(801c) Modeling of CO2 Absorption Into Aqueous Piperazine/Aminoethylpiperazine

A novel blend of piperazine (PZ) with aminoethylpiperazine (AEP) has been proposed as a superior solvent for CO2 capture from coal-fired flue gas.  Blending with AEP can remediate the precipitation issue of concentrated PZ while maintaining its high CO2 absorption rates, and high resistance to degradation.  A rigorous model accurately predicting thermodynamic and kinetic properties for CO2 absorption in PZ-AEP-H2O-CO2 aqueous amine solutions has been developed in Aspen Plus® using the electrolyte-Nonrandom Two-Liquid (e-NRTL) activity coefficient model.  Unavailable thermodynamic parameters of AEP-related species, including AEP, AEPH+, AEP(H+)2, AEPCOO-, AEP(COO-)2, H+AEPCOO-, (H+)2AEP COO- and H+AEP(COO-)2,  were estimated by built-in models in Aspen Plus®, or by referring to corresponding PZ-related species as the starting point, and then sequential regressions were applied using related experimental data.  The amine volatility data and heat capacity data for unloaded AEP solutions were regressed to determine Henry’s constants and NRTL parameters for the AEP/H2O pair.  The vapor-liquid equilibrium (VLE) data for AEP-H2O-CO2 were regressed to determine the standard-state properties (free energy of formation and heat of formation) and e-NRTL parameters of the AEP/AEP ion/H2O pairs.  The VLE data for PZ-AEP-H2O-CO2 were used to identify the e-NRTL interaction parameters for the PZ-AEP pairs.  The prediction of speciation by the model was validated by NMR measurement.  The heat capacity and heat of absorption for PZ-AEP-H2O-CO2 were also predicted using this model.  At 40 °C the heat of absorption of 6 m AEP is about 90–55 kJ/mol CO2 at operation loading range (0.27–0.33) and the heat of absorption of 5 m PZ/2 m AEP is about 78–70 kJ/mol CO2 at operation loading range (0.3–0.38).  A steep decrease of heat of absorption was found at loading near 0.3 for 6 m AEP and at loading near 0.35 for 5 m PZ/2 m AEP.