Experimental and modeling studies of bicarbonate forming amines for CO2 capture by NMR spectroscopy and VLE

Three tertiary diamines, N,N,N',N'-tetramethyl-1,2-ethanediamine, N,N,N,N' tetramethyl-1,3-propanediamine and N,N,N',N-tetramethy1-1,4-butanediamine, were investigated for application in post-combustion CO2 capture. The acid-base properties of the diamines were studied through measuring pH values of the acidified amine solutions to obtain reaction constants and corresponding enthalpies/entropies of the reactions. Competitive formation of monoprotonated and diprotonated diamines was observed. CO2 absorption using these diamine solutions was carded out through a continuous flow method. NMR spectroscopy was used to detect speciation of the bulk solution. Then a general evaluation of the diamines was made in comparison with the monoamines in terms of equilibrium CO2 solubility and pKa. The results demonstrated the potential of the structurally modified diamines to be promising bicarbonate forming absorbents in CO2 absorption. Finally, the equilibrium CO2 solubility of aqueous N,N,N',N'-tetramethyl-1,2-ethanediamine solution was measured at a wider range of temperatures, CO2 partial pressures and initial amine concentrations to establish a thermodynamic model. The model provided reasonable prediction of equilibrium CO2 solubility and species concentration profiles.