Thermodynamics of aqueous potassium carbonate, piperazine, and carbon dioxide

Abstract

CO 2 solubility was measured in a wetted-wall column in 0.6–3.6 molal (m) piperazine (PZ) and 2.5–6.2 m potassium ion (K +) at 40–110 °C. Piperazine speciation was determined using 1H NMR for 0.6–3.6 m piperazine (PZ) and 3.6–6.2 m potassium ion (K +) at 25–70 °C. The capacity of CO 2 in solution increases as total solute concentration increases and compares favorably with estimates for 7 m (30 wt.%) monoethanolamine (MEA). The presence of potassium in solution increases the concentration of CO 3 2−/HCO 3 − in solution, buffering the solution. The buffer reduces protonation of the free amine, but increases the amount of carbamate species. These competing effects yield a maximum fraction of reactive species at a potassium to piperazine ratio of 2:1. A rigorous thermodynamic model was developed, based on the electrolyte nonrandom two-liquid (ENRTL) theory, to describe the equilibrium behavior of the solvent. Modeling work established that the carbamate stability of piperazine and piperazine carbamate resembles primary amines and gives approximately equal values for the heats of reaction, Δ H rxn (18.3 and 16.5 kJ/mol). The p K a of piperazine carbamate is twice that of piperazine, but the Δ H rxn values are equivalent (∼−45 kJ/mol). Overall, the heat of CO 2 absorption is lowered by the formation of significant quantities of HCO 3 − in the mixed solvent and strongly depends on the relative concentrations of K + and PZ, ranging from −40 to −75 kJ/mol.