Zhigang Lei,Yaru Guo,Yanyan Guo,Xinxin Li,Chengna Dai*

State Key Laboratory of Chemical Resource Engineering,Beijing University of Chemical Technology,Beijing 100029,China

1.Introduction

Hydrogen peroxide(H2O2)is widely used in paper making,textile,water treatment and other fields as a strong oxidant[1–3].The only reduction product of H2O2is water,which makes it an important chemical oxidant in green chemistry[4].In industry,the synthesis processes of H2O2mainly include anthraquinone method,electrolysis,isopropyl alcohol method and other methods.Among them,hydrogenation/oxidation of anthraquinone in liquid phase is the most commonly used method in commercial production of H2O2[5,6].

The production process of hydrogen peroxide by anthraquinone method mainly consisted of the preparation of the working solution,alkyl anthraquinone hydrogenation,the oxidation of anthrahydroquinone,extraction of the reaction mixture,and recycle of the working fluid.Anthraquinone working solution required in the process is formed of organic solvent and anthraquinone.Catalytic hydrogenation of anthraquinone is the key step in the production of hydrogen peroxide[7,8].Hydrogen solubility in the working solution not only affects the process of hydrogenation directly,but also determines the production capacity of the plant.Thus,it is necessary to study the solubility of H2in different parameters such as pressure,temperature and solvent volume ratio.

Sebastianet al.[9]measured the solubility parameter of hydrogen by experimental method and got the correlations for predicting hydrogen solubility in hydrocarbon solvents.It was proved that the correlation formula for hydrogen was accurate and reliable which could be useful for engineering calculations.Scho fieldet al.[10]studied the H2solubility in white oil as a function of temperature and pressure.The results calculated by the Krichevsky–Ilinskaya and van't Hoff model exhibited a small deviation from experimental data,suggesting that the thermodynamic model was applicable.Saajanlehtoet al.[11]measured the solubility of H2in heavy oil systems with a continuous flow apparatus and the results showed that the hydrogen solubility increased with the increasing temperature and pressure.Ronzeet al.[12]studied hydrogen solubility in gas oil using a chromatographic method.It was found that the relationship between partial pressure of hydrogen and liquid phase composition seemed to be linear in experimental range.Zhouet al.[13]studied H2solubility in pyrolysis gasoline(pygas)by experiment and SRK(Soave–Redlich–Kwong)equation.The results showed that H2solubility in pygasincreased with the rise of temperature and pressure and the SRK equation had a good practicability in modeling H2solubility.However,H2solubility data in anthraquinone working solution are not available in open literature.

The experimental measurement is often difficult,tedious,and time consuming.Therefore,the predictive thermodynamic models for H2solubility are highly desirable to reduce the workload of experiments.The conductor-like screening model for real solvent(COSMO-RS)model based on unimolecular quantum chemical calculations and independent on experimental data is a reliable and efficient method for fluid equilibrium,which can be used to predict the thermodynamic properties of pure and mixed liquids[14–18].The COSMO-RS method was developed by Klamt and Eckert[19]to predict gas solubility,Henry's law constant[20],vapor pressure and so on[21,22].Thus,COSMO-RS model has been concerned in the prediction of gas solubility in organic solvents and ionic liquids[23–25].We will explore the possibility of COSMO-RS model to predict the solubility of H2in anthraquinone working solution.

In this study,H2solubility in anthraquinone working solution was measured at temperatures of 30.0–80.0 °C and pressures of 0.2–3.0 MPa.Besides,solvent volume ratio of trioctyl phosphate to trimethylbenzene was changed in order to find out whether and how to affect H2solubility.Gas–liquid mass transfer coefficient of H2in anthraquinone working solution was also studied to understand the interaction of gas and liquid phases.In addition,the solubility of hydrogen in anthraquinone working solution was predicted by the COSMO-RS model using the COSMOthermX software package(version C30_1301),and the predicted values were compared with the experimental data.

2.Experimental

2.1.Materials

Anthraquinone working solution was formed by dissolving a certain amount of 2-ethylanthraquinone(EAQ)into organic solvent consisting of trioctyl phosphate(TOP)and trimethylbenzene(C9H12)at a certain volume ratio.2-Ethylanthraquinone,trioctyl phosphate and trimethylbenzene with the mass fraction over 97%were all supplied by Aladdin Industrial Company.H2with purity of99.99%(mass fraction)was purchased from Beijing Beiwen Gas Factory.The amountofsamples was measured by an electronic balance(CPA 1003S,Sartorius)with the precision of±0.001 g.

2.2.Experimental apparatus and procedure

The main experimental apparatusused to measure the solubility and gas–liquid mass transfer coefficient of H2in anthraquinone working solution is a magnetic agitated view-cell,and the schematic diagram is shown in Fig.1.The pressures and temperatures of the system were measured by pressure sensor with an accuracy of±0.01 MPa and temperature sensor with an accuracy of±0.1°C,respectively.The overall uncertainty of pressure and temperature was controlled within 1%including the uncertainty caused by the measurement method and apparatus.

Before the experiment,a series of anthraquinone working solution with desired compositions were prepared by dissolving EAQ into organic solvent.Then,a certain volume(VL,0)of working solution was added into the equilibrium cell,and the system started heating up to the desired liquid temperature.Next,the equilibrium cell was evacuated with a vacuum pump until the pressure to?0.10 MPa(P=Pm).It must be sure that the experimental device had very good air tightness.Subsequently,H2was rapidly filled into the equilibrium cell and the initial pressure(t=t0,P=P0)of the system was recorded.The cell was stirred by a magnetic stirrer at a constant speed to expedite the gas–liquid equilibrium.With the dissolution of hydrogen into the working solution,the pressure of the equilibrium cell was gradually reduced.During the experiment,the system pressure(P=Pt)was recorded every minute until the system reached gas–liquid balance and the equilibrium pressure(P=Pe)should be recorded.

2.3.Calculation method

The solubility of H2(xH2)in anthraquinone working solution can be calculated as follows:

Fig.1.Schematic diagram of the experimental setup.1—hydrogen cylinder;2—constant temperature jacket;3—magnetic agitated view-cell reactor;4—magnetic stirring;5—thermocouple;6—vacuum pump;7—monitor of temperature and pressure.

wherenH2,nH2,0andnH2,erepresent the amount of hydrogen in the working solution,gas phase at initial and equilibrium states,respectively;Vm,0andVm,erepresent the mole volume of H2under the corresponding pressuresP0andPe,respectively,which can be found from NIST database;Vcellis the volume of equilibrium cell;VL,0andVL,eare the initial and equilibrium volumes of liquid phase in the equilibrium cell,respectively.The hydrogen solubility in anthraquinone working solution is very small,so the effect of H2volume in the working solution on the total liquid phase volume can be ignored,that is,VL,e=VL,0.The amount of trioctyl phosphate(nTOP)and trimethylbenzene(nC9H12)is calculated by

wheremi,Mi,ρiandViare the mass,molecular weight,density and volume of substancei,respectively.The amount of 2-ethylanthraquinone(nEAQ)is calculated bynEAQ=mEAQ/MEAQ.The accuracy and reliability of the experimental procedure and methods were verified by comparing the experimental data of H2solubility in toluene at 50.0°C with those reported in the literature[26],and the results can be found from our previous publication[27].

During the experiment,the system pressure at operating timet(Pt)was recorded every minute in order to determine the value of gas–liquid mass transfer coefficientkLa.There is a relationship between the pressures at initial time(P=P0)and at the time of equilibrium state(P=Pe)in gas and liquid phases[28]as the following:

It can be seen that it is a linear relationship between ln((P0?Pe)/(Pt?Pe))and the timet.Therefore,mass transfer coefficientkLacan be determined by the slope of the straight line[29].

3.COSMO-RS Model

H2solubility in solvents is estimated on the basis of activity coef ficient,which can be calculated by

where?1(T,P,y1)respects the fugacity coefficient of H2in the gasphase,as calculated using the Peng–Robinson(PR)equation[30];y1andx1are molar fraction of H2in gas and liquid phases,respectively;γ1is the activity coefficient of H2in liquid phase;Pis the system pressure andPs1is the saturated vapor pressure of H2,which can be calculated from the Antoine equation[31].During the calculation,gasphase was treated as pure gas(y1=1).Moreover,the solubility of H2is extremely small,thus the activity coefficient of H2in solvents at in finite dilution state(γ1≈γ1∞)was used.Then,the solubility can be obtained from the following equation:

The structure of 2-ethylanthraquinone and trioctyl phosphate does not exist in the database of COSMO-RS model,therefore,it is necessary to design and optimize the structure of these two substances in the COSMOthermX software.The optimized structures(COSMO files)were developed by Turbomole using the triple-ζ valence potential(TZVP)basis set with Becke and Perdew functional at the density functional theory(DFT)level.The molecular geometries of chemical materials used in this paper are shown in Table 1.

4.Results and Discussion

4.1.Solubility studies of hydrogen

At the beginning,the influence of 2-ethylanthraquinone concentrations(CEAQ)on the solubility of H2in anthraquinone working solution was studied at the temperature of50.0°Cby experiment,and the results indicate that H2solubility is relatively large when the concentration of 2-ethylanthraquinone is 0.06 g·ml?1as shown in Fig.2.Therefore,all the following experiments and simulations were carried out under the condition ofCEAQ=0.06 g·ml?1.

Table 1The molecular geometries of chemical materials

Fig.2.Effect of concentration of 2-ethylanthraquinone on H2 solubility at 50.0°C.

The data of H2solubility in anthraquinone working solution measured by experiment and COSMO-RS model at temperatures of 30.0–80.0 °C and pressures of 0.2–3.0 MPa are listed in Table 2.The study of pressure refers to the equilibrium pressure.TheT?xdiagram is shown in Fig.3.As can be seen from the diagram,temperature has no obvious effect on the solubility of hydrogen under low pressures,while under high pressures H2solubility increases with the increase of temperature.In addition,it clearly shows that the solubility of hydrogen increases with increasing pressure.H2solubility measured by experiment is smaller than that calculated by COSMO-RS model through the comparison of the experimental data and simulated values.The difference may be caused by the volatility of solvent.On the other hand,the non-idealities during the dissolution of EAQ into trioctyl phosphate(TOP)and trimethylbenzene(C9H12)solution as well as the dissolution of H2in anthraquinone working solution were not considered in COSMO-RS model.There is a certain gap between the virtual component optimized by COSMO-RS model and the real component.Generally,the variation tendency of hydrogen solubility obtained by experiment is the same with that calculated by COSMO-RS model.

The effect of the volume ratio of trioctyl phosphate(TOP)to trimethylbenzene(C9H12)on the solubility of hydrogen in the working solution was studied,and the experimental data are listed in Table 3.It can be seen from Fig.4 that the greater the amount of trimethylbenzene,the larger the solubility of hydrogen,indicating that trimethylbenzene is conducive to the dissolution of hydrogen in anthraquinone working solution.

Table 2Solubility of H2 in anthraquinone working solution at different temperatures and pressures by experiment and COSMO-RS model

Fig.3.H2 solubility in anthraquinone working solution at different temperatures and pressure points,experimental values;solid lines,the values calculated by COSMO-RS

The Henry's constant(H,MPa)was calculated according to the method of linear extrapolation atxH2→0.The relationship between Henry's constant and temperatureT(K)was fitted using the equation of lnH=A+B/T,and the results are illustrated in Fig.5.It can be seen that lnHhas a good linear relationship withT?1,and the values of A and B are shown in Fig.5.

4.2.Gas–liquid mass transfer

The variation of system pressure with timetof hydrogen dissolution process at temperature of 60.0°C and pressure of 2.0 MPa is shown in Fig.6.There is a linear relationship between ln((P0?Pe)/(Pt?Pe))and timetas expressed in Eq.(5)and the functional relation can be seen in Fig.6.Gas–liquid mass transfer coefficient(kLa)of hydrogen in anthraquinone working solution was determined by calculating the slope of the line.The effect of pressure on mass transfer coefficient at temperature of 60.0°C is illustrated in Fig.7.It can be seen that the mass transfer coefficient increases with the increase of pressure,indicating that in anthraquinone working solution hydrogen dissolves more rapidly in the condition of high pressure.

5.Conclusions

The hydrogenation process is the key step in the anthraquinone method to produce hydrogen peroxide,and it determines the product yield and energy consumption.Therefore,it is essential to study hydrogen dissolution in anthraquinone working solution as the material participated in the reaction.In this paper,the solubility of H2in anthraquinone working solution at different temperatures and pressures were measured by experiment and COSMO-RS model.The results showed that at low pressures,the temperature had little effect on the H2solubility;while under high pressures H2solubility increases with increasing temperature.Trimethylbenzene is conducive to the dissolution of hydrogen comparing with the trioctyl phosphate.There is a linear relationship between Henry's constant lnHand 1/T.Besides,gas–liquid mass transfer coefficient was calculated by recording the variation of system pressure as the absorption proceeded.The result shows that mass transfer coefficient increases with the increase of pressure,that is,high pressure is beneficial to both absorption and mass transfer of hydrogen in working solution.

Fig.4.H2 solubility in anthraquinone working solution at different volume ratios of TOP to

Fig.5.Diagram for Henry's constants of H2 in anthraquinone working solution at different temperatures.

Fig.6.System pressure as a function of time at temperature of 60.0°C and pressure of 2.0 MPa for the determination of gas–liquid mass transfer coefficient.

Fig.7.Effect of pressure P on gas–liquid mass transfer coefficient k L a at temperature of 60.0°C.

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