Yabing Qi,Jun Li

1 School of Chemistry and Chemical Engineering,Xi’an University of Architecture and Technology,Xi’an 710055,China

2 School of Chemical Engineering,Sichuan University,Chengdu 610065,China

Keywords:1,2-Diphenylethane Zone refining Varied zone size Effective distribution coefficient Thermodynamics

ABSTRACT Effective distribution coefficients of 9 impurities in 1,2-diphenylethane have been calculated by directional crystallization under different ambient frozen temperature.The effect of varied zone size,temperature difference between the melt and ambient frozen environment,number of zone on purity of 1,2-diphenylethane have been also investigated during the process of zone refining.The results indicate that the product purity in the intermediate purified region with varied zone size is higher 0.04%-0.2% than that with constant zone size.The product purity increases with temperature difference between the melt and ambient frozen environment.The appropriate temperature difference is adopted 50 °C.The product purity in the intermediate region of sample bar with 2 molten zones is higher 0.05%-0.43%than that with 1 molten zone.In addition,the change of enthalpy and entropy between impurities and 1,2-diphenylethane have been determined.

1.Introduction

In recent years,zone refining has been considered as an exceptional method for preparing high or ultrahigh pure materials [1-10] since it was first outlined by Pfann [11,12].As is shown in Fig.1,there are two solid-melt interfaces containing the freezing interface and melting interface.When the molten zone traverses the solid bar,at the melting interface,solid materials are merely melted and mixed with the material in molten zone;at the freezing interface,solid material are recrystallized from the melt.If the presence of impurity lowers the melting point of the material,the impurity concentration will decrease in the refrozen section of the material bar.However,if the impurity raises the melting point of the material,its concentration will increase in the solidified section of the material bar.Therefore,the freezing interface can reject or attract impurities in the molten zone.When the material contains several impurities which lower or raise the melting point of the main component,the former will travel against the mobile direction of molten zone and the latter will travel along the moving direction of molten zone during the process of molten zone passing through the material bar.As a result,the impurities will be pushed to the two ends of material bar.The similar process of zone refining will be repeated until the desired purity is realized.Then the two ends of the material bar are removed,the intermediate section of the material bar are the purified product [13-16].

1,2-Diphenylethane(C14H14;Molecular Weight 182.26;Melting Point 52°C;Boiling Point 284°C;CAS Registry No.103-29-7;Fig.2)is known as an important aromatic compound that can be considered the derivative of ethane,substituting two phenyl groups for two hydrogen atoms from different carbons respectively.As it has special molecular structure,1,2-diphenylethane is liable to undergo substitution,sulfonation,oxidation,and dehydrogenation reactions.The particular structure has aroused considerable interest in organic synthesis field and makes 1,2-diphenylethane become a significant intermediate of organic synthesis.It is used for synthesizing flame retardant named 1,2-bis(pentabromophenyl) ethane,fluorescent brightening agents,and forming the central core of some stilbenoid natural products and isoquinoline alkaloids,etc.[18,19].At present,the purity of raw industrial 1,2-diphenylethane is less than 98%.In order to realize the higher purity of 1,2-diphenylethane,the traditional purification methods,distillation and solvent crystallization are used for purification of raw 1,2-diphenylethane.However,both methods are hard to produce high or ultrahigh pure 1,2-diphenylethane.The distillation separation of 1,2-diphenylethane consumes excessive energy,needs heavy tower equipment,wastes many money,is not easy to remove the impurities of high boiling point and is very difficult to realize the purity of 1,2-diphenylethane excess 99.5%.The solvent crystallization of 1,2-diphenylethane consumes a lot of organic solvents,recycling of organic solvents exhausts many energies and causes problems of environmental pollution.Aiming at realizing high pure 1,2-diphenylethane,saving energy,and reducing the use of organic solvents,zone refining is borrowed to purify 1,2-diphenylethane.The obvious advantages of zone refining are no using solvents and achieving high purity.In order to investigate the process of zone refining of 1,2-diphenylethane,the effective distribution coefficients of a several main impurities have been calculated by directional crystallization.The impacts of varied zone size,temperature difference,and molten zone number on product purity have been also investigated.Finally,the change of thermodynamic property have been calculated.

Fig.1.The sketch of zone refining.

2.Experimental

2.1.Apparatus and materials

The experimental apparatus are shown in Fig.3.The zone refiner of 1,2-diphenylethane consists of driving motors,mobile structure,speed regulators,heaters,coolers,and quartz glass tube,and so on.The raw material and solvent are 1,2-diphenylethane and chromatographic grade dichloromethane respectively.GC-MS is used for analyzing the purity of 1,2-diphenylethane and impurity concentration.HP-5 (30 m × 0.32 mm × 0.25μmm) is designated as the pattern of chromatographic column.

Fig.3.The sketch of zone refiner.

2.2.Experimental and analytical methods

Experimental procedures:Firstly,the raw 1,2-diphenylethane was loaded into the quartz glass tube,then,the nitrogen was filled in the quartz glass tube,and finally the quartz glass tube was sealed with an airtight cork.The cooling temperature was controlled by the low temperature thermostatic bath while the heating temperature was controlled by the digital temperature controller.The molten zone moved at a specific rate with the help of driving units.When it ended one motion,the molten zone returned to the initial position and started another motion.After the process ended,the different samples were removed with the sampler.

Fig.2.Chemical structure of 1,2-diphenylethane.

Analytical method:First of all,the operator accurately weighed 0.1 g 1,2-diphenylethane and added it into a volumetric flask.Then the operator added chromatographic grade dichloromethane into the volumetric flask and made 1,2-diphenylethane dissolve completely.Finally,the operator started analytical experiment with GC-MS.The temperature program was that the starting column temperature was 120 °C,then the column temperature was kept for 3 minutes and finally increased to 280 °C at the rate of 10°C·min-1,the holding time was 15 minutes,the temperature of vaporizing chamber was 290 °C,and the connection tube temperature was 280 °C.The injection volume was 0.2 μl [17,18].

3.Therodynamics Analysis

Under the constant pressure,the Gibbs free energy of impurity in liquid is equal to that in solid when the impurity diffusion equilibrium is reached at the solid-liquid interface [6,9].

Where ΔHi,ΔHmc,ΔSiand ΔSmcare the variations of enthalpy and entropy between the liquid and solid phases during the process of solidification,respectively.

WhereCSandCLare mass fraction of impurity in solid and liquid respectively.Miis molar mass of every impurity.MSandMlare molar mass of solid and liquid respectively.From Eq.(7) and Eq.(8),it can been obtained as follows:

Neglecting the difference betweenMSandMl,it can be gotten that:

By rearranging Eq.(9) and Eq.(10),it can been obtained as follows:

4.Results and Discussion

4.1.Effective distribution coefficients

The effective distribution coefficient is expressed aske=CS/CL.According to the related theory[19],the directional crystallization equation is shown as follows:

WhereC0is the initial impurity concentration of the sample,xis the normalized distance from the starting end.Eq.(14)is gotten by Eq.(13)

ln(1-x) is set as abscissa,ln(CS/C0) is taken as vertical coordinate and a straight line is plotted.keis gained from the slope of the straight line.With the help of directional crystallization,the effective distribution coefficients of nine main impurities in 1,2-Diphenylethane have been shown in Table 1.The conditions of directional crystallization is that the temperature of melt is 60 °C,and the ambient frozen temperature isTfrozen,°C.From Table 1,it can be seen that the effective distribution coefficients depend on ambient frozen temperature,Tfrozen.kewhich is less than 1.0 increases withTfrozenwhilekewhich is more than 1.0 deceases withTfrozen.Lowering ambient frozen temperature can enlarge the difference betweenkeand 1.0.It means that low ambient frozen temperature is helpful to the impurities segregation.

Table 1The effective distribution coefficients under different experimental conditions

Table 2Change of enthalpy and entropy between impurities and 1,2-Diphenylethane

4.2.Effect of varied zone size on product purity

Fig.4 shows the profiles of product purity with different zone sizes after 15 zone passes.The experiments adopt two programs.The first one is constant zone size with normalized zone size of 0.21.The other is varied zone size that is 1.0 for the first zone pass,0.333 for the second to third zone pass,0.200 for the fourth to fifth zone pass,and 0.133 for the sixth to fifteenth zone pass.It can be seen that the product purity in the intermediate region of the sample bar with varied zone size is higher 0.04%-0.2% than that with constant zone size.It also provides an idea that varied zone size is better than constant zone size during zone refining process of 1,2-Diphenylethane.It also matches the results of the previous literature [18] about zone refining.In the literature [18],through numerical modeling and calculation,it is proved that the refining efficiency with varied zones from large molten zones at primarystage to small molten zones at final stage is higher that with constant zones after many zone passes.

Fig.4.Effect of varied zone size on the product purity (v=12.35 mm·h-1, Tmelt=60 °C, Tfrozen=20 °C).

Fig.5.Effect of temperature difference on product purity(v=12.35 mm·h-1,N=1,z= 0.21).

4.3.Effect of temperature difference on product purity

From Fig.5,it indicates that the product purity in the intermediate purified region increases with ΔT,which is temperature difference between the melt and ambient frozen environment after 1 zone pass.When the temperature difference ΔTis smaller,the product purity is lower but the increase of product purity is larger in the intermediate purified region.When the temperature difference ΔTis bigger,the product purity is higher and the increase of product purity becomes smaller in the intermediate purified region.When temperature difference ΔTincrease to 50 °C,the product purity in the intermediate purified region reach stability.However,bigger temperature difference is helpful to zone refining of 1,2-diphenylethane.In fact,the influence of temperature difference on product purity are due to two aspects.The one is free convection which is positive factor and the other is constitutional subcooling which is negative factor.On one hand,a larger temperature difference means stronger free convection and it strengthens the mixture of material in the melting zone and reduces the mass transfer resistance between the freezing and melt interface.On the other hand,a larger temperature difference inevitably intensifies constitutional subcooling,makes the freezing interface become rough,and rough freezing interface hinders the mass transfer between the freezing and melt interface to some extent.Within the range of experimental conditions,a larger temperature difference between the melt and frozen environment is prone to improving the product purity.Hence,the temperature difference of 50 °C is adopted.

4.4.Effect of number of zone on product purity

The effect of number of zone on product purity is shown in Fig.6.It can be seen that the product purity in the intermediate region of sample bar with 2 molten zones is higher 0.05%-0.43%than that with 1 molten zone.It can be concluded that increasing number of zone promotes the zone refining efficiency.But when the molten zone number is more than 2,it can hardly realize steady and smooth zones,and the zone refining process is out of control.Therefore,during zone refining of 1,2-Diphenylethane,two molten zones are recommended.

Fig.6.Effect of molten zone number on the product purity (N=1, Tmelt=60 °C,Tfrozen=20 °C, v=4.97 mm·h-1).

Fig.7.The plots of lnke versus 1/T for 5 impurities.

Fig.8.The plots of lnke versus 1/T for 4 impurities.

4.5.Change of thermodynamics property

5.Conclusions

The effective distribution coefficients of 9 impurities in 1,2-diphenylethane have been obtained by directional crystallization.The majority of effective distribution coefficients are below 1.0 and the minority of effective distribution coefficients are above 1.0.The product purity in the intermediate purified region with varied zone size is higher 0.04%-0.2%than that with constant zone size.The purity of 1,2-diphenylethane in the intermediate purified region increases as temperature difference decreases,but the increasing range decreases as temperature difference increases.The temperature difference between the melt and ambient frozen environment of 50 °C is preferred.The product purity in the intermediate region of sample bar with 2 molten zones is higher 0.05%-0.43% than that with 1 molten zone.The change of enthalpy and entropy between impurities and 1,2-diphenylethane are determined.

Nomenclature

C0initial impurity concentration of the sample,g·g-1

CLimpurity concentration in liquid phase,g·g-1

CSinitial concentration in solid phase,g·g-1

GlGibbs free energy of the system in liquid,J·mol-1

GsGibbs free energy of the system in solid,J·mol-1

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ΔHivariations of enthalpy of impurity between the liquid and solid phases,J·mol-1

enthalpy of main component in liquid,J·mol-1

enthalpy of main component in solid,J·mol-1

ΔHmcvariations of enthalpy of main component between the liquid and solid phases,J·mol-1

keeffective distribution coefficient

Nnumber of zone passes

Rgas constant,8.314 J·mol-1·K-1

R2square of linearly correlation coefficients

ΔSivariations of entropy of impurity between the liquid and solid phases,J·mol-1·K-1

ΔSmcvariations of entropy of main component between the liquid and solid phases,J·mol-1·K-1

Tthe ambient frozen temperature,K

ΔTtemperature difference between the melt and ambient frozen environment,°C

Tfrozenthe ambient frozen temperature,°C

TmeltThe temperature of melt,°C

vtranslation rate of molten zone,mm·h-1

mole fraction of main component in liquid

mole fraction of main component in solid

mole fraction of impurity in liquid

mole fraction of impurity in solid

xnormalized distance along the sample bar

znormalized size of molten zone

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.