| Benzene is an aromatic,flammable,carcinogenic,colorless and transparent liquid that is insoluble in water but soluble in organic solvents.Methanol is a toxic organic solvent with irritating odor.At atmospheric pressure,benzene and methanol will form an azeotrope(azeotropic point of 57.5 ℃),which is difficult to separate and purify by ordinary distillation.At present,extracive distillation is generally used to separate the benzene-methanol system in industry.The key of extracive distillation is to screen extractants.However,screening extractants by experimental methods has the disadvantages of time-consuming,large consumption of extractants,difficult recovery and more waste generation.Therefore,researchers at home and abroad have begun to study the mechanism of extracive distillation based on quantum chemistry theory and molecular dynamics simulation,so as to select extractant more effectively.This is a new,efficient and green research method.Therefore,this paper selected the extraction solvent based on quantum chemical calculation,molecular dynamics simulation and process simulation.The mechanism of extracive distillation was further studied,and the feasibility of the selected extractant was verified by process simulation.Firstly,the molecular configurations of extractant molecules(glycerol,ethylene glycol,1-hexyl-3-methylimidazolium acetate and 1,4-butanediol)and benzene-methanol were established by Gaussian and Gaussian View software.The geometric configuration optimization and frequency analysis were carried out based on SMD solvation model and B3 LYP / 6-31 + G(d,p)basis set.On the basis of configuration optimization,the interaction energy of the stable configuration between benzene-methanol and extractant molecules was calculated.The effect of extractant on the relative volatility of benzene-methanol system was reflected by the difference of Interaction energy.The results showed that ionic liquid(1-hexyl-3-methylimidazolium acetate)>ethylene glycol ≈ glycerol>1,4-butanediol.Then,the electrostatic potential analysis(ESP),atomic topology analysis(AIM)and reduced density gradient function analysis(RDG)were carried out by Multiwfn program to obtain the interaction force type and hydrogen bond strength of eachextractant molecule with benzene and methanol.The results show that there are hydrogen bonds between oxygen atoms on glycerol,ethylene glycol,ionic liquid and 1,4-butanediol and methanol hydroxyl group,and the van der Waals force is strong.Comprehensive analysis showed that the hydrogen bonding of ionic liquid [HMIM][OAC] was stronger than that of glycerol,ethylene glycol and 1,4-butanediol,indicating that ionic liquid [HMIM][OAC] was the best extractant.Subsequently,the azeotropic behavior of ionic liquids [HMIM][OAC] with methanol and benzene at different concentrations was further studied by molecular dynamics simulation.The radial distribution function(RDF)and spatial distribution function(SDF)show that the interaction between anions and methanol is the strongest in [HMIM][OAC],and the concentration of ionic liquid [HMIM][OAC] has a strong influence on anions in the separation process.With the increase of anion concentration,the separation effect is better.Although cations are very attractive to benzene and methanol,this effect is insensitive to the concentration of ionic liquids.Finally,Aspen Plus V 11 was used to simulate the separation of benzene-methanol azeotropic system in industrial process.When the required purity of benzene and methanol is reached,the parameters are optimized with the heat load as the optimization target.This study provides theoretical guidance for studying the mechanism of extracive distillation and the selection of solvents.The optimum operating parameters are: total number of trays N=25,reflux ratio R=2.1,raw material feeding position NF=21,extractant feeding position NS=4,extractant feeding amount Sm= 280 kmol/h,and the minimum heat load after process optimization is 10430.2KW. |
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