Effect of Viscosity of a Deep Eutectic Solvent on CO₂ Capture Performance in an Energy-Efficient Membrane Contactor-Based Process

Greenhouse gas contributions to climate change have driven intense interest in the separation of CO₂ from wet flue gas streams. Deep eutectic solvents (DESs) are an emerging class of highly selective CO₂ absorbents. A prototypical DES, reline, is a mixture of choline chloride and urea. Reline is a thermally stable, nontoxic, and biodegradable solvent with negligible volatility and is inexpensive. We demonstrate a scalable and energy-efficient hollow fiber membrane contactor (HFMC)-based process using a green solvent for CO₂ capture. This process uses reline in HFMC to provide close interfacial interactions and contact between DES and CO₂. This approach overcomes the disadvantages associated with direct absorption in DES and could potentially be applied to a variety of solvent-based CO₂ capture methods. Commercial, low-cost polymer hollow fiber membranes were evaluated for the capture of CO₂ with reline. From a mixed gas containing N₂ and CO₂, the DES-based HFMC separated CO₂ with a purity of 97 mol %. The effect of the viscosity of reline on the CO₂ capture performance was investigated by adding water to the reline. The addition of water to reline significantly reduced its viscosity, which led to a permeate flux of 170 mmol/(m²·h) at 35 °C, 4 bar, and 60 wt % water in solvent, which was approximately 8 times higher than that of the pure reline in the membrane contactor system. In situ Fourier transform infrared spectroscopy and nuclear magnetic resonance (NMR) revealed that reline absorbs CO₂ by physical absorption without forming new chemical compounds and that CO₂ separation by reline occurs via the pressure swing mechanism. This research provides fundamental insights about green physical solvent-based separation processes and a pathway toward industrial deployment.