Shan Cong ,Ying Liu,Hong Li,Xingang Li,Lvhong Zhang,*,Lei Wang

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

2National Engineering Research Center for Distillation Technology,Tianjin 300072,China

3Yunnan Jiehua Clean Energy Development Co.,Ltd.,Yunnan 650224,China

Keywords:

A B S T R A C T

1.Introduction

Durene is an important organic chemical raw material,generally used to produce pyromellitic dianhydride(PMDA)by catalytic oxidation at high temperature.PMDA is considered as the most economical and ideal raw material to produce polyimide film.Polyimide,as heterocyclic aromatic polymers,keeps an eminent position among heat-resistance engineering plastics.With the increasing demand on PMDA,durene attracts more and more attention.At present,the popular methods for producing durene include separation and synthesis.Many approaches have been developed to synthesize durene,such as pseudocumene benzene chloromethylation,pseudocumene dismutation and isomerization,tetramethylbenzene isomerization,pseudocumene and methanol alkylation,pseudocumene and formaldehyde(or dimethyl ether)condensation and cracking,and in conversion from methanol to hydrocarbons with ZSM-5 catalyst[1].However,the major source of durene is from the tetramethylbenzene distillation in catalytic reforming unit,in which durene is separated by freezing crystallization technique.The crude crystal obtained by freezing crystallization is centrifuged from the mother liquor,and durene is obtained from the filter cake by squeezer[2].The highest purity of the production is 95%and the yield of crystal is only 40%to 50%,which needs further purification.

Static melt crystallization has been widely used in purifying organic isomer solutions.It is considered as an environment friendly process due to its absence of organic solvents[3],low energy consumption,low temperature,and high selectivity.Mandriet al.[4]used melt crystallization in desalination,examined main kinetic parameters influencing sweating of ice,and developed a statistical model for ice weight and purity using fullfactor experimental design.Coconutoil[5]was purified by melt crystallization instead of complicated suspension crystallization.Melt crystallization was also used for ultra-purification of ionic liquids(ILs)[6],with excellent purification for EMIM-chloride.It is an efficient method to purify ionic liquids in different scales from 0.5 g up to 1000 kg with purity of IL＞99.99%.Erjawetz et al.[7]produced milk fat fractions in layer crystallization process.Tian et al.[8]purified 2,6-diisopropylnaphthalene(2,6-DIPN)by static melt crystallization from a mixture containing diisopropyl naphthalene isomers,with2,6-DIPN purity≥99% and yield of 87% through a process of three steps:crystallization,sweating.

Static melt crystallization is based on differences of melting points of materials,with the desired material being purified by decreasing the temperature below its freezing point but higher than those of impurities.In the eutectic system,pure matter is obtained in one step theoretically.In practice,crystals are contaminated by impurities,either on crystal surfaces as adhered “mother liquor”or inside crystals as inclusions[9-11].

Sweating is a useful method to purify crude crystalline layer with the effect of temperature gradient[12].In an isothermal sweating,the change of composition can be expressed as[9]

where ws0is the initial composition of crystal at θ =0,ws∞is the constant composition of crystal sweating for a long time,and kpis the purification rate coefficient,de fined as the ratio of purification rate to the excess amount of impurity in the crude crystal.

The purification of durene by freezing crystallization has been reported[13-15].In this study,crude durene of 94%is purified to more than 99%by static melt crystallization.It provides a green method to further purify durene.

2.Experimental

2.1.Materials

Table 1 Chemical composition of crude material

2.2.Experimental setup

Fig.1 shows the experimental apparatus used for static melt crystallization and sweating operations.It consists of a glass crystallizer,two circulators for controlling temperature,a precise thermocouple and temperature acquisition system for measuring the temperature of the melt liquid.The glass tube of the crystallizer has a jacket in which water is circulated to control the liquid temperature inside the crystallizer.The crystallizer has 30 mm inside dimension,50 mm outside dimension,and 60 cm length.The thermostatic baths have PID controllers to monitor the temperature and cooling rate.

Fig.1.Schematic diagram of experimental apparatus of crystallization.1—thermocouple;2—crystallizer;3—water bath 1;4—water bath 2;5—data acquisition unit.

2.3.Crystallization and sweating procedure

The experimental process included three steps:crystallizing,sweating and melting.First,200 g raw material was fed into the crystallizer and melted completely by the heated circulated water.The bath temperature was then decreased to 1°C,above the melting point of durene.Staying 30 min at such bath temperature,the melt liquid crystallized at a constant cooling rate,which was set by circulating the heat medium of thermostatic bath with a PID controller.The crude crystal and the mother liquor were kept at the crystallization temperature for half an hour and then the mother liquor was drained out naturally.The crude crystal was remained on the surface of the tube.Subsequently,the temperature of the running water was switched to the sweating temperature and kept at the temperature in the sweating step.Part of crude crystal melted during this process,while the impurity draining out was collected regularly,weighed and analyzed by gas chromatography.At the end of the sweating step,the remaining crystalline layer molted completely and drained as the final product,and it was weighed and analyzed.

2.4.Quantitative analysis method

The purity of durene was analyzed by GC(PE-XL,USA)with the FID detector.They were separated on a FFAP capillary column(30 m × 0.32 mm × 0.25 μm film thickness).Nitrogen was used as the carrier gas with the flow rate of 1.0 ml·min-1.The oven temperature was programmed as follows:the initial temperature was held at 80 °C for 1 min,then rose to 200 °C at the rate of 5 °C·min-1and held for 5 min.The temperatures of injection port and detector were set at 250 °C and 280 °C,respectively.The split ratio was 20:1.The acetone was the solvent and ethyl benzene was the internal standard material.The durene purity was determined by comparing the peak area with those of the durene standard under the same condition.

3.Results and Discussion

3.1.Effect of process parameters

3.1.1.Effect of cooling rate in crystallization

The crystal growth rate in layer melt crystallization is controlled by the cooling rate.The crystal growth rate decreases with crystallization time.Average crystal growth rate is calculated by dividing the thickness of layer by the crystallization time.Crystals grow until the coolant temperature decreases to 74°C.Fig.2 shows that the plot of the average crystal growth rate versus cooling rate is linear,in accordance with a previous study[16].The effects of cooling rate on the purity,mass fraction and yield of crude crystal are shown in Fig.3.The mass fraction and yield of the crude crystal increase as cooling rate increases,while the purity shows a reverse trend.As the cooling rate affects the crystal growth rate,the crystalline layer formed at different cooling rates has different inclusion amounts.Besides,the cooling rate affects the structural con figuration of crystalline layer.Higher cooling rate results in more complex crystalline layer,entrapping more impurities and decreasing the purity of crystalline layer.The change of purity of crystalline layer with cooling rate was faster at the beginning,then slows down and was quicker again in the experimental range.It may be resulted from the combination of two mechanisms in layer crystallization[17]:entrapment of liquid impurities during crystal growth and movement of inclusions due to the temperature gradient in crystalline layer in the whole growth process.

Fig.2.Average crystal growth rate versus cooling rate in layer crystallization.

Fig.3.Effect of cooling rate on the mass fraction and purity of crude crystal and yield of durene.

3.1.2.Effect of crystallization temperature

In the crystallization process,liquid impurities would be crystallized on the solid side,while the crystal solid would be melted into the liquid phase in the crystalline layer due to the thermal driving force.Different temperature represents different sub-cooling degree,soit results in different inclusion amounts and subsequently influences the purity and mass fraction of crude crystal.Fig.4 shows that the crystallization temperature influences the mass fraction and purity of crude crystal significantly.As temperature increases,the purity of crude crystal increases,but the mass of crude crystal reduces.The crystallization temperature and concentration are in an equilibrium state in the range of experiments[17].

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Fig.4.Effect of crystallization temperature on the mass fraction and purity of crude crystal.

3.1.3.Effect of sweating temperature and sweating time

In crystallization,crude crystalline layer forms on the wall of crystallizer,with a large amount of mother liquor included.Sweating is an effective way to purify the crude crystalline.The purification proceeds everywhere in the layer in a way that impure melt drains downward through the layer by gravity and is removed[9].

Fig.5.Purity of crystalline layer versus sweating time at different sweating temperatures.

Fig.5 shows the change of crystalline layer composition with sweating time at different sweating temperatures,with a cooling rate of 0.03 °C·min-1and a crystallization temperature of 74 °C.The sweat with entrapped impurities drained out through inter-crystal micro channels.In the early stage of sweating,many shallow ravines formed on the surface of crystal,then they would deepen gradually,so that compact crystalline layer became loose and porous,and even broken to small crystals[18,19].As the melted liquid from partially melted crystal moved outwards and drained down,the purity of crystalline layer increased,which suggests that purification proceeds during the sweating process.

Fig.6 shows the yield of crystalline layer versus sweating time at different sweating temperatures.The crystal purity increases with the loss of yield.The yield of crystal is lower at higher sweating temperature with the same sweating time.The purity of crystal increases rapidly in the first 30 min due to the largest temperature difference in the early stage of sweating.This indicates that the impurity entrapped in the crystallization phase of growing crystals is predominantly located on the surface of the crystalline layer[16].Impure inclusions outside the layer are easily drained away at the beginning of the sweating operation.Then the purification proceeds very slowly because the impurities inside the layer have to travel through the crystal layer or the structure of crude crystals.After some time,the crystal purity is approximately constant in the sweating operation.At this stage,the mass transfer mostly follows the diffusion mechanism and its rate is slow[16].

Fig.6.Change of yield of crystalline layer with sweating time at sweating temperatures.

The final purity of crystal at different sweating temperatures is also presented in Fig.5.The purity of crystals strongly depends on the sweating temperature.Higher sweating temperature improves the crystal purity significantly,but the mass loss is more.Therefore,it is necessary to monitor sweat by controlling sweating temperature to guarantee both the purity and mass of crystalline layer.

3.2.The orthogonal experimental design

The critical parameters affecting the purifying efficiency include crystallization temperature(Tc),cooling rate(Rc),sweating temperature(Ts)and sweating time(θ).From the economic point of view,higher growth rate of crystallization is desirable,which is beneficial to higher production efficiency.As the result,higher cooling rate and lower crystallization temperature are encouraged.However,this would entrap more impurities and decrease the purity of crystalline layer.Similarly,the sweating temperature and time have reverse effect on purity and mass of crystalline layer.Therefore,it is a compromise among these parameters to determine the optimal conditions.Since it requires an integral optimization for crystallization and sweating process,the orthogonal experimental design[20,21]is applied,with a factor number of 4,a level of 3,and a total experimental run of 9,as shown in Table 2.

Table 2 Variables and responses for the experimental design

Table 2 and Fig.7 show that the significance of individual parameter is ranked in the order of cooling rate＞sweating temperature＞crystallization temperature＞sweating time.Fig.7 also indicates that lower cooling rate favors the yield of durene.During crystallization,cooling rate controls the crystal growth rate and morphology of crystalline layer.Impurities contained in crystalline layer in the crystallization process are liquor pockets trapped inside the layer.Mandrietal.[4]showed mono-crystalline structure with low growth rate,in which impurities were mostly embedded inside the crystal,while crystalline layer with higher growth rate showed a poly-crystalline structure,in which impurities were mostly entrapped in the interstices between crystals.Therefore,impurities in crystalline layer obtained at higher cooling rate can be drained out more easily by sweating,resulting in more sweat in equilibrium with crystal.Thus the inclusion fraction in the crystalline layer and crystal structure may affect the sweating rate,implying that the crystallization conditions may be important to the purification in the sweating process[16].

Fig.7.Effects of 4 factors on the yield of durene.

The sweating temperature determines the equilibrium of crystal and sweat.With the increase of sweating temperature,the purity increases and the mass of crystal decreases,which makes the yield of durene decrease monotonously with sweating temperature under the experimental conditions.Since the mass of crude crystal increases as crystallization temperature decreases and since more impurities are entrapped in the crystal phase,less sweat will be drained out from the crude crystal and more crystal is obtained at lower crystallization temperature under the same sweating condition.Under the combined effect of crystallization temperature and sweating temperature,crystallization at 73°C generated the highest yield.

As for sweating time,both purity and mass fraction of crystalline layer changed quickly in the first 30 min and did not change much with sweating time,so the three levels of sweating time:30,60,and 120 min,had little effect on the yield of crystal.

From the average value of each level,the optimal conditions for the highest yield are as follows:crystallization temperature 73°C,cooling rate 0.03 °C·min-1,sweating temperature 77 °C and sweating time 30 min.

3.3.Purifying durene under the optimal conditions

Under the optimal conditions for durene purification,the ultimate yield of durene was 75.29%and the purity of crystal was 99.06%.The change of crystal purity with the sweating time is shown in Fig.8.

Fig.8.Purity of crystal versus sweating time and fitted curve.

With the purity of crystalline layer at certain sweating time,Eq.(1)becomes

with R2=0.9847.The purification rate coefficient kpis1.39×10-3s-1.The calculated results match well with the experimental data,as shown in Fig.8.

4.Conclusions

The purification of durene by static melt crystallization with singlestage process was investigated.The process involved a freezing step,followed by a sweating step to purify the crude crystalline layer.Orthogonal experimental design was applied to evaluate the effects of operating condition on the yield of crystal and determine the optimal conditions:crystallization temperature at 73 °C,cooling rate at 0.03 °C·min-1,sweating temperature at 77°C,and sweating time 30 min.Under the optimal conditions the purity of crystal was 99.06%and the yield was 75.29%.The purification rate coefficient was1.39×10-3s-1.No chemical reagent was used in the process,which provides an environment-friendly and energy-efficient method to purify durene for further use.Meltcrystallization is expected to be more widely used to purify chemical materials in the future.

Nomenclature

kppurification rate coefficient,s-1

Mcmass of crystal product,g

Rccooling rate,°C·min-1

Tccrystallization temperature,°C

Tssweating temperature,°C

wcpurity of final crystal,%

wscrystal purity during sweating,%

ws0initial mass composition of crystal,%

ws∞constant mass composition of crystal with sufficiently long time during sweating initial composition of crystal,%

θ sweating time,s