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

Kai Coenen,Fausto Gallucci,Emiel Hensen,Martin Van Sint Annaland

doi:10.1016/j.cej.2018.08.175

Abstract

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.

Highlights

Sorption-enhanced water-gas shift is a promising process based on in-situ CO2 capture during the water-gas shift reaction
We have developed a kinetic model to describe the adsorption and desorption kinetics of CO2 and H2O on a potassium-promoted hydrotalcite-based sorbent at elevated temperatures between 300 and 500 °C, including their complex mutual interactions, on the basis of an extensive TGA study
The model includes three adsorption sites, where two sites model the weaker chemisorption of H2O and CO2, which can be regenerated with N2, whereas a third site accounts for stronger bond CO2 and H2O on the adsorbent, which cannot be regenerated with N2 and where the adsorption of CO2 is accompanied with the desorption of H2O and vice versa

Summary

Introduction

Sorption-enhanced water-gas shift is a promising process based on in-situ CO2 capture during the water-gas shift reaction. Various authors [8,9,10,11,12] have tried to explain the complex adsorption behavior on hydrotalcite-based adsorbents by proposing different chemical phases and reaction pathways. Desorption rate constant on site A (H2O)(s−1) desorption rate constant on site B (CO2)(s−1) Freundlich constant for site A (mol kg−1). Adsorption rate constant on site A (H2O) (s−1) Freundlich constant for site B (mo kg−1). KB,Bulk,ads reaction rate constant for formation of bulk carbonates (s−0.5). KB,Bulk,dis distribution coefficient for formation of bulk carbonates (mol bar−1 kg−1). KC,ads adsorption rate constant on site C (bar−1s−1) kC,desCO2 desorption rate constant on site C (CO2) (s−1) kC,desH2O desorption rate constant on site C (H2O) (s−1)

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Conclusion