Evaluation and Modeling of Vapor-Liquid Equilibrium and CO Absorption Enthalpies of Aqueous Designer Diamines for Post Combustion Capture Processes.

Novel absorbents with improved characteristics are required to reduce the existing cost and environmental barriers to deployment of large scale CO capture. Recently, bespoke absorbent molecules have been specifically designed for CO capture applications, and their fundamental properties and suitability for CO capture processes evaluated. From the study, two unique diamine molecules, 4-(2-hydroxyethylamino)piperidine (A4) and 1-(2-hydroxyethyl)-4-aminopiperidine (C4), were selected for further evaluation including thermodynamic characterization. The solubilities of CO in two diamine solutions with a mass fraction of 15% and 30% were measured at different temperatures (313.15-393.15 K) and CO partial pressures (up to 400 kPa) by thermostatic vapor-liquid equilibrium (VLE) stirred cell. The absorption enthalpies of reactions between diamines and CO were evaluated at different temperatures (313.15 and 333.15 K) using a CPA201 reaction calorimeter. The amine protonation constants and associated protonation enthalpies were determined by potentiometric titration. The interaction of CO with the diamine solutions was summarized and a simple mathematical model established that could make a preliminary but good prediction of the VLE and thermodynamic properties. Based on the analyses in this work, the two designer diamines A4 and C4 showed superior performance compared to amines typically used for CO capture and further research will be completed at larger scale.