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 degrees 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.