The Basic Study Of CO₂ Liquid-solid Phase Change Absorption With Non-aqueous Solvent/polyamine System

CO₂capture and storage technology is considered to be the key technology to mitigate CO₂emission and atmospheric CO₂concentration.Chemical absorption using aqueous amine solutions is favored for CO₂capture due to its high reactivity at low CO₂partial pressures,large handing capacity and good applicability.However,aqueous amine-based processes have a major drawback of high energy penalty during the regeneration process.To tackle this challenge,phase-change solvent absorption technologies have emerged as one of the most promising alternatives for CO₂capture.These approaches can significantly reduce the energy consumption in the regeneration step since only CO2-rich phase needs to be regenerated.In this work,nonaqueous amine/solvent systems were investigated to obtain absorvents exhibiting liquid-solid phase-change features.Excellent ones were selected for CO₂removel from simulated biogas and flue gas,their performance were further evaluated,which includes the capacity and kinetics of CO₂absorption,as well as cyclic absorption/desorption efficiency.Solid products were analyzed,and more experiments combining with molecular dynamics simulation were conducted for the discussion of reaction mechanism and phase-change mechanism.From the exploratory experiments,it can be concluded that two conditions need to be satisfied for nonaqueous amine/solvent system featuring phase-change phenomena after CO₂absorption.For the amine in system,both ends of molecule should be primary or secondary amino group,and no tertiary amino group is permitted in the molecule.For the organic solvent,its polarity should be lower than methanol or no hydroxyl is contained in the molecule.Diamine/ethanol solutions were chosen for CO₂capture form biogas due to their volatility in addition to attractive phase separation charateristics and absorption peformance.Triethylenetetramine/polyethylene glycol?TETA/PEG200?not only exhibits high thermal stability and relatively low viscosity,but also has a highest absorption capacity within the tested thermal stable absorbents.Thus TETA/PEG200 solution was selected for CO₂absorption from high temperature flue gas.Ethylenediamine?EDA?/ethanol solutions and piperazine?PZ?/ethanol solutions were measured for CO₂absorption and the performance of corresponding aqueous solutions were also studied as comparison.Results show that diamine/ethanol solutions have faster absorption rate than diamine aqueous solutions.The overall average CO₂absorption rate of EDA/ethanol solutions is almost doubled in comparison to that of EDA aqueous solutions.The capacity of diamine/ethanol solutions is?1.1 times as much as that of diamine aqueous solutions.In addition,the solid products were identified to be the mixture of monocarbamate and dicarbamate from its ¹³C NMR spectrum.TG-DSC analyses show that EDA-carbamates and PZ-carbamates have a similar decomposition temperature of?90?.The regeneration heat of EDA-carbamate and PZ-carbamate is 25.6 and 20.5%less than traditional MEA solutions,respectively.It was also found that all chemically absorbed CO₂were captured in the solid phase,and there is only physically absorbed CO₂in the liquid phase,which was determined by CO₂solubility in ethanol.CO₂absorption/desorption performance of TETA/PEG200 solution from flue gas at higher temperature was studied,and the reaction mechanism and phase-change mechanism of CO₂absorption were explored by combining both experimental and simulation methods.Results from absorption experiments show that CO₂capacity of TETA/PEG200 solution increased with the increasing TETA concentration while decreased with the increasing temperature.Notably,when TETA concentration reached 1 M,the CO₂absorption capacity of the solution is only slightly decreased?8.4%as the temperature increases to 60?.1 M TETA/PEG200 solution shows a high CO₂capacity of 1.63 mol/mol TETA at 40 ?,which is comparable to TETA aqueous solution.Moreover,the overall average CO₂absorption rate of TETA/PEG200 solution decreased within 10%as temperature rises from 40 to 60 ?.CO₂distribution experiments indicate that CO₂content of per volume solid phase increases as TETA concentration increased.For instance,when 1 M TETA/PEG200 solution absorb CO₂to saturated at 60 ?,the final volume of solid phase only accounted for 29%of the total volume,while the CO₂content was as high as 92%.For regeneration processes,microwave heating was identified to be a more effective method than classic heating.The regeneration efficiencies of the whole solution and solid phase were as high as 96%and 90%after four absorption/desorption cycles,respectively.Results from the ¹³C NMR and FTIR analyses show that the main products of TETA/PEG200 solution after CO₂absorption are monocarbamates,dicarbamates and PEG200 based alkylcarbonates.During the phase-change process,no new species were found and monocarbamates were the main products that precipitate out from solution.The spectroscopic analysis also indicated that there were strong hydrogen bonding interactions between-NH₃⁺ and-COO^-in the solid products.Experimental work reveals that when the TETA concentration is in the range of 1 M to 2 M,precipitation will happen as CO₂loading reaches?1 mol/mol TETA,and the process of precipitation is reversible by adding extra TETA.Based on above fmdings,a phase-change mechanism was then proposed,in which zwitterionic monocarbamate?ZM?is the main form of precipitates.Evidence from molecular dynamics simulations further confirmed that ZM tends to form inter-and intra-molecular hydrogen bonds,while reducing the number of hydrogen bonds with the solvent as CO₂loading increases,leading to self-aggregation.