Youchang Wang,Shaoming Zhou,Jinhui Tang,Yun Chen*,Libo Li*

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

Keywords:Liquid–liquid equilibrium Mesityl oxide Resorcinol NRTL UNIQUAC

A B S T R A C T Liquid–liquid equilibrium(LLE)data were measured for the ternary system of water+resorcinol+mesityl oxide under atmospheric pressure at temperatures of(298.2,323.2,333.2,343.2 and 353.2)K.The reliability of the experimental data was verified by the Hand and Bachman equations.The distribution coefficient and selectivity were calculated from the experimental LLE data,which showed high efficiency of mesityl oxide extracting resorcinol from the aqueous solution.The NRTL and UNIQUAC models were employed to fit the measured experimental LLE data and yield corresponding binary interaction parameters.

1.Introduction

Phenolic compounds(also known as ‘phenols’),especially nonvolatile phenols(e.g.resorcinol),are priority pollutants with high toxicity even at low concentration[1].They exist in the wastewater from a large variety of industries,such as oil refining,pharmaceuticals,coking operations,resin manufacturing,plastics,paint,paper and wood plants[2,3].On one hand,resorcinol is even more toxicity than other phenols[4,5],thus discharging it to the environment without treatment may bring serious health risks to human beings and environment[6].The resorcinol concentration in typical industrial effluents could be hundreds of ppm[7],butit's concentration in surface water has been restricted to less than 0.5 mg·kg-1(0.5 ppm)in many countries[8].Thus there have been long standing efforts to remove resorcinol from industrial wastewater before discharging it[1].On the other hand,resorcinol is a valuable reagent for many applications,which can be used as polymerization inhibitors,antioxidants,photographic developing agents or used as raw material to synthesize value-added chemicals such as dyes,drugs,explosive materials and electronic chemicals[9].Therefore,it is of considerable environmental and economic significance to recover resorcinol from wastewater.

Solvent extraction is an efficient method in industry to recover phenolic compounds from wastewater[10],and liquid–liquid equilibrium(LLE)data are essential for simulating or designing an extraction process[11–14].Solvent extraction of phenolic wastewater had been performed below 328 K,but the paraffin wax(melting point ranges from 328 K to 335 K)in the wastewater would clog the extraction equipment[15].Therefore,plants,e.g.China Coal Erdos Energy&Chemical Co.,Ltd.in China,Xinjiang Guanghui Energy Co.Ltd.,have risen the operation temperature to above 333 K to avoid such severe problem.However,LLE data for the ternary systems,water+extractant+phenolic compound,above 333 K are quite scarce yet,though they are critical for screening a proper extractant to extract phenolic compounds from wastewater efficiently and economically in this temperature range.LLE data for dihydric phenols such as resorcinol,either above or below 333 K,are even more rare despite that,resorcinol is highly toxic[4,5],extremely intractable to biological degradation[3,4],and more difficult to extract due to its higher hydrophilicity[4].Methyl isobutyl ketone,a hydrogenated product of mesityl oxide,has been widely used in industry due to excellent efficiency(in terms of distribution coefficient and selectivity)in the extraction[16].And mesityl oxide has shown excellent efficiency in extracting a range of phenolic compounds,such as phenol[17]and cresols[18].However,LLE of the ternary system,{water+resorcinol+mesityl oxide},and corresponding extraction efficiency of mesityl oxide have not been studied yet.

In this work,liquid–liquid equilibrium(LLE)data for the ternary mixtures,{water+resorcinol+mesityl oxide},were determined at the temperature T=(298.2,323.2,333.2,343.2 and 353.2)K,whose reliability was verified by the Hand and Bachman equations.These LLE data were also correlated with the NRTL[19]and UNIQUAC[20]models to yield binary interaction parameters,which could be used for the relevant process simulations.

2.Experimental

2.1.Chemicals

Suppliers and purity grades(expressed as mass fraction)of the chemical reagents used in this work are shown in Table 1.The purity of these reagents was confirmed by gas chromatography,which were then used without further purification due to their purities over 98%.Deionized water was used in all the experiments.

Table 1Suppliers and purity grade(mass fraction)of chemical reagents in this work

Table 2Experimental LLE data(mass fraction)for ternary system{water(1)+resorcinol(2)+mesityl oxide(3)}at temperature T and pressure p=0.1 MPa①

Fig.1.LLE data for ternary systems{water(1)+resorcinol(2)+mesityl oxide(3)}at T=298.2 K:■,experimental data;□,calculated data from NRTL model;○,calculated data from UNIQUAC model.

2.2.Procedure

To obtain the LLE data,the mixture solution was prepared in a volume of 100 ml glass-sealed cell consisting of a thermostat water jacket.The temperature of the cell was controlled by a thermostatic bath with a precision of±0.1 K.The water+resorcinol+mesityl oxide ternary mixture with an approximate volume of85 mlwasloaded into the glass cell,stirred vigorously for more than 2 h and then was left to stand for at least 18 h to reach the phase equilibrium.After the above mixture formed two liquid phases,each of them was sampled by a syringe.The analysis of all LLE samples was performed on an Agilent 6820 gas chromatograph produced by Agilent Technologies with a DB-5MS capillary column(30 m×0.32 mm×0.25 μm)and a flame ionization detector(FID,the sensitivity was 10–100 ppb),and their compositions were analyzed by an internal standard method,with n-octanolas the internal standard for resorcinol and n-propyl acetate for mesityl oxide.Water mass fraction in each phase was calculated by deducting the mass fractions of resorcinol and mesityl oxide from 1.The column temperature was first kept at 313.15 K for 2 min,and then increased to 433.15 K at a rate of 30 K·min-1.Nitrogen was used as the carrier gas with a flow rate of 30 ml·min-1.The injector's and the detector's temperatures were set to 493.15 K and 543.15 K respectively.The samples and internal standards were weighed by an analytical balance(Shimadzu,AUW220D)whose accuracy was 0.0001 g.Each LLE sample was GC analyzed at least three times,and the standard deviation was less than 0.5%,thus the average value was reported.

Fig.2.LLE data for ternary systems{water(1)+resorcinol(2)+mesityl oxide(3)}at T=323.2 K:■,experimental data;□,calculated data from NRTL model;○,calculated data from UNIQUAC model.

3.Results and Discussion

3.1.LLE experimental data

The experimental LLE data for the ternary system,water(1)+resorcinol(2)+mesityl oxide(3),at temperatures of(298.2,323.2,333.2,343.2 and 353.2)K are listed in Table 2,with all concentrations expressed in mass fractions.Figs.1–5 show the corresponding triangular phase diagrams at different temperatures.The distribution coefficient(Di)for water(i=1)and resorcinol(i=2),and the selectivity(S),calculated from the experimental LLE data,were used to estimate the efficiency of mesityl oxide extracting resorcinol from the aqueous phase:

Fig.3.LLE data for ternary systems{water(1)+resorcinol(2)+mesityl oxide(3)}at T=333.2 K:■,experimental data;□,calculated data from NRTL model;○,calculated data from UNIQUAC model.

Fig.4.LLE data for ternary systems{water(1)+resorcinol(2)+mesityl oxide(3)}at T=343.2 K:■,experimental data;□,calculated data from NRTL model;○,calculated data from UNIQUAC model.

Table 3Distribution coefficients(Di)of water(i=1)and resorcinol(i=2)and Selectivity(S)along the resorcinol concentrations in the aqueous phase(w23)at(298.2,323.2,333.2,343.2,and 353.2)K

Fig.6.Distribution coefficient of resorcinol,D2,plotted against mass fraction of resorcinolin the aqueous phase at different temperatures:–▲–,at298.2 K;–△–,at333.2 K;–★–,at333.2 K;–☆–,at 343.2 K;–●–,at 353.2 K.

where subscript j in wijmean organic phase(j=1)and aqueous phase(j=3),respectively.These distribution coefficients and selectivities at different temperatures are listed in Table 3,also are shown in Figs.6 and 7.According to these results,the distribution coefficients of resorcinol,D2,decrease with the resorcinol concentration in the aqueous phase increasing,especially when the aqueous phase resorcinol concentration is below 0.02(note that the resorcinol concentration in a typical coal industry wastewater is about hundreds ppm,well below 0.02[7]).D2decrease with the temperature increasing,though such effect become insignificant for T above 298.2 K.The influence of the temperature or resorcinol concentration on selectivity is similar:the increasement of temperature or concentration leads to selectivity decreasing.In comparison with other phenolic compounds,resorcinol is more difficult to extract than phenol or cresols:e.g.when MIBK is used as the extractant at 333.2 K,the D and S for resorcinol,phenol,m-cresol are(12.18–14.74,97.30–235.75)[5]＜ (41.87–83.73,274.47–1546.12)[15]＜ (76.63–205.53,630.79–6918.62)[21],respectively.Such sequence agrees with their lg Kow:0.8＜1.46＜1.9,indicating that phenolic compounds with higher water affinity,e.g.resorcinol,are more difficult to extract,as reported in our previous publications[4,5].Thus we need screen extractants more thoroughly when extracting resorcinol,especially when treating highly concentrated phenolic wastewater at elevated temperatures.

Fig.7.Selectivity of resorcinol,S,plotted against mass fraction of resorcinol in the aqueous phase at different temperatures:–▲–,at 298.2 K;–△–,at 333.2 K;–★–,at 333.2 K;–☆–,at 343.2 K;–●–,at 353.2 K.

Fig.8.Distribution coefficient(D)and selectivity(S)of mesityl oxide and MIBK for resorcinol versus the mass fraction of resorcinol in aqueous phase at different temperatures.

Fig.9.Distribution coefficient(D)and selectivity(S)of mesityl oxide and MIPK for resorcinol versus the mass fraction of resorcinol in aqueous phase at different temperatures.

A few of other solvents,such as diisopropyl ether(DIPE)[21,22],methyl isobutyl ketone(MIBK)[5]and methyl isopropyl ketone(MIPK)[4],have been studied to extract resorcinol.The distribution coefficient(D)of resorcinol in DIPE(~2.06,resorcinol concentration~4000 mg·kg-1,298.2 K)is far below mesityl oxide.The distribution coefficient(D)and selectivity(S)of mesityl oxide are compared with MIBK and MIPK whenever applicable in Figs.8&9(note that the temperature range in this study may not be fully covered in other works).In the temperature range of 298.2 K to 323.2 K,the D and S of mesityl oxide are considerably higher than other solvents(e.g.MIPK and MIBK)when the resorcinol concentration varies from 200 to 4000 mg·kg-1(Figs.8&9).From 333.2 to 353.2 K,D of mesityl oxide is higher or close to MIBK for typical resorcinol concentration in industrial effluent(mass fraction ＜2000 mg·kg-1),while S is considerably higher for the resorcinol concentration range of 200–2500 mg·kg-1(Fig.8).The high selectivity of mesityloxide indicates that,fewer stages,consequently smaller apparatus and lower costs,are required when designing a separation process.It also indicates less water in the extract phase,and lower difficulty to separate water from the extractant.The high distribution coefficient of mesityl oxide means less solvent usage and lower operating costs for a given separation process.In addition,mesityl oxide's boiling point(402.2 K)is considerably higher than ethers(e.g.DIPE 341.7 K)and ketones(e.g.MIBK,391.2 K;MTBK,379.2 K;MIPK,390.0 K),thus a mesityl oxide based extraction process could be performed at higher temperature,which is quite necessary for treating wastewater with highly concentrated paraffin.Furthermore,the toxicity of mesityl oxide(LD50,rats oral,1120 mg·kg-1)is lower than other solvents,e.g.Pinacolone(LD50,rats oral,610 mg·kg-1),MIPK(LD50,rats oral,148 mg·kg-1,);Heptane(LD50,rats oral,222 mg·kg-1),etc.The toxicity data were obtained from web page(https://en.wikipedia.org/wiki/).Moreover,a large variety of other organic solvents have been investigated for extracting phenols,such as ethylbenzene[23],(hydrocarbon solvent);1-,2-or tert-butanol[24],(alcoholic solvent);isopropylether(ether solvent)[25],isobutylacetate(esters solvent)[26].The distribution coefficients of phenolin these solvents are much lower than mesityl oxide,which could be attributed their weak interactions with phenolic compounds[27].Thus,it could be expected that,their extracting performance for resorcinol should also be lower than mesityl oxide.Additionally,these solvents usually suffer from other flaws,such as high solubility in water(e.g.1-octanol),high volatility,easy to hydrolyze,or requiring complex process to recover,etc.In summary,all these results show that mesityl oxide is a promising solvent to extract resorcinol from the aqueous phase in the temperature range of 298.2–353.2 K,due to its high D and S,high boiling point and low toxicity.

The reliability of the experimental liquid–liquid equilibrium data was verified by the Hand[28]and Bachman[29]equations,as shown in Eqs.(3)and(4):

where w21and w23are the mass fractions of resorcinol in the organic and the aqueous phase,respectively,w31is the mass fraction of mesityl oxide in the organic phase and w13is that of water in the aqueous phase,a1,b1and a2,b2are fitting parameters.The straight lines plotted by these two equations are shown in Figs.10 and 11,respectively.Table 4 presents the fitting parameters(a's and b's)and regression coefficients of Eqs.(3)and(4)for the studied system.All linear correlation coefficients(R2)are very close to 1,indicating a good concurrency of our experimental LLE data.

3.2.Data correlation

The experimental LLE data were correlated by the NRTL and UNIQUAC activity coefficient models(all concentrations were converted to mole fractions in the correlation).The non-randomness parameter(αij)in the NRTL model was set as 0.2 or 0.3[30].The structural parameters(r and q)of the UNIQUAC model were taken from literatures[31,32],as listed in Table 5.

Fig.10.Hand plots for{water(1)+resorcinol(2)+mesityl oxide(3)}system:☆,resorcinol at 298.2 K;▲,resorcinol at 323.2 K;●,resorcinol at 333.2 K;★,resorcinol at 343.2 K;○,resorcinol at 353.2 K.

Fig.11.Bachman plots for{water(1)+resorcinol(2)+mesityl oxide(3)}system:☆,resorcinol at 298.2 K;▲,resorcinol at 323.2 K;●,resorcinol at 333.2 K;★,resorcinol at 343.2 K;○,resorcinol at 353.2 K.

Table 4Fitting parameters in hand or Bachman equations

Table 5Structural(area and volume)parameters for the UNIQUAC model

Binary interaction parameters(gij–gjiand gji–gij,uij–ujiand uji–uij)in the NRTL or UNIQUAC correlation were obtained by minimizing the objective function(OF)[33]:

where n refers to the number of tie-lines,wexpand wcaldenote experimental and calculated mass fractions,and Tcalare experimental and calculated temperatures,subscripts i refer to the component,j and k refer to the phase and the tie-line,respectively,σTand σware standard deviations of the experimental temperatures and mass fractions,respectively.The regressed parameters of two activity coefficient models for the water+resorcinol+mesityl oxide ternary system,are shown in Table 6.

The quality of the correlation between the model and experimental data,was assessed by the root-mean-square-deviation(RMSD):

Table 6Binary interaction parameters and RMSDs calculated from NRTL and UNIQUAC model for ternary systems:Water(1)+Resorcinol(2)+Mesityl Oxide(3)

where i,j,k,n,wexpand wcalshare similar meanings as those in Eq.(5).These RMSDs are all below 0.011,as listed in Table 6.The calculated tie line data,either by NRTL or UNIQUAC models,are compared with the experimental data in Figs.1–5(all concentrations were converted to mass fractions in Figs.1–5),showing good agreement.All these results indicate both NRTL and UNIQUAC models could represent the experimental LLE data accurately,and could be used to simulate the process of extracting resorcinol from water with mesityl oxide.

4.Conclusions

Liquid–liquid equilibrium(LLE)data for the ternary system,{water+resorcinol+mesityl oxide}at(298.2,323.2,333.2,343.2 and 353.2)K under atmospheric pressure were measured.The experimental tie-line data are quite reliable,as confirmed by the Hand and Bachman equations.The distribution coefficients and selectivity values indicate that,mesityl oxide could recover resorcinol from the aqueous phase rather efficiently.A comparison with other extractants showed promising application potential of mesityl oxide to extract resorcinol,particularly when treating industrial effluents at elevated temperature.Both NRTL and UNIQUAC models correlated the experimental LLE data successfully and yielded binary interaction parameters for relevant process simulation.