), assessed using an error function (Err) and the coefficient of determination (R2), Avrami's equation was the most appropriate to describe the kinetics of CO2 adsorption on the 40%TEPA/CTAB-DE adsorbent and suggested that more than one reaction pathway occurred in the CO2 adsorption.