Economics and controllability of conventional and intensified

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Process Systems Engineering

Economics and controllability of conventional and intensified extractive distillation configurations for acetonitrile/methanol/benzene mixtures Chao Wang, Chen Wang, Yue Cui, Chao Guang, and Zhishan Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01875 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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Industrial & Engineering Chemistry Research

Economics and controllability of conventional and intensified extractive distillation configurations for acetonitrile/methanol/benzene mixtures Chao Wang, Chen Wang, Yue Cui, Chao Guang, Zhishan Zhang* College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China ABSTRACT: For the industrial separation problem of acetonitrile/methanol/benzene involving three azeotropes, this article proposes two novel intensified configurations with chlorobenzene as solvent by extending extractive dividing-wall column (EDWC) and side-stream extractive distillation column (SEDC) from single azeotropic to multi-azeotropic mixtures, and makes a comparison with a conventional extractive distillation (CED) configuration in aspects of economic and controllability. Steady-state designs are optimized via using the sequential iterations search based on minimum total annual cost (TAC). The results show that extended-EDWC and extended-SEDC compared to CED can reduce TAC by 6.32%, 14.39% and energy consumption by 4.26%, 20.59% respectively. As far as controllability is concerned, all proposed control structures can hold products at high purities with acceptable deviations and short settle time after introducing feed flow and composition disturbances. Overall, apparent economic benefits and high energy-efficiency provided by the extended-SEDC configuration can be achieved without a deterioration of control behavior for some multi-azeotropic mixtures. 1. INTRODUCTION In recent years, the separation problem involving two or more azeotropes in the chemical and pharmaceutical industries has arouse extensive interest in the study,1,2 and it is fairly challenging in two aspects of optimal design and dynamic control due to the system complexity. A few special distillation processes have been widely used for separating azeotropic mixtures, such as azeotropic distillation,3-7 extractive distillation,8-13 pressure-swing distillation.14-16 Pressure-swing distillation is very efficient only for pressure-sensitive mixtures and azeotropic distillation is likely to bring some challenges like multiple steady states and larger energy demand. By contrast, extractive distillation is the most promising method via introducing a solvent which can break all azeotropes and eliminate distillation boundaries for this separation issue, and also a 1

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number of solvent selection methods have been proposed by some researchers,17,18 but at the same time it is also the energy intensive process. Generally, the remixing of internal streams with different compositions that occurs at feed points and along the column is an intrinsic source of thermodynamic inefficiency of this separation process, which exhibits concentration peaks of middle boiling components either above or below the feed stage.19, 20 To reduce or even eliminate the remixing effect for improving energy efficiency, extractive dividing-wall column (EDWC) and side-stream extractive distillation column (SEDC) were studied,21-27 and it has been proven that these thermally coupled configurations are hopeful alternative energy solutions. Certainly, any process intensification will inevitably bring the complicated dynamic behavior and control structure. Studies on the dynamic and control properties of EDWC have been reported by many literatures,28-33 and but there have been few reports of SEDC. Moreover, up to now most researches on extractive distillation concentrated on the separation of binary mixtures only containing one azeotrope. The focus of this work is to illustrate the possible applications of these intensified alternative configurations in the separation of multi-azeotropic mixtures with as an example of acetonitrile/methanol/benzene. To the best of our knowledge, only a few studies on pressure swing distillation has been reported for this system containing three binary azeotropes.34-38 In this work, two novel thermal coupled configurations, namely extended-EDWC and extended-SEDC, were developed for this specific separation with chlorobenzene as a solvent, and compared with the configuration of conventional extractive distillation (CED) from two aspects of economic and controllability. The optimization of sequential iterative search was carried out to minimize total annual cost (TAC). The steady state study showed promising results in economic benefits for two proposed configurations. The control structures of three configurations were established, and they represented satisfactory results in the feed flow and composition disturbances rejection capability. In addition, a comparison among them was presented to further evaluate operational aspects of extended EDWC and extended-SEDC implementations. 2. DESIGN BASIS 2.1 Thermodynamics. The wilson thermodynamic model is chosen to describe the vapor-liquid equilibrium in the system being studied because of the agreement between the predicted and experimental data in the literatures34,35, 39-41. All binary interaction parameters built in the Aspen 2


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Plus (V8.4) are represented as a supplementary material in Supporting Information. From the ternary composition diagram shown in Figure 1, there are three binary homogenous azeotropes in the system of acetonitrile/methanol/benzene.

Figure 1. The ternary composition diagram for the acetonitrile/methanol/benzene system. Chlorobenzene is selected as a solvent for this separation process of extractive distillation. Figure 2 describes vapor-liquid equilibrium behaviors in connected with chlorobenzene at 1 atm. It can be seen that all azeotropes are broken at the solvent-to-feed ratio (S:F) of 1.0, and the separation in a solvent recovery column is straightforward, and the light-heavy component order becomes methanol-acetonitrile-benzene.

(a) 1

(b) 1 S:F=0 S:F=1

0.8

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ymethanol

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ymethanol

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0.4 0.6 xbenzene

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Figure 2. The y-x plots of different components: (a) methanol-benzene, (b) methanol-acetonitrile, (c) benzene-acetonitrile, and (d) chlorobenzene-different component. 2.2 Economics. In this article, TAC is used as the objective function to be minimized by adopting the global sequential iterative optimization methods to screen the optimum design, including annual capital costs and operating costs. Major pieces of equipment are column vessels (including column internals) and heat exchangers (condenser and reboiler). Small items such as reflux drums, pumps, valves, and pipes are usually not considered at the conceptual design stage because of their lower costs compared with column vessels and heat exchangers. The costs of cooling water and solvent makeup are not included in operating costs because they are much lower than the costs of heat duties. The specific calculation procedure for the economic evaluation42-44 is given in Supporting Information. 3. STEADY-STATE DESIGN The commercial software Aspen Plus V8.4 is applied to simulate these separation processes of CED, extended-EDWC and extended-SEDC. A case study presented in the reference35 is used as the basis for our study. In that reference, a fresh feed is 1000 kg/h (28 kmol/h) with composition 70 wt% (78 mol%) methanol, 20 wt% (17.4 mol%) acetonitrile and 10 wt% (4.6 mol%) benzene. The product purities are specified as 99.5 mol% methanol, 99.5 mol% acetonitrile, 99.8 mol% benzene, and 99.995 mol% solvent, respectively. In the economic optimization, the number of stages (NT), feed location (NF), solvent location (NFS), side stream location (NSD) and solvent flow (SF) are chosen as optimal variables. The key design variables including reflux ratios, distillate flowrates and side stream flowrates are adjusted to meet the specified purity and recovery 4


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of key components. Additionally, the temperature of the recycle solvent and the operation pressure of distillation column are often fixed to a constant and not optimized in most of the literatures to date.45-48 Therefore, the temperature of the recycle solvent is fixed at 320 K and the operation pressure is set at 1 atm for simplification in this work, which is line in the work of Luyben.49,50 3.1. Design of CED. Figure 3 gives the flowsheet of the CED process with two extractive distillation columns and a solvent recovery column. The fresh feed is fed to the first extractive distillation column (EDC1) from the lower tray, and one recycle stream of solvent is fed to EDC1 from the upper tray. The distillate of EDC1 is the methanol product. The bottom from EDC1 is fed to the second extractive distillation column (EDC2) from the lower tray, and meanwhile the other recycle stream of solvent is fed to EDC2 from the upper tray. The distillate of EDC2 is the acetonitrile product. The bottom from EDC2 enters the solvent recovery column (SRC). The benzene product is obtained at the top of SRC. The high-purity solvent from the base of SRC after being cooled is split and recycled back to EDC1 and EDC2. A very small solvent makeup stream is required because of very small solvent losses in three product streams.

Figure 3. The optimal flowsheet of the CED configuration. The sequential iterative optimization procedure for the CED configuration is shown in Supporting Information. The key design variables including reflux ratios (RR1, RR2 and RR3) and distillate flowrates (D1, D2 and D3) of EDC1, EDC2 and SRC are adjusted to reach the separation requirements. There are still ten optimization variables in this configuration as follows: the number of stages (NT1, NT2 and NT3) and the fresh feed locations (NF1, NF2 and NF3) of EDC1, 5



EDC2 and SRC, the solvent locations (NFS1 and NFS2) and the solvent flowrates (SF1 and SF2) of EDC1 and EDC2. The vapor and liquid composition profiles of three columns in the CED configuration are shown in Figure 4. It can be clearly seen from Figure 4a and Figure 4b that the purities of acetonitrile and benzene achieve the maximum value on a certain tray near the bottom of EDC1 or EDC2, after that tray they decrease toward the bottom of EDC1 or EDC2. The effect of remixing with the heaviest solvent increases the energy consumption of the following separation process in order to repurify the acetonitrile and benzene. From Figure 4c, the distillate is the high purity benzene and the bottom is the high purity solvent in SRC.

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Figure 4. Vapor and liquid composition profiles of the CED configuration: (a) EDC1, (b) EDC2, and (c) SRC. 3.2. Design of extended-EDWC. Figure 5 shows the flowsheet of the newly proposed extended-EDWC configuration. This complex configuration features an extended-EDWC in which extractive distillation is carried out in both main column (MC) and side column (SC). The distillates of MC and SC are the methanol product and the acetonitrile product, respectively. The bottom from MC, which is mostly benzene and solvent, is fed to the solvent recovery column (SRC). The distillate of SRC is the benzene product. The high-purity solvent from the base of SRC after being cooled is split and recycled back to MC and SC.

Figure 5. The optimal flowsheet of the extended-EDWC configuration. The sequential iterative optimization procedure for the extended-EDWC configuration is given in Supporting Information. The key design variables including reflux ratios (RR1, RR2 and RR3) of 7



MC, SC and SRC, distillate flowrates (D1 and D3) of MC and SRC, and side vapor flowrates (VR) of MC are adjusted to reach the separation requirements. There are still ten optimization variables in this configuration as follows: the number of stages (NT1, NT2 and NT3) of MC, SC and SRC, the feed locations (NF1 and NF3) in MC and SRC, the side vapor withdrawn location (NSD) of MC, the solvent flowrates (SF1 and SF2) and the solvent locations (NFS1 and NFS2) of MC and SC. Figure 6 describes the vapor and liquid composition profiles of the extended-EDWC configuration. As shown in Figure 6a and Figure 6b, the high purity methanol can be obtained at the top of MC and the acetonitrile product can be obtained at the top of the SC. Moreover, the remixing phenomenon is alleviated, which results in the reduction of energy consumption. From Figure 6c, the high purity benzene and recycle solvent are obtained in the distillate and the bottom of SRC, respectively.

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Acetonitrile

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Figure 6. Vapor and liquid composition profiles of the extended EDWC configuration: (a) MC, (b) SC, and (c) SRC. 3.3. Design of extended-SEDC. Figure 7 gives the flowsheet of the newly proposed extended-SEDC process. This complex configuration features two side-stream extractive distillation columns. The methanol product is removed from the top of the first side-stream extractive distillation column (SEDC1). A liquid side stream (LS1) is withdrawn near the bottom of SEDC1 and fed to the second side-stream extractive distillation column (SEDC2), which contains all benzene and acetonitrile and partial solvent. The distillate of SEDC2 is the acetonitrile product. A liquid side stream (LS2) is withdrawn near the bottom of SEDC2 and fed to the solvent recovery column (SRC), which contains all benzene and partial solvent. The benzene product is obtained at the top of SRC. The high-purity solvent from the base of three columns after being cooled is split and recycled back to SEDC1 and SEDC2.

9



Figure 7. The optimal flowsheet of the extended-SEDC configuration. The sequential iterative optimization procedure for the extended-SEDC configuration is shown in Supporting Information. The key design variables including the reflux ratios (RR1, RR2 and RR3) and the distillate flowrates (D1, D2 and D3) of SEDC1, SEDC2 and SRC, and the side liquid flowrates (LS1 and LS2) of SEDC1 and SEDC2 are adjusted to reach the separation requirements. There are still twelve optimization variables in this configuration as follows: the number of stages (NT1, NT2 and NT3) and the feed locations (NF1, NF2 and NF3) of SEDC1, SEDC2 and SRC, the solvent locations (NFS1 and NFS2) and the solvent flowrates (SF1 and SF2) of SEDC1 and SEDC2 and the side liquid withdrawn locations (NSD1 and NSD2) of SEDC1 and SEDC2. The vapor and liquid composition profiles of the extended-SEDC configuration are shown in Figure 8. As shown in Figure 8a and Figure 8b, three high purity products can be obtained at the top of the SEDC1, SEDC2 and SRC, respectively. Moreover, the remixing effect is apparently alleviated, which results in the dominant reduction of energy consumption. From Figure 8c, the high purity benzene and solvent can be obtained in the distillate and bottom of SRC, respectively.

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(c) 1 Methanol Benzene Acetonitrile Solvent

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Figure 8. Vapor and liquid composition profiles for the extended-SEDC configuration: (a) SEDC1 (b) SEDC2, and (c) SRC. Table 1 presents the head-to-head comparison of three configurations in terms of optimized results and economic costs. It can be seen that two proposed thermal coupled configurations have obvious advantages in economic benefits and energy consumption compared with the CED configuration. This high energy efficiency in two intensified configurations reflects the reduction or elimination of the remixing effect, especial for the extended-SEDC configuration. The reason for this difference is that two side liquid streams (LS1 and LS2) in SEDC1 and SEDC2 can reduce the remixing effect in the compositions of acetonitrile and benzene, but one vapor split (VR) in MC can reduce partly the remixing effect in the compositions of acetonitrile and not work for the component of benzene. Table 1. Optimal Results for Three Separation Configurations CED

extended-EDWC

extended-SEDC

EDC1

EDC2

SRC

MC

SC

SRC

SEDC1

SEDC2

SRC

NT

50

44

35

56

48

33

58

57

31

NFS

29

21

—

27

21

—

29

24

—

NF

43

33

12

41

49

10

43

35

11

NSD

—

—

—

49

—

—

50

43

—

ID(m)

0.82

0.58

0.67

0.664

0.949

0.541

0.236

RR

2.09

2.43

43.94

2.12

1.86

43.83

2.11

2.11

5.3

Side stream (kmol/h)

—

—

—

18.43

—

—

25.53

15.13

—

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100

40

—

105

33

—

100

53

—

1.13

11



Capital cost (105$/a)

1.87

1.00

1.21

2.57

1.18

2.22

1.09

0.34

Energy cost (105$/a)

1.48

0.52

0.73

1.91

0.72

1.55

0.53

0.10

TAC (105$/a)

3.35

1.52

1.94

4.48

1.90

3.77

1.62

0.44

Total (105$/a)

6.81

6.38

5.83

4. DYNAMIC CONTROL The proposed configurations in this paper have the advantages of more energy efficiency and economic benefits than other distillation alternatives in separating the multi-azeotropic system, but control complexity and less robust dynamic behaviors are undesirable side effects since more control variables are introduced. Therefore, to examine operational aspects of three configurations, their control structures are developed and the dynamic performances are tested when disturbance occur in feed flowrate and composition. For the control implementation, the steady state design is exported to a pressure-driven simulation in Aspen Dynamics. Reflux drum and base volume are specified to provide 5 min of holdup for 50% liquid level. Pumps and valves are sized to give proper pressure drops to handle changes in flowrate.42 4.1 Control of CED. In order to select properly a temperature control location, the open loop sensitivity analysis is carried out with ±0.1% changes in the reboiler duty (QR1, QR2 and QR3) of EDC1, EDC2 and SRC and with ±0.1% changes in the reflux ratio (RR3) of SRC, respectively. As shown in Figure 9, the temperature control point of each column is selected as the following: the 46th tray for EDC1 (T1,46), the 36th tray for EDC2 (T2,36), and the 7th tray for SRC (T3,7). Note that only one temperature control tray that occurs in the rectifying can be determined by using the sensitivity criteria of reflux ratio (RR3) and reboiler duty (QR3), as shown in Figure 9c and Figure 9d, thus one composition controller has to be installed to control the impurity in the bottom.

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Figure 9. The open-loop sensitivity analysis of the CED configuration: (a) EDC1, (b) EDC2 and (c, d) SRC. Figure 10 shows the control structure developed for the CED configuration. Flow controllers are set as Proportional-Integral (PI) controller with a gain (KC) of 0.5 and an integral time (τ1) of 0.3min. All pressure controllers are tightly tuned with KC =20 and τ1 = 12 min. All level controller are proportional with KC = 2 and τ1 = 9999 min. All temperature and composition controllers have deadtimes of 1 min and 3 min, respectively, and are tuned using relay-feedback testing and Tyreus-Luyben tuning rules.42 All control loops are described as follows: (1) Fresh feed is flow controlled. (2) The column pressure is controlled by manipulating the condenser duty. (3) The base level of EDC1 and EDC2 are held by manipulating the bottom flowrates, and that of SRC is held by manipulating the small solvent makeup. (4) Reflux drums levels of EDC1 and EDC2 are held by manipulating the distillate flowrates respectively, and that of SRC is held by manipulating the reciprocal of reflux flowrate because of the high reflux ratio. (5) All reflux ratios are kept constant. (6) Two proportional controllers that solvent flowrates are ratioed to feed flowrates are introduced because the recycle solvent needs to split properly for two extraction distillation processes. (7) The control point T1,46 is controlled by manipulating heat input to the EDC1 reboiler. (8) The control point T2,36 is controlled by manipulating heat input to the EDC2 reboiler. (9) The control point T3,7 is controlled by manipulating the reciprocal of the reflux ratio of SRC. 13



(10) The benzene composition in the bottoms of SRC is controlled by manipulating heat input to the SRC reboiler. (11) The temperature of the recycle solvent is controlled by manipulating the cooling water flowrate to the solvent cooler.

Figure 10. The overall control structure of the CED configuration. In order to demonstrate the effectiveness of the developed control structure, a small magnitude of 5% disturbances in the feed flow is introduced since valves saturation is easy to occur in the control of the complex extraction distillation process. The magnitude of the disturbances in the feed composition is similar with the reported literatures,51, 52 that is, increasing from 20 % to 22 wt % acetonitrile, 10 % to 11 wt % benzene, and decreasing from 20 % to 18wt % acetonitrile, 10 % to 9 wt % benzene. Figure 11a gives the close loop responses for feed flow disturbances. All temperature controllers responded relatively fast and the product compositions are maintained quite close to the specified values. The time required to reach the steady state again is approximately 5 h. 14


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Figure 11b gives the close loop response results for feed composition disturbances. All temperature controllers handled the disturbance reasonably well by bringing the temperatures back to their setpoints. The purities of acetonitrile and benzene after 5h have a small deviation from their desired values. Overall, the proposed control structure shows good dynamic behavior when facing feed flow and composition disturbances.

99

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0.998

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Figure 11. The dynamic responses for disturbances of the CED configuration: (a) feed flow (b) feed composition. 4.2 Control of extended-EDWC. As shown in Figure 12, temperature control locations of MC and SRC are selected also by the open loop sensitivity analysis, which is carried out with ±0.1% changes in the reboiler duty (QR1 and QR2) of MC and SRC, and with ±0.1% changes in the reflux ratio (RR2 and RR3) of SC and SRC, respectively. MC have two temperature control points of the 43th and 50th tray (T1,43, T1,50), and the temperature control point of SRC is the 6th tray (T3,6). However, it is hard to identify the temperature control point for SC based on Figure 12b, showing the open loop sensitivity analysis with ±0.1% changes in the reflux ratio (RR2) of SC. Therefore, the slope criterion shown in Figure 12e is used for selecting a temperature control 16


Page 17 of 33

location of SC, that is, the 36th tray (T2,21).

(a) 30

(b) 0.2 +0.1%QR1

20

0

0

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(e) 120 110 T(℃)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 90 80 70 0

10

20

30 Stage

40

50

Figure 12. The temperature sensitive tray of the extended-EDWC configuration: (a) open-loop sensitivity analysis for MC, (b) open-loop sensitivity analysis for SC, (c, d) open-loop sensitivity analysis for SRC and (e) temperature profile of SC. Figure 13 shows the control structure developed for the extended-EDWC configuration. There are several differences except for basic control loops compared to that of traditional extractive distillation as follows: (1) A cascade combination of composition and temperature control is used in SC, which achieves both fast control and the maintenance of acetonitrile product purity. (2) The temperature control point of T1,43 is controlled by manipulating the ratio of the side vapor 17



flow (VR) to the reboiler heat duty (QR1), with the reflux ratio (RR1) being held constant.

Figure 13. The overall control structure of the extended-EDWC configuration. In order to demonstrate the effectiveness of the developed control structure, the same feed disturbances as the CED configuration are introduced. Figure 14a gives the close loop responses for feed flow disturbances. All temperature control points return quite effectively to their set points and all the product compositions are maintained quite close to the specified values. The time required to reach the steady state again is approximately 5 h. Figure 14b gives the close loop responses for feed composition disturbances. All temperature controllers also handle the disturbance reasonably well by bringing the temperatures back to their setpoints. The purity of acetonitrile after 5h has a small deviation from its desired value. These results demonstrate fully the proposed control structure can achieve the controllable operating of the extend-EDWC configuration. 18


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(a) 113

92

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0.999 0.998 0.997 0.996 0

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Figure 14. The dynamic responses for disturbances of the extended-EDWC control structure: (a) feed flowrate and (b) feed composition. 20


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4.3 Control of extended-SEDC. Similarly, the open loop sensitivity analysis is carried out with ±0.1% changes in the reboiler duty (QR1, QR2 and QR3) of EDC1, EDC2 and SRC and with ±0.1% changes in the reflux ratio (RR3) of SRC, respectively, for the temperature control tray. It can be seen from Figure 15 that the temperature control tray of SEDC1 is the 47th tray (T1,47), and that of SEDC2 is the 37th tray (T1,37) and that of SRC is the 7th tray (T3,7). Figure 16 shows the overall control structure. Special instructions except for basic control loops are required as follows: (1) A cascade combination of composition-flow control (CC3-FC2) is installed to keep the methanol composition in the side stream LS1 constant, and as well a cascade combination of composition-flow control (CC4-FC3) is settled to keep the acetonitrile composition in the side stream LS2 constant. (2) The CB impurity in the distillate of SEDC1 is controlled by the reflux ratio (RR1), and that of SEDC2 is controlled by the reflux ratio (RR2). (3) The solvent flow (SF2) is ratioed to the initial feed flow (F1).

(a) 300

(b) 40 +0.1%QR1 -0.1%QR1

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20 0

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0

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Figure 15. The open-loop sensitivity analysis of the extended-SEDC: (a) SEDC1, (b) SEDC2, and (c, d) SRC.

21



Figure 16. The overall control structure of the extended-SEDC configuration. The effectiveness of the developed control structure is demonstrated by introducing the same feed disturbances as the CED configuration as well. Figure 17a gives the close loop responses for feed flow disturbances. All temperature control point return quite effectively and all the product compositions are maintained quite close to the specified values. The time required to reach the steady state again is approximately 15 h. Figure 17b gives the close loop responses for feed composition disturbances. All temperature controllers handle the disturbance reasonably well by bringing the temperatures back to their setpoints. The purity of acetonitrile after 10h has an acceptable deviation from its desired values. Likewise, these results prove that the extend-SEDC configuration can implement with the good controllability based on the proposed control structure.

(a)

22


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117.48

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0.998 0.9975 0.997

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0.995 0.994

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Figure 17. The dynamic responses for disturbances of the extended-SEDC configuration: (a) feed flowrate and (b) feed composition. The proposed control structures for three complex configurations are effective in managing these operations to achieve the separation objectives. But there are some differences between them that need to be highlighted. First, the control structure for the CED configuration uses only one composition controllers that is slow and expensive, thus it has the advantages of fast responses and economy in the industrial application. Second, the control structures for both intensified configurations have more degrees of freedom, and adopt more composition controllers to maintain the product purities and complex cascade controls for improving the control quality. At last, the control structure for the extended-SEDC configuration is a bit less responsive to feed disturbances 24


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than other two control structures, and this is not to be expected and needs to be further improved. The data for all temperature and composition controllers of three control structures, such as transmitter ranges, tuning parameters and controller output ranges can be found in Supporting Information. 5. CONCLUSION

This article demonstrates three newly proposed extractive distillation configurations including CED, extended-EDWC and extended-SEDC for the separation of acetonitrile/methanol/benzene with chlorobenzene as solvent. The optimization results of the steady-state design show that two intensified configurations can provide great savings in TAC and energy consumption compared to the CED configuration, especially for the extended-SEDC. Dynamic control studies represent that they are also comparable to CED in terms of controllability under the same feed disturbances, although requiring more complex control structures. To sum up, all of the proposed configurations in this article are the essential alternatives without any potential dynamic control problems for separating the similar multi-azeotropic mixtures with acetonitrile/methanol/benzene, and particularly attractive is the extended-SEDC flowsheet from the energetic and economic standpoints. ■ ASSOCIATED CONTENT S

Supporting Information

The Supporting Information is available free of charge on the ACS publications website at DOI: (1) Correlation parameters for Wilson model for acetonitrile/methanol/benzene. (2) The necessary formula and parameter for economic evaluation. (3) Temperature-composition controllers tuning parameters. (4) The sequential iterative optimization procedure for the CED configuration. (5) The sequential iterative optimization procedure for the extended-EDWC configuration. (6) The sequential iterative optimizaton procedure for the extended-SEDC configuration. ■ AUTHOR INFORMATION *E-mail: [email protected]. ORCID Zhishan Zhang: 0000-0001-8291-4727 25



Notes The authors declare no competing financial interests. ■ NOMENCLATURE AC = heat transfer area of condenser (m2) AR = heat transfer area of reboiler (m2) CED = conventional extractive distillation CSCinst = installed cost of column shell ($) EDC = extractive distillation column EDWC = extractive dividing-wall column extended-EDWC = the configuration of extended extractive dividing-wall column extend-SEDC = the configuration of extended side-stream extractive distillation column F = feed flow (kmol/h) Hcol = column height (m) Htray = column height between top and bottom tray (m) ID = column diameter (m) KC = gain coefficient KU = ultimate gains LS = side liquid flow(kmol/h) NF = fresh feed location NFS = solvent location NSD = side stream location NT = the number of stages PU = periods QC = condenser duty (kW) QR = reboiler duty (kW) RR = reflux ratio SEDC = side-stream extractive distillation column S:F = solvent to feed ratio SF = solvent flow (kmol/h)

26


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SRC = solvent recovery column T = stage temperature (℃) TAC = total annual cost ($/a) TCinst = installed cost of trays ($) U = heat transfer coefficient (kW/(K·m2)) VR = side vapor flow (kmol/h) x = liquid mole fraction τI = integral time constant (min)

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ABSTRACT GRAPHIC

31



TABLE OF CONTENTS

ABSTRACT 1. INTRODUCTION 32


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2. DESIGN BASIS 2.1 Thermodynamics 2.2 Economics 3. STEADY-STATE DESIGN 3.1. Design of CED 3.2. Design of extended-EDWC 3.3 Design of extended-SEDC 4. DYNAMIC CONTROL 4.1 Control of CED. 4.2 Control of extended-EDWC 4.3 Control of extended-SEDC 5. CONCLUSION ASSOCIATED CONTENT Supporting information

AUTHOR INFORMATION NOMENCLATURE REFERENCES ABSTRACT GRAPHIC

33


Economics and controllability of conventional and intensified

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