Comparison of heterogeneous azeotropic distillation and energy-saving extractive distillation for separating the acetonitrile-water mixtures

Comparison of heterogeneous azeotropic distillation and energy-saving extractive distillation for separating the acetonitrile-water mixtures

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Journal Pre-proofs Comparison of heterogeneous azeotropic distillation and energy-saving extractive distillation for separating the acetonitrile-water mixtures Jun Qi, Yafang Li, Jiaxing Xue, Ruiqi Qiao, Zhishan Zhang, Qunsheng Li PII: DOI: Reference:

S1383-5866(19)32000-3 https://doi.org/10.1016/j.seppur.2019.116487 SEPPUR 116487

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

13 May 2019 22 October 2019 24 December 2019

Please cite this article as: J. Qi, Y. Li, J. Xue, R. Qiao, Z. Zhang, Q. Li, Comparison of heterogeneous azeotropic distillation and energy-saving extractive distillation for separating the acetonitrile-water mixtures, Separation and Purification Technology (2019), doi: https://doi.org/10.1016/j.seppur.2019.116487

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© 2019 Published by Elsevier B.V.

Comparison of heterogeneous azeotropic distillation and energy-saving extractive distillation for separating the acetonitrile-water mixtures Jun Qia, Yafang Lia, Jiaxing Xuea, Ruiqi Qiaoa, Zhishan Zhangb, Qunsheng Lia* a: State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Box 266, Beijing, 100029, China b: Department of Chemical Engineering, College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao, China

ABSTRACT: The separation of acetonitrile-water mixtures is a challenging and significant task due to the presence of azeotrope, for which three separation alternatives are proposed, including an energy-efficient extractive distillation strategy (ED), an pressure-swing distillation strategy (PSD) and an novel azeotropic distillation strategy (EAD). Concept design based on ternary phase diagrams of various solvents demonstrates that benzene is a suitable solvent for the newly proposed EAD scheme. The separation sequence and operating parameters are optimized via using different sequential iterative procedures based on the minimal total annual cost (TAC). Under temperature-enthalpy (T-H) diagram guidance, heat integration applied to this three sequences significantly is completed, which reduces TAC by 27.45%, 30.42%, and 46.67%, respectively. Comprehensive evaluation of the three separation alternatives from the economic and environmental perspectives indicates that the heat-integrated EAD scheme is more attractive, since its TAC decreases by 52.03% and 55.30%, respectively, and CO2 emissions decreases by 56.63% and 61.63%.compared with ED and PSD schemes. It is worth noting that the conceptual design of the EAD scheme also provides an energy efficient alternative for other systems that form azeotropes with water. Keywords: Extractive distillation; Azeotropic distillation; Heat integration; Acetonitrile / water; Comprehensive evaluation.

1. Introduction Acetonitrile (ACN), an important organic intermediate in fine chemicals, is widely used in fabric dyeing, lighting, fragrance manufacturing, and photosensitive material manufacturing. The presence of carbon-nitrogen triple bonds makes ACN can perform typical nitrile reactions for preparing a variety of nitrogen-containing compounds.1-3 In addition, the most important application of ACN is as a solvent, such as extracting butadiene and fatty acid.4 However, the formation of ACN-water azeotrope poses a major challenge for this energy-intensive separation process.5-7 Therefore, it is important to find a suitable separation method. Special distillation alternatives (including extractive distillation8-10, pressure-swing distillation11-16, azeotropic distillation17-18, membrane separation19-20) are effective methods for separating azeotropes, where ED is the first special method to separate ACN-water azeotrope.21-25 Acosta-Esquijarosa et al.26 reported a method of ACN dehydration with butyl acetate as an effective entrainer, which combined batch distillation and extraction scheme. Zhou et al.27 proposed a method for separating ACN-water by ED scheme and validated the separation performance of ethylene glycol (EG). It was found that EG can eliminate the azeotropic point of ACN-water. You et al.28 further optimized the operating parameters of ED scheme to minimize the TAC and the energy consumption using a muti-objective genetic algorithm. Based on the traditional ED scheme. With the deterioration of the global environment, environmentally friendly ionic liquids have received much attention as solvents for separating azeotropes.29-35 Kurzin et al.36 pointed out that tetrabutylammonium bromide can effectively eliminate the azeotrope of ACN-water and obtain high purity ACN by corresponding ED process. The pressure-swing distillation (PSD) process, used to achieve azeotrope separation by adjusting the pressure to change the azeotropic composition, is suitable for separating pressure sensitive azeotropes.37-39 The most prominent advantage of the PSD scheme is that no new entrainer enters the separation column compared to other methods. Coincidentally, the azeotropic composition of ACN-water exhibits sensitivity to pressure. Repke et al.40 proposed a PSD process, but didn’t analyze TAC and energy costs. There is no relevant

literature reporting on the comparison of energy consumption and economic evaluation between partial or full thermal integrated PSD scheme and ED scheme. Azeotropic distillation is an established and effective way to separate azeotropes, it exhibits lower energy consumption than the ED and PSD schemes in some systems that form azeotropes with water. Chien et al.41 reported an alternative to the IPA dehydration process by azeotropic distillation. Qiu et al.18 investigated heterogeneous azeotropic distillation for ethanol/toluene/water separation. At present, almost no literature has reported azeotropic distillation for the ACN dehydration process. Therefore, it is of great significance to find a viable entrainer and study the application of azeotropic distillation strategy in the ACN dehydration process. For the separation task of ACN-water mixtures, three special alternatives are investigated in this article, including an energy-efficient ED scheme, an PSD scheme and an novel EAD strategy using benzene as entrainer. The optimal operating variables are given via the sequential iterative procedure based on the minimal TAC. The innovation of this work is as follows: (1) explore the energy saving potential of ED scheme through combining vapor-liquid stream between solvent recovery column and preconcentration column; (2) screen the most suitable entrainer from a variety of solvents based on conceptual design of ternary phase diagram; (3) propose a new energy-efficient EAD scheme with benzene as an solvent; (4) carry out thermal integration of the three schemes under the guidance of the temperature-enthalpy (T-H) diagram; (5) complete a comprehensive evaluation of the three alternatives from the perspectives of economy and environment, obtain the optimal separation configuration. 2. Design basis 2.1 Thermodynamic model Thermodynamic model is extremely important for accurately simulating the separation process of complex azeotropes. The comparison between experimental data and predicted values of vapor-liquid equilibrium by thermodynamic model can be used as the basis for model selection. Fig. 1. displays the experimental values and predicted values based on the UNIQUAC model42-43. It is found that the UNIQIAC model can accurately describe the phase

equilibrium characteristics of ACN-water azeotrope. Table 1 displays the binary interaction parameters of all chemical solvents involved in this paper.. 375

1.0

370

0.8

365

360



vapor



liquid

yACN

T/K

0.6

0.4

wEG=0.6 wEG=0.4 ▲ wEG=0.2 ■

355

● ■

0.2

350

345 0.0

0.2

0.4

xACN

0.6

0.8

1.0

0.0 0.0

0.2

0.4

0.6

0.8

1.0

xACN Fig. 1. Experimental and calculated VAE data at p = 101.3 kPa: (a) Txy diagram without extractant. (b) effect of EG on VAE of ACN-water. Table 1 Interaction parameters of the UNIQUAC model Component i

Component j

Aij

Aji

Bij

Bji

ACN

water

-0.317

0.226

-40.598

-239.292

ACN

EG

0.000

0.000

-191.305

-126.305

EG

water

0.602

-0.602

-18.671

120.779

ACN

benzene

0.000

0.000

57.423

-249.759

water

benzene

0.000

0.000

-369.010

-860.810

EG

benzene

0.000

0.000

-113.530

-649.050

2.2 Economic evaluation indicator This paper still uses TAC as evaluation objective to achieve economic analysis of all process design schemes, which is composed of equipment costs and operating costs. The relevant computing method is provided by Luyben's literature.44-45 The capital costs only contain the column shell, tray, condenser and reboiler. The investment of reflux drums, pump and valves are not taken into account. Unless chilled water is used in the separation process, operating costs only consider steam costs. Table 2 shows the goal function and necessary parameters of economic evaluation.41-43 Table 2

Basis of economic evaluation. Basis of economics and equipment sizing. column diameter (ID): Aspen tray sizing height(H) of a distillation column, H= 1.2×0.61×(NT−2) vessel (ID and H in meters) column shell cost ($) = 17640×(D)1.066×(H)0.802 condenser (area in m2 ) QC AC  K C ΔTC heat transfer coefficient (Kc ) = 0.852 kW/m2 K capital cost ($) = 7296 (Ac )0.65 reboiler (area in m2 ) AR 

QR K R ΔTC

heat transfer coefficient (KR ) = 0.568 kW/m2 K capital cost ($) = 7296 (AR)0.65 energy costs LP steam (433 K) = $7.72/GJ MP steam (457 K) = $8.22/GJ HP steam (537 K) = $9.88/ GJ capital cos t TAC   energy cos t paybackperiod Payback period = 3 years

2.3 Environmental evaluation indicator CO2 emissions directly determine the environmental benefits of the separation process, whose calculations are related to the amount of steam required for different processes, and the calculation formula is as follows:

CO2 emiss

 Q fuel   NHV 

  C%       100 

 Q proc      h proc  419  TFEB  T0  Qfuel   T T    stack   FEB  proc 

The Qfuel [kW] represents the amount of fuel burnt, α (3.67) is the ratio of molar masses of CO2 and C, NHV is the abbreviation for net heating value and C% represents carbon content. If heavy fuel oil is selected as the fuel for heating steam, NHV and C% can be set to 39,771 kJ/kg and 86.5 kg/kg, respectively. Qproc [kW] is is the sum of the head load of the separation

system. In addition, the λproc [kJ/kg] is the latent heat and hproc [kJ/kg] represents enthalpy of utility steam, TFTB and Tstack represent the temperature of the boiler flue gases and stack.46-47 3. Energy-efficient ED scheme 3.1 Non-heat integrated ED scheme The ED process for separating ACN-water mixture is simulated via Aspen Plus (V 8.4) based on UNIQUAC model. The feed flow rate of industrial wastewater containing 20 mol% ACN is initially set at 500 kmol/h. EG is selected as the entrainer to eliminate azeotrope of ACN-water, the feasibility of which has been confirmed in the second section.44 Product purity is specified to be 99.9 mol% and the impurity content in the entrainer is only 0.01%. Considering that the water content in the system is much higher than ACN, a feasible energy-saving method for ED scheme is to add a preconcentration column to remove most of the water. Therefore, the main equipment contains preconcentration column (PDC), ED column (EDC) and solvent recovery column (ERC). The purpose of process optimization is to achieve the lowest TAC and energy consumption. A typical optimization program is chosen to search optimal operating parameters as shown in Fig. 2. Variables that need to be optimized contain total stages (NT1, NT2, NT3), feed stage (NF1, NF2, NFE2, NF3), solvent flow rate (S1). Design specifications are met by adjusting distillate rate (D) and reflux ratio (RR). Fig. 3 displays the optimization results of the conventional ED process. Among them, the optimal total stages are NT1=13, NT2=35, NT3=15, the optimal feed stage are NF1=8, NF2=28, NF3=8, NFE2=5 and the optimal solvent flow rate is S1=140 kmol/h. The flowsheet of non-heat integrated ED scheme with optimal operating parameters and steam data is illustrated in Fig. 4.

Fix P1=0.4atm, P2=1atm, P3=1atm

Give NT3

Give S1

Give NF3

Give B1

Give NT2

Give NT1

Give NF2,NFE2

Give NF1

Vary RR3,D3 to meet two design specifications of T3

No

Vary RR2,D2 to meet two design specifications of T2

Is QR3 minimal with NT3 fixed? No Yes

No

Vary RR1 to meet one design specifications of T1

Is QR2 minimal with NT2,S1 fixed?

Calculate TAC

Yes

No Is QR1 minimal with NT1 fixed?

No

No Is TAC minimal with S1 fixed?

Is TAC minimal ?

Yes Yes

Yes

No Is TAC minimal ? Yes

Get the optimal NT1,NT2,NT3,NF1,NF2,NF3, NFE2,S1 and TAC

No

Is TAC minimal ?

Get optimal NF1、NT1

Yes Over

Get the optimal NT2 ,NFE2,NF2,S1

QR4 (kW)

QR1 (kW)

Fig. 2. Iterative procedure for ED scheme.

NF1

NF2

NFE2

QR4 (kW)

TAC ($/y)

NT1

S1

NT3

NT4

Fig. 3. The optimal results of design variables in conventional ED scheme.

NF4

Fig. 4. Flowsheet for non-heat integrated ED alternative. 3.2 Partial heat-integrated ED alternative The most efficient method of thermal integration is to exchange heat between the overhead vapor and the reboiler by adjusting the pressure of column. In addition, preheating the feed also increases energy efficiency. In this section, increasing the pressure of EDC and ERC would result in expensive steam consumption due to the presence of high boiling solvent. Therefore, the heat-integration of the ED scheme could be effectively realized by reducing the pressure of the PDC. In order to ensure that the overhead vapor of the ERC can provide energy for the bottom reboiler of the PDC while the overhead vapor of the PDC could be condensed with normal temperature water, the pressure of the PDC is determined to be 0.4 atm. As is shown from the flowsheet and the temperature-enthalpy diagram (T-H) in Fig. 5, the overhead vapor of the ERC can provide 609.26 kW of heat for the reboiler of PDC. high temperature EG from the bottom of the ERC could also be used to provide 683.76 kW of energy to the reboiler of the PDC.

Fig. 5. The flowsheet for the heat-integrated ED process and the relevant T-H diagram. 3.3 Energy-efficient ED scheme The existence of the PDC offered another potential to savings and equipment investment reduction. The composition and temperature of the bottom stream of the PDC are consistent with the top stream of the ERC. so the overhead vapor of the ERC can be transported to PDC, which provided part of the steam reflux for the PDC, part of the bottom stream (water) of the PDC can also be used as the top reflux of the ERC. In addition, the bottom stream of the ERC provides 551.60 kW of energy to the reboiler of the PDC and further preheat the feed of the PDC. This energy-efficient ED process has three advantages: (1) avoiding the use of vacuum column; (2) achieving partial heat-integration; (3) reducing equipment investment in reboiler of the PDC. Fig. 6 displays the optimal flowsheet for the energy-efficient ED scheme.

Fig. 6. Flowsheet of the energy-efficient ED alternative. As an important indicator of process evaluation. TAC is used to make a comparison

between the traditional ED scheme and the heat-integrated ED scheme. The data of Table 3 indicates that TAC and energy consumption of thermal integrated ED process decreases by 27.26% and 36.92%, respectively, compared with conventional ED scheme, the energy-efficient ED scheme has 27.45% reduction of TAC and 34.17% energy savings, which means it has greater potential for energy savings. Table 3 Economic analysis of three different ED processes. Conventional ED

Heat-integrated ED

Energy-efficient ED

PDC

EDC

ERC

PDC

EDC

ERC

PDC

EDC

ERC

column

100.61

210.42

75.03

94.76

206.57

62.83

81.69

206.57

62.83

reboiler

102.44

105.31

53.19

137.44

108.48

47.71

90.79

105.17

53.08

condenser

107.16

94.15

56.71

185.24

103.03

--

108.83

105.58

--

Investment (103$)

other heat exchanger heat duty (kW)

48.13 2282.44

1811.96

32.34 1086.16

452.20

1896.56

30.98 918.95

519.13

1808.31

Qtotal saving

--

36.92%

34.17%

TIC(105$)

9.53

9.78

8.45

operation cost

5.87 12.45

8.11

8.52

5$/year) (10 5$/year) TAC(10

15.63

11.37

11.34

--

27.26%

27.45%

TAC saving

1082.85

4. Steady-state design of PSD scheme The sensitivity of the ACN-water azeotropic composition to pressure indicates that PSD is also an effective choice for separating ACN-water azeotrope. Compared to ED process, PSD has the advantage of avoiding the introduction of heavy entrainers to increase VOCs emission. The key issue with PSD process is how to determine the pressure of the column. The relationship between azeotropic composition and pressure is illustrated Fig. 7, which reveals that the change trend of azeotropic composition gradually slows down when the pressure exceeds 5 atm. Therefore, for the pressure of the high pressure column (HPC), 5 atm is a suitable choice. The primary purpose of setting the pressure of the low pressure column (LPC) to 0.4 atm is to avoid the application of expensive chilled water.

440

0.4 atm

420

0.75

400 0.70 380 0.65 360 0.60

5 atm

340

0.55

Temperature / K

Azeotropic composition / ACN

0.80

320

0.50

300 0

1

2

3

4

5

6

7

8

9

Pressure / atm

Fig. 7. The relationship between azeotropic composition and pressure. 4.1 Process simulation and optimization of PSD The feed flow rate, temperature, composition are consistent with the ED process. The water content in the feed and the latent heat of water indicate that the first column should be used to remove water. In addition, the azeotropic composition of ACN is higher at lower pressure, so setting the first column to LPC can reduce distillate rate, thereby reducing energy consumption. Variables that need to be optimized contain total stages (NT1, NT2), feed stage (NF1, NF2).The optimal operating parameters for the PSD process are given by a sequence iterative procedure (Fig. 8). The optimal total stages are NT1=12, NT2=20 and the optimal feed stage are NF1=10, NFR1=9 NF2=10 (Fig. S1). The flowsheet of the PSD scheme containing stream data and operating parameters is displayed in Fig. 9. From the economic analysis results, the energy cost of PSD process is $ 9.00×105/y and the TAC is $ 17.49×105/y.

Fix P1=0.4atm,P2=5atm Give NT2 Give NT1

Give NF2 Give NF1

Vary RR2,D2 to meet two design specifications of HPC Vary RR1,D1 to meet two design specifications of LPC

No Is QR2 minimal with NT2 fixed?

No Is QR1 minimal with NT1 fixed?

Yes Yes

Calculate TAC

Calculate TAC No No

Is TAC minimal ? Is TAC minimal ? Yes Yes Get the optimal NT1 ,NF1

Get the optimal NT1 ,NF1,NT2,NF2

Over

Fig. 8. Optimization program of PSD scheme.

Fig. 9. Flowsheet of the non-heat-integrated PSD scheme. 4.2 Heat-integrated PSD process The difference in pressure between the two columns provides the potential for heat integration of the PSD configuration. The heat-integrated separation process and T-H diagram are presented in Fig. 10. First, the overhead vapor of the HPC can provide 64.66 % heat to the reboiler of the LPC, the remaining 1125.34 kW heat load is supplied by external steam. Second, unsaturated liquid feed provides another possibility for heat integration: preheating, the simulation results of the heat exchanger indicate that water from the bottom of LPC can

be used to lift the temperature of feed to 317.15K, the bottom stream of HPC further lifted the temperature of feed to 335.15K, which reduces the reboiler duty of the LPC by almost 500 kW. The results of the economic analysis shown in Table 4 indicate that the TAC and energy consumption of heat-integrated PSD process has 30.42% and 39.20% reduction compared to non-heat integrated PSD scheme. Although the energy-saving effect of heat-integrated PSD scheme is better than the ED scheme, the excess azeotrope from the top of HPC needs to be recycled back to LPC, which makes the thermal integrated PSD scheme still not dominant in terms of energy consumption and TAC.

Fig. 10. Flowsheet of the heat-integrated PSD scheme and the relevant T-H diagram. Table 4 The economic evaluation of the PSD scheme. PSD

Heat-integrated PSD

LPC

HPC

LPC

HPC

column

157.90

147.84

157.90

147.84

reboiler

96.32

126.01

159.57

126.01

condenser

294.70

76.78

294.66

0

Investment

(103$)

other heat exchanger heat duty (kW)

-3610.41

77.00 2729.46

1125.34

2729.39

Qtotal saving

--

39.20%

TIC(105$)

9.00

9.63

operation cost (105$/year)

5.87 14.49

8.96

TAC(105$/year)

17.49

12.17

--

30.42%

TAC saving

5. Novel azeotropic distillation The introduction of a heavy third component improves the energy cost and CO2 emissions of the ED scheme. Although the PSD scheme need not introduce new components, the pressure adjustment can not achieve a significant change in the azeotropic composition (the mole fraction of ACN is 0.7434 at 0.4 atm and 0.5581 at 5 atm), which directly leads to an increase in the energy cost. Therefore, it is increasingly important to explore more efficient and energy-efficient separation methods with the aggravation of environmental pollution. 5.1 Concept design and solvent selection In this section, considering that ACN and water are completely miscible, if a suitable solvent can extract ACN from water through a decanter and then further purified by distillation to obtain high purity ACN, the energy cost may be significantly reduced. This design method can reduce overhead circulation compared to the PSD scheme and avoid the introduction of heavy entrainers compared to the ED process. It is worth noting that ACN acts as a co-solvent in water and immiscible solvent system, which makes it difficult to achieve clear separation of the ACN-water mixtures during the extraction process. Four solvents that may meet the design requirements are selected among many water-immiscible solvents, including cyclohexane (C6H12), toluene (C7H8), benzene (C6H6) and chloroform (CHCL3). Fig. 11 displays the ternary diagrams of four candidate solvents. From the phase diagram, the first solvent to be excluded is cyclohexane, because the partition coefficient of ACN in water is much higher than that in cyclohexane, the addition of cyclohexane can not achieve the purpose of extracting ACN from water. Secondly, the area of the region I in the ternary phase diagram of toluene-ACN-water represents the possibility of obtaining ACN through decanter to across the distillation boundary line. Obviously, the extraction effect of toluene on ACN is not satisfactory. The phase diagrams of benzene and chloroform indicate that the two solvents are suitable for concept design. However, preliminary simulations found that ACN and chloroform have strong interaction forces, which poses a great challenge to the separation process. Therefore, benzene is ultimately selected as the solvent most suitable for this separation process.







Fig. 11. Ternary phase diagrams of candidate solvents. 5.2 Process simulation of EAD scheme The concept design using ternary phase diagram is shown in Fig. 12. First, the organic phase (B) is stripped from the aqueous phase (C) by introducing benzene to the decanter, and the aqueous phase (C) that removes most of the ACN is sent to the dehydration column. The ACN-water azeotrope (E) from the distillate of the dehydration column with the organic phase containing most of ACN and all of the benzene are transported to the T2 column for further separation. ACN meeting the purity requirement (H) is acquired at the bottom of the T2 column. The ternary azeotrope (G) from distillate is recycled to decanter. The feed flow rate, temperature, composition, product purity and thermodynamic model are consistent with the ED process. Two distillation columns (EAC1, EAC2) and one decanter (S-1) constitute the main equipment for the EAD scheme. The pressure of EAC1 and EAC2 is set to 1atm. The temperature of the decanter is set to 298.15K to avoid the introduction of chilled water.

H

B

E

F

G

D

A

C

Fig.12. Process design based on ternary phase diagram The design variables of the EAD process contain total stages (NT1, NT2), feed stage (NFS1, NFS2, NFD1), entrainer flow rate (S1). Fig. 13 provides an optimization procedure for finding optimal operating parameters. The optimal operating parameters are NT1=17, NT2=28, NF1=13, NFD1=13, NF2=16, S1=48.15 kmol/h (Fig. S3.). Flowsheet containing optimal parameters and stream data is shown in Fig. 14. Fix P1=1atm,P2=1atm

Give NT2 Give S1

Give NFS2,NFD1 Give NT1

Vary RR2,D2 to meet two design specifications of EAC2 Give NFS1

No Is QR2 minimal with NT2 fixed?

Vary RR1,D1 to meet two design specifications of EAC1

Yes No

Calculate TAC Is QR1 minimal with NT1,S1 fixed?

Yes

No Is TAC minimal with S1 fixed ?

Calculate TAC Yes No Is TAC minimal ?

No

Is TAC minimal ?

Yes Yes Get the optimal NT1 ,NFS1 Get the optimal NT1 ,NFS1,NT2,NFS2,NFD1,S1

Over

Fig. 13. Optimization procedure for EAD scheme.

Fig. 14. Flowsheet for non-thermal integrated EAD scheme 5.3 Full heat-integrated EAD process Similar to heat-integrated PSD scheme, the thermal integration of EAD scheme is still carried out by heat exchange between the overhead vapor of EAC2 and the bottom reboiler of EAC1. The pressure of the EAC1 and EAC2 are set to 0.4 atm and 4 atm, respectively, which not only ensures that water is removed as an ternary azeotrope (at this time, the reflux flow is minimal), but also facilitates the implementation of heat-integration. The flow sheet of the thermal-integrated EAD scheme containing optimal operating parameters and the relevant T-H diagram are displayed in Fig. 15. The temperature difference of 27.7 K makes it possible for the overhead vapor of EAD2 to provide 1033.17kW for the reboiler of EAD1. In addition, the unsaturated feed stream is preheated to 315.15 K using the bottom stream of EAC1 and further preheated to 335.15 K using the bottom stream of EAC2. Considering that the cool duty of EAC2 is higher than the heat duty of EAC1, the distillate of EAC2 can be used to provide heat for the organic phase to achieve energy matching between the two columns. The economic analysis results in Table 5 indicate the heat-integrated EAD scheme with benzene as entrainer has 46.67% reduction of TAC and 57.53% energy savings.

Fig. 15. Flowsheet of the heat-integrated EAD process and the relevant T-H diagram. Table 5 The economic analysis of the EAD process. EAD

Heat-integrated EAD

EAC1

EAC2

EAC1

EAC2

column

95.64

210.40

136.23

114.38

reboiler

69.71

68.14

84.50

106.68

condenser

74.06

154.02

138.50

0

Investment (103$)

other heat exchanger heat duty (kW)

-1564.17

64.99 1934.55

0

1478.88

Qtotal saving

--

57.53%

TIC(105$)

6.72

6.45

operation cost (105$/year)

7.78

3.29

TAC(105$/year)

10.02

5.44

--

46.67%

TAC saving

6. Evaluation of the three alternatives 6.1 Economic evaluation Optimal design variables are obtained using three sequential iterative procedures and the same economic evaluation indicator. In order to search the most economical scheme to separate ACN-water azeotrope, a comprehensive comparison of energy consumption and TAC is achieved. Table 6 provides the energy cost, capital cost and TAC of the three

separation methods. From the results, the novel EAD process shows greater potential in terms of energy consumption and TAC, which has 52.03% reduction of TAC and 56.63% energy savings compared to the energy-efficient ED scheme. The direct result of high circulation to the heat-integrated PSD process is an increase in energy consumption. Compared with energy-saving ED scheme, TAC increased by 7.32% and energy consumption increased by 13.03%. Table 6 The economic analysis of the three separation methods. Energy-efficient ED PDC

EDC

Heat-integrated PSD ERC

HPC

LPC

Heat-integrated EAD EAC1

EAC2

ERC

Investment (103$) column

81.69

206.57

62.83

157.90

147.84

136.23

114.38

reboiler

90.79

105.17

53.08

159.57

126.01

84.50

106.68

condenser

108.83

105.58

0

294.66

0

138.50

0

30.98

other heat exchanger heat duty (kW)

519.13

77.00 1808.31

77.00 1082.85

1125.34

64.99 2729.39

0

1478.88

Qtotal saving

--

-13.03%

56.63%

TIC(105$)

8.45

9.63

6.45

operation cost

8.52

8.96

3.29

(105$/year) 5$/year) TAC(10

11.34

12.17

5.44

--

-7.32%

52.03%

TAC saving

6.2 Environmental evaluation Severe climate and environmental conditions have caused more attention to greenhouse gas emissions. Therefore, CO2 emissions should be used as an important indicator to evaluate three separation schemes. It is well known that CO2 emissions depend on energy consumption in the process, and its reduction will inevitably lead to a reduction in CO2 emissions. the reduction in energy consumption will in turn lead to a reduction in CO2 emissions. Fig. 16. visually shows a comparison of the key evaluation indicators for the three schemes. Full thermal integrated EAD scheme is more attractive than the other two schemes, because it can reduce CO2 emissions by 56.63% and 61.63% compared to the energy-saving ED configuration and the thermally integrated PSD configuration.

Fig. 16. Comparison of three schemes based on key indicators. 7. Conclusion For the separation of ACN-water azeotrope, a suitable separation method can significantly reduce TAC and energy consumption. This article explored three special distillation methods, including an energy-efficient ED strategy, an heat-integrated PSD strategy and an novel EAD strategy with benzene as solvent. Based on minimal TAC, the separation schemes and operating variables were optimized via three sequential iterative procedures. The feasibility of thermal integration of three separation schemes was analyzed by temperature-enthalpy (T-H) diagram. It was found that the improved energy-efficient ED scheme can be achieved by combining the vapor-liquid stream of the PDC and the ERC, which has 27.45% and 34.17% reduction of TAC and energy consumption compared to non-heat integrated ED scheme. Concept design based on ternary phase diagrams of various solvents demonstrated that benzene is a suitable entrainer for the newly proposed EAD scheme. The comprehensive evaluation results based on key performance indicators of TAC and CO2 emissions indicated that the EAD scheme is more attractive, TAC decreased by 52.03% and 55.30%, respectively, and CO2 emissions decreased by 56.63% and 61.63%, respectively, compared to ED and PSD schemes. In addition, the design method of EAD scheme is equally suitable for the separation of other systems that form azeotropes with water, if a suitable entrainer is found, the energy consumption and environmental pollution will be greatly reduced. Acknowledgment

The research was funded by the National Key R&D Program of China (No. 2018YFB0604902).

Append A. Supplementary material Supplementary data to this article can be found online at

Corresponding Author *Qunsheng Li. Tel.: +86-18801488553. E-mail:[email protected]

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Highlights (1) Heat-integrated energy-efficient ED and PSD schemes are completed with temperature enthalpy diagram. (2) A more energy-efficient azeotropic distillation strategy is explored. (3) Three evaluation indicators are employed to rank the different configurations.

Conflict of interest statement

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. This manuscript (Comparison of energy-efficient extractive distillation,pressure-swing distillation

and

heterogeneous

azeotropic

distillation

for

separating

the

acetonitrile-water mixtures) is approved by all authors for publication. I would

like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.

Dr. Qunsheng Li E-mail: [email protected]