Thermodynamics of CO2/2-Methylpiperazine/Water

Aqueous 2-methylpiperazine (2MPZ) is an attractive solvent for CO2 capture from coal-fired power plants. Quantitative NMR spectroscopy was used to speciate at 40 °C 8 m 2MPZ loaded with CO2. 13C NMR spectra were acquired from 0 to 0.37 mol CO2/mol alkalinity (or per amino group). 13CO2 was used for solution preparation to amplify signals from carbamate and bicarbonate. The NMR spectra show that the unhindered 2MPZ monocarbamate is the dominant carbamate species and accounts for more than 75% of the total dissolved CO2; bicarbonate and 2MPZ dicarbamate each only account for up to 10% of the total absorbed CO2. Hindered 2MPZ was not detected. This speciation and CO2 solubility were represented by an electrolyte nonrandom two liquid thermodynamic model developed through data regression. At a CO2 loading giving 1.5 kPa CO2 partial pressure at 40 °C the heat of CO2 absorption varies from 50 to 75 kJ/mol at 40?140 °C. At 40 °C the equilibrium CO2 partial pressure was 0.5 kPa at 0.265 mol CO2/mol alkalinity and 5 kPa at 0.356 mol CO2/mol alkalinity. These values give an intrinsic CO2 operating capacity of 0.81 mol/kg (2MPZ + water) compared to 0.47 for 7 m MEA and 0.79 for 8 m piperazine.