VAPOR-LIQUID EQUILIBRIA OF BICARBONATE FORMING SOLVENTS FOR POST-COMBUSTION CO2 CAPTURE: EXPERIMENTAL AND MODELING

Abstract

Bicarbonate forming solvents have been proposed in the literature as promising solvents for capturing CO2 in flue gases due to their high CO2 loading capacities and low heats of absorption. Key solvent properties are indicated by vapor-liquid equilibria (VLE) and thus, equilibrium measurements are crucial in the search for novel absorbents. In this work, experimental VLE data of CO2 in aqueous solutions of 2-piperidineethanol (2-PE, 50 mass %), 1-(2-hydroxyethyl)pyrrolidine (12-HEPRLD, 40 mass %), and monoethanolamine (MEA, 30 mass %) were obtained (not for amine mixtures). Two different apparatuses were used to provide equilibrium measurements at 40, 60, 80, 100 and 120 oC and pressures from atmospheric pressure to 6 bar. The methods were verified by comparing the results of the bench-mark solvent MEA with values from the literature, and the experimental results showed satisfactory consistency. Based on the VLE data, soft models were created to describe the equilibrium behavior of the amine systems. The average absolute relative deviation (AARD) of the models were 20.1 % for 2-PE, 30.9 % for 12-HEPRLD, and 29.8 % for MEA. The equilibrium models made it possible to estimate cyclic capacities, temperature sensitivities of CO2 partial pressure (pCO2) and heats of absorption. 2-PE was a sterically hindered secondary amine and 12-HEPRLD a tertiary amine, and their bicarbonate forming natures were confirmed to have significantly higher capacities to bind CO2 than the primary amine MEA. It was found that the equilibrium cyclic capacities at 10 kPa pCO2 were 0.70 mol CO2/mol Am (2.41 mol CO2/kg solution) for 2-PE, 0.67 mol CO2/mol Am (2.10 mol CO2/kg solution) for 12-HEPRLD, and 0.36 mol CO2/mol Am (1.64 mol CO2/kg solution). The temperature sensitivity was higher for 12-HEPRLD than 2-PE. MEA was most temperature sensitive at high loadings, but 12-HEPRLD became more sensitive at loadings below 0.51, while 2-PE became more temperature sensitive below 0.45. The estimated heats of absorption were calculated from the equilibrium model function, and it was found to be lower for the bicarbonate forming solvents than MEA. However, indicators suggested that the estimated values from this method were uncertain and thus, the values for heats of absorption should not be emphasized.