The present study has established the direct pseudo–first-order reaction kinetics of different aqueous-based single and blended amines over the temperature range of 298.15-313.15 K using stopped-flow techniques. The single amines include one primary amine (monoethanolamine, MEA), two secondary amines (diethanolamine, DEA and 2-ethyl(amino)ethanol, 2EAE), four tertiary amines (N-methyldiethanolamine, MDEA, 1-dimethylamino-2-propanol, 1DMA2P, 3-dimethylamino-1-propanol, 3DMA1P, and 2-dimethylaminoethanol, 2DMAE), one sterically hindered amine (2-amino-2-methyl-1-propanol, AMP), and one cyclic diamine (piperazine, PZ). The blend systems used are MEA/PZ, DEA/PZ, MDEA/PZ, AMP/PZ, MEA/AMP, MDEA/2EAE, 1DMA2P/2EAE, 3DMA1P/2EAE, and 2DMAE/2EAE. Different reaction mechanisms for the reaction of CO₂ with aqueous solutions of amines, such as base-catalysis, zwitterion, termolecular, hybrid of zwitterion, hybrid of base-catalysis-zwitterion, and hybrid of base-catalysis-termolecular reaction mechanisms, are used to correlate the experimental data. For the single amines, the zwitterion mechanism is well suited to fit the experimental data of primary, secondary, sterically hindered and cyclic amines with an absolute average deviation (AAD%) less than 5%. The base-catalysis mechanism fits the experimental data of all the tertiary amines well with an AAD less than 5%. For the blends, the hybrid of zwitterion mechanism fits the experimental data of MEA/PZ, DEA/PZ, AMP/PZ, and MEA/AMP well with an AAD less than 5%, whereas the hybrid of base-catalysis-zwitterion mechanism fits the experimental data of MDEA/PZ, MDEA/2EAE, 1DMA2P/2EAE, 3DMA1P/2EAE, and 2DMAE/2EAE well with an AAD less than 5%.

This article presents a thermodynamic and kinetic characterization of CO₂ absorption by 30 aqueous amine solutions. A solvent screening setup (S.S.S.) was used to find the CO₂ loading (α) for 30 different aqueous amine solutions (30 wt%) at a pressure of 1 bar with feed gas containing 15 vol% CO₂ and 85 vol% N₂ at 313.15 K to provide reliable absorber parameters. The structures of various amines (linear, non-linear, polyamines, sterically hindered, etc.) were tested and the S.S.S. results showed that hexamethylenediamine (HMD) has higher CO₂ loading at 1.35 moles of CO₂/mole of amine, and triethanolamine (TEA) has the lowest at 0.39 mole of CO₂/mole of amine. The heat of absorption indicates that MDEA has the lowest and HMD has the highest at −52.51 kJ/mole of CO₂ and −98.39 kJ/mole of CO₂, respectively. The combined data for the CO₂ loading and the absorption heat generated 6 amines that have good properties for the post-combustion CO₂ capture process in comparison with that of MEA. These amines are made up of one secondary amine (2-ethylaminoethanol, 2EAE) and 5 tertiary amines (N-methyldiethanolamine, MDEA, 1-dimethylamino-2-propanol, 1DMA2P, 2-dimethylaminoethanol, 2DMAE, 3-dimethylamino-1-propanol, 3DMA1P and N,N,N′,N′-tetramethyl-1,3-propanediamine, TMPDA). In comparison with the amine reference MEA (ΔH = −85.13 kJ/mole of CO₂ and α = 0.58 mole CO₂/mole of amine), the 6 amines have heats of absorption that are between −68.95 kJ/mole of CO₂ and −52.51 kJ/mole of CO₂, and their CO₂ loading is between 0.52 and 1.16 mole of CO₂/mole amine. The third important parameter, namely the reaction kinetics between aqueous amine solutions and CO₂, was studied by using stopped-flow equipment to measure the pseudo-first order reaction (k₀, s⁻¹) at different amine concentrations. A determination of the second order rate constants (k₂, m³ mole⁻¹ s⁻¹) at 298.15, 303.15, 308.15 and 313.15 K was performed by using the proposed reaction mechanisms. The base catalysis mechanism was used to correlate the experimental data of the tertiary amines and the zwitterion mechanism was used to correlate the experimental data of the primary/secondary amines. The kinetics results show that 2EAE, a secondary amine, was very high in comparison with MDEA, 1DMA2P, 2DMAE, 3DMA1P and TMPDA, all of which are tertiary amines that are very close to MEA, which is the reference case. This result shows that 2EAE is a good candidate for CO₂ capture as an alternative to MEA because it has good CO₂ absorption, a low heat of absorption and high kinetic reaction with CO₂. Moreover, the outcomes for the thermodynamic and kinetic properties indicate that the following 5 amine blends have emerged from this study: 2EAE/MDEA, 2EAE/1DMA2P, 2EAE/2DMAE, 2EAE/3DMA1P and 2EAE/TMPDA.