Carbon Dioxide Capturing in a Deep Eutectic Solvent (DES) of Choline Chloride/Methyldiethanolamine (MDEA) and a DES-Water System at High Pressure–Experimental Study and Thermodynamic Modeling Using Perturbed-Chain Polar Statistical Associating Fluid Theory (PC–PSAFT) Equation of State

Abstract

Deep eutectic solvents (DESs) have emerged as promising alternatives to conventional alkanolamines for CO2 capture because of their low volatility, tunable molecular interactions, and ease of preparation. This study investigates CO2 solubility in a ChCl/MDEA (1:6) DES and its aqueous mixtures (5, 10, and 15 wt % water) at 323.15–343.15 K and pressures up to 5000 kPa. Water addition markedly enhances CO2 uptake; for example, at 323.15 K and a loading of approximately 0.15 mol CO2/mol DES, the equilibrium pressure decreases from 937 kPa for the pure DES to 666 kPa with 10 wt % water, reflecting reduced viscosity and disrupted hydrogen-bond networks. Increasing temperature from 323.15 to 343.15 K reduces CO2 loading by 25–30% at a given pressure, confirming physical-absorption-dominated behavior. The PC–PSAFT equation of state, which incorporates explicit dipole–quadrupole and quadrupole–quadrupole interactions, accurately reproduces all experimental vapor–liquid equilibrium data for the pure DES at 323.15 K, with an overall AARD of 5.7% (RMSE = 48.2 kPa, R2 = 0.994). These results establish hydrated ChCl/MDEA DESs as energy-efficient CO2 capture solvents, offering negligible vapor pressure and significantly lower latent heat requirements compared to conventional aqueous amine systems.