Single-component and binary H2O and CO2 co-adsorption isotherm model on amine-functionalised Mg-Al mixed metal oxides

Abstract

Development of amine-functionalised CO₂ adsorbents for negative emissions is a popular research topic in the field of direct air capture (DAC). While most studies aim to improve the adsorption capacities of DAC adsorbents, it is imperative to accurately model the DAC process to understand its roles and reduce operating costs. To this end, a comprehensive understanding and systematic modelling of the adsorption behaviour of amine-functionalised materials is essential. This includes examining the effect of H₂O on CO₂ adsorption under air conditions and desorption by steam purging. In this study, a fundamental analysis of single-component and binary H₂O and CO₂ adsorption by amine-functionalised Mg-Al mixed metal oxides (MMOs) was performed. Single-component H₂O and CO₂ adsorption experimental data were obtained using Guggenheim Anderson De Boer and modified Sips models, respectively. To fit the CO₂ uptake at different temperatures (25–75 °C), CO₂ isotherm models take into account both thermodynamic and diffusive factors. Subsequently, a novel mechanistic H₂O and CO₂ co-adsorption isotherm model is developed and calibrated with the breakthrough experiments. The mechanistic co-adsorption isotherm model captured the improvement in the equilibrium CO₂ capacity in the presence of H₂O. Moreover, the co-adsorption model considers the synergistic effects of H₂O and heat. Overall, the proposed isotherm models are expected to be useful in modelling DAC processes based on novel amine-functionalised adsorbents under complex conditions and ultimately guiding DAC process design and optimisation.