Investigating the performance and thermodynamic efficiency of CO2 reactive absorption - A solvent comparison study

The capture of the CO2 released in the atmosphere by various sources is a key issue included in all environmental related European new legislation and roadmaps. Among Carbon Capture Systems (CCS), post-combustion reactive absorption is one of the most efficient and easily deployable methods. In specific, amine aqueous solvents ensure high absorption efficiency due to their reactivity with CO2. Monoethanolamine (MEA) and a mixture of Monoethanolamine and Piperazine, 30 MEA/18 PZ % wt., were found to achieve very satisfying levels of CO2 capture. For these solvents, a simulation of the entire CO2 capture unit was performed in Aspen Plus software. For the MEA case, a parametric analysis was performed to determine the impact of the absorber inlet liquid temperature, gas and liquid flow rates on CO2 mole fraction, reboiler heat duty and stripper temperature profile. The absorber inlet liquid temperature was found to be the most important factor influencing the absorption performance as well as the reboiler heat duty required for regeneration. The absorber inlet gas and liquid flow rates had also impact on the liquid temperature profiles in the absorber and on the energy cost. Comparing the two solvents, the MDEA/PZ solvents captured 99% of the CO2 of the flue gas while MEA only 80% for the conditions of the simulation. For the reference system with MEA, overall exergy efficiency was calculated considering the carbon capture unit as an integrated thermodynamic system. The overall exergy efficiency of the MEA system was calculated 23.8%, while the respective value for the MDEA/PZ system was calculated 26.1%. Evaluating the two main columns of the system in terms of exergy efficiency, the absorber showed low values of efficiency for both solvents, in specific 16.3% for MEA and 22.6% for MDEA/PZ, while the desorber had higher efficiencies, 27.8% for MEA and 34.3% for MDEA/PZ. In general, MDEA/PZ system was characterized by higher thermodynamic efficiencies both in system and individual column level. Furthermore, the analysis for the MDEA/PZ system resulted to a 5% lower reboiler heat duty than in the MEA case.