Reactive dividing-wall column for the co-production of ethyl acetate and n-butyl acetate

Accepted Manuscript Reactive Dividing-Wall Column for the Co-Production of Ethyl Acetate and n-Butyl Acetate

Hongshi Li, Tong Li, Chunli Li, Jing Fang, Lihui Dong PII: DOI: Reference:

S1004-9541(17)31482-9 doi:10.1016/j.cjche.2018.02.023 CJCHE 1063

To appear in: Received date: Revised date: Accepted date:

1 November 2017 20 January 2018 1 February 2018

Please cite this article as: Hongshi Li, Tong Li, Chunli Li, Jing Fang, Lihui Dong , Reactive Dividing-Wall Column for the Co-Production of Ethyl Acetate and n-Butyl Acetate. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Cjche(2018), doi:10.1016/j.cjche.2018.02.023

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ACCEPTED MANUSCRIPT

Separation Science and Engineering

Reactive Dividing-Wall Column for the Co-Production of Ethyl Acetate and n-Butyl Acetate*

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Hongshi Li, Tong Li, Chunli Li **, Jing Fang, Lihui Dong National-Local Joint Engineering Laboratory for energy conservation of chemical process Integration and resources

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utilization, Hebei University of Technology, Tianjin 300130, China

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Abstract Reactive dividing-wall column (RDWC) technology plays a critical role in the energy saving and high efficiency of chemical process. In this article, the process of co-producing ethyl acetate (EA)

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and n-butyl acetate (BA) with RDWC was studied. BA was not only the product, but also acted as entrainer to remove the water generated by the two esterification reactions. Experiments and simulations

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of the co-production process were carried out. It was found that the experimental results were in good agreement with the simulation results. Two kinds of RDWC structures (RDWC-FC and RDWC-RS)

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were proposed, and the co-production process operating parameters of the two types of RDWC were

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optimized by Aspen Plus respectively. The optimal operating parameters of the RDWC-FC were determined as follows: 0.6 of the reflux ratio of aqueous phase (RR), 0.66 of the vapor split (RV) and 0.51 of the liquid split (RL). And the optimal operating parameters of the RDWC-RS were shown as

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follows: RR was 0.295 and RV was 0.61. Furthermore, the energy saving analysis of the co-production

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process was based on the annual output of 10000 tons of EA, compared with the traditional reaction distillation (RD) to prepare EA and BA, the reboiler duty of the RDWC-FC column could save 20.4%, TAC saving 23.6%; RDWC-RS reboiler energy consumption could save 17.0%, TAC 22.2%。 Keywords reactive dividing-wall columns, ethyl acetate, n-butyl acetate, coproduction, energy-saving

1 INTRODUCTION Distillation is a main energy consumer in the production process of petroleum and chemical industry. At present, the traditional distillation technology has been widely applied for chemical

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production, which has the key problems of low thermodynamic efficiency, high energy consumption and high investment cost. Therefore, the development of novel distillation technology and equipment to achieve the chemical process of energy saving and social sustainable development is of great significance.

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The dividing-wall column (DWC) was conceptualized in 1930 and did not achieve the first

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industrialization until 1984. Due to its advantages of low energy demand, low equipment investment, high thermal efficiency and high ability to simultaneous separation for multi-component mixture in a

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single column [1-5], the DWC has been studied extensively since 1992, and the number of DWC in the world had already reached more than 300 by 2016. The reactive distillation (RD) is carried out

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simultaneously with the reaction and separation, and promotes the reaction forward by timely removal

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of the product, which can improve the reaction conversi on and selectivity. As shown in Fig.1, the reactive dividing-wall columns (RDWC) integrates RD with DWC into a column, which has the both advantages of the RD and DWC. It is widely applied to the synthesis and decomposition of esters, the

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prospects for development.

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hydrogenation of hydrocarbon [6-9], the preparation of biodiesel [10-12] and so on, which has good

Figure 1 Developing process of the reactive dividing wall column

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The energy-saving methods of esterification process for the preparation of ethyl acetate (EA) have been widely studied, including extractive distillation [13-15], membrane pervaporation [16-18], homogeneous / heterogeneous reactive distillation [19] and other methods. Zhang et al. [20] simulated the process of azeotropic-reactive distillation to prepare EA, in which n-butyl acetate (BA) was used as

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an entrainer, the reaction zone was located at the bottom of the column, the reactant ethanol (ETOH) was fed to the bottom of the reaction zone, acetic acid (AA) was fed to the top of the reaction zone, and

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the entrainer was added to the column in advance for recycling. Compared with the traditional RD, a

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large amount of water generated by the reaction in the column was taken out from the side line through the BA in the process, which effectively reduced the backflow of the ester phase in the top and achieved

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good energy-saving effect. Raúl Delgado-delgadoa et al. [10], for the first time, published the pilot test results of producing EA by RDWC. The experimental results were consistent with the steady state

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simulation results, which provided an important basis for verifying the accuracy of the previous simulation work. The experimental results were in good agreement with the steady-state simulation

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results and provided an important basis for verifying the accuracy of previous simulation work. Miguel

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A. Santaella et al. [6] compared different EA production routes (traditional two-column distillation, RD, reactive pressure rectification, RDWC) based on several parameters such as conversion rate, yield and

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E-factor. The results showed that the RDWC energy-saving effect was very good. Meng et al. [21] proposed a co-production simulation of EA and BA in a reactor and column. First,

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ETOH and n-BUOH with an excess of AA were first esterified in the reactor, and then the production entered the column to be separated, the azeotropic mixture of EA was distilled overhead and the azeotropic mixture of BA and water containing a small amount of EA was withdrawn on the side of the column. Optimization of the parameters such as the number of plates and the reflux ratio of the column was carried out to determine the operating conditions for the best separation effect. Liu et al. [22] studied the acidity control of the co-production process by simulation and determined the operating conditions for controlling the content of AA in the product. Tian Hui et al. [23-24] carried out a

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simulation and experimental research of co-producing EA-BA through two esterification columns. The experimental results showed that the co-production process had a good energy-saving effect. The author’s group investigates the azeotrope-reactive distillation to prepare the EA. The water is separated from the side line by using BA as the entrainer to form azeotrope with water. According to

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this, a co-production process [22-27] that n-BUOH and ETOH react with AA at the same time to get

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BA and EA in the RDWC is proposed. The BA is not only the reaction product, but also the entrainer. Experiments and simulations are carried out to verify the feasibility of the process, and then the process

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is optimized and the energy-saving is analyzed by Aspen Plus.

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2 MODEL DESIGN

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2.1 Properties parameters

In this esterification system, six substances are involved, and their character data is shown in Tab.1.

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The interaction between the six substances not only forms a variety of azeotropes, but also makes the

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existence of liquid-liquid equilibrium phase-separation phenomenon, which involves a total of eight azeotropes, the azeotropic temperature and composition are shown in Tab.2.

Component ID

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Table 1 The character data for AA, ETOH, n-BUOH, EA, BA and water Formula

Relative density

Melting point /℃

Boiling point /℃

Molecular weight

C2H4O2

1.05

16.66

118.1

60.05

C2H6O

0.79

-114.5

78.32

46.07

n-BUOH

n-C4H10O

0.81

-89.8

117.7

74.12

EA

C4H8O2

0.9

-83.8

77.11

88.07

BA

n-C6H12O2

0.88

-73.5

126.11

116.16

water

H2O

1

0

100

18.02

AA

AC

ETOH

Table 2 Azeotropic data for ETOH, n-BUOH, EA, BA and water Azeotropic composition /wt% Azeotrope

Azeotropic point /℃ ETOH

n-BUOH

EA

BA

water

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78.17

96

-

-

-

4

EA-water

70.38

-

-

91.53

-

8.47

ETOH-EA

71.81

30.98

-

69.02

-

-

ETOH-EA-water

70.23

8.4

-

82.6

-

9

n-BUOH-water

92.7

-

57.5

-

-

42.5

BA-water

90.2

-

-

-

71.3

28.7

n-BUOH-BA

116.2

-

63.3

-

36.7

-

n-BUOH-BA-water

90.7

-

8

-

63

29

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2.2 Vapor-Liquid Equilibrium equation

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ETOH-water

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For systems with gas-phase association, using a single physical method can not accurately calculate the interaction parameters between the systems, so a complex physical method is needed for

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the corresponding calculation. The Hayden-o ' Connel state equation [28] is a virial equation up to two,

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which can accurately predict the blend and two polymerization of polar components at less than 1 MPa,

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especially for mixtures containing carboxylic acids.

2.3 Thermodynamic model

The paper [29] calculated the azeotropic point and composition of the systems using the binary

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interaction parameters of the NRTL and UNIQUAC in the Aspen Plus database. The NRTL was found to be superior to UNIQUAC and gave a detailed azeotropic data, see Tab.3, so the NRTL

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thermodynamic method is chosen in this paper. The binary interaction parameters of the NRTL are given in Tab.4. In addition, there is a dimer phenomenon in the vapor phase of AA molecule, which needs the Hayden-o ' Connel state equation, so the physical property model NRTL-HOC is applied to the thermodynamic calculation.

Table 3 The data from NRTL Composition /wt%

Boiling Components

EA-water

point /℃

AA

ETOH

n-BUOH

EA

BA

water

71.39

-

-

-

90.97

-

9.03

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71.78

-

29.76

-

70.24

-

-

ETOH-water

78.15

-

95.62

-

-

-

4.38

ETOH-EA-water

70.33

-

12.62

-

78.64

-

8.75

BA-water

90.95

-

-

-

-

71.7

28.59

n-BUOH-BA-water

90.49

-

-

17.35

-

53.38

28.3

n-BUOH-water

92.53

-

-

57.98

-

-

42.02

n-BUOH-BA

116.95

-

-

68.88

-

31.12

-

AA

AA

AA

ETOH

ETOH

EtAc

BA

Component j

ETOH

EA

water

EA

water

water

water

aij

0

0

-1.9763

0

0

0

0

aji

0

0

3.3293

0

0

0

0

bij

-252.4821

-235.2789

609.8886

216.3048

-55.1698

470.8551

100.659

bji

225.4756

515.8212

-723.8881

95.0457

670.444

1165.6357

cij

0.3

0.3

0.3

0.3

0.3031

0.4104

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Component i

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Table 4-1 Parameters of NRTL activity coefficient model

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ETOH-EA

2333.838 0.2071

Component j

n-BuOH

aij

0

aji

0

bij

AA

ETOH

ETOH

n-BuOH

n-BuOH

n-BuOH

BA

n-BuOH

BA

EA

water

BA

0

0

0

0

0

0

0

0

0

0

0

0

550.1623

17.3333

33.483

467.8983

-7.1253

211.6319

313.8322

bji

-381.5959

261.2017

8.4365

-105.0851

289.9343

1319.7672

-40.8184

cij

0.3

0.3

0.3467

0.1842

0.3008

0.4269

0.3

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AA

AC

CE

Component i

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Table 4-2 Parameters of NRTL activity coefficient model

2.4 Reaction kinetics equation Amberlyst15 is a new type of strong acidic ion exchange resin, which has the advantages of non-corrosive, reusable and selectivity, and is widely used in catalysis of various chemical reactions, so the Amberlyst15 is selected as the catalyst for esterification reaction. 2.4.1 Reaction Kinetics of Synthesis of EA

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Kinetic equation of catalytic synthesis of EA catalyzed by Amberlyst15 catalyst [30] is expressed by Eq. (1). （1）

r1=mcat (k+xAAxETOH·k-xEAxH2O)

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k+=2.881× (-28490/RT)，kmol·(kg cat)-1·s-1 k-=0.051× (-26700/RT)，kmol·(kg cat)-1·s-1

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where r1 is the reaction rate, kmol·k-n·s-1 (holdup unit); k is the exponential factor, kmol·(kg cat)-1·s-1; x is the activity. 2.4.2 Reaction Kinetics of Synthesis of BA

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Kinetic equation of catalytic synthesis of BA catalyzed by Amberlyst15 catalyst [31] is expressed

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by Eq. (2).

r2=mcat (k+xAAxBuOH·k-xBAxH2O)

（2）

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k+=3.3856×106 (-70660/RT)，kmol·(kg cat)-1·s-1

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k-=1.0135×106 (-74241.7/RT)，kmol·(kg cat)-1·s-1 where r2 is the reaction rate, kmol·s-1; k is the exponential factor, kmol·(kg cat)-1·s-1; x is the activity.

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2.5 Two structures of the RDWC

As is well-known, the AA content in waste acid water is strict in the industry. In order that

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wastewater discharge reaches the standard, the AA content should be controlled below 0.002 wt%. A structure of RDWC, RDWC-FC (Fig.2), which is the equivalent to full thermal coupling distillation column and another structure of RDWC, RDWC-RS (Fig.3), which is the equivalent to the lateral line distillation column are designed to study the process of co-producing EA and BA. The RDWC-FC is simulated by the four-column model [32] (Fig.4), while the RDWC-RS (Fig.5) is simulated by the three-model for the absence of a public distillation section.

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Figure 2 The process of RDWC-FC for co-producing ethyl acetate and n-Butyl acetate

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Figure 3 The process of RDWC-RS for co-producing ethyl acetate and n-Butyl acetate

Figure 4 The simulation process of the RDWC-FC by four radfrac models for co-producing ethyl acetate and n-Butyl acetate

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Figure 5 The simulation process of the RDWC-RS by three radfrac models for co-producing ethyl acetate and

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n-Butyl acetate

NP

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On the basis of the literature [22, 24], the number of theoretical plates of each section by the simulation was determined (see Tab.5).

NR

NM

NS

RDWC-FC

30

10

30

10

RDWC-RS

30

-

30

10

Table 5 The number of theoretical plate of the RDWC-FC and RDWC-RS by simulation Number

of

theoretical

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3 Experiments

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plates

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3.1 Experimental device

The experimental device of RDWC for the co-production of EA and BA is displayed in Fig.6, and

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the RDWC-FC structure is investigated. The material of the packing column is 316 L stainless steel and the column is packed with 3 × 3θ ring (316 L stainless steel). The total height of the column is 4 m, the effective packing height is 2.1 m, the public distillation section is 600 mm, the public stripping section is 500 mm, and the pre-fraction section and the main column section are both 1500 mm. The reaction zone is located in the pre-fraction section with a total height of 1000 mm. The diameter of the public distillation section and the public stripping section are 41 mm, the main column section and the non-reaction zone of the pre-fraction section are 28 mm and the diameter of the reaction zone of the pre-fraction section is 38 mm. 9

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Figure 6 The experiment device of the reactive dividing-wall column

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1. Voltage regulator 2.electrically heated wire 3.reboiler 4.public stripping section 5.pre-fraction section 6.reaction section 7.pre-fraction section 8. public distillation section 9. raw material feed tank 10. reflow ratio controller 11.the top product 12 the side product 13.valve 14.the reboiler product

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The Amberlyst15 is selected as the catalyst, and the catalyst packing method is shown in Fig.7. The swing hopper is controlled by the solenoid valve, and the collected liquid is divided into two separate flows to the catalyst and packing loading section. The liquid phase contacts with the catalyst in

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the section filled with catalyst and enters the section filled with packing, while the vapor phase contacts

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with the falling liquid phase for mass transfer in the packing loading section from bottom to top.

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Furthermore, the height of the upper layer of the catalyst is 40 mm.

Figure 7 The catalyst packing mode 1.The funnel 2.the electromagnetic valve 3.the section filled with packing 4. the section filled with catalyst

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The driving process of the RDWC is as follows: 4 L of AA, a mixture of 2 L of BA (63 wt%), n-BUOH (8 wt%) and water (29 wt%) were preliminarily added into the reboiler. The reboiler was kept heating, and then ETOH, n-BUOH and AA were simultaneously fed at a certain rate when the ternary mixture of BA, n-BUOH and water reached the position of the liquid dispenser. The top product was

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withdrawn in accordance with a certain proportion, so that the top temperature was maintained at 69.5℃,

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and the public distillation section T1 was stable at 71℃. In order to ensure that the ternary mixture of BA, n-BUOH and water was taken from the side-draw, the liquid split and side line ratio were adjusted

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to make the outlet temperature of the side withdrawing keep at 90.5℃。

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3.2 Analysis method

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The samples were detected through Shimadzu GC-2014 gas chromatograph equipped with a HayeSep Q column. In the process of analysis, the column temperature was kept at 180℃. The

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3.3 Results and discussion

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calibration area normalization method is chosen as the detection method.

The operating parameters after one hour stabilization of the system are as follows: the flow of the

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mixture of AA and n-BUOH (92 wt% acetic acid) was 8ml· min-1 and the flow of ETOH (95 wt%) was

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5ml · min-1, reflux ratio (R) was 5, the liquid split (RL) was 0.59, and the side line ratio (RS) was 0.2. The top, side line and reboiler product composition in the experiment were given in Tab.6, the temperature measurement point of the position of the distribution was shown in Fig.6, and the temperature distribution was illustrated in Fig.8. The results of simulation and experiment agree with each other very well, hence, the model is correct and reliable, and the model optimization can be carried out. Table 6 The results of RDWC-FC by experiment and simulation

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simulation

Top temperature /℃

69.9

70.5

Side line temperature /℃

90.5

89.7

Bottom temperature/℃

111.5

111.1

Component

content

content

EA

76

74.3

ETOH

16.5

14

Water

7.5

11.7

Component

content

content

BA

54.3

Water

20.2

n-BUOH

24.1

ETOH

1.4

Component

content

AA

89.2

Water

6

BA

3.6

n-BUOH

1.2

composition /wt%

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Top product

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experiment

57.7

Side line product

18.4

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composition /wt%

22.7 1.2

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content

Reboiler product

7.7 2.1 0.9

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D

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composition /wt%

89.3

AC

Figure 8 The temperature distribution of RDWC-FC by experiment and simulation

4 Optimization

The theoretical tray number of each section in the simulation process are shown in Tab.5. The feed flow of ETOH (95 wt%) and n-BUOH (100 wt%) were 40kmol·h-1 and 10kmol·h-1, respectively, and the AA excess by 20%. With the content of EA in the crude ester more than 90 wt%, and the AA content in the side-draw below 20μg·g-1 as the restrictions, and the lowest reboiler duty as the goal, two 12

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different structures of RDWC, RDWC-FC and RDWC-RS, were studied. The optimum operating conditions were determined by the analysis of the reflux ratio of aqueous phase (RR), the vapor split (RV), RL and other factors in the process of co-producing EA and BA. The design specs, optimization,

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sensitivity in Aspen Plus are used to optimize the parameters.

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4.1 The optimization to the RDWC-FC

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As RDWC-FC structure is demonstrated in Fig.2, EA, BA and water are produced by the

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esterification reaction in the reaction distillation section. Next, the vapor phase of the public distillation section is condensed by the condenser and then the condensed liquid is divided into light phase and

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water phase in the phase separator. In order to control the content of AA in the product, the part of the water phase should be refluxed. Since the boiling point of AA is high, the content of AA in the side

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product is larger than that in the product at the top of the column. Therefore, it is only necessary to

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make the content of AA in the crude BA less than 20μg·g-1. The content of AA in the crude BA is positively influenced by side withdrawing position, and the content of AA in the crude BA is changed

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as depicted in Fig.9. The lower the position of the side withdrawing is, the higher the content of the AA

400 350

Mass fraction of AA / ppm

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in the crude BA is. Hence, the position of the side withdrawing is selected at the 25th theoretical plate.

300 250 200 150 100 50 0 18

20

22

24

26

28

30

32

34

36

Stage number of RDWC

Figure 9 Effect of the side withdrawing position on the content of acetic acid in the side-draw

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The operating variables of the system include RR, RV and RL. 4.1.1 Effect of the RR From the simulating in Fig. 10, it is clear that the content of AA in the crude BA is significantly

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affected by RR. The AA content decreases with the increase of RR. When the RR is more than 0.6, the

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content of AA in the BA is up to standard. We can clearly see the same trend of the conversion of

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ETOH and n-BUOH as the AA content by the effect of the RR. Because the rise of the RR makes the increase of the concentration of the water in the column, which, to some extent, inhibits the positive

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esterification reaction of ETOH and n-BUOH. In order to ensure a higher conversion of ETOH and

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D

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n-BUOH and the content of AA in the crude BA to standard, the RR of the column is set as 0.6.

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Figure 10 Effect of the reflux ratio of aqueous phase on the content of acetic acid in the side-draw and the conversion of ETOH and n-BUOH

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4.1.2 Effect of the RV

As can been seen from the Fig.11, a considerable increase of mass fraction of AA occurs from 0.66 to 0.68 of RV, but the AA content is close to zero when the vapor split is less than 0.66. As the RV increasing, the heat distributed to the main column decreases and the separation efficiency of the main column decreases, so that the intermediate component and the heavy component can not be effectively separated, which results in an increase of the AA content in the side-line BA. The conversion of ETOH

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shows an increasing trend, while n-BUOH remains almost unchanged. This may be related to the greater the RV, the more heat distributed to the pre-fraction section. According to the figure, the RV of the structure is chosen to be 0.66 for the purpose of ensuring that a higher conversion ratio of ETOH

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and n-BUOH and the qualified content of AA in crude BA.

250

n-BUOH

98

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AA ETOH

96

200

94

100 50 0 0.60

0.62

0.64

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0.58

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150

0.66

92 90

Conversion / %

Mass fraction of AA / ppm

100

300

88 86 0.68

The vapor split

Figure 11 Effect of the vapor split on the content of acetic acid in the side-draw and the conversion of ETOH and

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n-BUOH

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4.1.3 Effect of the RL

The mass fraction of AA in the side-draw sharply falls to near zero when the liquid split reaches

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0.51 in the Fig.12. In addition, the AA content is up to standard when the RL is more than 0.51. When the RL is less than 0.51, the liquid reflux in the pre-fraction section decreases, and the heavy component

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AA can not be separated clearly in the pre-fraction section, so that the AA crosses the bottom of the public distillation section and enters the main column section, which will make the AA content in the side-draw increase. The conversion of ETOH and n-BUOH follow the same downward trend, since the increase of RL leads to the growing content of AA and BA in the pre-fraction section, and then the esterification of n-BUOH is carried out in reverse. Therefore, the optimal RL is 0.51 so that we can get a higher conversion ratio of ETOH and n-BUOH and a standard mass fraction of AA.

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99.6 AA ETOH n-BUOH

99.0 98.4 97.8

400

97.2 96.6

300

96.0 95.4

200

94.8 94.2

100

Conversion / %

93.6 93.0

0

92.4 91.8 0.49

0.50

0.51

0.52

0.53

0.54

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The liquid split

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Mass fraction of AA / ppm

500

Figure 12 Effect of the liquid split on the content of acetic acid in the side-draw and the conversion of ETOH and

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n-BUOH

4.2 The optimization to the RDWC-RS

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The Fig.3 illustrates the RDWC-RS structure, whose partition is placed at the top of the distillation

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column, including two condensers and a reboiler. Owing to the clear separation of the light and the intermediate component in the pre-fraction column, the content of AA in the top product of the column

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is larger than that in the top of the main column. Thus, the AA content of EA in the top of the

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pre-fraction is controlled to meet the standard of AA content of all products. The operating variables in the system include: RR and RV.

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4.2.1 Effect of the RV

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The Fig.13 describes the effect of the vapor split on the content of AA in the side-draw and the conversion of ETOH and n-BUOH. The RV from 0.59 to 0.61 sees a sharp drop in the mass fraction of AA from 500 to near zero until the RV is greater than 0.61, which AA content is a leveling out at near zero. This is because the increase of RV causes more heat to be distributed to the pre-fraction section, so that the separation efficiency of the pre-fraction section is increased, the heavy component AA is efficiently separated, and the content of AA in the overhead crude EA is decreased. The conversion of ETOH and n-BUOH are in the opposite direction. This may also be related to the heat of the 16

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pre-fraction section. Consequently, 0.61 is a suitable option for RV to make sure a higher conversion

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ratio of reactant and qualified product.

Figure 13 Effect of the vapor split on the content of acetic acid in the side-draw and the conversion of ETOH and

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n-BUOH

4.2.2 Effect of the RR

D

As is exhibited in the Fig.14, the RR plays an important role in the content of AA in the crude EA

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of the pre-fraction section. When the RR is more than 0.295, the content of AA in the side-draw is up to standard. A slight reduction in ETOH conversion is caused by the RR, because the increase of the RR

CE

promotes the esterification of ETOH and AA to a certain extent in the reverse direction. 0.295 is

AC

determined as the value of RR for the sake of gaining a higher conversion ratio of ETOH and n-BUOH and a qualified content of AA.

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Figure 14 Effect of the reflux ratio of aqueous phase on the content of acetic acid in the side-draw and the conversion of ETOH and n-BUOH

The effects of RR, RL and RV on the reaction of two different structures were studied. The optimum

Table 7 More suitable operating conditions of RDWC-FC and RDWC-RS RL

RDWC-FC

0.6

0.51

RDWC-RS

0.295

-

RV 0.66 0.61

SC

RI

RR

PT

operating conditions were determined. The results are shown in Tab.7.

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4.3 Energy Saving Analysis of RDWC-FC and RDWC-RS

MA

The distribution of the mass composition of the components in the RDWC-FC is shown in Fig. 15. The light component of EA has the highest concentration at the top of the public distillation

D

section, the heavy component AA at the bottom of the column reaches the highest point, and the BA

PT E

and water are withdrew at the highest concentration in the main column section. The Fig.16 provides the mass fraction distribution regarding the components in the RDWC-RS. The concentration of the

CE

heavy component AA at the bottom of the column reaches a peak, and the concentration of BA and

AC

water at the top of the main column was maximum.

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1.0

AA EA BA WATER

Mass fraction

0.8

0.6

0.2

0.0 10

20

30

40

Stage number of RDWC-FC

SC

Figure 15 The mass fraction distribution of the RDWC-FC

1.0

NU

AA EA BA WATER

0.8 0.6 0.4

MA

Mass fraction

50

RI

0

PT

0.4

0.2 0.0 5

10

15

20

25

30

35

40

D

0

Stage number of RDWC-RS

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Figure 16 The mass fraction distribution of the RDWC-RS

Table 8 Energy consumption and TAC of A-RD, RD and RDWC RD (BA)

RDWC-FC

RDWC-RS

10000

3300

10000

10000

Number of theoretical plate

38

23

40

40

Reaction plate position

15-28

6-17

15-25

15-25

1017

180

948

916

Reboiler energy demand / kW

1019

186.4

959

994

Column diameter / m

0.6

0.34

0.81

0.81

Equipment costs /CNY

428376

107943

380175

380175

Operating costs /CNY

2872609

644457

2695917

27487714

3015401

680438

2822642

2875439

-

-

20.4%

17.0%

AC

EA production/ t·a-1

CE

A-RD (EA)

Condenser energy demand / kW

-1

TAC /CNY·a

Energy savings

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TAC savings

-

-

23.6%

22.2%

The Tab.8 compares the energy consumption and TAC of A-RD [33], RD and RDWC, details as follows: With an annual output of 10,000 tons EA as the base, relative to the traditional A-RD (azeotropic-reactive distillation column) and RD for preparation of EA and BA, the RDWC-FC

PT

structural reboiler duty and TAC respectively savings by 20.4% and 23.6%; the RDWC-RS can save reboiler energy demand 17.0% and save TAC 22.2%. The equipment costs of the condenser and reboiler

RI

are obtained using Eq. (3), and the column equipment cost formula is expressed in Eq. (4). And the

(3)

C=1764D1.066L0.802

(4)

NU

C=7293A0.65

SC

author's group previously had did the A-RD and RD economic accounting.

Where C is the equipment cost, CNY; A is the heat exchange area, m2; D is the column height, m; L is

MA

the column diameter, m.

Because of its good separation and energy saving effect, the next step we will carry out pilot test.

D

5 Conclusions

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Two RDWC structures for the co-production process of EA and BA were proposed in this paper. The experiment and simulation of the process were carried out by the RDWC-FC, whose results were in

CE

good agreement with each other, and the correctness of the model selection and the reliability of the

AC

simulation were verified. Based on the simulation of the two kinds of RDWC structures for the process, the optimal operating parameters about RR, RV and RL of the RDWC-FC and RR, RV of the RDWC-RS were determined. Moreover, compared with the preparation of EA and BA by A-RD and RD respectively, the co-production process of the two RDWC structures was better, which used the good water-carrying property of the product BA to make the water produced by the esterification reaction separated from the side-draw of the main column, thereby, the reflux rate of the ester phase and the

20

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components back to mix could be reduced and the thermodynamic efficiency would be improved, so as to realize the energy saving and consumption reduction of the process.

AA

acetic acid

A-RD

azeotropic-reactive distillation column

A-RDWC

azeotropic-reactive dividing-wall column

n-BUOH

n-butyl acetate

NU

BA

RI

heat exchange area, m2

SC

A

PT

NOMENCLATURE

n-butanol equipment cost, CNY

D

column height, m

DWC

dividing-wall column

EA

ethyl acetate

ETOH

ethanol

k1

exponential factor, kmol·(kg cat)-1·s-1

L

column diameter, m

NM

number of theoretical plate of the main column section

NP

number of theoretical plate of the pre-fraction section

NR

number of theoretical plate of the public distillation section

Nr

number of theoretical plate of the reaction section

NS

number of theoretical plate of the public stripping section

R

reflux ratio

RL

liquid split

RS

side line ratio

AC

CE

PT E

D

MA

C

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vapor split

r1

reaction rate, kmol·k-n·s-1 (holdup unit)

r2

reaction rate, kmol·s-1

RD

reactive distillation

RR

reflux ratio of aqueous phase reactive dividing-wall column

RDWC-FC

equivalent to full thermal coupling distillation column

RDWC-RS

equivalent to the lateral line distillation column total annual cost, CNY·a-1

x

the activity

NU

TAC

SC

RDWC

RI

PT

RV

MA

REFERENCES

1 Dwivedi, D., Strandberg, J.P., Halvorsen, I.J., Preisig, H.A., Skogestad, S., “Active Vapor Split Control for

D

Dividing-Wall Columns”, Ind. Eng. Chem. Res., 51 (46), 15176-15183 (2016).

PT E

2 Asprion, N., Kaibel, G., “Dividing wall columns: Fundamentals and recent advances”, Chem. Eng. Process., 49 (2), 139-146 (2010).

3 Ömer Yildirim, Kiss, A.A., Kenig, E.Y., “Dividing wall columns in chemical process industry: A review on current

CE

activities”, Sep. Purif. Technol., 80 (3), 403-417 (2011). 4 Fang, J., Hu, Y.Q., Li, C.L., “Energy-Saving Mechanism in Heat Transfer Optimization of Dividing Wall Column”,

AC

Ind. Eng. Chem. Res., 52 (51), 18345–18355 (2013). 5 Xiong, X.R., Wu, C.L., M, C., H, Y.Y., “Comparing the influences of double feeds and double-side streams on the operation and control of the dividing wall columns”, Modern Chemical Industry, 37 (01), 184-187 (2017). 6 Santaella, M.A., Orjuela, A., Narváez, P.C., “Comparison of different reactive distillation schemes for ethyl acetate production using sustainability indicators”, Chem. Eng. Process., 96, 1-13 (2015). 7 Qian, X., Jia, S.K., Luo, Y.Q., Yuan, X.G., Yu, K.T., “Selective hydrogenation and separation of C3 stream by thermally coupled reactive distillation”, Chem. Eng. Res. Des., 99, 176-184 (2015). 8 Kiss, A.A., David, J.P.S., “Innovative dimethyl ether synthesis in a reactive dividing-wall column”, Comput. Chem.

22

ACCEPTED MANUSCRIPT

Eng., 38 (10), 74-81 (2012). 9 Dai, X., Ye, Q., Yu, H., Suo, X.M., Li, R., “Design and Control of Dividing-Wall Column for the Synthesis of n-Propyl Propionate by Reactive Distillation”, Ind. Eng. Chem. Res., 54 (15), 3919-3932 (2015). 10 Delgado-Delgado, R., Hernández, S., Barroso-Muñoz, F.O., Segovia-Hernández, J.G., Castro-Montoya, A.J., “From

PT

simulation studies to experimental tests in a reactive dividing wall distillation column”. Chem. Eng. Res. Des., 90 (7), 855-862 (2012).

Eng. Res. Des., 91 (9), 1760-1767 (2013).

SC

RI

11 Ignat, R.M., Kiss, A.A., “Optimal design, dynamics and control of a reactive DWC for biodiesel production”, Chem.

12 Kiss, A.A., Segovia-Hernández, J.G., Bildea, C.S., Miranda-Galindo, E.Y., Hernández, S., “Reactive DWC leading

NU

the way to FAME and fortune”, Fuel, 95, 352-359 (2012).

13 Li, Y.H., Liu, J.Q., Ma, P.S., “Study on the Synthesis of Pure Ethyl Acetate by an Extractive-Reactive Distillation

MA

Process”, Petrochemical Technology, 25 (10), 719-723 (1996).

14 Gu, Z.G., Zhi, H.Z., Ma, Z.F., Yao, H.Q., “Study on composite extractive distillation of ethyl acetate-ethanol-water”,

D

Computers and Applied Chemistry, (06), 58-60 (2005).

Engineering

PT E

15 Qiu, X.Q., Cai, J.T., Xu, Q.C., Chen, H.Q., "The Study of a New Method to Purify Ethyl Acetate", Chemical (China), 25 (1), 41-44 (1997).

16 Guo, S.W., He, B.Q., Li, J.X., Zhao, Q., Cheng, Y., “Esterification of Acetic Acid and Ethanol in a Flow-Through

CE

Membrane Reactor Coupled with Pervaporation”, Chem. Eng. Technol., 37 (3), 478-482 (2014). 17 Yuan, H.K., Xu, Z.L., Ma, X.H., Wei, Y.M., Qi, J., Zeng, Y.H., “PVA-PFSA/PAN Composite Membrane for

AC

Pervaporation Dehydration in the Enhanced Esterification of Ethyl Acetate Production and Process Simulation”, Journal of East China University of Science and Technology (Natural Science Edition), (02), 174-179 (2009). 18 Fang, J., Zhao, H.M., Qi, J.C., Li, C.L., Qi, J.J., Guo, J.J., “Energy conserving effects of dividing wall column”, Chinese. J. Chem. Eng., 23 (6), 934-940 (2015). 19 Li, B.C., Li, X.H., Zhong, H.W., Liu, C.R., “A study on catalytic distillation synthesis of methyl acetate”, Modern Chemical Industry, (04), 37-40 (2010). 20 Hu, S., Zhang, B.J., Hou, X.Q., Li, D.L., Chen, Q.L., “Design and simulation of an entrainer-enhanced ethyl acetate reactive distillation process”, Chem. Eng. Process., 50 (11-12), 1252-1265 (2011).

23

ACCEPTED MANUSCRIPT

21 Meng, Q.P., Chen, Q.L., Hua, B., Xie, X.A., Chen, X.H., “Simulation Research of Co-production of Ethyl Acetate and Butyl Acetate”, Chemical Industry and Engineering Progress, 22, 34-38 (2003). 22 Liu, X.L., Chen, Q.L., Meng, Q.P, Yin, Q.H., “Simulation on product acidity control of co-production flowsheet of ethyl acetate and butyl acetate”, Journal of North China Electric Power University, 31 (6), 104-107 (2004).

PT

23 Tian, H., Zheng, H.D., Huang, Z.X., Qiu, T., Wu, Y.X. “Novel Procedure for Coproduction of Ethyl Acetate and n-Butyl Acetate by Reactive Distillation”, Ind. Eng. Chem. Res., 51 (15), 5535–5541 (2012).

SC

reactive distillation”, Chinese. J. Chem. Eng., 23 (4), 667-674 (2015).

RI

24 Tian, H., Zhao, S.Y., Zheng, H.D., Huang, Z.X., “Optimization of coproduction of ethyl acetate and n-butyl acetate by

Chinese. J. Chem. Eng., 23 (6), 934-940 (2015).

NU

25 Fang, J., Zhao, H.M., Qi, J.C., Li, C.L., Qi, J.J., Guo, J.J., “Energy conserving effects of dividing wall column”,

26 Wu, Y.C., Lee, H.Y., Tsai, C.Y., Huang, H.P., Chien, I.L., “Design and control of a reactive-distillation process for

MA

esterification of an alcohol mixture containing ETOH and n –butanol”, Comput. Chem. Eng., 57 (29), 63-77 (2013). 27 Li, C.L., Song, Y.Y., Fang, J., Liu, Y., Su, W.Y., Hu, Y.Q., “Separation process of butanol-butyl acetate-methyl

D

isobutyl ketone system by the analysis to residual curve and the double effect pressure-swing distillation”, Chinese.

PT E

J. Chem. Eng., 25 (3), 274-277 (2017).

28 Lee, H.Y., Huang, H.P., Chien, I.L., “Control of reactive distillation process for production of ethyl acetate”, J. Process. Contr., 17 (4), 363-377 (2007).

CE

29 Wu, Y.C., Lee, H.Y., Tsai, C.Y., Huang, H.P., Chien, I.L., “Design and control of a reactive-distillation process for esterification of an alcohol mixture containing ethanol and n –butanol”, Comput. Chem. Eng., 57 (29), 63-77 (2013).

AC

30 Calvar, N., González, B., Dominguez, A., “Esterification of acetic acid with ethanol: Reaction kinetics and operation in a packed bed reactive distillation column”, Chemical Engineering & Processing: Process Intensification, 46 (12), 1317-1323 (2007).

31 Gangadwala, J., Mankar, S., Mahajani, S., Kienle, A., Stein, E., “Esterification of acetic acid with butanol in the presence of ion-exchange resins as catalysts”, Ind. Eng. Chem. Res., 42 (10), 2146-2155 (2003). 32 Fang, J., Qi, J.C, Li, C.L., Guo, J.J., “Design and Calculation for Four-Column Model of Dividing Wall Column”, Petrochemical Technology, 43 (5), 530-535 (2014). 33 Li, C.L., Dong, L.H., Duan, C., Chang, L.F., Peng, F., “Experiment and simulation of azeotropic-reactive distillation

24

ACCEPTED MANUSCRIPT

AC

CE

PT E

D

MA

NU

SC

RI

PT

for production of Ethyl acetate”, Journal of Hebei University of Technology, 46 (3), 68-72 (2017).

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