From CO ₂ Solubility to the Carbon Capture Process: Thermodynamic Modeling, Electrolyte Speciation, Phase Behavior, Solidification, and Regeneration Energy in the CO ₂ –NH ₃ –H ₂ O System

This study investigates and validates a thermodynamic model for aqueous ammonia (NH₃) in postcombustion CO₂ capture, focusing on both vapor-liquid equilibrium (VLE) and NH₄HCO₃ precipitation. Experimental VLE data at 4-8 wt % NH₃ (40-80 °C) and existing literature data sets were used to refine an electrolyte nonrandom two-liquid (e-NRTL) model. The results highlight that lower NH₃ concentrations (under 7 wt %) eliminate solid precipitation (solidification) risk but may raise regeneration energy, while higher concentrations provide reduced circulation flow rates yet risk solid formation at high CO₂ loadings. By applying a simplified regeneration energy analysis, we illustrate how stripper temperature, stripper pressure, and NH₃ concentration influence the components of regeneration energy: reaction heat, sensible heat, and latent heat. For a 10 wt % NH₃ solution, an optimal stripper temperature of about 130.0 °C and total pressure of 800 kPa are identified to minimize the total reboiler duty (2.90 GJ/tCO₂). However, a further pressure increase reduces reaction and latent heat but simultaneously boosts lean loading, raising the sensible heat requirement. Overall, the thermodynamic model and parametric study provide operational strategies to reduce CO₂ capture costs, highlighting temperature control and NH₃ concentration as dominant factors.