Carbon dioxide absorption in aqueous alkanolamine blends for biphasic solvents screening and evaluation

Using biphasic solvents for carbon dioxide (CO₂) capture is regarded as a promising technology to drastically reduce the regeneration energy. However, this technology hasn't been put into commercial application due to lack of the biphasic solvents with high stabilities and low costs. To overcome this challenge, we developed a novel type of biphasic solvent based on alkanolamine-alkanolamine blends, i.e., aqueous blends of 2-(diethylamino)-ethanol (DEEA, 50 wt%) and monoethanolamine (MEA, 25 wt%) or 2-((2-aminoethyl) amino) ethanol (AEEA, 25 wt%). The aqueous DEEA-AEEA solvent demonstrated a cyclic capacity as high as 0.64 mol CO₂/mol which is significantly higher than those of 30 wt% MEA and other reported biphasic solvents. In addition, a phase split time as short as a few minutes is obtained. The phase split dynamics were analyzed to investigate the effects of CO₂ loading, temperature, gas flow rate and height-to-diameter ratio. Quantitative ¹³C NMR analysis was then performed to reveal the reaction and phase split mechanism of CO₂ absorption in the DEEA-AEEA solvent. It indicated that the reaction of AEEA with CO₂ dominated at the beginning of CO₂ absorption and the products are formed as carbamates. Then, DEEA began reacting with CO₂ to yield bicarbonate and carbonate. The phase split mechanism indicated that the reaction products of carbamates and HCO₃⁻/CO₃²⁻ migrated to the lower phase while DEEA and unreacted AEEA migrated to the upper phase. Finally, the cyclic tests were repeated and the energy consumption was evaluated. The regeneration energy of the biphasic solvent, without process optimizations, is 2.58 GJ/tCO₂, which is 32% lower than that of the traditional 30 wt% MEA solvent.