Energy-Efficient CO2 Capture Using Nonaqueous Absorbents of Secondary Alkanolamines with a 2-Butoxyethanol Cosolvent

Nonaqueous amine-based absorbents by replacing water with organic solvents have potential advantages of low energy penalty and reduced amine degradation for CO2 capture. An advanced nonaqueous system of 2-(butylamino)ethanol (BAE) with 2-butoxyethanol (EGBE) was proposed for low-energy consumption CO2 capture. EGBE was selected as a green cosolvent because of its superior properties over the reported organics and water. A comprehensive study on CO2 capture performance has been performed for secondary alkanolamine nonaqueous systems such as 2-(methylamino)ethanol (MAE), 2-(ethylamino)ethanol (EAE), and BAE and their aqueous counterparts. Product species in CO2-loaded solutions were identified by 13C nuclear magnetic resonance and Fourier transform infrared spectroscopy. Regeneration energy consumption was also evaluated using a modified method and compared with that of conventional aqueous monoethanolamine (MEA). Results demonstrated that the aqueous 5.0 M secondary amine absorbents had higher absorption capacity and larger cyclic capacity than their nonaqueous counterparts. The desorption efficiency for both systems was in a descending order: BAE > EAE > MAE. Surprisingly, the cyclic capacity of aqueous and nonaqueous BAE systems was about 180 and 100% higher than that of aqueous MEA, respectively. The underlying reasons were also discussed. CO2 can react with these amines, forming unstable carbamates and protonated amines in the EGBE solvent, whereas it forms bicarbonate species in addition to ionic couples in aqueous solutions. The regeneration heat duty of nonaqueous EGBE-based secondary amine absorbents was about 45–55% lower than that of aqueous MEA (3.82 MJ kg–1 CO2). In addition, the feasibility of low-temperature desorption makes the BAE/EGBE absorbent a compelling solution for energy-saving CO2 capture technology using low-grade industrial waste heat.