Amine solvent development for carbon dioxide capture
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36 novel aqueous piperazine (PZ)-based amine blends for CO2 capture from flue gas were screened for their thermal degradation, amine volatility, CO2 cyclic capacity, and CO2 absorption rate at normal operating conditions. These amines include 7 imidazoles, 8 cyclic and long-chain diamines, 12 tertiary amines, 4 hindered amines, 3 hindered and tertiary amino acids, and 2 ether amines that were selected based on known amine structure-property relationships and their potential for industrial application. 18 thermally stable PZ-based amine blends were identified with proposed degradation mechanisms. 14 novel tertiary and hindered amines were found to have a lower volatility than 2-amino-2-methyl-1-propanol (AMP). A group contribution model to predict amine volatility was developed. In a PZ/tertiary amine, the optimum pKa of the tertiary amine was around 9.1 to give the highest CO2 cyclic capacity. A generic model for PZ/tertiary amines was developed in Aspen Plus®, which can predict the CO2 vapor-liquid-equilibrium based on the pKa of the tertiary amine in blend. To a lesser degree than pKa, the polarity of the tertiary amine also affects the CO2 solubility of the blend. CO2 absorption rates of most 2.5 m PZ/2.5 m tertiary amines are slightly lower than 2.5 m PZ itself, due to the higher viscosity of the blends, but they still absorb CO2 much faster than 7 m monoethanolamine (MEA). 2 m PZ/3 m 4-hydroxy-1-methylpiperidine (HMPD) is the blend that shows the best overall properties for thermal stability, amine volatility, CO2 cyclic capacity, and CO2 absorption rate. 2 m PZ/3 m HMPD also has a much better solid solubility than 5 m PZ. The capital and energy cost for flue gas CO2 capture using 2 m PZ/3 m HMPD is expected to be much lower than that using 7 m MEA, while comparable to that using 5 m PZ. Thermally degraded diglycolamine® (DGA®)/dimethylaminoethoxyethanol (DMAEE) was found to have a better performance for CO2 capture than the original solvent. At high temperature, DGA®/DMAEE reaches equilibrium with its major degradation product, methylaminoethoxyethanol (MAEE). The production of MAEE enhances the CO2 absorption rate, while maintaining the CO2 capacity of the original solvent.