Intensification Of CO₂ Capture By Mixed Solvent Systems Based On Alkanolamines Or Ionic Liquids With Polyethylene Glycols As Co-solvent

The large amount of CO2 produced by coal-derived power plants is one of the major contribution to global climate issues, and how to effectively reduce CO2 emissions becomes a hot academic and industrial research topic in recent years. By introducing a CO2 capture set-up for the power plant could significantly reduce greenhouse gas emissions, thereby avoiding climate change issues. However, the chemical absorbents that have been applied in current power plant still have many drawbacks in practical industrial applications. Therefore, developing high efficient CO2 absorbents becomes the core issue and key technology for the large-scale industrial application of CO2 capture.Focused on drawbacks of conventional alkanolamine aqueous solutions and advanced ionic liquids, the mixed solvents systems based on alkanolamines or task-specifc ionic liquids (TSILs) with polytheylene glycols (PEGs) as co-solvent were firstly proposed for the post-combustion and pre-combustion CO2 capture processes, in order to establish high-efficient CO2 capture technology and provide technical references for industrial application.For the purpose of assessing the availability of PEGs as co-solvent, their physical and chemical properties, CO2 solubility and dissolution thermodynamic properties were investigated. The results showed that PEGs had a comparable low vapor pressure of the ionic liquids, excellent thermal stability at high temperature, and only a half viscosity of ionic liquids. In addition, under the conditions of temperatures range of 303.15-333.15 K and pressures up to 1.2 MPa, the gas-liquid equilibrium between CO2 and PEGs indicated CO2 solubility was competent to ionic liquids. Thus, PEGs can be applied as an ideal co-solvent for alkanolamines or ionic liquid mixed system in this study.For post-combustion process, ethanolamine (MEA), diglycolamine (DGA), diethanolamine (DEA) and N-methyl-diethanolamine (MDEA) were slected as chemical absorbents with PEG200 as co-solvent according to practical absorption conditions. Physicochemical properties, CO2 absorption and desorption capacity, absorption mechanism and absorption kinetics of mixed solvents were studied. According to the physicochemical properties analysis, the bad thermal stability and high vapor pressure of MEA impede its application in CO2 capture process. As a result, DGA, DEA and MDEA were intensively investigated. DEA-PEG200 and DGA-PEG200 solution could achieve completely regeneration at 353 K, and their cyclic CO2 capacity increase 25% when compared with aqueous alkanolamins. Moreover, the mixed solvents were more energy efficient. By employing pressure-drop method, the absorption kinetic was studied. The result showed the reaction rate and the enhancement factor of DGA were higher than that of DEA. According to the calculated reaction rate constant and Hata number, it was comfirmed that CO2 reaction in DEA/DGA-PEG200 was fast reaction. Based on above results, it indicated that DGA-PEG200 showed the superiority among studied alkanolamine-PEG200 solutions.Focused on high temperature and pressure absorption process in pre-combustion capture, the mixed solvents of amino acid ionic liquid (AAILs) with PEG400 as co-solvent were developed. According to the thermal stability analysis of nine kinds of TSILs, AAILs showed the best performance. Decomposition temperatures of the mixed systems were above 530 K, and at 373 K under N2 purging no weight loss could be found. CO2 absorption process in AAILs-PEG400 consisted of physisorption and chemisorption. Moreover, AAILs-PEG400 showed high selectivety towards CO2; under low pressure conditions, CO2/H2 ideal selectivity coefficient was higher than 1000, thereby being able to effectively avoid the loss of H2. CO2 absorption mechanism in AAILs-PEG400 was studied by FTIR, indicating that the reaction will be 1:1 stoichiometric. For the reaction rate constant, it ranked as [P4444] [Gly]> [P4444] [Pro]-PEG400> [P4444] [Ala]-PEG400 at 333.15 K. According to absorption kinetic parameters, CO2 absorbed by AAILs-PEG400 were quick reactions.