Study Of The Relationship Between The Amine Molecular Structure And The Carbon Dioxide ?CO₂? Absorption-desorption Performance

Climate change has caused strong concern worldwide and excessive CO₂emission is considered to be the main contributor to global warming,therefore CO₂capture and storage has become an important research subject.Normally,there are three major technologies to capture CO₂ are post-combustion,pre-combustion,and oxy-fuel combustion CO₂ capture.Among different CO₂ capture methods,post-combustion capture using chemical absorbents,especially amine absorbents,is regarded as a feasible method for mitigating the CO₂ emissions from many industrial sources.Chemical absorption using aqueous amine-based solutions for post-combustion is the leading method for large-scale CO₂ capture in industrial plants.This technology,however,still faces many challenges,in particular the high energy requirements for solvent regeneration,which limit the economic viability of the technology.The main content of this work are lised as follows:?1?The formation of bicarbonate ions in an amine solution during CO₂ absorption results in lowering the heat duty for amine solvent regeneration in the CO₂ capture process because bicarbonate breakdown needs the lowest energy input to release CO₂.With more bicarbonate in rich amines will lead to lower energy cost in CO₂ desorption process.To get a preciously knowledge of the formation of bicarbonate in tertiary amines for designing a better absorbent for CO₂ capture,a series of tertiary amines such as N-diethylethanolamine?DEEA?,1-dimethylamino-2-propanol?1DMA2P?,1-diethylamino-2-propanol?1DEA2P?,3-dimethyl-amino-1-propanol?3DMA1P?,N-methyldiethanolamine?MDEA?,dimethylmonoethanolamine?DMMEA?and triethanolamine?TEA?with various CO₂ loading at 1 mol/L were investigated using¹³C NMR technology.The amount of bicarbonate was calculated by the chemical shift of bicarboante/carboante in ¹³C NMR spectra.And the results show that the order of the amount of bicarbonate in those tertiary amines is DMMEA>MDEA>3DMA1P>1DMA2P>TEA>DEEA>1DEA2P.With consider of the effects of electron density of nitrogen atom?N?and the steric hinderance in those tertiary amines to the formation of bicarbonate in those aqueous tertiary amines solution,it can be concluded that:the aqueous 3DMA1P solution produce more bicarbonate compare to DMMEA for it's nearer distance of-OH to N;less hydroxyalkyl and one more methyl in amine molecular structure connected to N in MDEA compared to TEA lead to more bicarbonate generation;a smaller alkyl connected to N in DMMEA molecular structure compare to DEEA lead to more bicarbonate was generated in aqueous DMMEA solution;one more methyl branch exists in 1DMA2P,1DEA2P molecular structure compared to DMMEA,DEEA respectively lead to less bicarbonate generation.?2?In addition,bicarbonate formation was conducted for two mixed solvents consisting of tertiary amines?1DMA2P or MDEA?blended with MEA in order to determine both formation rate and capacity of bicarbonate ions as compared to MEA alone.The amines and concentrations used in the study were:MEA?5 mol/L?,MEA-MDEA?5:1 molar ratio,6 mol/L total?and MEA-1DMA2P?5:1 molar ratio,6 mol/L total?at various CO₂ loadings.The formation of bicarbonate ions was evaluated using¹³C NMR technique.The results show that,for the single tertiary amine system,higher concentrations of bicarbonate ions were formed for MDEA than for 1DMA2P for the same CO₂ loading.The results for the blended amine systems showed that bicarbonate ions were generated at a lower CO₂ loadings than MEA alone,with MEA-1DMA2P generating bicarbonate ions at a lower CO₂ loading?0.34 mol CO₂/mol amine?than MEA-MDEA?0.38 mol CO₂/mol amine?.Thus,as an additive in MEA,1DMA2P has a better potential than MDEA to generate bicarbonate ions at a leaner CO₂ loading with the attendant lowering of the regeneration energy.?3?To guide the development of more energy efficient amine solvents,we also studied the effect of molecular characteristics of diamines,including carbon chain length and type of amino functional group,on CO₂ absorption and desorption performance.Six linear terminal diamines?NH₂CH₂CH₂-R,R=NH₂,NHCH₃,N?CH₃?₂,CH₂NH₂,CH₂NHCH₃ and CH₂N?NH₃?₂?were investigated and two monoamines,monoethanolamine?NH₂CH₂CH₂OH,MEA?and 3-aminopropanol?NH₂CH₂CH₂CH₂OH,3AP?,were also tested as benchmarks.The CO₂ absorption capacity in each amine was measured at 40°C under atmospheric pressure using different CO₂ gas partial pressures.¹³C and ¹H NMR spectroscopy were used to identify and quantify species present in the CO₂-amine-H₂O system.The experimental results showed that the chain length extension from C₂ to C₃ led to a higher CO₂ absorption capacity and more bicarbonate formation during the CO₂ absorption process.In addition,the experimental results also demonstrated that increasing the substitution on one N atom in the tested diamines is favorable for a higher CO₂ absorption capacity and more bicarbonate formation under a CO₂ partial pressure of 101 kPa.Both chain length extension from C₂ to C₃ and an increase in the number of substituents on one N atom yield better performance in the CO₂ desorption with regards to the CO₂ higher cyclic capacity and faster initial CO₂ release rate for the tested amines.?4?3-Dimethylaminopropylamine?DMAPA?,3-Diethylaminopropylamine?DEAPA?and 3-Piperidinopropylamine?3PDPA?were selected to investigate the influence of substituent type to the CO₂ capture performance.The molecular structure of selected diamines can be described as H₂NCH₂CH₂CH₂N-R₂?R=CH₃,CH₂CH₃ and-?CH₂?₅-?.The experimental results shows that the-CH₂CH₃ as a substituent connected to N-atom is better than-CH₃ and-?CH₂?₅-for the higher CO₂ equilibrium solubility,more bicarbonate formation,lower heat of CO₂ absorption,higher CO₂ cyclic capacity,faster CO₂ initial release rate and higher CO₂ removal efficiency.In addition,the mass transfer performance of DEAPA and DMAPA also are investigated by using a wetted wall column and the results displays that both of them shows better mass transfer performance compared to MEA and MEA-DMEA,MEA-DEEA and MEA-AMP and it further reveal that the-CH₂CH₃ is better than-CH₃ as a substituent on N-atom for CO₂ transfer.DEAPA has a lower heat of absorption and higher CO₂ solubility than most conventional amines.Therefore,it can conclude that the-CH₂CH₃ has more advantages than-CH₃ and-?CH₂?₅-as a substituent at N-atom for designing an efficient CO₂ capture solvent.?5?we investigated CO₂ absorption peformance of 2 mol/L DEAPA,2 mol/L MEA and 4 mol/L MEA-MDEA?mole ratio=1:1?.The experimental results show that the intramolecular tertiary amino group of DEAPA can promote the CO₂ absorption rate of the intramolecular primary amino group and enhance its CO₂ absorption capacity.The study results using ¹³C NMR technique also showed that the intermolecular tertiary amine in MEA-MDEA system was more favored to promote the primary amine in the blend to form bicarbonate at an earlier CO₂ equivalent loading stage and produced less carbamate than the intramolecular tertiary amino group of DEAPA did to its primary amine.Morevoer,the CO₂ equilibrium solubility of DEAPA was measured at different temperatures with various CO₂ partial pressures,and then an empirical model was developed based on this experimental data.The results predicted by this model were in a good agreement with the experimental results.The study results demonstrates that DEAPA has potential to be an alternative solvent with high absorption rate,high CO₂capacity in CO₂ capture processes.