CO₂ Absorption into Aqueous Solutions Containing 3‑Piperidinemethanol: CO₂ Mass Transfer, Stopped-Flow Kinetics, ¹H/¹³C NMR, and Vapor–Liquid Equilibrium Investigations

Abstract

Global efforts to reduce carbon dioxide emissions stemming from the combustion of fossil fuels have acknowledged and focused on the implementation of post combustion capture (PCC) technologies utilizing aqueous amine solvents to fulfill this role. The cyclic diamine solvent piperazine has received significant attention for application as a CO₂ capture solvent, predominantly for its rapid reactivity with CO₂. A thorough investigation of alternative but simpler cyclic amines incorporating a single amine group into the cyclic structure may reveal further insight into the superior kinetic performance of piperazine and the wider applicability of such cyclic solvents for PCC processes. One such example is the cyclic monoamine 3-piperidinemethanol (3-PM). To facilitate the evaluation of 3-PM as a capture solvent requires knowledge of the fundamental chemical parameters describing the kinetic and equilibrium of the reactions occurring in solutions containing CO₂ and 3-PM. Additionally, in parallel with the preceding, experimental measurements of CO₂ absorption into 3-PM solutions, including mass transfer and vapor–liquid equilibrium measurements, can be used to validate the CO₂ absorption performance in 3-PM solutions and compared to that of monoethanolamine (MEA) under similar conditions. The present study is focused in two parts on (a) determination of fundamental kinetic and equilibrium constants via the analysis of stopped-flow kinetic and quantitative equilibrium measurements via ¹H/¹³C nuclear magnetic resonance (NMR) spectroscopy and (b) experimental measurements of CO₂ absorption into 3-PM solutions via wetted wall column kinetic measurements, vapor–liquid equilibrium measurements, and corresponding physical property data including densities and viscosities of the amine solutions over a range of concentrations and CO₂ loadings. Fundamental kinetic rate constants describing the reaction of CO₂ with 3-PM are significantly faster than MEA at similar temperatures (3-PM = 32 × 10³ M^–1 s^–1, extrapolated to 40 °C from kinetic data between 15.0 and 35.0 °C; MEA = 13 × 10³ M^–1 s^–1, 40 °C). Conversely, the equilibrium constants describing the reaction between bicarbonate and amine, often termed carbamate stability constants, are significantly lower for 3-PM than MEA at similar temperatures. Overall CO₂ absorption rates in 3.0 M solutions of 3-PM and MEA, assessed in overall CO₂ mass transfer coefficients, are lower in the former case over the entire range of CO₂ loadings from 0.0 to 0.4 mol of CO₂ per mol of amine. The reduced absorption rates in the 3-PM solutions can be attributed to higher solution viscosities and thus corresponding reductions in CO₂ diffusion. CO₂ absorption and cyclic capacities in 3.0 M solutions of 3-PM and MEA were found to be significantly higher in the case of 3-PM. The larger CO₂ capacities are attributed to the lower stability 3-PM carbamate and the formation of larger amounts of bicarbonate compared to MEA. Overall, the larger CO₂ absorption capacity, cyclic capacity, and rapid kinetics with CO₂ position 3-PM as an attractive CO₂ capture solvent