NMR Analysis of Various Amine-CO2-H2O Systems Interactions for Studies of Vapour-Liquid Equilibrium and Catalyst Aided and Unaided Solvent Regeneration

Abstract

The removal of CO2 by chemical absorption and regeneration has been researched for decades. CO2-amine interactions in solvent chemistry have been studied via the Vapour-Liquid Equilibrium (VLE) model. Databases for VLE models have been generated either through software simulators or conventional pH meter + Nuclear magnetic resonance (NMR) experiments. Studies have focused mostly on single amines solutions (MEA, MDEA, BEA, AMP, etc.) below 40 degrees C. limitations have restricted research in VLE modeling. Solvent regeneration or CO2 stripping studies have also been of research interest for years. However, the heat duty is very high, even with stripping performed at reboiler temperations around 120-140 degrees C. Two methods, the conventional (pH + NMR) method and a newly developed NMR calibration method presented, were used to obtain and compare the ion speciation data of VLE model in terms of both accuracy and a wider range of operating conditions. The novel NMR method involved both 1D and 2D NMR methods. The 1D NMR analyses include 13C chemical shifts test samples and area intebration tests of the specific 13C NMR peaks for carbanate and bicarbonate/carbonate. The 2D NMR involved 1H ve 1H and 1H vs 13C tests to detect intermolecular interactions. NMR analyses of single and blended solvents showed typical but more accurate concentrations of major ions that changed with CO2 loading. The application of NMR analysis widened the operating range to quaternary amine systems and high temperature regions, for which the ion speciation of the blended amine systems could be completed elucidated and quantified of various MEA-DEAB-CO2-H2O solutions in 24 degrees C, from 40 to 70 degrees C. In solvent regeneration, the apparatus and procedures were used for amine regeneration studies with single and blended amine systems + two specific solid acid catalysts, AL2O3 and H-ZSM-5. The primary amine was not energy efficient because of few HCO3 ions to reduce the energy requirement for MEAH+ deprotonation. The effect of adding a tertiary amine (in blended amine) facilitated the heat duty reduction because of adding two components, R3N and HCO3 to cut the free energy gaps. DEAB generated more HCO3 with tertiary amines resulting in a much lower heat duty, even though MDEA is less energy demanding than DEAB as it is less basic as per the energy diagram. The addition of solid acid catalysts during regeneration results in a further reduction of the energy cost because strong acidic catalysts facilitated the release of medium acidic oxides CO2. Al2O3 (Lewis acid) was more effective in the lean region because it replaced the role of HCO2 (negligible < 0.25); while H-ZSM-5 (Bronsted acid) was effective throughout the loading range within (0.15-0.50) because it donated protons. The combination of a blended amine (e.g., MEA-DEAB-CO2-H2O) with solid acid catalysts (especially H-ZSM-5) for regeneration was able to cut the energy cost by up to 60% compared to MEA even at 90 degrees C, which implies that hot water (95-98 degrees C) can be used as the heat source instead of steam, reducing plant disruption.