Modeling Differential Enthalpy of Absorption of CO₂ with Piperazine as a Function of Temperature

Temperature-dependent correlations for equilibrium constants (ln K) and heat of absorption (ΔH_abs) of different reactions (i.e., deprotonation, double deprotonation, carbamate formation, protonated carbamate formation, dicarbamate formation) involved in the piperazine (PZ)/CO₂/H₂O system have been calculated using computational chemistry based ln K values input to the Gibbs-Helmholtz equation. This work also presents an extensive study of gaseous phase free energy and enthalpy for different reactions using composite (G3MP2B3, G3MP2, CBS-QB3, and G4MP2) and density functional theory [B3LYP/6-311++G(d,p)] methods. The explicit solvation shell (ESS) model and SM8T solvation free energy coupled with gaseous phase density functional theory calculations give temperature-dependent reaction equilibrium constants for different reactions. Calculated individual and overall reaction equilibrium constants and enthalpies of different reactions involved in CO₂ absorption in piperazine solution are compared against experimental data, where available, in the temperature range 273.15-373 K. Postcombustion CO₂ capture (PCC) is a temperature swing absorption-desorption process. The enthalpy of the solution directly correlates with the steam requirement of the amine regeneration step. Temperature-dependent correlations for ln K and ΔH_abs calculated using computational chemistry tools can help evaluate potential PCC solvents' thermodynamics and cost-efficiency. These correlations can also be employed in thermodynamic models (e.g., e-UNIQUAC, e-NRTL) to better understand postcombustion CO₂ capture solvent chemistry.