Zhongpeng Xing ,Yujie Gao ,Hui Ding *,Xianqin Wang Lujun LiHang Zhou

1 School of Chemical Engineering and Technology,Tianjin University,Tianjin 300072,China

2 Tianjin Academy of Environmental Sciences,Tianjin 300191,China

3 Tianjin Huanke Environmental Planning Technology Development Company Limited,Tianjin 300191,China

4 School of Environmental Science and Engineering,Tianjin University,Tianjin 300072,China

1.Introduction

Ethyl propionate,a kind of widely-used organic synthetic raw material and solvent,is applied in the field of pharmacy,antifungal agents,edibles,plasticizers,spice,dyes and even biomass[1,2].In industry,the traditional method of producing ethyl propionate is esterification of propanoic acid and ethanol with sulfuric acid as catalyst at 101.3 kPa,and then cut fraction from 369.15 K to 373.15 K is collected to gain pure product in distillation[3].However,the literature has reported that ethyl propionate and ethanol form an azeotrope at 101.3 kPa,which reduces the yield and purity of ethyl propionate[4].Therefore,it is a challenge for researcher to find out a way to improve the purity of ethyl propionate in the operation process.

For azeotrope or similar boiling point system,extractive distillation is an effective and widely-used process to separate the mixture[5],and entrainer needs to be added into the original azeotrope system to increase the relative volatility of the azeotrope.There are several reported entrainers used for breaking binary azeotrope system,such as ionic liquid[6,7],dimethyl sulfoxide(DMSO)[8]and N,N-dimethyl formamide(DMF)[9].Because of high viscosity,ionic liquid is not widely used in extractive distillation and the extraction capacity of DMSO,and DMF is lower than para-xylene in the field of separating ethanol and ethyl propionate azeotropic system[10].Due to wide availability,low cost,high boiling point,less causticity,good thermal stability and miscibility with organic solvent[11],para-xylene has been reported as an effective additive in traditional extractive distillation,especially for the separation of mixture of ethylic acid and water[12–13].Zhang[14]selected para-xylene as solvent of methanol and trimethoxysilane in extractive distillation and found that para-xylene can break the azeotrope.However,up to now,the utilization of paraxylene as extraction agent to separate azeotropic system of ethanol+ethyl propionate has not been reported,which may be due to the lack of VLE data.The binary system VLE data,including ethanol(1)and ethyl propionate(2)and ethanol(1)and p-xylene(3),have been reported in the literature[4,15],but lack of VLE data of ethyl propionate(2)and p-xylene(3).In addition,the ternary system of ethanol(1)+ethyl propionate(2)+para-xylene(3)also has no VLE data reported openly.Since the lack of VLE data and its preferable prospect in extractive distillation process,measurement of isobaric VLE data for binary of ethyl propionate(2)+p-xylene(3)and ternary system of ethanol(1)+ethyl propionate(2)+para-xylene(3)is meaningful.

In this paper,the VLE data for binary system of ethyl propionate+para-xylene and ternary system of ethanol+ethyl propionate+para-xylene were determined at 101.3 kPa.The Wilson and UNIQUAC activity coefficient models were employed to correlate the binary VLE data and to obtain the interaction parameters,thus by use of the interaction parameters to predict the ternary VLE data.

2.Experimental

2.1.Chemicals

Three chemicals are ethanol,ethyl propionate and para-xylene,respectively,which are always miscible in all measurements.Their corresponding information ab out molecular formula,purity grade and source is listed in Table 1.The purity of all the chemicals was measured by gas chromatograph(GC)with a FID detector.All the reagents were used without further purification.

Table 1 Chemical reagents information

2.2.Procedure

A circulation vapor–liquid equilibrium was used to measure the isobaric VLE data of binary and ternary system[16].The volume of the chamber was about 50 ml,of which 40 ml was taken up by liquid.A mercury thermometer below the liquid level was used to detect experimental temperature.The accuracy of the mercury thermometer is±0.1 K.The pressure was measured by a transducer(Digiquartz 2300A)connected to a Digiuartz 740 intelligent display unit(Paroscientific)whose accuracy is 0.01%.More details about the apparatus were referred to our previous papers[17,18].

About 40-ml liquid samples were fed into the chamber during each experiment,then heated at 101.3 kPa.The system was considered to reach equilibrium state when the temperature of the mercury thermometer was not change for about 1 h.Then,the samples of vapor and liquid phase were collected at the same time for analysis.To minimize the effect of sample amount on the equilibrium,three analyses were in parallel conducted and the amounts of the analyses were taken at 0.1 ml.

2.3.Analysis

Component analysis of the equilibrium vapor and liquid phase were conducted by GC2060 with a FID detector and SE-54 column(30 m×0.32 mm × 0.5 μm).High purity nitrogen was carrier gas at flow rate of 30 ml·min?1.The temperatures of injector,detector and oven were 473.15 K,473.15 K and 393.15 K,respectively.Standard solutions were applied to calibrate the GC,which were prepared gravimetrically by an electronic balance(FA2004N,uncertainty of±0.0001 g).In addition,calibration factor of pure substance was determined ahead of time.The final composition of each sample was determined upon the average of three analyses.

3.Results and Discussion

3.1.Experimental data

The VLE data for binary systems,including ethanol(1)+ethyl propionate(2)and ethanol(1)+para-xylene(3),are measured at 101.3 kPa.The activity coefficient(γi)is determined by the following equation[19–20]:

where xiand yirepresent the liquid and vapor content of component i,respectively;obtained according to the extended Antoine equation,is the saturation vapor pressure of pure component i;is the liquid molar volume of pure liquid i and R is the gas constant;φiandare the fugacity coefficient of component i in the heterogeneous vapor phase and in homogeneous saturated vapor phase,separately.At low pressure,the gas phase can be regarded as ideal gas,andis approximately equal to 1.Meanwhile,φiandare equal to 1,respectively[21,22],so the activity coefficient equation can be simplified as follows:

The extended Antoine equation is defined by Eq.(3).

where C1,i?C7,i,Tminand Tmaxare pure component constants which are listed in Table 2.

Table 2 The constants C1,i-C7,i,T min and T max for the pure components?

The isobaric VLE data for ethanol(1)+ethyl propionate(2)and ethanol(1)+para-xylene(3)are presented in Tables 3 and 4.To check the stability of experimental device,the experimental VLE data were compared with literature data[4,15],which are shown in Figs.1and 2.Meanwhile,the absolute and relative errors in temperature and vapor phase mole fraction are listed in Table 5.Obviously,the experiment results show a good agreement with literature data.Therefore,it is confirmed that the experimental device and operation process are reliable.

Table 3 VLE data and activity coefficients for the binary system of ethanol(1)+ethyl propionate(2)at 101.3 kPa?

Table 4 VLE data and activity coefficients for the binary system of ethanol(1)+para-xylene(3)at 101.3 kPa?

Fig.1.T vs x1,y1 diagram for the ethanol(1)+ethyl propionate(2)system at 101.3 kPa(●,experimental vapor phase composition y1; ■,experimental liquid phase composition x1;—,literature liquid phase composition x1;…,literature vapor phase composition y1[4]).

Fig.2.T vs x1,y1 diagram for the ethanol(1)+para-xylene(3)system at 101.3 kPa(●,experimental vapor phase composition y1;■,experimental liquid phase composition x1;—,literature liquid phase composition x1;…,literature vapor phase composition y1[15]).

Table 5 The mean absolute and relative deviations of vapor phase mole fraction and equilibrium temperature for system of ethanol(1)+ethyl propionate(2)and ethanol(1)+para-xylene(3)

The isobaric VLEdata for the binary system of ethyl propionate(2)+para-xylene(3)and ternary system of ethanol(1)+ethyl propionate(2)+para-xylene(3)were obtained at 101.3 kPa,which are listed in Tables 6 and 7.For the ternary system,para-xylene was added into the still at a constant content(50 mol%).In Table 7,x′and y′denote the mole fraction of corresponding liquid and vapor phase on the basis of free para-xylene,respectively.The relative volatility of ethanol to ethyl propionate is determined by the following equation[23]:

Table 6 VLE data and activity coefficients for the binary system of ethyl propionate(2)+paraxylene(3)at 101.3 kPa?

In extractive distillation,relative volatility is an important factor to evaluate the performance of extraction agent[24,25].When the relative volatility is greater than 1,the two components can be separated by distillation[21].Table 7 shows that the minimum relative volatility of ethyl alcohol to ethyl propionate is 3.4141 after para-xylene was added,which demonstrates the complete separation of ethanol and ethyl propionate can be achieved by extractive distillation.

Table 7 VLE data for the ternary system of ethanol(1)+ethylpropionate(2)+para-xylene(3)at 101.3 kPa?

3.2.Data regression

The acquired VLEdata was correlated with the Wilson and UNIQUAC models by Aspen Plus to gain the interaction parameters of the ternary system of ethanol(1)+ethyl propionate(2)+para-xylene(3)[26].To obtain the minimizing maximum likelihood objective function,the binary VLE data was regressed,which was described as:

where σ is the standard deviation of the corresponding parameters.The standard deviations of pressure σP,temperature σT,liquid composition σxand vapor composition σyused in this VLE data correlation are 0.1013 kPa,0.1 K,0.001 and 0.001,respectively.

The correlated parameters and the root-mean-square deviations(RMSD)of temperature and vapor phase mole fraction are given in Table 8.Meanwhile,the contradistinction between experimental data and calculated data is presented in Fig.3,revealing that all the values calculated by the two models fit well with the experimental data.

Table 8 Correlated parameters and RMSD for systems of ethanol(1)+ethyl propionate(2),ethanol(1)+para-xylene(3)and ethyl propionate(2)+para-xylene(3)

3.3.Consistency tests of experimental data

The Van Ness test method,a point consistency method put forward by Fredenslund et al.[27],was quoted to verify the reliability of experimental data[28].The criterion is expressed by the following eq.[29]:

Fig.3.T vs x2,y2 diagram for the ethyl propionate(2)+para-xylene(3)system at 101.3 kPa(■,□ experimental data;…,calculated data with Wilson model;—,calculated data with UNIQUAC model).

where n is the number of experimental data points;the superscript exp represents experimental data;the superscript cal represents values determined by Wilson and UNIQUAC models.If the value of Δyiis lower than 1,the VLE data can be confirmed to be thermodynamically consistent.Table 9 shows the results of binary and ternary systems applying the above expression,revealing that all the experimental data obtained in this work is thermodynamically consistent.

Table 9 The results of thermodynamic consistency test of Van Ness method for the binary and ternary systems

3.4.Data prediction

The ternary VLE data of ethanol(1)+ethyl propionate(2)+para-xylene(3)were predicted with the correlated binary parameters which were obtained by Wilson and UNIQUAC models.The maximum and mean absolute deviations of equilibrium temperature and vapor mole fraction for each system are listed in Table 10.The results illustrate that the predicted data agrees well with the experimental data,which indicate that both Wilson and UNIQUAC models can predict the experimental data accurately.A vapor–liquid residue curve map is constructed by residue in a simple distillation in time,which is a geometric analysis method for distillation system to explain the composition of an azeotropic system[30,31].In order to further check ternary system VLE data,the residue curve of the ternary system was predictedby UNIQUAC model using correlated binary parameters,which was shown in Fig.4.The connecting lines of the vapor phase points and liquid phase points are tangent very well with the residue curves atthe liquid phase points,indicating that the prediction data are coincident with experimental data[32,33].

Table 10 Maximum and mean absolute deviations of equilibrium temperature and vapor-phase mole fraction for system of ethanol(1)+ethyl propionate(2)+para-xylene(3)

Fig.4.Residue curves of the ternary system ethanol(1)+ethyl propionate(2)+paraxylene(3)(■,experimental liquid phase composition;○,experimental vapor phase composition;—,pairs of VLE data;…,residue curves).

Fig.5.x1 vs y1 diagram for the comparison of VLE behavior of binary system ethanol(1)+ethyl propionate(2)with and without para-xylene(▲,experimental VLE data without para-xylene;●,experimental VLE data with para-xylene).

To investigate the effect of with and with out para-xylene on the system of ethanol(1)+ethyl propionate(2),isobaric VLE data of the binary system are presented in Fig.5.As shown in Fig.5,one can note that the azeotropic phenomenon is disappeared when the mole ratio of para-xylene and binary system of ethanol and ethyl propionate is 1:1.The reason why para-xylene can change the relative volatility of azeotrope in our work may be explained by the fact that attraction of para-xylene for alcohols is larger than that for esters[34],which demonstrates that para-xylene is a potential extraction agent for this system.

4.Conclusions

Isobaric VLE data of the binary system ethyl propionate+para-xylene and ternary system of ethanol+ethyl propionate+para-xylene were determined at 101.3 kPa.The thermodynamic consistency test indicated that both binary and ternary VLE data passed the Van Ness test.Wilson and UNIQUAC activity coefficient models were used to correlate experimental data to obtain binary interaction parameters.The comparison between the experimental data and VLE data predicted by the two models reveals that predicted data fits well with experimental date.The azeotropic phenomenon vanishes when the mole ratio of the azeotrope and para-xylene is 1:1.The experimental and prediction results imply that para-xylene is an available additive to separate the binary system of ethanol and ethyl propionate in extractive distillation.

Nomenclature

aij,aji,bij,bjithe correlated parameters of Wilson and UNIQUAC models

C1,i?C7,ipure component constants

F term defined by Eq.(6)

n the number of experimental data points

P the total pressure,kPa

saturated vapor pressure of pure component i,kPa

R universal gas constant

T tempreture,K

ΔT mean absolute deviations of equilibrium temperature

δT relative deviations of equilibrium temperature

u standard uncertainty

liquid molar volume of pure liquid i,m3·mol?1

xi,yiliquid and vapor content of component i,respectively

Δy mean absolute deviations of vapor-phase mole fraction

δy mean relative deviations of vapor phase mole fraction

σy root-mean-square deviations of vapor phase mole fraction

α12relative volatility of ethanol to ethyl propionate

γ liquid activity coefficient

σ the standard deviation

fugacity coefficient of component i in the mixture vapor phase and pure saturated vapor

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