Experimental Study On Chemical Desorption Of CO₂ Absorbed By Alkanolamine In Rich Solution

Under the condition of CO₂ emission reduction,the alkanolamine absorption is widely used because of its fast absorption rate and high absorption capacity.After absorbing CO₂ by alkanolamine absorption,the desorption of CO₂-rich solutions needs to be carried out.The problems of high desorption energy consumption,low desorption efficiency,and high economic cost are still existed in the current desorption methods,such as thermal desorption and membrane desorption.CO₂ is desorbed and mineralized in the form of carbonate by the carbonation reaction between the chemical desorbents and the CO₂ in the rich solution through the new chemical desorption,with high desorption efficiency,simple operation,low energy consumption.The desorption effect is directly related to the absorbents.So absorbents with better desorption performance are tested for chemical desorption in this paper.In this paper,five absorbents of single alkanolamine（TEA,MDEA,DEA,AMP,MEA）and four absorbents of mixed alkanolamine（MDEA+TEA,MDEA+DEA,MDEA+AMP,MDEA+MEA）were tested by chemical desorption.Ca（OH）₂ was selected as the chemical desorbent.First,the effects of single factors,such as Ca（OH）₂ dosage（Ca:C）,p H,temperature,CO₂ loading,stirring rate,stirring time and standing time on the desorption rates were investigated.Then the best operation conditions for the desorption of each CO₂-rich alkanolamine solutions and the desorption rates under the best operation conditions were obtained in the orthogonal experiments.Then the repeated regeneration performances of absorbents were studied by multiple absorption experiments.Finally,the optimum alkanolamine absorbent was selected through the comparison of the best desorption rates,repeated regeneration performances and absorbent costs.The effects of different influencing factors on absorbents were tested in the single factor experiments.With the increases of Ca:C,the desorption of CO₂-rich solutions were promoted.But as Ca:C>1:1,the promotion effects were slowed down.Properly increasing the p H was beneficial to the desorption of the CO₂-rich solutions,but when it was increased to a certain extent,the inhibitory effects on the desorption were produced.Lower temperature ranges such as 20℃and 30℃were considered to be more economical for the chemical desorption of rich solutions.The best chemical desorption effects were shown when the CO₂ loading of the rich solutions were 0.6 mol·L^-1(TEA was 0.4 mol·L^-1).With an increase of the stirring rate and an extension of the stirring time,the desorption rates of the CO₂-rich solutions first increased and then leveled off.Long standing times（approximately 20 min）were required by MEA,and the states of complete precipitation of the other eight CO₂-rich solutions could be reached within15 minutes.For the mixed alkanolamine CO₂-rich solutions,with the proportions of TEA and MEA increasing,the desorption rates of MDEA+TEA and MDEA+MEA rich liquid increased.With the proportion of DEA increasing,the desorption rates of MDEA+DEArich solutions first increased and then decreased.With the proportion of AMP increasing,the desorption rate of MDEA+AMPrich solution decreased.Orthogonal experiments were performed on each absorbent by combining the results of single-factor experiments.The desorption rates of each absorbents under optimal conditions were as follows:TEA（82.85%）≈MDEA（82.49%）=AMP（82.49%）>MDEA+AMP（81.23%）≈MDEA+TEA（81.11%）>DEA（80.12%）>MDEA+DEA（71.56%）>MDEA+MEA（63.21%）>MEA（51.33%）.It was indicated that the best desorption rates of all absorbents were above 50%.Desorption rate of about 80%could be achieved by six absorbents except MDEA+DEA,MDEA+MEA and MEA.The repeated absorption-regeneration performances of each absorbents were studied by 5repeated absorption tests.Under the respective optimal conditions,the results of the repeated absorption-regeneration performances of each absorbents（the ratio of the fifth desorption rate to the initial desorption rate）were as follows:MDEA+DEA（98.78%）≈MDEA（97.70%）≈MDEA+TEA（97.34%）≈TEA（96.83%）>MDEA+AMP（92.88%）>DEA（82.20%）≈AMP（81.53%）>MEA（60.30%）>MDEA+MEA（42.89%）.Except for the two absorbents of MEA and MDEA+MEA,the fifth desorption rates of absorbents could reach more than 80%of the initial desorption rates.Among them,MDEA+DEA,MDEA,MDEA+TEA and TEA could reach more than 95%of the initial desorption rates,almost maintaining the original desorption effects.The nine absorbents of TEA,MDEA,DEA,AMP,MEA,MDEA+TEA,MDEA+DEA,MDEA+AMP,MDEA+MEA were comprehensively compared in three aspects:the best desorption rates,repeated absorption-regeneration performances and absorbent costs.Conclusions are as follows:In the chemical desorption method of adding Ca（OH）₂,except for MEA and MDEA+MEA absorbents,good desorption performances were shown in the other absorbents.Compared with mixed absorbents,single absorbents were more suitable for chemical desorption of Ca（OH）₂.Among the nine absorbents,it was recommended to choose TEA absorbent first,and MEA,MDEA+MEA absorbents were not recommended.