Thermodynamics of CO2/2-Methylpiperazine/Water

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