Alcohol Regulated Phase Change Absorbent for Efficient Carbon Dioxide Capture: Mechanism and Energy Consumption

Abstract

Phase change absorbent based on amine chemical absorption for CO2 capture with energy saving potential, but generally suffer from difficulties in CO2 regeneration. As a protic reagent with a low dielectric constant, alcohol can provide free protons for the rich phase of absorbent, reduce the basic strength of the solution, that is expected to promote the regeneration of acid gas CO2. In this study, N-aminoethylpiperazine (AEP)/sulfolane/H2O was represented as the liquid-liquid phase change absorbent and alcohol was used as the regulator. First, by constructing suitable ion pair models, the solvent effect of the CO2 products in different alcohol solutions was simulated. The results showed that these ion pair products reached the maximum solvation free energy (ΔEsolvation) in the rich phase containing ethanol (EtOH); the desorption experiment results showed that the maximum regeneration rate after adding EtOH reached 0.00763 mol/min, which was much higher than 0.004 mol/min without adding EtOH, confirming that EtOH was the most suitable regulator. Through the quantum chemical (QC) calculations and 13C NMR characterization, it was found that with the addition of EtOH, the product AEP-carbamate (AEPCOO-) was partially converted into a new product ethyl carbonate (C2H5OCOO-), which reduced the number of H bonds and enhanced the regeneration reactivity. In addition, the decomposition paths of different CO2 products were simulated visually, the snapshots of transition states (TS) were searched, and the activation energy (ΔEact) of every reaction was calculated. It was found that the ΔEact of decomposition of C2H5OCOO- (9.465 kJ/mol) was lower than that of the AEPCOO- (26.163 kJ/mol), implying that CO2 was more likely to be released when C2H5OCOO- was regenerated. Finally, the regeneration energy consumption of the alcohol-regulated absorbent was estimated to be only 1.92 GJ/ton CO2, which had excellent energy saving potential.

Keywords: Biphasic solvent, Organic alcohols activator, Regenerability, Energy consumption