Design And Screening Of Ionic Liquids And Analogues As Absorbents For Carbon Dioxide Capture

Up to now,the fossil fuels still play an important role in the generation of heat and power,leading to billions of tons of annual CO2 emissions,thereby attracting great attention.Currently,the most widely used CO2 capture technology in the industry is the chemical absorption by aqueous solutions of alkanolamines,however,such process suffers from serious disadvantages,such as high energy penalty for solvent regeneration,severe corrosion of equipment and large solvent loss.Therefore,the most promising ionic liquids(ILs)and IL analogues,deep eutectic solvents(DESs)are studied as alternative solvents for CO2 absorption in this work.Considering the possible combinations of cation-anion for ILs and hydrogen bond acceptor(HBA)-hydrogen bond donor(HBD)for DESs,and gas-in-IL/DES behaviors varying remarkably from case to case,the experimental trial-and-error approach is expensive,time-consuming,and and even unrealistic.In this regard,a theoretical design and screening of ILs and DESs towards different CO2 capture tasks is conducted in this work from the following aspects:Most studies on computational IL selection only take into account the molar capacity of ILs for the target gas,leading to the optimal ILs not satisfactory from the practical point of view.To solve this problem,a systematic computer-aided ionic liquid design(CAILD)method is developed,where a mass-based Absorption-Selectivity-Desorption index(ASDI)integrating the most important thermodynamic properties of ILs(i.e.,gas solubility,selectivity,and desorption capacity)is proposed as the objective function,and physical properties(melting point and viscosity)of ILs are implemented as optimization constraints.The formulated mixed-integer nonlinear programming(MINLP)is solved by a combined simulated annealing-genetic algorithm.Through the comparative CAILD for the flue gas separation,the mass-based ASDI is demonstrated to be able to identify practically promising IL absorbents by decreasing the molar weight of ILs and reaching a good trade-off among different desired thermodynamic properties,thus giving rise to low energy consumption.The proposed CAILD method is further applied to other CO2 capture cases of syngas(CO2/H2)and sour gas(CO2/H2S)separation,and all the designed ILs show promising performance than the referenced solvents.The differences of design results for the three CO2 capture tasks are well interpreted by the σ-profile.Considering that the mass-based ASDI defined only for two-component gas mixture at the infinite dilution condition,cannot reflect the IL performance at the specific global composition of interest as well as in the continuous absorption process,a multilevel screening method is further proposed exemplified by the simultaneous removal of CO2 and H2S from natural gas(NG).In this method,the ASDI is employed to prescreen potential ILs that have promising target properties at infinite dilution condition.Following this,their simultaneous CO2 and H2S removal performances are further evaluated from the vapor-liquid equilibrium(VLE)of {IL+NG} systems at the specific composition of interest.After the thermodynamic screening,key physical properties of the obtained ILs are estimated by group contribution methods to find out solvents suitable for practical application.Finally,continuous acidic gas removal processes based on the remaining ILs are simulated and compared by Aspen Plus.By comparing the required column height with IL and energy consumptions in the continuous processes,[BeMPYO][H2PO4]and[EMIM][H2PO4]are identified as the best two absorbents for the simultaneous removal of CO2 and H2S from NG.In the view of the fact that there are so far no generally applicable thermodynamic prediction models for the behaviors of CO2-in-DES,a quantitative structure-property relationship(QSPR)model based on 1011 available solubility data collected from literature is developed using the random forest method with COSMO descriptors of HBA and HBD as input parameters.The QSPR model is found be able to accurately predict the CO2 solubility in DESs with an average relative deviation of 7.76%,and the effects of the DES nature as well as the absorption conditions on CO2 solubilty are well demonstrated,indicating the efficiency of the proposed model.Moreover,the effects of the input parameters on CO2 solubility are compared,following the ranking:pressure>HBA>HBD>HBA/HBD ratio>temperature.To investigtate the CO2 absorption mechanism in DESs,a study combining the experimental measurement and molecular dynamics(MD)simulation is performed.Firstly,four phosphonium-based DESs,namely,TBPB:PhOH(1:4),TBPB:DEG(1:4),ATPPB:PhOH(1:4),and ATPPB:PhOH(1:6),are prepared,and their CO2 solubility are measured experimentally and analyzed.By comparing with the DESs and ILs previously reported in the literature,the DESs studied in this work are found to have a competitive CO2 solubility as physical absorbents,Then,MD simulations are performed to study the microscopic behaviors of the DESs and {DES+CO2} mixtures.Through the analyses of radial distribution functions(RDFs),spatial distribution functions(SDFs)and intermolecular interaction energy from the molecular dynamics(MD)simulation,the eutectic formation and CO2 absorption mechanisms,as well as the effect of HBA/HBD type and molar ratio are well interpreted.In conclusion,a modified IL design method and a systematic IL screening method are successively proposed,which could be extended to other gas absorption systems.Moreover,a QSPR model is developed for the prediction of CO2 solubility in DESs,and the CO2 absorption mechanism is investigated by MD simulation,building the blocks for the design or screening of DESs.