Reversible Reaction-Assisted Intensification Process for Separating

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Reversible reaction-assisted intensification process for separating the azeotropic mixture of ethanediol and 1,2-butanediol#Reactants screening Xingang Li, Rui Wang, Jian Na, Hong Li, and Xin Gao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04012 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Industrial & Engineering Chemistry Research

Reversible reaction-assisted intensification process for separating the azeotropic mixture of ethanediol and 1,2-butanediol：：Reactants screening Xingang Lia, Rui Wanga, Jian Naa, Hong Li a, Xin Gao a, * a

School of Chemical Engineering and Technology, National Engineering Research Center of

Distillation Technology, Collaborative Innovation Center of Chemical Science and Engineering

(Tianjin), Tianjin University, Tianjin 300072, China * Corresponding Author E-mail: [email protected].

Abstract The feasibility of employing reversible reactions to convert the separation of ethanediol (EG) and 1,2-butanediol (1, 2-BDO) azeotropic system was analyzed in our previous work. This article aims at systematically screening the feasible reactants for the reaction-assisted separation process. A screening principle was brought up and applied. Through preliminary screening, acetic acid, acrylic acid, acetaldehyde, propionaldehyde, propanol and butanone were obtained as potential reactants. Propionaldehyde was chosen as the proper one after fully comparing the reaction selectivity difference and conversion of EG and 1,2-BDO in reaction process, the separation efficiency in purification process as well as final EG yield. The separation efficiency was predicted based on the relative volatility of EG, 1,2-BDO and their 1

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corresponding products and was verified by distillation experiments. The stratification appeared in the reaction was found beneficial to the coupling of reaction and separation. Keywords: ethylene glycol; 1,2-butanediol; reaction-assisted; azeotrope; separation

1. Introduction Ethylene glycol (1,2-ethanediol, EG) has wide applications such as antifreeze, precursor of manufacturing polyester fibers and resins, solvents as well as a variety of chemical intermediates for its excellent physical and chemical properties. Owing to the increasing demand for polyester fibers, as the largest consumer of mono EG by far, China accounts for about half of the Mono EG consumption globally with an estimated increase of 4% on average per year1. Meanwhile, a new Coal-To-MEG route has been developed in China because of the increasing environmental concerns in EO hydration process as well as the abundance of coal rather than oil resources2, 3. This new route contains two steps: dimethyl oxalate synthesis through syngas (usually produced by coal or natural gas) coupling and then the hydrogenation of dimethyl oxalate

to

get

EG4-7.

However,

1,2-propanediol

1,2-butanediol(1,2-BDO) are coproduced as the by-products

(1,2-PDO)

and

5, 8, 9

. Because of the

close-boiling points and forming azeotrope at a fixed mole ratio of 1:1 between EG and 1,2-BDO, the separation of 1,2-BDO and EG is hard10-12. Conventional distillation process has two disadvantages. Firstly, in order to produce EG that can be used in production of polyester fibers, high reflux ratio is needed, which results in

2


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high-energy consumption. Secondly，as equimolar EG will be entrained along with the removal of 1,2-BDO,the yield of EG is low9. In addition, the operation conditions are limited because EG is sensitive to heat. The similar difficulty also exists in the production of EG by biomass8, 13-15. To overcome the separating issues, some researchers committed to employing azeotropic distillation or extractive distillation to separate the mixture of 1,2-BDO and EG16. However, as there is a large amount of EG in the industrial feeding, large amounts of entrainer are required and will be circulated in the purification process. The energy consumption is still high. The selective removal of 1,2-propanediol and 1,2-butanediol from bio-ethylene glycol by catalytic dehydration reaction to form volatile aldehydes, ketones and acetals was studied17. In addition, adsorption might also be a convenient and energy-saving separation method. However, few industrial applications have ever been reported. A reversible reaction-assisted intensification separation method was proposed in our previous work while employing acetaldehyde as the third reactant. The feasibility of the method was proved by thermodynamic study.18,19, 20. Figure 1 illustrates the general idea of the method, in which PEG and P1, 2-BDO represent the corresponding product of EG and 1, 2-BDO, respectively. The whole process was separated into three sub-processes: forward reaction, products purification and backward reaction. The separation of the heat-sensitive close-boiling mixture was converted into the separation of their corresponding products or the separation of one diol with the product of the other diol via forward reactions process. The purification of the 3



products was easy to achieve due to effective conversion. The original EG and 1,2-BDO can be obtained through back reaction. The reactant proposed in our previous work may not be the most suitable and economical one, so the reactant screening process needs carrying out systematically.

Figure 1. The general idea of reversible reaction-assisted intensification separation for EG and 1,2-BDO azeotropic mixture.

This paper aims to screen out the feasible reactants to achieve the separation of EG and 1,2-BDO system in an energy-saving and economical way. A screening standard was proposed and applied in the EG and 1,2-BDO system. Batch reaction experiments were carried in the presence of KRD001 catalyst to study the reaction conversions and reaction selectivity difference of a potential reactant toward EG and 1,2-BDO.Batch distillation experiments were proceeded to testify the feasibility of products purification. A final rank of all the potential reactants was given after going through the whole screening process.

2. Screening principle 2.1 Reactant screening standards A pre-screening is designed to narrow down the screening scope of the reactants 4


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before carrying out experiments. It starts from the screening of the reaction types. The reaction networks of a feasible reaction type can effectively convert the separation system. Then the potential reactants were chosen out within the reaction type. In order to achieve the effective separation, the following factors should be taken into account when conducting the pre-screening process. i.

The reactions types should be reversible. Thus, the substances can be

obtained through backward reactions. In addition, the reaction networks can effectively convert the separation, avoiding the cross reaction of the original substances. ii.

The reactions are carried out in mild conditions, which can decrease the

energy consumption and the instrument investigation. iii.

High value-added products allow the follow-up separation process have more

selectivity and feasibility. iv.

Environment-friendly bulk chemicals with price, transportation and storage

advantages make the whole process more economical. The reaction conversion and reaction selectivity difference were defined as Eq.1 and Eq.2.： The conversion of component i in the mixture is calculated as follows:

Con. = 1 −

(1)

Where i means the component in the azeotropic mixture. The reaction selectivity difference of component i to component j in the mixture towards the reactant is defined as follows : 5



Reaction Selectivity Difference $RSD% =

&.' &.(

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(2)

Where i and j means the components in the azeotropic mixture. The RSD reflects the separation degree of i and j. Based on the reaction selectivity difference and conversions of a potential reactant towards the substances in the azeotropic system, the reactant was divided into three categories: A. The RSD is a lot greater or less than 1, indicating that the reactants mainly react with one substance of the azeotropic system. However, as the properties of the substances are similar, it is usually hard to find the reactant that only reacts with one substance. B. The RSD is a little greater or less than 1, but the reaction conversions are high. It’s the second best. The selective remove of one substance is realized by altering the reaction conditions to achieve the more conversion of one substance and the less conversion of the other substance. C. The RSD is around 1 and the reaction conversion is not high, which is the last option. The separation of the azeotropic system is converted to the separation of their corresponding products. The original substances can be obtained through the backward reaction process. The purification efficiency of the products and the yield of the target substance are important points and need paying attention to. The following flow sheet, shown in Figure 2, gives the complete screening procedures. If the separation needs solving regardless of the cost and effort, the screening procedures can start directly from the 6


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reaction step without the pre-screening step.

Figure 2. Reactant-screening procedures to get the feasible reactants.

2.2 The preliminary screening for potential reactants This part is designed to pick out some potential reactants for the separation of EG and 1,2-BDO. Based on literature survey, the networks of esterification, aldolization and ketalation reactions can achieve the feasible conversion of the separation.17, 21-26 The preliminary screening was carried out based on the standards mentioned above. Esterification. Although the corresponding products of EG and 1,2-BDO are more than one, there does not exist the cross reaction of EG with 1,2-BDO in the reaction system. Furthermore, the reaction kinetics and thermo-dynamics of the EG esterification system has been thoroughly studied. In addition, the application of 7



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esterification has been widely reported in every possible way 26, 27. Ethylene glycol diacrylate is applied in the bi-functional acrylate polymer modification and other fields, as adhesives, coatings as well as chemical irrigation 28

materials due to its high reactivity and low toxicity

.The esterification of ethylene

glycol with acetic acid is regarded as a suitable method to produce monoacetylated ethylene glycol as well as diacetylated acetylene glycol29,

30

. The diacetylated

acetylene glycol is considered as popular organic solvents and non-polluting fuel-additives for its excellent physical properties

31

. Although few literatures were

reported about the reaction of 1, 2-BDO with HAc or AA, its corresponding products may show great potential in real utility considering their similarity with the products of EG. The esterification of acetic acid or acrylic acid with the mixture of EG and 1,2-BDO can be carried out in mild condition32. The corresponding products can either hydrolyze back to get the original components under specific conditions or sell as a commodity. Therefore, we choose acetic acid and acrylic acid as the potential reactants. Aldolization and ketalation. EG corresponds to one product after aldolization or ketalation reaction, and it is the same with 1,2-BDO17. The boiling points of their corresponding products are much lower than the originals. Besides, aldolization and ketalation reactions have excellent applications in mixture recovery and separation. The recovery of propylene glycol from dilute aqueous solutions via acetalization reaction with formaldehyde forming 4-methyl-l, 3-dioxolane or with acetaldehyde 8


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forming 2,4-dimethyl-1,3-dioxolane were studied experimentally. Ketalation of EG with acetone and butanone is widely studied for their corresponding products have broad industrial application 33. Propionaldehyde, acetone and butanone are widely used in industry for propionaldehyde is an important raw material in fine chemicals and acetone and butanone are important raw materials in organic synthesis. The prices of them are stable and the transportation and storage are relatively mature. In addition, their corresponding cyclic components are important solvents and intermediates. So, we took acetaldehyde, propionaldehyde, acetone and butanone as representatives to testify whether they can effectively convert the separation of EG and 1,2-BDO mixture and obtain some regularities to guide better reactant selections. Acetaldehyde had been proved to be an effective reactant to solve the separating puzzle of EG and 1,2-BDO mixture in our previous work9. However,it might not be the most economical and feasible one, so the screening process is essential to optimize the choice of reactants.

3. Experimental section 3.1 Materials Chemicals. The suppliers and specifications of the chemicals used in this research are listed in Table 1. The mass purities of all the chemical substances were tested by gas chromatography (GC, PerkinElmer) equipped with a HP-innowax capillary column (30 m × 0.25 mm × 0.25 µm, Agilent). 9



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Table 1. Suppliers and purity of the chemicals used in the paper. Name

Supplier

Purity(mass) by GC

Ethylene glycol 1,2-Butanediol Acetic acid Acrylic acid Propionaldehyde Acetone Butanone

Tianjin Jiangtian TCI Tianjin Jiangtian Tianjin Jiangtian Aladdin Tianjin Jiangtian Tianjin Jiangtian

99.5% 98.0% 99.0% 95.0% 99.5% 99.5% 99.5%

Catalyst. KRD006-1（Cary environmental technology co., LTD）was used as the catalyst in our study. The relevant characteristics of the KRD006-1 catalyst are summarized in Table 2. Before used in reactions, the resin was distributed flat on a tray and dried up at atmosphere at 378.15K for 4 h in an electric thermostatic drying oven. Table 2. The characteristics of the KRD006-1 catalyst. KRD006-1 Appearance Total Exchange Capacity mmol/g (mmol/ml)

Opaque beads ≥4.80[H+] (≥1.5mmol/ml)

Moisture holding capacity %

50-58

Shipping weight (g/ml)

0.70-0.85

Specific gravity (g/ml)

1.15-1.25

Particle Size Range

(0.315mm-1.250mm)

Mechanical Strength %

≥95

Max. operation Temperature ℃

120

Ionic Form as shipped

H

10


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3.2 Experimental Procedures Allowing for the dilution heat of the third reactant mixing with the diols, the reactants were weighed into the flask first with the magnetic stirring and heat turned on. A precise amount of the KRD006-1 catalyst was added after the desired temperature was attained and this was recorded as the start of the reaction. As for acetic acid and acrylic acid, the experimental temperature differs in order to testify the effect of temperature. Samples (about 0.2ml) were withdrawn from the liquid mixture at specified intervals and the composition were analyzed. As for acetone, butanone and propionaldehyde, the corresponding reaction products are dioxetane compounds, which are partially or even completely miscible with water34. Stratification appeared during the reaction may cause the heterogeneity of sampling. Thus, we decided to measure the final conversion without tracing the instantaneous change. The operation procedures before the start of the reaction were same. After sufficient reacting time, the after-reaction liquid was transferred into a separatory funnel. Stratification would appear after enough time of standstill. The quantities of the upper and lower layers were accurately weighed and the composition of each layer was analyzed. The graphic representation of the batch-reaction apparatus and the follow-up purification distillation separation apparatus are provided in Supporting Information for better understanding.

3.3 Analysis method The structures of the reaction products were identified using gas chromatography– 11



mass spectrometry (GC-MS, GC-7890A, MS-5975C, Agilent). Gas chromatography (GC, PerkinElmer) equipped with a flame ionization detector (FID) were used to quantitatively analyze the samples. The specific information about the samples’ analyzing methods as well as their corresponding capillary columns were given in Supporting Information. Each sample was analyzed at least three times and the averaged data was used to ensure that the mole fraction standard uncertainty was within ± 0.001. Response curves of EG and 1,2-BDO with peak area to concentration were determined by a series of calibration standards that spanned the range of sample compositions. For aldolization and ketalization samples, the water content was measured by Karl Fischer titration (ZSD-1, Shanghai Anting Electric Instruments Co. Ltd., China).

4.Results and discussion Acetic acid. Figure 3 gives the conversions and RSD over time. The conversion of EG and 1,2-BDO with HAc reached 0.48 and 0.34 in 1 hour, respectively. The reaction equilibrium of EG was obtained within 2 hours with final conversion reaching 0.5. The reaction equilibrium of 1,2-BDO was 6 hours with final conversion reaching 0.46. However, the RSD decreased with the time, indicating that the reaction rate of EG decreases faster than that of 1,2-BDO with decrease of EG concentration because of the preferential esterification of EG. The final RSD is around 1, showing that there is no obvious reaction equilibrium difference, thus, HAc is divided into category C. Raising the reaction temperature accelerates the reaction rate of 1, 2-BDO

12


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with HAc, for the reaction equilibrium time is shortens to 3 hours. However, the change of temperature has little effect on the RSD and equilibrium conversion of EG and 1, 2-BDO as can be seen in Figure 3(a), (b) and (c). Because of the relatively small standard enthalpy of reaction in the case of esterification reactions, the chemical equilibrium constant slightly depends on temperature, the same results were also found in other studies.30, 35 .

(a) T=333.15K

(b) T=343.15K

(c) T=353.15 Figure 3. Effect of temperature on the conversion of EG and 1,2-BDO over time. (Reaction conditions: Cat: 5w%, MS=300r/min, MolHAC:(MolEG + Mol1,2-BDO) =1:1, MolEG: Mol1,2-BDO =1:1).

There is nine substances in the after-reaction system. The boiling points of the products are high and close (elaborated in the Supporting Information). Multiple sets of azeotropes would form in the after-reaction system36, 37. Thus, the purification 13



process is energy-intensive. Besides, each diol generates at least two products, the sub-sequential processes will be complex.

Acrylic acid. Figure 4 gives the conversions and the RSD over time. According to figure 4 (a), the conversions of EG and 1,2-BDO increase along with time, whereas the RSD decreases, indicating that acrylic acid prefers to react with EG. According to figure 4 (a) and (b), the conversions of EG and 1,2-BDO increase along temperature, whereas the RSD decreases. Because of the raising of reaction temperature, the reaction rates of EG and 1,2-BDO with acrylic acid were significantly accelerated, causing a dramatically decrease in EG concentration because of the preferential esterification of EG. Acrylic acid can be divided into category A for little 1,2-BDO took part in the reaction. However, as we aim to get the EG, all the EG has to be recovered from the hydrolysis of its corresponding products. The separation problems are the same with that of HAc. (elaborated in the Supporting Information).

(a) T=333.15K (b) T=353.15K Figure 4. Effect of temperature on the conversion of EG and 1, 2-BDO over time. Reaction conditions: Cat:5w%, MS=300r/min, Molacrylic acid:(MolEG + Mol1,2-BDO) =1:1, MolEG: Mol1,2-BDO =1:1).

Acetone and Butanone. Table 3 gives the equilibrium conversions and the RSD of 14


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acetone and butanone towards EG and 1,2-BDO. The equilibrium conversion of EG and 1,2-BDO with acetone are 22% and 40%, respectively, and the final RSD is 0.55. As the final conversion is small, plenty of acetone is needed in order to achieve the effective conversion, which will cause the increase of the cost. The similar problem also exits when choosing butanone as the reactant. So, acetone and butanone are divided into category C. Table 3. The conversion of EG and 1,2-BDO with acetone and butanone.

Reactant Acetone Butanone

Con

Con

.EG

22% 29%

RSD

.1，2-BDO

40% 45%

0.55 0.644

Reaction conditions: T=313.15K, Molacetone or butanone :(MolEG + Mol1, 2-BDO) =1:1, MassEG: Mass1, 2-BDO =4:1, Cat: 5w%, MS=300r/min.

Table 4 gives the ketalization products and their corresponding boiling points. The boiling points of the products are lower than that of the diols. In addition, there is significant boiling point difference between the products. Thus, the separation process would be easy to perform. Moreover, each diol only corresponds to one product, so the backward reaction process getting the original substances would be easy to achieve.

Table 4. The ketalization reaction products of EG and 1, 2-BDO. Reactant Acetone

Butanone

Main product

CAS number

TB

EG

2,2-dimethyl-1,3-dioxane

2916-31-6

365.15K

1,2-BDO

2,2-dimethyl-4-ethyl-1,3-dioxane

145125-18-4

406.15±8K

EG 1,2-BDO

2-methyl-4-ethyl-1,3-dioxane 2-methyl-2,4-diethyl-1,3-dioxane

126-39-6 1938136-04-9

388.55-389.35K 429.5±8K

The boiling point data is obtained from the SciFinder estimated at conditions: 101.3kPa, 298.15K.

Propionaldehyde. Figure 5 gives the equilibrium conversions together with the 15



RSDs along with temperature. The conversion of 1,2-BDO and EG are around 1 and 0.9, respectively. As temperature does little effect on final conversions, the RSD stays nearly the same. While, as the mole ratio of propionaldehyde to the mixture of EG and 1,2-BDO increases, a minimum SRD appeared, given in Figure 6. Nearly all the 1,2-BDO was converted to its product and less than 30% of EG was converted when the mole ratio is around 0.5, which dividing propionaldehyde into category B. It is favorable to the azeotropic mixture of EG and 1,2-BDO. EG can be recovered from two ways: the hydrolysis of its corresponding product and the EG left in the reaction system.

Figure 5. Effect of temperature on the conversion of EG and 1,2-BDO. (Reaction conditions: Molpropionaldehyde:(MolEG + Mol1,2-BDO) =1:1, MassEG: Mass1,2-BDO =4:1, Cat:5w%, MS=300r/min.).

16


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Figure 6. Effect of the mole of propionaldehyde to the mole of mixture of EG and 1,2-BDO as well as the mole ratio of EG to 1,2-BDO on the conversion of EG and 1,2-BDO. (Reaction conditions: Cat:5w%, MS=300r/min, T=303.15K,3h.).

The boiling points of 2-ethyl-1,3-dioxane(2ED) and 2,4-diethyl-1,3-dioxane (24DMD) are 379.18K and 424.15K±8K, respectively, shown in Table 5. Due to the lack of literature report, distillation experiments were carried out to prove the feasibility of purifying 2ED and 24DED from the reaction system. 99.7% mass purity of 2ED and 99.8% mass purity of 24DED were obtained (given in the Supporting Information), which demonstrates the advantage of propionaldehyde as the reactant. Table 5. The acetal reaction products of EG and 1, 2-BDO. Reactant Propionaldehyde

EG 1,2-BDO

Main product（abbreviation）

CAS number

TB

2-ethyl-1,3-dioxane(2ED) 2,4-diethyl-1,3-dioxane(24DED)

2568-96-9 133279-48-8

379.18K 424.15K±8K

The boiling point data were obtained from the SciFinder estimated at conditions: 101.3kPa, 298.15K.

The RSDs of acetic acid and acrylic acid toward EG and 1,2-BDO are mainly due to the reaction rate difference, and the advantage of acrylic acid is greater than acetic acid for 1,2-BDO nearly does not participate in the reaction in the first two hours. However, the subsequent purification of the products as well as the hydrolysis process obtaining the pure EG are tricky, especially when the proportion of EG is large. The RSDs of propionaldehyde, acetone and butanone toward EG and 1,2-BDO 17



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are resulted from the reaction equilibrium difference. Propionaldehyde has greater advantage than acetone and butanone for it can achieve the direct recovery of large amount of EG. The products purification processes of propionaldehyde, acetone and butanone as reactants are easier to achieve and more energy-saving comparing to that of acetic acid and acrylic acid, which can be predicted from the distribution of their boiling points. The compositions of upper and lower layers of the reaction system were analyzed. As is shown in table 6, 2ED and 24DED are enriched in the upper layer and the EG is enriched in the lower layer, which is beneficial to coupling of reaction and separation. (more data information is given Supporting Information).

Table 6. The materials and their mass content in upper and lower layers after reaction. Total mass (g) Upper layer Lower layer

Prop （W）

2ED (W)

24DED (W)

EG (W)

1,2-BDO (W)

Water (W)

24.5993

0.0122

0.5813

0.3822

0.0195

0.0003

0.0044

90.6650

0.0239

0.1748

0.0283

0.5572

0.0073

0.2084

W means the mass content of the component. Reaction conditions:Molpropionaldehyde:(MolEG + Mol1,2-BDO) =0.5:1, MassEG: Mass1,2-BDO =4:1 Cat:5w%, MS=300r/min, 303.15K ,3h.

The final evaluation of the potential reactants for the azeotropic system of EG and 1, 2-BDO separation is given in Table 7. Considering the RSDs and the conversions in reaction process, the separation efficiency in purification process as well as EG yields, acetaldehyde and propionaldehyde were chosen as feasible reactants. The stratification

is

found

beneficial

to

reaction

and

18


purification,

therefore

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propionaldehyde was finally chosen for further study. Table 7. Summary of the reactant screening results for the azeotropic system of EG and 1,2-BDO. Acetic acid

Acrylic acid

Acetone

Butanone

Acetaldehyde

Reaction Conversion

‐

‐

‐

‐

√

√

RSD (category)

-(C)

√(A)

√(C)

√(C)

√(C)

√(B)

Separation efficiency

‐

‐

√

√

√

√

Additional advantages

‐

‐

‐

‐

‐

√

Rank

5

4

3

3

2

1

Propionaldehyde

5. Conclusions A screening principle to get the feasible and economical reactants via reversible reaction-assisted intensification separation process was proposed and applied in the azeotropic mixture of EG and 1, 2-BDO. Acetic acid (HAc), acrylic acid (AA), acetaldehyde, propionaldehyde, propanol and butanone were picked out as potential reactants after pre-screening step. Propionaldehyde turned out to have appropriate RSD and conversions towards the EG and 1, 2-BDO system after carrying out the batch experiments. The separation efficiency was predicted based on the relative volatility and verified by distillation experiments. 99.7% mass purity of 2ED and 99.8% mass purity of 24DED were obtained. The stratification in the reaction system was found beneficial to the reaction and separation process. Propionaldehyde was evaluated as the effective reactant to separate the 19



azeotropic mixture of EG and 1, 2-BDO in an economical way. The reaction selectivity difference between EG and 1, 2-BDO with propionaldehyde is very important and worth further studying.

Acknowledgement The authors are grateful for the financial support from the National Natural Science Foundation of China (Nos. 21690084, 21336007, 21776202), Key Technology R&D Program of Tianjin (No.15ZCZDGX00330), and International S&T Cooperation Program of China, ISTCP (No. 2015DFR40910).

Supporting Information for Publication Schematic of the experimental setups, Tables of some results discussion and some sets of validation experimental data are available in the Supporting Information.

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Reversible Reaction-Assisted Intensification Process for Separating

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