Kinetic model for adsorption and desorption of H2O and CO2 on hydrotalcite-based adsorbents

A comprehensive kinetic model was developed for the description of the sorption behavior of CO₂, H₂O and their interactions on potassium-promoted hydrotalcite-based sorbents. An Elovich-type equation has been used to describe the interactions of gas molecules with heterogeneous surface sites on the sorbent, accounting for the change in activation energy of desorption as a function of the surface coverage. The ability of the sorbent to desorb more CO₂ while adsorbing H₂O and vice versa was modeled with an additional adsorption site, assuming equimolar exchange between CO₂ and H₂O. A Freundlich isotherm was used to describe the change in adsorption capacity of CO₂ and H₂O as a function of the partial pressure of CO₂ and H₂O respectively, while the sorption capacity of the exchange site depends on both the CO₂ and H₂O partial pressures and could be described with a Freundlich type dependency for both CO₂ and H₂O. A good description of the desorption kinetics of both CO₂ and H₂O measured by extensive thermogravimetric analysis experiments has been obtained. The model is shown to adequately describe the complex sorption behavior at various experimental conditions between 300 and 500 °C and different partial pressures of the sorbate species CO₂ and H₂O and their mixtures. This is the first time that a kinetic model has been developed that can describe the adsorption and desorption kinetics of both CO₂ and H₂O including their complex interactions.