Tiefeng Wang, Jinxiang Dong,2,*

1 College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China

2 School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China

Keywords:Separation Crystallization Purification Ethylene glycol 2-Methoxyethanol

ABSTRACT The coal (syngas)-to-ethylene glycol (CTEG), is contaminated with the naughty impurity 2-Methoxyethanol (ME) generated during the hydrogenation stage, which affect the quality of EG for fiber-grade polyester production.Distillation, is the employed separation process in industrial, which makes production complicated because of the heat sensitivity of the impurities system.Melt crystallization has been regarded as an effective technology to obtain high-purity organic compounds based on the melting points difference,which could avoid the problems by heating.In this work,we have explored the feasibility of the static melt crystallization on the separation of EG/ME in a jacketed crystallization tube.The experimental parameters were investigated,which covers crystallization and sweating stage in each step.The results showed that the purity of EG could reach ≥99.8%from the binary system studied via the quaternary separation process.

1.Introduction

Ethylene glycol (EG), as the simplest diol, covers almost all aspects of daily life which involve the chemicals,automobile,manufacturing technologies and other fields [1].In particularly, EG is an indispensable commodity with an increasing demand[2],which underpins the polyester fiber industry[3].Throughout the existing synthesis process, the conventional technology still occupies the major position in commercial production [4], which driven by direct hydration of ethylene epoxide and depends on its precursor-ethylene that is mainly from the catalytic pyrolysis of petroleum hydrocarbons [5].In recent years, with the concerns of petroleum resources dwindling and regarding global sustainability, the alternative strategy is urgently needed.Various efforts are made to seek the suitable route, such as the technology from formaldehyde[6,7],the conversion of polyols or cellulose from biomass [8,9], and syngas-dimethyl oxalate (DMO) route [10].The indirect synthetic route from coal to syngas(CO+H2)is important in C1 chemistry with high atom economy[11], has been of particular interest with a high conversion and lower energy consumption [12], and demonstrates stronger market vitality and competitiveness due to the development of technology [13], the production capacity has grown rapidly in the production structure of EG, which is promising and interpreted as a reliable alternative approach.For this process, DMO is an intermediate product, and the chemoselective hydrogenation reaction of DMO is the crucial step to produce EG [14], which adopted the copper-based catalyst generally [15,16].However, methanol is co-produced as a byproduct during the hydrogenation procedure [16], which can be further undergoes intermolecular dehydration reaction with the main product EG to 2-methoxyethanol(ME)in the presence of acid catalyst[17].The undesired ME is part of the impurities system for the coal-to-EG (CTEG), which set a resistance to the production of fiber-grade EG and affect the quality of polyester.Therefore, the separation operation of ME is necessary to performed.

As far as we know,distillation is still the mainstream method in industry at present.However, the temperature required for distillation is unfavorable for the liquid product in actual production,because the heat sensitivity of the system [18]could cause the occurrence of other side reactions, which complicates production.It is of great significance to find a novel way to remove impurities that can avoid the above problems and environmentally-benign.

Melt crystallization is a type of industrial crystallization separation, which depends on the melting point difference between the components rather than the change in solubility, the basic principle is to control the conditions to crystallize the desired pure fraction in melt [19].It is favored by practitioners because of the unique advantages compared to the conventional separation processes, such as no need additive(s), high purity products, suitable for narrow boiling point mixture (like isomers) and heat sensitive systems.In recent years, the fundamental research on separable species is gradually increasing and the cases of successful application are widely in chemical, food and pharmaceutical industry [20], for instance, the refining of mono-chloroacetic,bisphenol, and p-xylene [21].It is proved that melt crystallization is a promising technology to obtain high-purity organic compounds,whether from practical application or theoretical research.

In our previous work [22]of removal the 1,2-butanediol and 1,2-propanediol which also belong to the impurities system of coal-to-EG, melt crystallization is employed as a feasible method.Here in, the effectiveness of static melt crystallization for the separation of this binary system(EG/ME)which has a similar molecular structure(Fig.1)is verified according to the difference between the melting points (EG, -13 °C; ME, -85 °C).The experiments is conducted in a double-jacketed crystallization tube, the experimental parameters of each separation step covering the crystallization and sweating stage are investigated in detail and the contents of EG is monitored by gas chromatogram.Finally,the IV separation process is determined,and EG is purified successfully.In this work,the removal of ME expands the research on the purification of CTEG via melt crystallization, and the potential of its industrial applications is implied.

2.Experimental

2.1.Chemicals

Ethylene glycol (Anhydrous grade, 99.8%), 2-Methoxyethanol(AR),methanol(chromatographic grade,≥99.9%),the above chemicals were purchased from Shanghai Aladdin Biochemical Technology Co.,Ltd.High purity argon were obtained from Beijing Beiyang Special Gas Research Institute Co., Ltd.

2.2.Experimental set-up

The equipment used in experiment as illustrated in Fig.2.The crystal tube is borosilicate glass and divided into two layers: the inner layer (22 mm o.d., and a length of 300 mm) placement the melt liquid to be separated, and the outer layer jacket (45 mm o.d.)is used to circulate the cooling medium.Temperature controlled by the cryostat(DHX-6008,Nanjing Karma Instrument Equipment Co., Ltd.).The discharge of mother liquor is realized by a vacuum pump (Edwards RV 5).The thermocouple is used to record the temperature change of the melt in the inner tube.

2.3.Analytical methods

The content of EG and ME were monitored using the gas chromatography (GC-2014 C, Shimadzu) equipped with an on-column injector (230 °C), FID detector (230 °C), DB-624 capillary column(30 m, 0.53 mm i.d., 3 μm film thickness, Agilent) and automatic injector.High purity argon was used as the carrier gas at a flow rate of 1.0 ml·min-1.Methanol(10 ml)was used as a solvent to dissolve the crystal sample (0.05 g).The column temperature was initially at 50 °C for 2 min, then gradually increased to 100 °C at a heating rate of 5 °C·min-1, and then increased to 200 °C at 10 °C·min-1,held at this temperature for 3 min.For each sample, the composition was determined by the average of three analyses.

Fig.1.The molecular structure of EG and ME.

Fig.2.Melt crystallization equipment: (1) jacketed crystal tube,(2) thermocouple,(3) cryostat, (4) vacuum pump, (5) collection bottle.

3.Results and Discussion

The mixture to be separated could be undergoes the process of pre-cooling, seed generation in-situ, crystallization, discharge of mother liquor, sweating and final melting.The crystallization and sweating stage was set, and the parameters used were investigated.

The feed melt is pre-cooled to -30 °C first, then continuing to cool down, the original crystal nucleus are produced in the super-cooled state of the system and grow into primary crystals on this basis,but the controlled temperature lower than the freezing point with a large deviate.Thus,an immediate heating program is necessary direct to the initial crystallization temperature (near the equilibrium temperature),the primary crystals may be melted and secondary crystallization during this constant temperature,and are dispersed to form seed crystals for next cooling process,the system is in a state of solid-liquid coexistence at this time.Then, the driving force of crystallization depend on the controlled cooling program, the molecules in the melt adhere to the crystal interface, new crystals are constantly produced and grown on the seed crystals with a static non-fluid flow process,which entrapped impurity inevitably.The crystallization process continues until to the final crystallization temperature, the crude crystals are obtained by draining the mother liquor.

In the crystallization stage, the discharge of mother liquor creates narrow pores between crystals, the resulting pores are described by porous fractal theory, and the equilibrium status of system is assumed, the effective and average porosity (φave) [23]and tortuosity (τave) [24]can be approximate expressed as Eq.(1)and Eq.(2):

Kc[25](the distribution coefficient) is to quantify the separation efficiency for the crystallization, which is defined as Eq.(3), wcand w0is the average impurity concentration in the crude crystals and initial melt, respectively.K is corresponds to the process include crystallization and sweating; therefore, replace wcwith wf(the concentration of impurity after sweating).

Refer to our previous work,based on laboratory research about the purification with a wider impurity content, the concentration of EG in initial feeding was 94.66% detected on gas chromatography (95% (mass)).

3.1.Crystallization

3.1.1.The effect of the initial crystallization temperature

The initial crystallization step was carried out at a constant temperature for 30 min,which is a necessary process as a junction of seed generation and cooling crystallization:on the one hand,the crystal formed under super-cooled conditions was melt, and the crystal may be melted and re-growth in this stage, the entrapped impurities were released from the primary crystals; on the other hand,the crystal seeds were prepared.The different initial crystallization temperatures were studied at a constant cooling rate(0.05 °C·min-1) and determined final crystallization temperature(-22 °C), the seed crystals were not obtained well when the temperature was -15 °C.From Table 1 and Fig.3, it can be seen the ΔWcshowing a slight increase and the yield of crude crystals was decrease upon decreasing the initial crystallization temperature, Kcwas also decreasing, but there were no clear difference at the temperature ≤-16 °C, the pore parameters of the obtained crude crystals were also basically similar, -16 °C was chosen as the beginning point and the crystal slurry was provided for continued crystallization.

3.1.2.The effect of the cooling rate on the crystallization process

Under the condition of determining the same final crystallization temperature (-22 °C), the different cooling rate was studied.Table 2 shows that the crystallization time becomes shorter as rcincreases, but the φaveand τavewere basically similar except for 0.2 °C·min-1, it also can be seen in the yield from Fig.4, which show a decrease at 0.2 °C·min-1, this was due to the crystallinity deteriorated caused by the limited crystallization time and incomplete crystallization.The increase of EG purity has obvious changes with the decrease of cooling rate, as it brings a gentler crystallization driving force, the crystal growth was slower and the crystallization rate decreased accordingly, which reduces the entrapped of impurities in the crystal, the crystal was more perfect, and the content of EG in crude crystals was higher.Fig.4 shows Kcwasminimum when the cooling rate was 0.01 °C·min-1, which represents a better purity improvement.At the same time, the difference of Kcvalue between 0.02 and 0.01 °C·min-1was small,0.02°C·min-1was a more suitable choice with considering the factor of operating time.As can be seen from ΔWcin Table 2 that the influence of the cooling rate was significant,and it can be regarded as the most important factor effect the crystallization process.

Table 1 The status of crude crystals formed under the different initial crystallization temperatures studied

Table 3 The status of the crude crystals formed under the different final crystallization temperatures studied

Fig.3.The effect of the initial crystallization temperature on the yield of the crude crystals and Kc.

Table 2 The status of the crude crystals formed under the different cooling rates studied

Fig.4.The effect of the cooling rate on the yield of the crude crystals and Kc.

3.1.3.The effect of the final crystallization temperature

Fig.5.The effect of final crystallization temperature on the yield of the crude crystals and Kc.

The most intuitional effect of the final crystallization temperature (Tfct) was the change of yield.From Fig.5, the yield of crude crystals shows a decreasing trend as Tfctincreases;there were both the effect of temperature and cooling time(tc);With Tfctdecreases and tcextend,the degree to which the liquid was crystallized deepens caused the porosity decreases and the tortuosity increases(Table 3).What need to be explained was that the crystal column structure in the inner tube was destroyed at-18°C due to the negative pressure environment after the mother liquor was discharged at a constant final crystallization temperature for 40 min.However,ΔWcshowed a tendency become larger first and then smaller,analogously, the inflection point of Kcwas appeared at -21 °C.This may be because EG was continuously crystallized as the temperature decreases when Tfct≥-21°C,which was beneficial for purification; With Tfctcontinues to decrease when the temperature＜-21 °C, more impurity present in the new generated crystals,the amount of residual impurity in crude crystals was constantly increasing, the efficiency become to worse accordingly and Kcwas turn to increase.It is suggested that the final crystallization temperature was also an important factor in crystallization process.

Sweating was considered as a post-purification process of crystallization,which was to further discharge the remnant impurities in crude crystals by heating.The fusion liquid flows down through the channels under the action of gravity,and the maintained crystals were purified.In this work, the sweating temperature was studied after determining the crystallization conditions, and the heating rate was 0.05 °C·min-1, in order to discharge the sweat completely, the constant temperature time for sweating was set to 120 min.

Fig.6.The effect of the sweating temperature on the yield,ΔW and K.

Fig.7.Crystal layer changes during sweating: (a) Before sweating; (b-f) Crystal layer fracture and shedding; (g) Crystal layer retained at the end of sweating.

Fig.6 shows the changes of EG content in final crystalline with the different sweating temperatures (Tsw).As can be seen from K,sweating has almost does not work on purification at -17 °C (K was 0.511, and the best Kcwas 0.513).When Tswincreases to-16.5 and -16 °C, the yield reduction, although ΔW was gradual increases and K was decreases,the change was also not significant compared to the optimal ΔWcand Kcfrom the crystallization step.With the sweating temperature continue rises to -15.5 °C, the value of K was minimum(0.405),while the yield was decline sharply and undesired at this time, in other words, purification comes at the expense of yield.Actually, we observed the changes in the crystal layer during sweating at -15.5 °C (Fig.7), the crystal layer was fracture and shedding with the sweating process progresses,which was continuously melted and discharged.At the end, the lower part in tube was emptied, and the crystals remaining in the upper part were also loose.The effect of sweating was limited,and the decision to abandon the sweating stage was made.

3.2.Removal of ME from EG using a further separation process

The original melt was purified to a certain extent via the primary purification process (I), which running the crystallization stage only, and the optimum experimental parameters were obtained.However, the separation task was not achieved; therefore, the further purification steps were adopted.

3.2.1.The separation process of II step

For the secondary step(II)of separation,the final crystallization temperature and the sweating temperature were investigated mainly according to the reference data in I step.In the crystallization stage, a similar conclusion was drawn that the optimal temperature appeared at -19 °C from Fig.8(a), and set this level as the starting point of sweating stage.As shown in Fig.8(b),the content of EG showed a maximum increase at -14.5 °C, but the yield was slumped to 19%, which was still not a satisfactory result.The crystallization process plays a major role even its effect on separation was weakening with the EG purity improvement.Therefore,the sweating process was not suitable for adoption.

Fig.8.(a) The effect of final crystallization temperature in step II; (b) The effect of sweating temperature in step II.

Fig.9.(a) The effect of final crystallization temperature in step III; (b) The effect of sweating temperature in step III.

3.2.2.The separation process of III step

The same parameters as step II were studied in the subsequent experiments.From the Kcvalue in Fig.9(a),the effect of crystallization on purification is further weakened compared to the crystallization process in step II even at the best point (-18 °C), and the dependence on the final crystallization temperature was limited when ＜-16°C.The function of sweating cannot be ignored because the difficulty of purification increases with the further improvement of purity.Although the higher sweating temperature was beneficial to purity as Fig.9(b), excessive temperature (-13.5 °C)caused the loss of EG, which did not bring about further increase of ΔW than that in-14°C,and the factors of yield were considered comprehensively,-14.5°C seems to be a more appropriate choice,the yield was 67%.In this way,the sweating process was started to be run from the tertiary step (III).

THERE was once a Prince who wished to marry a Princess; but then she must be a real Princess.1 He travelled all over the world in hopes of finding such a lady; but there was always something wrong. Princesses he found in plenty; but whether they were real Princesses it was impossible for him to decide, for now one thing, now another, seemed to him not quite right about the ladies.2 At last he returned to his palace quite cast down, because he wished so much to have a real Princess for his wife.

3.2.3.The determination of the separation process

The purity of EG after the tertiary treatment was increase to about 99.12%,compared to that prior to purification.A crystallization process was tested firstly, when entering the quaternary step(IV).The result showed that the purity of EG was only 99.34%after crystallization,implying the realization of target on purity was full of obstacle.Refer to our previous experiment of separating miscel-laneous glycols, the crystallization-crystallization-crystallization& sweating method was considered in order to the impurity in mother liquid and sweat to be fully removed, when EG was purified to more than 99%.The parameters used were shown in Table 4.

Table 4 The parameters used in IV step

Fig.10.EG content in follow separation steps.

Fig.11.(a) The separation process determined; (b) Changes of the composition in each step observed on gas chromatography.

In the quaternary purification process,the Tictand Tfctwas keep constant at -14 and -17 °C, respectively.In the final sweating stage, the crystal structure was collapsed, the yield was affected for the product obtained (～29%), and the purity of EG increased to ≥99.8%.

From Fig.10,we can see the changes of EG content for each step of separation clearly, which involving crystallization and sweating operations,the EG content was gradually increased with the purification process executed and the purification task was achieved finally.However,as considered above,in order to take into account the yield, some steps have been discarded.Therefore, the separation process was determined as shown in Fig.11(a), I and II step were crystallization only; step III was crystallization and then sweating; crystallization three times and sweating once was employed in step IV.The gas chromatogram corresponding to each step of separation was shown in Fig.11(b), the ME impurity content was gradually reduced, which exhibited a retention time of 2.8-3.2 min, and the EG content was increased from 94.66% to 99.83% during the fourth purification processes.

We were committed to an attempt to separate one of the impurity systems via melt crystallization technology under the background of CTEG production.For the purpose in laboratory research, we proposed that seed crystals were manufactured insitu based on physical properties.The crystallization process and the effect of crystallization parameters were analyzed, which reveals that a smaller cooling rate was beneficial to purification.In the sweating stage, the fracture of the crystal layer was observed, but the sweating function to be implemented by sacrificing yield,explain that the form of sweating still needs improvement.The quaternary purification steps were proposed through systematic investigation of the separation process and parameters,ME impurity was removal successfully and EG purity ≥99.8%finally.The effectiveness and feasibility of melt crystallization can be confirmed,which provide experimental basis for innovative industrial applications.

4.Conclusions

In this work,with the considered of diversified impurities from CTEG need to be removed, and in order to prevent the problem of heat-sensitivity, the purification of EG via the static melt crystallization from EG/ME system was studied.The material in the inner tube has gone through the typical stages of seed crystals manufacturing, cooling crystallization and warming up to sweating, the parameters were investigated to improve crystallization and sweating, some steps were discarded to protect the yield and the final process was determined.The experiment results indicated that EG was obtained with high purity.We showed that melt crystallization was a potential method to remove impurities containing ether bonds from EG.However, this experiment is based on the research of batch operation only,which was not achieved the recycle of material, a continuous process is the direction of industrial production and more reasonable design is required.Overall, melt crystallization could be a promising technique for the separation of these similar systems,and increased the possibility of industrial applications.

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.

Acknowledgements

This work was financially supported by the Program for Sanjin Scholars of Shanxi Province of China, and Ten Thousand Talents Program: Millions of Leading Engineering Talents.

Nomenclature

Kcdistribution coefficients in crystallization process

K distribution coefficients in total process

m0mass of feed melt

mmlmass of mother liquid discharged from the crystallizer

mswmass of perspiration

rccooling rate

tccooling time

Tictinitial crystallization temperature

Tfctfinal crystallization temperature

Tswsweating temperature

ΔWcincrease of EG purity during the crystallization process on GC

ΔW increase of EG purity observed using GC

w0impurity concentration in feed melt

wcaverage impurity concentration in crude crystals

wfaverage impurity concentration in final melt

φaveaverage porosity

τaveaverage tortuosity