Reactions of CO₂ with Aqueous Piperazine Solutions: Formation and Decomposition of Mono- and Dicarbamic Acids/Carbamates of Piperazine at 25.0 °C

Abstract

Piperazine (PZ) is widely recognized as a promising solvent for postcombustion capture (PCC) of carbon dioxide (CO(2)). In view of the highly conflicting data describing the kinetic reactions of CO(2)(aq) in piperazine solutions, the present study focuses on the identification of the chemical mechanism, specifically the kinetic pathways for CO(2)(aq) in piperazine solutions that form the mono- and dicarbamates, using the analysis of stopped-flow spectrophotometric kinetic measurements and (1)H NMR spectroscopic data at 25.0 °C. The complete set of rate and equilibrium constants for the kinetic pathways, including estimations for the protonation constants of the suite of piperazine carbamates/carbamic acids, is reported here using an extended kinetic model which incorporates all possible reactions for CO(2)(aq) in piperazine solutions. From the kinetic data determined in the present study, the reaction of CO(2)(aq) with free PZ was found to be the dominant reactive pathway. The superior reactivity of piperazine is confirmed in the kinetic rate constant determined for the formation of piperazine monocarbamic acid (k(7) = 2.43(3) × 10(4) M(-1) s(-1)), which is within the wide range of published values, making it one of the faster reacting amines. The corresponding equilibrium constant for the formation of the monocarbamic acid, K(7), markedly exceeds that of other monoamines. Kinetic and equilibrium constants for the remaining pathways indicate a minor contribution to the overall kinetics at high pH; however, these pathways may become more significant at higher CO(2) loadings and lower pH values where the concentrations of the reactive species are correspondingly higher.

CO 2 chemical absorption into aqueous solutions of piperazine: modeling of kinetics and mass transfer rate

“…18 Nevertheless, our proposed kinetic constant correlation predictions are in good agreement with some previous published values, especially those reported in (Conway et al, 2013;Liu et al, 2012;Sun et al, 2005).…”

“…At high ‫‬ ை మ ranges and consequently high CO 2 loading regions as in our work, PZ concentration would no longer remain constant in the liquid film region, i.e. its value at the gas-liquid interface remarkably differs its bulk concentration, and the P.F.O simplifying assumption breaks down (Conway et al, 2013;Derks et al, 2006;Sun et al, 2005). Therefore, in development of the proposed correlation, the real second-order reactions rates kinetics, were applied, and ܰ మ was correlated as a function of distinctive dimensionless groups of chemical absorption system, which makes it independent of the equipment sizes.…”

“…Additionally, Conway et al (Conway et al, 2013) have confirmed that the contribution of the reactions represented in Eqs. (3) and (5) are significant for PZ, which is a highly basic amine, since the concentration of hydroxide increases with amine basicity.…”

“…Liu et al (Liu et al, 2012) proposed a mass transfer model, namely the interface concentration M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 4 corrected P.F.O, to compensate deviation from P.F.O regime, due to low PZ concentration and high ‫‬ ை మ driving force, in their study. More recently, Conway et al (Conway et al, 2013) obtained kinetic rate constants values for CO 2 reactions rates, only at 298 K.…”

“…However, a relatively large discrepancy is observed between the published values for the second-order kinetic rate constant of the main reaction between CO 2 and free PZ ሺ݇ ሻ in the literature, as already mentioned in the introduction section which is usually attributed to different measurement techniques, CO 2 partial pressure and PZ concentration ranges (Bishnoi and Rochelle, 2000;Conway et al, 2013;Derks et al, 2006;Liu et al, 2012;Sun et al, 2005;Xu et al, 1992). Therefore, in the present work, it was intended to propose two new correlations for temperature dependency of ݇ and ݇ ைை ష as two Arrhenius expressions as expressions in the power-law form as follows:…”