CO₂ Absorption in a Dual-Functionalized Ionic Liquid-Blended N-Methyldiethanolamine Aqueous System: A Thermodynamics Study

Currently, the applicability of various aqueous absorbent blends is being researched for the CO2 absorption process. Thermodynamic studies and equilibrium thermodynamics modeling of blend absorbents are important aspects of the absorption process optimization. This work is aimed at the synthesis of two categories of dual-functionalized ionic liquids (DFILs), namely, T-Im and D-Im, and subsequently combine them with a low-reactive aqueous methyldiethanolamine (MDEA) and finally assess the solubility of CO2 in these blends across a broad spectrum of pressures (210–710 kPa) and temperatures (303–328 K). The Gibbs free energy change showed that CO2 absorption in the aqueous T-Im/MDEA blend was feasible at lower temperatures. The analysis of 13C NMR spectra suggested that the D-Im/MDEA absorbent had approximately 24% more carbamate formation compared to T-Im/MDEA. The results suggest that the T-Im outperforms the D-Im promoter, and the CO2 solubility increases by approximately 23.10% in the aqueous MDEA absorbent at high temperatures (328 K). Further, the modified Kent–Eisenberg thermodynamic concept was employed to develop a temperature- and pressure-dependent semiempirical model for the equilibrium absorption constant for the CO2/DFIL/MDEA/H2O system. The accuracy of the developed model, in terms of % Average Absolute Deviation (%AAD), was 1.88 and 1.64% for the T-Im/MDEA absorbent and the D-Im/MDEA absorbent, respectively. The entropy change was found to be more negative for D-Im/MDEA (−165.28 J/mol K) than for the T-Im/MDEA absorbent (−159.76 J/mol K). The heat of absorption for T-Im/MDEA and D-Im/MDEA was found to be 50.32 and 52.06 kJ/mol, respectively. A comparative study suggests that DFIL-blended MDEA offers a lower heat of absorption than other commonly used absorbents, such as MEA, PZ/MDEA, and TETA/MDEA.