Phase Separation Behavior of an Activated Methyldiethanolamine-Based Biphasic Solvent for CO2 Capture

A CO2 phase change absorbent (CPCA) is of interest because it has the potential to reduce energy consumption for CO2 capture. Controllable phase separation behavior was expected in the development and application of a CPCA. In this study, the effects of CO2 loading under different activators and physical solvents on the phase separation behavior of an activated methyldiethanolamine (MDEA)-based CPCA were investigated. By adjustment of the CO2 loading, controllable phase separation behavior was achieved and the absorbent comprising an activator with more −NH– groups and a physical solvent with low ET(30) was favored to exhibit phase separation. Of special interest was that, with the increasing of CO2 loading, phase behaviors, from the liquid phase to solid–liquid phase and then to the liquid phase, were observed for CPCA comprising MDEA/piperazine (PZ)/N-methyl-2-pyrrolidone (NMP), which was different from all of the other known CPCAs. The solid compound was identified as PZ carbamate by X-ray diffraction (XRD), Fourier transform infrared (FTIR), and elemental analysis, and the liquid-phase samples were detected by 13C nuclear magnetic resonance (NMR). The phase change mechanism of this special phenomenon was proposed.