Liejin Luo,Dong Liu,Libo Li,Yun Chen

Key Laboratory of Heat Transfer Enhancement and Energy Conservation of Ministry of Education,School of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou 510640,China

1.Introduction

Liquid-liquid extraction plays an important role in removing phenolic compounds from wastewater.Phenolic compounds,such asp-cresol,are major toxic pollutants usually detected in the effluent of various industries.Some solvents including ether,ketone,ester and alcohol can be used to extract phenols from wastewater,because phenols show much higher solubility in these solvents than in water.Liquid-liquid equilibria(LLE)data are essential for proper understanding and simulating the solvent extraction processes[1,2].There have been a lot of liquid-liquid equilibrium studies about ternary systems of water+phenolic compound+extraction solvent,such as 2-methoxy-2-methylpropane+phenol or hydroquinone+water[3,4]and methyl butyl ketone+phenol or hydroquinone+water[5].

p-Cresol is widely used as antioxidants,herbicides,surfactants,pigments and dyes[6,7].Waste water from relevant industries usually contains such compound with high concentration,whose water solubility is above 24.4 g·L-1at298.15 K[8].p-Cresolis highly toxic,potentially carcinogenic,and can bring negative effect to the nervous,cardiovascular and respiration systems even at very low concentrations[9,10].For such reason,p-cresol is classified as a pollutant of Group C(possible human carcinogens)and is listed as a priority pollutant by USA-EPA[11,12].Therefore,we studied 2-methoxy-2-methylpropane as an organic solvent to extractp-cresol from water.2-Methoxy-2-methylpropane is chosen as the solvent due to its high distribution coefficients[13]and low energy consumption in phenol recovery process.

In this work,the LLE data for the ternary system,2-methoxy-2-methylpropane+p-cresol+water,were measured at 298.15 K and 313.15 K under atmospheric pressure.Also,this system's phase behavior was studied.The reliability of these tie-line data was assessed by the Hand[14]and Bachman[15]equations.They were also correlated by the NRTL[16]and UNIQUAC[17]models,which both yield LLE data in good agreement with experimental results.To the best of our knowledge,no similar LLE data has been reported for the 2-methoxy-2-methylpropane+p-cresol+water ternary system.

2.Experimental

2.1.Chemicals

Commercial analytical grade chemicals were used in this work.2-Methoxy-2-methylpropane was supplied by Sinpharm Chemical Reagent Co.,Ltd.p-Cresol and 1,3,5-trimethylbenze were obtained from Xiya Reagent Research Center.Methanol was purchased from ShangHai LingFeng Chemical Reagent Co.,Ltd,andn-butanol from JiangSu Qiangsheng Functional Chemistry Co.,Ltd.Distilled water was prepared in our laboratory and used throughout all experiments in this work.The purity of these chemicals was verified before use by gas chromatography and by comparing their densities with reported values[18,19].The purities and densities of these chemicals are shown in Table 1.

2.2.Procedure

In this work,all chemicals' densities were measured by a digital vibrating glass tube densitometer(Anton Paar,DMA 4500 M,Austria)to verify their purities.This densitometer was automatically calibrated with distilled water and dry air.Its temperature was set at 293.15 K with an uncertainty of±0.03 K.According to the user manual,the uncertainty of measured density was 5 × 10-5g·cm-3in the density range of0-3 g·cm-3.Please refer to other literatures[20,21]for the details of density measurement.

In LLE experiments,a 100 ml glass-sealed cell surrounded by a thermostat water jacket was used to prepare the samples.The cell temperature was controlled by a thermostatic bath,whose fluctuation was less than 0.1 K.An～55 ml2-methoxy-2-methylpropane+p-cresol+water ternary mixture which contained～5 ml 2-methoxy-2-methylpropane,～10 mlp-cresol and～40 ml water was fed into this glass cell,followed by a more than 2-hour vigorous agitation with a magnetic stirrer,then was left to stand for 18 h or more until the phase equilibrium was reached and the ternary mixture formed two liquid phases.Samples of both phases were collected by syringes,and were analyzed by a gas chromatography(GC6820,Agilent Technologies)equipped with a flame ionization detector(FID)and a DB-5MS capillary column(30 m × 0.32 mm × 0.25 μm).After GC analysis,we added 3-5 ml of 2-methoxy-2-methylpropane into the glass cell and repeated the above procedures to get a new tie-line data.In GC analysis,the injection volume for the organic phase was 0.5 μl while that for the aqueous phase was 1 μl,so the split ratio was 20:1.The sample's composition was determined by an internal standard method,where an internal standard was added into the LLE sample,collected by syringes from the LLE system,before GC analysis.All chemicals in GC analysis were weighed by an analytical balance(Shimadzu,AUW220D)whose accuracy was 0.1 mg.In the experiments,1,3,5-trimethylbenze and methanol were used as internal standards forp-cresol and 2-methoxy-2-methylpropane,respectively.n-Butanol was used as solvent to dissolve GC analysis samples with internal standards.The mass fraction of water was calculated from the mass balance equation:the total mass fraction of the extractants,p-cresol and water is 1.In GC analysis,the initial temperature of the oven was keptat313.15 Kfor2 min,then was increased to 473.15 Kata rate of30-K·min-1.The carrier gas was nitrogen,with a flow rate of30 ml·min-1.The temperatures of the injector and the detector were set at 523.15 K and 543.15 K respectively.Each sample was measured at least 3 times and the standard deviation was less than 0.5%,so the average value was reported in this work.

Table 1Purities,densities and UNIQUAC parameters of the components used in this work

Table 2Experimental LLE data for ternary system 2-methoxy-2-methylpropane(1)+p-Cresol(2)+Water(3)at T=298.15 K and T=313.15 K①

3.Results and Discussion

3.1.LLE experimental data

Table 2 shows the LLE data for the ternary system:2-methoxy-2-methylpropane+p-cresol+water at 298.15 and 313.15 K,and all concentrations were expressed as mass fraction.Fig.1 shows corresponding phase behavior for this ternary system.The distribution coefficient(D)and the selectivity(S)were calculated to estimate the ability of 2-methoxy-2-methylpropane to extractp-cresol from water:

where superscripts O and W means organic solvent phase and aqueous phase respectively,w2is the mass fraction ofp-cresol andw3is that of water.The distribution coefficient(D)reflects the solvent capacity of 2-methoxy-2-methylpropane while the selectivity(S)provides a measurement for the efficiency of 2-methoxy-2-methylpropane used in extraction.The distribution coefficient and selectivity forp-cresol at both temperatures are also shown in Table 2,which indicate that,2-methoxy-2-methylpropane is an excellent solvent to extractp-cresol from water.In addition,the dependence of distribution coefficients on the temperature is insignificant.

Fig.1.Ternary diagram for experimental LLE(mass fraction)of the ternary system,2-methoxy-2-methylpropane+p-cresol+water at T=298.15 K(a)and T=313.15 K(b).

The experimental tie-line data's reliability was verified by the Hand(H)and Bachman(B)correlation equations,given by Eqs.(3)and(4),respectively:

wherea1,b1anda2,b2are parameters for Hand and Bachman equations respectively,w1,w2andw3are mass fractions for 2-methoxy-2-methylpropane,p-cresol and water,respectively.The straight lines calculated from Hand and Bachman equations are shown in Figs.2 and 3.The fitting parameters,together with corresponding linear correlation coefficientR2,are listed in Table 3.All correlation coefficient,R2,are greater than 0.995,indicating a high consistency of our LLE data.

Fig.2.Hand plot for LLE data measured in this work.●The 2-methoxy-2-methylpropane+p-cresol+water system at T=298.15 K;■the 2-methoxy-2-methylpropane+p-cresol+water system at T=313.15 K.

Fig.3.Bachman plot for LLE data measured in this work.●The 2-methoxy-2-methylpropane+p-cresol+water system at T=298.15 K;■the 2-methoxy-2-methylpropane+p-cresol+water system at T=313.15 K.

Table 3Fitting parameters in Hand and Bachman equations

3.2.Data correlation

The experimental LLE data in this work were correlated by using the NRTL and UNIQUAC models.The equations for the NRTL model are:

where τji=(gji-gii)/RT

HereGEis the excess Gibbs energy,Ris the idealgas constant andTis the absolute temperature.

The equation for the UNIQUAC model is:

where

Here θiand ψiare area fraction and segment fraction for speciesi,qiandristand for UNIQUAC area and volume for this species,and τ is an adjustable parameter.Valuesqandr,the so called pure component structural parameters,were taken from literatures[22,23]and were listed in Table 1.

Binary interaction parameters in NRTL or UNIQUAC models were calculated from minimizing the objective function(OF):

wherenis the number of tie-lines,wexpandTexprefer to experimental mass fraction and temperature,wcalandTcalstand for calculated mass fraction and temperature.Subscriptsi,jandkare the number of components,phases and tie-lines,respectively.σTand σware standard deviations for experimental temperatures and mass fractions.The resulting binary interaction parameters are shown in Tables 4 and 5 for the NRTL and UNIQUAC models,respectively.

The quality of the NRTL or UNIQUAC correlation was estimated by the root-mean-square-deviation(RMSD),the difference betweenexperimental and calculated mass fractions as given by the following equation:

Table 4NRTL binary interaction parameters and RMSD values for the ternary system:2-methoxy-2-methylpropane(1)+p-cresol(2)+water(3)

Table 5UNIQUAC binary interaction parameters and RMSD values for the ternary system:2-methoxy-2-methylpropane(1)+p-cresol(2)+water(3)

In this work,the above RMSD values are very close to 0,as listed in Tables 4 and 5.This indicates that both NRTL and UNIQUAC models agree with experiments very well for the studied ternary system.We plot experimental and calculated mass fractions from NRTL or UNIQUAC models forp-cresolin Fig.4,which also revealgood agreement between them.All these results show that both NRTL and UNIQUAC models can simulate thep-cresol extraction process accurately.

Fig.4.Mass fraction of p-cresol in organic phase versus that in aqueous phase for the 2-methoxy-2-methylpropane+p-cresol+water system.●Experimental data at 298.15 K;…calculated results from the NRTL model at 298.15 K;--calculated results from the UNIQUAC model at 298.15 K.■Experimental data at 313.15 K;--calculated results from the NRTL model at 298.15 K;---calculated results from the UNIQUAC model at 313.15 K.

4.Conclusions

LLE data for the ternary system,2-methoxy-2-methylpropane+p-cresol+water,were measured at temperatures of(298.15 and 313.15)K and atmospheric pressure.Our experimental tie-line data show high reliability,as all regression coefficients(R2)from the Hand and Bachman equations are close to 1.The distribution coefficients and selectivity values indicate that 2-methoxy-2-methylpropane is very promising to serve as a solvent to extractp-cresol from the wastewater.Additionally,the experimental LLE data were correlated with the NRTL and UNIQUAC models to yield binary interaction parameters for simulatingp-cresolextraction processes.These 2 models agree with experiments very well,as indicated by the RMSD values and comparison figures.Both experimental LLE data and calculated binary interaction parameters in this work may be useful for simulating or optimizingp-cresol extraction processes.

Nomenclature

aijbinary interaction parameter for NRTL

a1,b1Hand correlation parameters

a2,b2Bachman correlation parameters

bijbinary interaction parameter for UNIQUAC

nnumber of tie-lines

Rideal gas constant

R2linear correlation coefficient

RMSD root-mean-square-deviation

r,qUNIQUAC parameters

Sselectivity factor

Tabsolute temperature

wmass fraction

θ area fraction for UNIQUAC

ρ density

σ standard deviation

τ adjustable parameter for UNIQUAC

ψ segment fraction for UNIQUAC

Subscripts

icomponenti

jphasej

ktie-linek

Superscripts

cal calculated value

exp experimental value

O organic solvent phase

W aqueous phase

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