Precipitating Amine Absorption Systems for Carbon Capture

Abstract: Carbon capture using amine-based absorption is an established technology for the separation of gaseous CO2. It is used in the upgrading of gaseous fuels, and it has also been suggested that it could play a crucial role in the mitigation of climate change through post-combustion carbon capture and sequestration (CCS), and by creating carbon sinks through bioenergy CCS. One drawback of the existing technology is the high energy requirement associated with the regeneration of conventional aqueous amine solutions, which must be addressed in order to make amine absorption a viable option for carbon capture in the industrial sector.The work presented in this thesis focuses on amine solutions where a sterically hindered primary amine (AMP) was studied in mixtures with organic solvents as possible alternatives to conventional aqueous amine solutions. The reason for replacing the water component is the potential reduction in temperature and energy requirement in the regeneration step. The solubility of CO2 and heat of absorption were studied using reaction calorimetry, the chemical species in solution were characterized using nuclear magnetic resonance, and the absorption rate was studied using a wetted wall column.The results showed that AMP in non-aqueous solutions with NMP or DMSO can be regenerated at lower temperatures (<90 °C) than with conventional aqueous amine solutions (≥120 °C). During absorption, precipitation of the AMP carbamate allowed more CO2 to be absorbed in the solution, and precipitation was promoted by increasing the AMP concentration. However, precipitation resulted in a higher heat of absorption than for non-precipitating solutions, which adds to the overall cooling demand of the capture process. When absorption was performed at 40 °C, a CO2 partial pressure of 20 kPa was needed to obtain a similar cyclic capacity to that in 30 wt% MEA, indicating that the studied non-aqueous AMP solutions were suitable for biogas upgrading and industrial carbon capture applications. The less toxic properties of DMSO, and the overall slightly higher solubility of CO2 in AMP/DMSO than in AMP/NMP, suggest that further studies should be carried out using 25 wt% AMP in DMSO.The influence of water accumulation in AMP/DMSO was also studied, due to the hygroscopic properties of DMSO. It was found that the formation of bicarbonate increased with increasing water content. However, some water accumulation may not be problematic as the capacity of the absorption solutions was not reduced in the temperature range studied (40-88 °C) in solutions with a water:AMP molar ratio of 2:1. Preliminary results also indicated that water accumulation could lead to higher AMP degradation rates, but further studies are required to investigate this.