Evaluation On Diamine Based Water Lean Solvents For Post-combustion CO₂ Capture

Huge ambitions for greenhouse gas control have been shown all over the world.More than 110 countries and areas have pledged to schedules of carbon neutrality by 2050and China is committed to zero CO₂ emission by 2060.CO₂ Capture,Utilization and Storage（CCUS）will play an important role in achieving this ambitious goal.Carbon capture based on chemical absorption is the leading capture approach for power plants and many other industries and is expected to be deployed more broadly in coming decades.The high energy consumption of solvent is the main bottleneck that retards the further industrialization of chemical absorption method.Nowadays,water lean solvents,which mix amines with organic diluents and contain low or no water,have gained considerable interests due to their lower energy requirement for regeneration and ability to be used in existing processes with minimal modification.In this work,through the the introduction of organic solvent 1-methyl-2-pyrrolidinone（NMP）and sulfolane（SFL）,diamine based water lean solvents after screening were investigated for physical properties,thermodynamics and dynamic performances,corrosion and degradation performances.The diamines N,N-dimethyl-1,3-propanediamine and N,N-dimethyl-1,2-ethanediamine with one primary and one tertiary amino group remained homogenous during CO₂uptake with the addition of cosolvents（NMP/SFL）and were further investigated for their physical properties.It is shown that ENH-5%H₂O（mass ratio DMEDA:NMP:H₂O=3:6.5:0.5）achieved highest density among all the solvents,which was 6%lower than MEA-H₂O aqueous solvent.ENH-5%H₂O with a CO₂ loading of 0.767 mol CO₂·mol amine^–1 had a viscosity of 7.603 m Pa·S,which is the lowest viscosity among reported water-lean solvents so far and comparable with traditional blended solvents.Considerable improvements of absorption and desorption rate,cyclic capacity and regeneration energy were gained in diamine based water lean solvents compared with MEA-H₂O.ENH-5%H₂O shown a 140%improvement in cyclic capacity compared with MEA-H₂O.Regeneration energy of ENH-5%H₂O at 90℃ was estimated to be2.418 GJ·t CO₂^-1 which was 36%lower than the aqueous MEA-H₂O solvent.Moreover,ENH-5%H₂O of low viscosity shown a similar regeneration energy no matter the value of heat exchangers parameter（α）was,and this can significantly reduce the size/surface area requirement of heat exchangers.Diffusivity of water lean solvent were evaluated using nuclear magnetic resonance（NMR）spectroscopy.It is found that diffusion coefficients of species（H₂O,NMP,amine and carbamate）increased with increasing temperature for a given solvent system and loading.Besides,it is found that the diffusion coefficient was proportional to 3/2power of temperature.For the effect of CO₂ loading,there was a general trend of decreased diffusion coefficient with increasing CO₂ loading.The major variation of the solvent system among CO₂ absorption process was the increasing amount of absorption product:carbamate,which was considered as the key affecting factor on the diffusivity of all the species.The diffusion coefficients of four species were ranked in the order of DMEDA>DMEDA>>MEA in water lean systems and MEA>DMEDA>DMEDA in aqueous systems.Regarding to the hydrodynamic radii of four species,it is illustrated that each of the species remained unimolecular across the temperatures studied both in aqueous and water lean solvents.Diamine based water lean solvents ENH and PNH had weaker molecular interaction due to the substitution of NMP for water at conditional absorption temperature 313 K,resulting in smaller hydrodynamic radii of species,which shows positive impact on mass transfer in the absorption process.Carbon steel:HA3 performed differently in aqueous and water lean solvents in immersion corrosion test.Protective films were coated on HA3 discs immersed in aqueous solvents,while no film was found on HA3 immersed in water lean solvents.HA3 immersed in DMPDA-H₂O formed more dense siderite（Fe CO₃）layer achieved a lower iron concentration and corrosion rate than the found on that in MEA-H₂O and DMEDA-H₂O formed looser chukanovite（Fe₂（CO₃）（OH）₂）layers.Water lean solvents with higher iron concentration and corrosion rate shown more serious corrosion.The feasibility of improving anti-corrosion ability by increasing the water content of water lean solvents was preliminarily verified in this works.Homogeneous diamine based water lean solvents and biphasic solvents DAH（50 wt.%N,N-diethylethanolamine（DEEA）-25 wt.%2-（2-Aminoethylamino）ethanol（AEEA）-25 wt.%H₂O）were investigated for their different performance on degradation behavior by enhanced oxidative degradation reactors.It is found that DEEA of main composition in upper phase of biphasic solvent DAH shown satisfied anti-degradation performance.Only 15.96%of degradation rate was achieved in 10%CO₂ condition through four weeks degradation process.While AEEA of main composition in lower phase degraded seriously.Approximately half of the free amine was transferred to the degradation product which was hard to be regenerated.The degradation rate of AEEA was 47.3%higher than aqueous MEA-H₂O.PNH with carbamate as main absorption product shown more serve degradation than corresponding aqueous solvents.Only17.019%of degradation rate was achieved in ENH-5%H₂O due to the strong steric hindrance effect of DMEDA,which was 25.2%lower than that of MEA-H₂O.Homogeneous water lean solvents ENH and PNH achieved lower degradation rate than that of AEEA in the lower phase of biphasic solvent DAH,showing advantages on degradation behavior.The main oxidative degradation products including piperazine,imidazolidone,pyridine and oxazolidone derivatives were obtained and their generation mechanisms were also speculated.