Effect of methylimidazolium-based ionic liquids on vapor-liquid equilibrium behavior of tert-butyl alcohol+water azeotropic mixture at 101.3 kPa☆

Zhigang Zhang,Qiang Zhang,Tao Zhang,Qinqin Zhang,Wenxiu Li

Liaoning Provincial Key Laboratory of Chemical Separation Technology,Shenyang University of Chemical Technology,Shenyang 110142,China

1.Introduction

Water is the main by-product in the production of tert-butyl alcohol(TBA)in chemical and pharmacy industries,and it is difficult to separate{TBA+water}azeotrope by conventional distillation techniques.Extractive distillation is a well-known technology to reduce operational and capital expenditures for separation of close boiling point mixtures such as{alcohol+water}system[1-3],with solvent added to the mixtures to promote the effect of the separation.Traditional solvents such as organic substance and metal salt have the disadvantage of high cost,lower efficiency,and pollution.A suitable solvent is very demanding for this purpose[4,5].

Ionic liquids(ILs),also known as liquid salts or ionic fluids,are a new chemical substances consisting of an organic cation and an inorganic or organic anion[6].With the outstanding physicochemical properties of low vapor pressure and melting point,high stability,full of design ability,and environment-friendly,ILs are under intensive investigation to determine their potential as replacement solvents for extractive distillation[7,8].ILs are used in the separation of many azeotropic mixtures,such as{alcohol+water},{alcohol+ketone},and{alcohol+ester}[9-15].The search for task-specific ionic liquids has generated a substantial amount of research in recent years[16-18].The ILs composed of 1-alkyl-3-methylimidazolium and halogen can well dissolve in{alcohol+water}mixtures and receive wide attentions[17-20].Zhanget al.[21]have studied the relative volatility of TBA to water with ILs composed of an anion from[OAc]-or[Cl]-and a cation from 1-ethyl-3-methylimidazolium([EMIM]+),1-butyl-3-methylimidazolium([BMIM]+),or 1-hexyl-3-methylimidazolium([HMIM]+)at a fixed TBA mole fraction of 0.95.The effect on enhancement of relative volatilities of TBA to water is[EMIM]Cl＞[EMIM]OAc＞[BMIM]Cl＞[BMIM]OAc＞[HMIM]OAc＞[HMIM]Cl.The interactions of ILs with TBA and water are discussed by activity coefficients calculated using the NRTL model equation from the vapor-liquid equilibrium(VLE)data for{TBA+water+IL}ternary systems.

Usually,with the addition of ILs,azeotropic components may change their non-ideality to a different extent.It is desirable that these changes remove the azeotropy[22].Therefore,it is necessary to understand the effects of ILs on the VLE behavior of azeotropic mixtures and this can be reflected by VLE data.Information from the phase equilibrium study is the fundamental for ILs to be effectively used as replacement solvents in extractive distillation[23-25].

The objective of this work is to explore the suitability of three ILs,[EMIM]Br,[BMIM]Br and[HMIM]Br,for use as the solvent for separation of{TBA+water}system.The three ILs and their molar mass,M,are listed in Table 1,and their structures are shown in Fig.1.In this work,new experimental VLE data for{TBA+water+IL}ternary systems are reported.The water mole fraction on an IL-free basis varies from 0.0 to 1.0,and the IL mole fraction is fixed at 0.05,0.10or 0.20.The effect of ILs on the VLE behavior of{TBA+water}system is evaluated.The experimental VLE data,T,x,y,are correlated with the NRTL model equation,and the interactions of ILs with TBA and water are discussed.

Table 1Three ILs and their molar mass

2.Materials and Methods

2.1.Chemicals

The CAS No.,supplier,and purity in mass fraction ofthe chemicals are as follows:bromoethane(74-96-4,Sinopharm,＞99%);bromobutane(109-65-9,Sinopharm,＞99%);bromohexane(111-25-1,Sinopharm,＞99%);n-alkylimidazole(616-47-7,Kaile,＞99%);tert-butyl alcohol(75-65-0,Sinopharm,＞99%);and distilled water(Sinopharm).

The ILs used in this work were synthesized by the method described in the literature[26].The synthesized ILs were purified by subjecting the liquid to a very low pressure of about 8×10-3MPa at a temperature about 353 K for approximately 3 h,and were analyzed by Agilent 1260 In finity liquid chromatography.The purity of[EMIM]Br,[BMIM]Br,and[HMIM]Br is 99.1%,99.1%,and 99.0%,respectively.

2.2.Apparatus

The apparatus used were all-glass dynamic recirculating still(NGW,Wertheim,Germany)described by Hunsmann[27];Agilent 7890A gas chromatography(GC),and HS-9 headspace sampler.

2.3.Procedure

The temperature was measured using a Beckmann thermometer.After the VLE temperature was maintained at a constant value for 1 h,the amounts of TBA and water in the liquid and vapor phases were determined by the GC with a headspace sampler.The uncertainty of the temperature is±0.15 K,including the visual detection and the purity of ILs.The precision of the electronic balance is±1×10-4g.

2.4.Analysis

A thermalconductivity detector was used with a Agilent19091J-413 capillary column(30 m × 0.32 mm × 0.25 μm)in the GC.The GC response peaks were integrated using an Agilent Chemstation.The temperatures of column,injector,and detector were 373 K,473 K,and 473 K,respectively.Every example was analyzed at least three times,and the standard deviation in the mole fraction was less than 0.002.

3.Results and Discussion

Fig.2.The comparison between the experimental VLE data for{TBA(1)+water(2)}binary system and the data in literature.■experimental data;solid line-data in literature[28].

To test the performance of the equilibrium apparatus,VLE data for{TBA(1)+water(2)}binary system were measured at 101.3 kPa and compared with those in literature[28],as shown in Fig.2.The experimental data are in good agreement with the data in literature,so the equilibrium apparatus is qualified.The experimental VLE data for{TBA(1)or water(2)+IL(3)}binary systems are listed in Table 2.

Table 2Experimental VLE data for{TBA(1)or water(2)+IL(3)}binary systems at 101.3 kPa

Fig.1.Structures of the three ILs.

The NRTL model equation proposed by Renon and Prausnitz[29,30]has been reported a good expression for the VLE data of systems containing ILs[31-34],which is

of which the parameters are determined by minimizing of the following objective function(OF):

The model parameters are regressed by the Levenberg-Marquardt method.Activity coefficients are calculated by

in which the saturated vapor pressure is calculated by Antoine equation.

The Antoine constants of TBA and water are from reference[35],as listed in Table 3.

Table 3Antoine constants of TBA and water

The empirical coefficients and binary interaction parameters for{TBA(1)or water(2)+IL(3)}binary systems obtained and those for the{TBA(1)+water(2)}binary system from reference[36]are listed in Table 4.

Table 4Empirical coefficients and binary interaction parameters

With the experimental VLE data for the{TBA(1)+water(2)+IL(3)}ternary systems at 101.3 kPa,the relative volatilities of TBA to water are calculated

The experimental VLE data for the ternary systems containing[EMIM]Br,[BMIM]Br,or[HMIM]Br and the relative volatilities of TBA to water are listed in Tables 5-7,respectively.The variation range ofx3is 0.049-0.052,0.096-0.105,or 0.197-0.204,for the approximate value of 0.05,0.1,or 0.2,respectively.

Table 5Experimental VLE data for{TBA(1)+water(2)+[EMIM]Br(3)}ternary system at 101.3 kPa

The isobaricT-x-yVLE diagrams for the ternary system containing[EMIM]Br,[BMIM]Br,or[HMIM]Br are shown in Figs.3-5,respectively.With the addition of ILs,the VLE temperature increases and the azeotropic point moves towardx1′=1.0 obviously.With the IL mole fraction fixed at 0.20,azeotropic points are removed from the whole concentration range.Thus all the three ILs have an obvious effect on the VLE behavior of{TBA+water}system.The isobaricx-yVLE diagrams for the ternary systems containing ILs are shown in Fig.6.With the IL mole fraction fixed at 0.05,0.10 or 0.20,the strength by the three ILs on the isobaricx-yVLE diagram is:[EMIM]Br＞[BMIM]Br＞[HMIM]Br.The experimental VLE data are in good agreement with those calculated using the NRTL modelequation.The relative volatilities of TBAto water with the TBA mole fraction on an IL-free basis are shown in Fig.7.With the addition of ILs,the relative volatilities of TBA to water are enhanced considerably.[EMIM]Br has the most significant effect on the enhancement of relative volatilities of TBA to water,and[BMIM]Br has more significant effect than[HMIM]Br.The relative volatilities of TBA to water are all higher than 1.0 in the whole concentration range at IL mole fraction of 0.20.

Mean absolute deviations and standard deviations between experimental and calculated values of VLE temperatures and vapor phase mole fractions are calculated as follows and listed in Table 8.

Table 6Experimental VLE data for{TBA(1)+water(2)+[BMIM]Br(3)}ternary system at 101.3 kPa

In order to explain the results,the interactions of ILs with TBA and those with water are investigated independently.The activity coef ficients of TBA,γ1,in the{TBA+IL}binary systems and those of water,γ2,in the{water+IL}binary systems as a function of IL mole fraction,x3,are shown in Fig.8.The values of γ2decrease from 1.00 and there is no obvious difference between them.The values of γ1in the{TBA+[HMIM]Br}system decrease from 1.00,while those in{TBA+[EMIM]Br or[BMIM]Br}systems increase from 1.00,and the values of γ1in the{TBA+[EMIM]Br}system are higher.

The values of γ2are below 1.00 and decrease as the IL mole fraction increases reflect that[EMIM]Br,[BMIM]Br and[HMIM]Br produce absorptive effect on water and the effect enhances with the IL mole fraction.The values of γ2are almost in the same range,indicating that the absorptive effect produced by ILs is mainly determined by the anion Br-,so the difference in absorptive effect is not obvious with the variation of cations.The values of γ2in the{water+[EMIM]Br}system are the least,which reflectsthat[EMIM]Brhasa strongerabsorptive effect on water comparing to[BMIM]Br and[HMIM]Br,and the main reason is that[EMIM]Br has a stronger hydrophilicity than[BMIM]Br and[HMIM]Br.The increase of γ1value in the{TBA+[EMIM]Br or[BMIM]Br}systems from1.00 and its decrease in the{TBA+[HMIM]Br}system from 1.00 reflect that[EMIM]Br and[BMIM]Br produce repulsive effect on TBA,while[HMIM]Br produces absorptive effect on TBA.The values of γ1in the{TBA+[EMIM]Br}system are higher thanthose in the{TBA+[BMIM]Br}system,which reflects that the repulsive effect on TBA produced by[EMIM]Br is stronger than that produced by[BMIM]Br.With the increase of alkyl chain length of the imidazolium ring,the repulsive effects on TBA are:[EMIM]Br＞[BMIM]Br＞[HMIM]Br.This is just on the opposite of the effect produced on water,which resulted from the integrative effect of IL with both hydrophobic and hydrophilic functional groups in TBA molecule.The relatively strong absorptive effect of[EMIM]Br on water and its relatively strong repulsive effect on TBA illustrate the reason why[EMIM]Br,among the three ILs,has the most significant effect on enhancing the relative volatilities of TBA to water.

Table 7Experimental VLE data for the{TBA(1)+water(2)+[HMIM]Br(3)}ternary system at 101.3 kPa

To further study the interaction of ILs with TBA and water,the experimental VLE temperature data for the{TBA+IL}and the{water+IL}binary systems with the IL mole fraction are shown in Fig.9.The temperatures of the{water+IL}binary systems are all higher than that of the ideal liquid water calculated using Raoult's law equation,but they are not in an obvious difference.With the addition of ILs,the deviation from Raoult's law of the water in the{water+IL}binary systems is negative,which reflects that[EMIM]Br,[BMIM]Brand[HMIM]Brhave a similar absorptive effect on water[37].The temperatures of the{TBA+[HMIM]Br}binary system are higher than those of ideal liquid TBA,while those of the{TBA+[EMIM]Br or[BMIM]Br}binary systems are lower.The deviation from Raoult's law of TBA in the{TBA+[EMIM]Br or[BMIM]Br}binary systems is positive,and this reflects that[EMIM]Br and[BMIM]Br produce a repulsive effect on TBA.The temperatures of the{TBA+[EMIM]Br}system are lower than those of the{TBA+[BMIM]Br}system,so that[EMIM]Br produces a stronger repulsive effect on TBA than[BMIM]Br.The deviation from Raoult's law of TBA in the{TBA+[HMIM]Br}system is negative,which reflects that[HMIM]Br produces an absorptive effect on TBA.The repulsive effects produced by different ILs on TBA are:[EMIM]Br＞[BMIM]Br＞[HMIM]Br,which is in the same trend of the absorptive effect on water.Thus the effects produced by different ILs on the enhancement of relative volatilities of TBA to water are:[EMIM]Br＞[BMIM]Br＞[HMIM]Br,and this is identical with the explanations from the activity coefficients.

Fig.3.Isobaric T-x-y VLE diagrams for the{TBA(1)+water(2)+[EMIM]Br(3)}ternary system at 101.3 kPa.-——without IL;▲x3≈0.05;◆x3≈0.10;● x3≈0.20;solid lines— calculated using the NRTL model equation.

Fig.4.Isobaric T-x-y VLE diagrams for the{TBA(1)+water(2)+[BMIM]Br(3)}ternary system at 101.3 kPa.-—— without IL;▲ x3≈0.05;◆ x3≈ 0.10;● x3≈ 0.20;solid lines—calculated using the NRTL model equation.

4.Conclusions

Isobaric VLE data for the{TBA+water+IL}ternary systems containing different ILs,[EMIM]Br,[BMIM]Br,or[HMIM]Br,at 101.3 kPa were measured.The results indicate that all the three ILs produce an obvious effect on the VLE behavior of the{TBA+water}system in the whole concentration range and eliminate the azeotropy.Their effect on the enhancement of the relative volatilities of TBA to water is:[EMIM]Br＞[BMIM]Br＞[HMIM]Br,so[EMIM]Br is the most favorable candidate solvent for the separation of{TBA+water}azeotropic mixture by extractive distillation among the three ILs.The experimental VLE data are correlated with the NRTL model equation,and good correlations are obtained.Further explanations are given by both activity coefficients and the temperatures of the binary systems.

Fig.5.Isobaric T-x-y VLE diagrams for the{TBA(1)+water(2)+[HMIM]Br(3)}ternary system at 101.3 kPa.-——without IL;▲ x3≈ 0.05;◆ x3≈ 0.10;● x3≈ 0.20;solid lines—calculated using the NRTL model equation.

Fig.6.Isobaric x-y VLE diagrams for{TBA(1)+water(2)+IL(3)}ternary systems at101.3 kPa.((a)x3≈0.05;(b)x3≈0.10;(c)x3≈0.20;-——without IL;△[HMIM]Br;◇[BMIM]Br;●[EMIM]Br;solid line—calculated using the NRTL model equation).

Fig.7.Relative volatilities of TBA to water with the TBA mole fraction on an IL-free basis at 101.3 kPa.((a)[EMIM]Br;(b)[BMIM]Br;(c)[HMIM]Br;without IL;△ x3 ≈ 0.05;◇x3≈0.10;○x3≈0.20).

Table 8Mean absolute deviations and standard deviations between experimental and calculated values

Nomenclature

Ai,Bi,CiAntoine constants of componenti

Gij,Gji,Gmj,Gki,Gkjinteraction of two different components

Mmolar mass,g·mol-1

Nquantities of experimental data

P0atmosphere pressure,Pa

Pi* saturated vapor pressure of componenti,Pa

Rideal gas constant,8.314 J·mol-1·K-1

Fig.8.Activity coefficients of TBAin the{TBA(1)+IL(3)}binary systems and those of water in the{water(2)+IL(3)}binary systems with the IL mole fraction at101.3 kPa.○[EMIM]Br;◇[BMIM]Br;△[HMIM]Br.

Fig.9.Experimental VLE temperature data for the binary systems with the IL mole fraction at 101.3 kPa.((a){TBA(1)+IL(3)};(b){water(2)+IL(3)};● [EMIM]Br;◆ [BMIM]Br;▲[HMIM]Br;solid line—temperatures of the ideal liquid calculated using the Raoult's law equation).

TVLE temperature,K

x1mole fraction of TBA in liquid phase

x1′ mole fraction of TBA in liquid phase on an IL-free basis

x2mole fraction of water in liquid phase

x3mole fraction of ILs in liquid phase

xi,xj,xm,xkmole fraction of componentiin liquid phase

y1mole fraction of TBA in vapor phase

yimole fraction of componentiin vapor phase

Δgij,Δgjibinary interaction parameter,J·mol-1

αijempirical coefficients

α12relative volatility of TBA to water

γ1activity coefficient of TBA

γ2activity coefficient of water

γiactivity coef12:07 2018-6-7ficient of componenti

δ mean absolute deviation

τji,τmjbinary interaction parameter

σ standard deviation

Superscript

calcd calculated

exptl experimental

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