A review of semi-empirical equilibrium models for CO2-alkanolamine-H2O solutions and their mixtures at high pressure

The knowledge of vapour-liquid equilibrium (VLE) of the carbon dioxide-alkanolamine-water system at high pressure plays a critical role in the design of natural gas sweetening systems. Semi-empirical thermodynamic models have been widely used to estimate the VLE at high pressure. However, the understanding of their correlation behaviour at high pressure is limited. Therefore, this study reviews the chronological development and analyse the performance of the two types of semi-empirical equilibrium models for the high pressure-high gas loadings. The effect of process parameters is first analysed on a theoretical basis and then modelled with use of correction factors, defining the systematic contribution of different parameters used in such models. The analysis shows that their VLE require small corrections for good correlation at high pressure, if the equilibrium constants are well-determined. Therefore, the study reports both reliable and improved values of equilibrium constants for alkanolamines, where applicable. In addition, the combined equilibrium constant based semi-empirical models are modified for estimating high pressure VLE of the said system, which were previously limited to low pressure equilibria and low loadings only. The study presents a comprehensive review of the semi-empirical equilibrium models, highlighting their key issues, challenges and future recommendations.