The Effect of Piperazine (PZ) on CO2 Absorption Kinetics into Aqueous Ammonia Solutions at 25.0 °C

Piperazine (PZ) has been reported as an effective rate promoter in the aqueous ammonia-based solvent process for the post combustion capture (PCC) of CO2. However, the detailed promotion effect of PZ on CO2 absorption into partially loaded ammonia solutions and the mechanism of this process are still unclear. In an effort to determine the detailed promotion effect of PZ in aqueous ammonia-based solvents, overall mass transfer coefficients (KG) describing the absorption of CO2 into aqueous PZ/NH3 solutions were determined using a wetted-wall column apparatus at 25 °C. The effect of added PZ (from 0 to 0.5 M) on the mass transfer of CO2 into 3.0 M NH3 solutions over a range of pre-loaded CO2 concentrations of 0.9 M at 25 °C are reported in this work. The fast kinetic reactions of CO2(aq) with blended solutions containing PZ/NH3 were investigated using stopped-flow spectrophotometry at 25.0 °C. Analysis of the kinetic measurements using a chemical model which incorporates the complete reaction sets of the individual amines with CO2 (i.e., NH3-CO2-H2O and PZ-CO2-H2O) resulted in good agreement with the experimental data. The contribution distribution from each reactive species was calculated based on the proposed reaction scheme of the PZ-NH3-CO2-H2O system. Results show that both the PZ/PZH+ and PZCO2− /PZCO2H pathways make contributions to the promotion of CO2 absorption into PZ promoted aqueous NH3 solutions. Importantly, the reactive piperazine mono-carbamate species, PZCO2− /PZCO2H, which is present in the CO2-loaded mixtures of PZ/NH3, plays an important role in the promotion of CO2 absorption into CO2-loaded aqueous NH3 solutions. The mass transfer simulation results reveal that there are additional reactions occurring in the gas–liquid interface and gas phase due to the volatility of NH3, which requires further improvement on the simulation model