Comparison of the CO₂ Absorption Characteristics of Aqueous Solutions of Diamines: Absorption Capacity, Specific Heat Capacity, and Heat of Absorption

Aqueous amine solutions have been widely used for the absorption of carbon dioxide (CO₂) from the gas mixtures. An understanding of the physical and chemical properties of aqueous amine solutions is important for the successful design and operation of CO₂ absorption processes. Particularly, the absorption capacity, absorption rate, and heat of absorption of CO₂ are major factors that affect the CO₂ absorption and stripping performance. A comparison study of the aqueous piperazine (PZ), 2-methylpiperazine (2-MPZ), homopiperazine (HomoPZ), and hexamethylenediamine (HMDA) solutions was conducted in this study. Absorption capacities and heats of absorption of these diamine solutions were measured using a semibatch-type reactor and a differential reaction calorimeter (DRC). The species distributions of the absorbents were investigated using a nuclear magnetic resonance spectroscopy (NMR), and the CO₂ absorption mechanism was also discussed. Aqueous PZ and PZ derivative solutions (2-MPZ and HomoPZ) displayed excellent characteristics as CO₂ absorbents. Aqueous 10 wt % PZ and PZ derivative solutions had higher absorption capacities and lower heats of absorption than that of aqueous 10 wt % monoethanolamine (MEA) at 313 K (−ΔH_abs of the CO₂-saturated PZ, 2-MPZ, HomoPZ, and MEA solutions: 62, 58, 68, and 80 kJ/mol CO₂).