Salting-out effect of ionic liquids on isobaric vapor-liquid equilibrium of acetonitrile-water system☆

Jing Fang ,Rui Zhao ,Hui Wang ,Chunli Li,*,Jing Liu

1 School of Chemical Engineering,Hebei University of Technology,Tianjin 300130,China

2 Environmental Monitoring Central Station of Suqian City,Jiangsu Province,Suqian 223800,China

Keywords:Salting-out effect Vapor–liquid equilibrium Separation Ionic liquid Acetonitrile

ABSTRACT This paper presents the vapor–liquid equilibrium(VLE)data of acetonitrile–water system containing ionic liquids(ILs)at atmospheric pressure(101.3 kPa).Since ionic liquids dissociate into anions and cations,the VLE data for the acetonitrile+water+ILs systems are correlated by salt effect models,Furter model and improved Furter model.The overall average relative deviation of Furter model and improved Furter model is 5.43%and 4.68%,respectively.Thus the salt effect models are applicable for the correlation of IL containing systems.The salting-out effect theory can be used to explain the change of relative volatility of acetonitrile–water system.

1.Introduction

At room temperature,ionic liquids(ILs)are one of novel“green solvents”.They have many special properties,such as negligible vapor pressure,good thermal stability,and environmental friendliness,so they have more potential to be used in some special distillation processes.Recently,extractive distillation with ionic liquids as entrainers is more and more popular.The phase behavior of the systems containing ionic liquids in extractive distillation has been studied[1–10],but the thermodynamic data in this field are still insufficient,so it is necessary to investigate the phase behavior of more IL containing systems.For analyzing the VLE data,activity coefficient equations are usually used to simulate ionic liquid systems,such as the Wilson equation,NRTL equation,UNIQUAC equation,UNIFAC equation,and modified UNIFAC equation,which are based on the concepts of local composition and group contribution methods.Since various components in the mixture can be considered as a whole molecular group,these activity coefficient equations are usually obtained by the excess Gibbs free energy equation of the liquid mixture.However,ILs as a kind of molten salt can naturally dissociate into anions and cations,so these equations cannot fit ionic liquid systems efficiently.

Salt effect is a phenomenon that the composition of the vapor phase in equilibrium in a binary solution usually changes by adding a salt with volatile components to the system due to interactions between the salt and solvent components[11–13].Salt effect is mainly caused by electrostatic interactions of ions.When ILs dissociate into anions and cations,the hydration between these ions and water molecules takes place due to electrostatic interactions,the result of which is that the number of free water molecules as solvent is reduced,the relative volatility of acetonitrile to water is improved,and the separation of materials is promoted.T the salt effect theory is often applied to predict vapor-liquid equilibrium with inorganic salt solution,and the prediction is acceptable[14–17].Literally ionic liquids are a kind of molten salt with similar nature as inorganic salt solution.

In this work,the isobaric VLE data for ternary systems of acetonitrile+water+different ionic liquids are measured at 101.3 kPa,which are 1-butyl-3-methylimidazolium tetra fluoroborate([Bmim][BF4]),1-butyl-3-methylimidazolium dibutyl phosphate([Bmim][DBP]),and 1-butyl-3-methylimidazolium chloride([Bmim][Cl]).They have the same cation but different anions.Their structures are as follows.

The salting-out effect of ILs on the separation performance of acetonitrile-water system will be discussed.The salt effect models,Furter model and improved Furter model,are used to correlate the vapor-liquid equilibrium of acetonitrile-water system containing IL.

2.Materials

The chemical reagents used in this work were acetonitrile,redistilled water,Karl-Fischer reagent and ILs.The analytical grade acetonitrile with purity of 99%was purchased from Jiangtian Chemical Reagents Co.,Tianjin,China and used without further purification.ILs[Bmim][BF4],[Bmim][DBP]and[Bmim][Cl]were provided by Chengjie Chemical Co.,Shanghai,China,with a minimum mass fraction of 99%.The purity of reagents was checked by gas chromatography(GC SP-3420,China).The properties of materials are shown in Table 1.

Table 1 Properties of the pure components

3.Results and Discussion

3.1.Experimental data

In order to check the reliability of the experimental set-up,the isobaric VLE data for acetic acid–water system at atmospheric pressure(101.3 kPa)were measured and compared with those in literature[18].The apparatus and adjusted results were given in our previous work[19].The isobaric VLE data for acetonitrile-water system containing different ILs at various IL contents(20%and 30%,by mass)were measured.

Fig.1.Isobaric VLE diagram for system acetonitrile(1)+water(2)+IL(3)at 20%mass fraction.■ IL free;□ [Bmim][BF4];○ [Bmim][DBP];Δ[Bmim][Cl].

Fig.2.Isobaric VLE diagram for system acetonitrile(1)+water(2)+IL(3)at 30%mass fraction.■ IL free;□ [Bmim][BF4];○[Bmim][DBP];Δ [Bmim][Cl].

The x–y diagrams are plotted in Figs.1 and 2 to describe the saltingout effect caused by ILs,where x3represents the mass fraction of IL in the liquid phase,x1is the mole fraction of acetonitrile in the liquid phase excluding IL,and y1is the mole fraction of acetonitrile in the vapor phase.[Bmim][BF4]is ineffective to enhance the relative volatility of the acetonitrile-water system,while[Bmim][DBP]and[Bmim][Cl]increase the relative volatility of acetonitrile in the whole concentration range.The salting-out effect for acetonitrile with[Bmim][Cl]is more effective than that with[Bmim][DBP].The azeotropic point for acetonitrile–water binary mixture shifts upwards with the addition of ILs and the azeotropic phenomenon even disappears at specific IL content in the cases of[Bmim][DBP]and[Bmim][Cl].

As organic molten salts,the influence of ionic liquids on the VLE of acetonitrile–water system can be explained by the hydration theory and electrostatic interaction theory.The hydration theory indicates that the salting-out effect is the interaction between water molecules and ions,which is reduced if the salt is hydrated by water molecules.In this experimental system,the polarization parameter of water is greater than that of acetonitrile,so the dipole moment of water molecules is towards to the hydrate of salt in the formation of hydrates.The relative volatility of acetonitrile to water is enhanced because of the precipitation of acetonitrile as a result of weak polarity.According to the electrostatic interaction theory,ILs can produce an electric field in the solution.Due to different polarities and dielectric constants of acetonitrile and water,water molecules tend to gather in the neighborhood of ions while acetonitrile molecules are expelled from the ions.In this way,the composition of acetonitrile–water system should change with the addition of ionic liquids and the relative volatility of acetonitrile to water is enhanced as observed in the experiments.

Vapor–liquid equilibrium is generally expressed by the relative volatility α,

where y and x represent the mole fraction of specific component in the vapor and liquid phases,respectively.

To further investigate the salting-out effect of ILs on acetonitrile–water system,the relative volatilities of acetonitrile to water with different ILs are plotted in Fig.3.The results show that the salting-out effect of ILs follows the order:[Bmim][Cl]＞[Bmim][DBP]＞[Bmim][BF4].

3.2.Correlation with the salting-out effect models

Furter[20]gave the first quantitative correlation of salt effect on VLE in the system with two solvents and one-salt electrolyte in terms of the relative volatility

where αsrepresents the relative volatility in the presence of salt,α represents the relative volatility in salt-free solution,and k is the parameter of salting-out effect.

Wu et al.[21]proposed an improved Furter model

where k1and k2are the parameters of salting-out effect.

The Furter model and improved Furter model are used to correlate the VLE data obtained in this study.The resulting relative volatilities are used to calculate the vapor phase composition in the IL containing systems.

where xis=xiandαis=αiexp(kx3)represent the Furter model;αis=αiexp(k1x3+k2x32)represents improved Furter model;and i indicates the light component.

The calculated and experimental data are compared in Tables 2–4.The salt effect models are applicable to correlate IL containing systems,with the overall average relative deviation of Furter model and improved Furter model as 5.43%and 4.68%,respectively.

The phenomenon of azeotropic point of acetonitrile–water system shifting upward or even disappearance with the addition of ILs as shown in Figs.1 and 2 can be attributed to the polarity of water,which is stronger than that of acetonitrile.The polar salt tends to hydrate with water molecules more than acetonitrile,increasing the relative volatility of acetonitrile.Thus it is possible to separate acetonitrile and water in a traditional distillation with the addition of ILs.This study confirms the capability of ILs as an entrainer for the separation of acetonitrile and water.

4.Conclusions

The isobaric VLE data of acetonitrile–water systems containing IL[Bmim][BF4],[Bmim][DBP]or[Bmim][Cl]were measured.Theresults show that[Bmim][BF4]is ineffective to enhance relative volatility of acetonitrile–water system,while[Bmim][DBP]and[Bmim][Cl]produce notable salting-out effects on acetonitrile and even eliminate the azeotropic phenomenon following the order of[Bmin][Cl]＞[Bmin][DBP].The Furter model and improved Furter model for salt effect are used to correlate the VLE data in this work,with the overall average relative deviation of 5.43%and 4.68%,respectively.The results indicate that the salt effect models are applicable for IL containing systems.

Table 2 Comparison of calculated results by Furter model and improved Furter model with experimental data(IL=[Bmim][DBF])

Table 3 Comparison of calculated results by Furter model and improved Furter model with experimental data(IL=[Bmim][Cl])

Table 4 Correlated results with Furter model and improved Furter model

Nomenclature

k parameter of salting-out effect

MWmolecular mass,g·mol?1

pccritical pressure,Mpa

Tbnormal boiling point,K

Tccritical temperature,K

x mole fraction in the liquid phase

y mole fraction in the vapor phase

α relative volatility

αsrelative volatility in the presence of salt

Subscripts

i,j component

s salt