Predictive Thermodynamics Models For Ionic Liquids And Their Application In The Separation Of Methylal And Methanol Mixture By Extractive Distillation

Methylal is an important chemical material.Because of its high solubility,low boiling point,good water solubility,and high evaporation rate,it is widely used in cosmetics,pharmaceuticals,household products,car polishes,diesel additives and pesticides.At present,methylal is mainly synthesized by the catalytic reaction of methanol and formaldehyde.However,the reaction is a reversible reaction and there exists the mixture of methanol,formaldehyde,and methylal in the production.Methylal and methanol can form a minimum-boiling azeotrope at atmosphere pressure.Therefore,in order to obtain a high purity methylal product,special separation methods are required to separate the methylal and methanol azeotrope.In this thesis,the separation of methylacetal and methanol azeotrope by extractive distillation with the organic solvent and ionic liquid(IL)mixture which combines the advantages of organic solvents(low viscosity,low cost,etc.)and ILs(high separation performance,green,etc.)was proposed.ILs are numerous due to different combinations of anions and cations.Determining the phase equilibrium behavior of systems containing ILs by experiments is very tedious.Therefore,it is necessary to establish a suitable predictive thermodynamic model to provide a solid theoretical support for the application of ionic liquids.Currently,the UNIFAC model is the most widely used predictive thermodynamic model in the petroleum and chemical industry.This work extends the application range of UNIFAC model in ionic liquid-containing systems by correlating a large amount of experimental data in the literature.4 new ionic liquid groups([MIM][DBP],[MIM][DCA],[MIM][TCM],[MPY][TCM])and 63 pairs of new interaction parameters were added,and 8 pairs of old parameters were modified.In addition,for the parameters in the UNIFAC model for ionic liquid are vacant due to lack of experimental data,the COSMO-based models are proposed to assist in filling the vacant UNIFAC model parameters by predicting infinite activity coefficients.The predictive phase equilibrium data of systems containing ILs with missing UNFIAC parameters show that the model parameters obtained by this method can provide relatively accurate predictions.This greatly expands the scope of application of the UNFIAC model.The key of extractive distillation is the choice of entrainer.Dimethylformamide(DMF)was selected as the organic solvent part of the mixed entrainer(organic solvent+ionic liquid)because of its high solubility,low viscosity and low price.The ionic liquid part in the mixed entrainer was screened by a predictive thermodynamic model.The COSMO-RS model was used to predict the selectivities and solubilities of methylal and methanol in 255 ILs consisting of 15 cations and 17 anions,and other physical parameters(toxicity,viscosity,etc.)were considered.[EMIM][Ac]was selected as the ionic liquid part of the mixed entrainer.In addition,the microscopic interaction between the separated components and the entrainer was investigated at the molecular scale:the σ-profile analysis showed that methanol is both a strong hydrogen bond donor and a strong hydrogen bond acceptor,and DMF and[Ac]-have extremely strong hydrogen bond acceptability;the mixed enthalpy analysis results show that hydrogen bonding dominates the mixing enthalpy of methanol and the mixed entrainer systems,but only minor effects in the methylal and mixed entrainer system;and the binding energy analysis results show that the binding energy between methanol and the mixed entrainer is obviously higher than that between methylal and the mixed entrainer.Based on the screening results by the COSMO-RS model,the vapor-liquid equilibrium data of the methylal-methanol-organic solvent ternary system and the methylal-methanol-organic solvent-ionic liquid quaternary system were measured at atmospheric pressure.The results show that DMF can significantly increase the relative volatility of methylal to methanol and break the azeotropic point of the system of methylal and methanol;DMF+[EMIM][Ac]mixed entrainer can further improve the separation of methylal and methanol azeotrope,and this effect increases with the increase of the content of[EMIM][Ac].In addition,the experimental values of the ternary and quaternary systems are correlated through UNFAC.The results show that the model predictions are consistent with the experimental values,and the obtained UNIFAC model parameters can be input into process simulation softwares to simulate the extractive distillation.The rigorous equilibrium stage model was used to compare the separation ability of pure DMF and the mixed entrainers for extractive distillation of methylal and methanol azeotrope.The results show that compared with pure DMF,the mixed entrainer not only reduces the amount of entrainer,but also reduces energy consumption,and satisfies the economic benefits of energy saving and emission reduction,which has great industrial application prospects.Investigating the interaction mechanism of the IL and separated components by molecular dynamics at the molecular level.The calculation results of the mixing process between methylacetal/methanol and entrainers(DMF/IL/DMF+IL)show that methylal can’t be mixed with pure[EMIM][Ac],but can be mixed with DMF and mixed entrainer respectively,indicating that a small amount of ionic liquid does not affect the solubility of DMF.Methanol can form hydrogen bond with the entrainer,but methylal can’t.The results of radial distribution function of methanol and[EMIM][Ac]mixture with different concentrations show that the increase of methanol content can destroy the interaction between the cation and the anion to a certain extent;the interaction between methanol and[Ac]-anion is significantly stronger than that between methanol and[EMIM]+cation;and the hydrogen bond formed between methanol and[Ac]-anion can destroy the hydrogen bond formed by the methanol molecule itself.The spatial distribution functions of the mixture of methanol and[EMIM][Ac]show that[Ac]-anion or methanol molecules tend to aggregate near the three H atoms of the imidazole ring of[EMIM]+cation,mainly due to each of these three H atom has a strong positive charge and is easy to attract negatively charged O atoms;the two oxygen atoms on the[Ac]-anion tend to form strong hydrogen bonds with methanol molecules.