Thermodynamics of aqueous piperazine/aminoethylpiperazine for CO₂ capture

Aqueous piperazine (PZ) blended with N-(2-aminoethyl) piperazine (AEP) is an attractive solvent for CO₂ capture from coal-fired power plants. Blending PZ with AEP can remediate the precipitation issue of concentrated PZ while maintaining its high CO₂ absorption rate, and high resistance to degradation. 5 m PZ/2 m AEP also shows a milder nitrosamine issue than concentrated piperazine. A rigorous thermodynamic model was developed in Aspen Plus® to predict properties of PZ/AEP/H₂O/CO₂, using the electrolyte-Nonrandom Two-Liquid (eNRTL) activity coefficient model. A sequential regression was performed to represent CO₂ solubility, speciation, and amine volatility data over operationally significant loading and temperature ranges. The model predicts a CO₂ cyclic capacity of 0.78 mol/kg (PZ + AEP + water) for 5 m PZ/2 m AEP, compared to 0.50 mol/kg for 7 m MEA and 0.86 mol/kg for 8 m PZ. The predicted heat of absorption is 75 to 80 kJ/mol CO₂ at the operating loading range (0.290-0.371 mol CO₂/mol alkalinity). Although 5 m PZ/2 m AEP has a slightly lower CO₂ capacity than 8 m piperazine, its higher heat of absorption may offset the negative effect on energy consumption. Speciation for PZ/AEP/H₂O at various CO₂ loading and temperature was also predicted, from which behavior of CO₂ in the amine system was proposed.