Study On Absorption Of Condensable Gas And Fluorinated Gas In Chemical Separation Process With Ionic Liquids

The components of condensable gases mainly include water(H2O)and volatile organic compounds(VOCs),which often exist in some chemical processes in the form of gas impurities.Due to factors such as affecting the quality of chemicals and causing pollution to the atmosphere,they are usually further treated in the chemical separation process.In addition,as a kind of greenhouse gas,the emissions of fluorinated gas(F-gas)have been increasing year by year due to its wide application.Although the emissions of F-gas are far lower than that of carbon dioxide(CO2),it has extremely high greenhouse effect potential.Therefore,it is highly important to develop a sustainable process for F-gas recovery.The absorption and separation of condensable gas and F-gas through the green medium is in line with the concept of sustainable development of traditional chemical industry.For this purpose,a new process for absorbing condensable gas and F-gas from gas mixture with ionic liquids(ILs)was proposed in this study.Firstly,the research on the application of triethylene glycol(TEG)-IL hybrid solvent to CO2 drying was carried out.The hydrophilic[BMIM][BF4]was adopted as the IL component of hybrid solvent through the screening method of COSMO-RS model combined with other factors.Then,the thermodynamic behaviors of CO2 in TEG,[BMIM][BF4],binary mixture of TEG+[BMIM][BF4]and ternary mixture of TEG+[BMIM][BF4]+H2O were determined.The CO2 drying experiment was performed in the laboratory scale,and the results show that the hybrid solvent presents excellent performance on gas drying.In addition,the mechanism of gas drying was revealed by excess enthalpy analysis and reduced density gradient(RDG)analysis.The results demonstrate that the removal of H2O in gas mixture is attributed to the different interactions between CO2,H2O and solvent molecules;the former is dominated by van der Waals(vdW)interaction,while the latter is dominated-by hydrogen bond(HB)interaction.Then,in order to provide guidance for the design of special ILs in the chemical process related to the H2O-IL system,the structural effects on thermodynamic behavior and HB interaction of the H2O-IL system were investigated.Taking into account the alkyl chain length of the cation and the type of anion,the vapor pressure of the H2O-IL system was measured.The experimental results show that the structural effect of anions is dominant compared to the structural effect of cations.In order to explain this phenomenon,quantum chemistry(QC)calculations and wave function analysis systematically studied the formation,strength and properties of HB in the H2O-IL system.The results indicate that the difference of HB strength under the microscopic view directly reflects the difference of the vapor pressure value under the macroscopic view.In addition,the intermolecular interaction between short-chain cations and H2O is stronger than that between long-chain cations and H2O in the structural effect of cations;nitrogen and oxygen atoms are more suitable as HB acceptors than fluorine atoms in the structural effect of anions.Next,the absorption performance of ILs for specific VOCs components was systematically studied.In order to recover the byproduct hydrogen(H2)in the methyl ethyl ketone(MEK)process,[EMIM][Tf2N]is used as a candidate to absorb MEK and sec-butanol(SBA)in the mixed gas stream.The thermodynamic behaviors of MEK+[EMIM][Tf2N]and SBA+[EMIM][Tf2N]systems were determined.In this process,the UNIFAC model demonstrated reliable predictive ability.The absorption experiment was conducted in a specially designed absorption system.The experimental results show that under a certain absorbent flow rate,the absorption ratios of[EMIM][Tf2N]to MEK and SBA are as high as 98.80%and 99.58%,respectively.A conceptual design of the process flow was performed on idustrial scale through process simulation.It can be found that[EMIM][Tf2N]maintains a high separation effect on MEK and SBA even on the industrial scale.The absorption mechanism of specific VOCs components was revealed by excess enthalpy analysis,QC calculation and wave function analysis.Due to the high molecular polarity of H2O and some VOCs,the dominant interaction between H2O and solvent molecules is HB interaction.Therefore,the molecular design of cations and anions to enhance the intermolecular HB interaction is the key to the whole separation process.Finally,the structure-property relationship between thermodynamic behavior and weak interaction of F-gas+ FIL system was deeply explored.Hexafluoroethane(R116)and 1,1,1,2-tetrafluoroethane(R134a),which are widely used in refrigeration system and microelectronics industry,are selected as representative F-gases.The solubility results show that compared to non-FILs,FILs exhibit a more efficient F-gas absorption capacity.In this process,fluorinated cations play a more critical role than fluorinated anions.QC calculation and wave function analysis provide a comprehensive and intuitive explanation for this plienomenon.Energy decomposition analysis indicates that the huge difference on solubility of R116 and R134a in ILs is due to the different nature of weak interactions;the former is characterized by dispersion interaction as the dominant role,and the latter is characterized by electrostatic interaction as the dominant role.Electrostatic potential(ESP)and vdW potential analysis suggest that the fluorination of cations simultaneously enhances the electrostatic and dispersion interactions between the cations and surrounding molecules.The semi-empirical method was used to optimize the molecular structure of the large system,and the results further confirmes that the solute(R116 or R134a molecule)is surrounded by a cage formed by[FOMIM]+with the long side chain through vdW interactions.This structural effect creates a large weak interaction region,which makes the thermodynamic behavior of F-gas+ FIL system predominantly dependent on the type of cation.