toluene azeotrope with intermediate-boiling entrainer

toluene azeotrope with intermediate-boiling entrainer

Chemical Engineering & Processing: Process Intensification 149 (2020) 107862 Contents lists available at ScienceDirect Chemical Engineering & Proces...

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Chemical Engineering & Processing: Process Intensification 149 (2020) 107862

Contents lists available at ScienceDirect

Chemical Engineering & Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Control of energy-efficient extractive distillation configurations for separating the methanol/toluene azeotrope with intermediate-boiling entrainer

T

Chao Wanga, Yu Zhuanga,b, Linlin Liua, Lei Zhanga, Jian Dua,* a b

Institute of Chemical Process Systems Engineering, School of Chemical Engineering, Dalian University of Technology, Liaoning, 116024, China Key Laboratory of Liaoning Province for Desalination, School of Energy and Power Engineering, Dalian University of Technology, Liaoning, 116024, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Extractive distillation Intermediate-boiling Process intensification Dynamic control Methanol/toluene

The steady-state design and dynamic performances of three extractive distillation configurations including conventional extractive distillation (CED), thermally integrated extractive distillation (TIED) and extractive dividing-wall column (EDWC) are investigated and compared for separating the methanol/toluene azeotrope with intermediate-boiling triethylamine (Et3N) as solvent. The results show that TIED and EDWC configurations can save 21.28 % and 52.70 % energy consumption compared to CED, respectively, but the difficulty of controllability will be increased due to a more complex flowsheet. Towards this end, two control structures for EDWC are established and tested via introducing feed throughput and compositions disturbances as well as one control structure for CED and five control structures for TIED. Similar to CED, the control structures of TIED and EDWC configurations can also maintain products purities and integral squared error (ISE) within the acceptable range in resisting feed disturbances. Furthermore, EDWC configuration still reflects high energy-efficiency compared to CED and TIED after smoothing.

1. Introduction Extractive distillation is one of the common separation methods for azeotropic mixtures. The search for high-efficiency solvents that can increase relative volatility, break azeotropes and not form new azeotrope is the most critical step in the extractive distillation process [1]. Up to now, a considerable amount of references has studied the extractive distillation process for separating the binary even multi-azeotropes system with heavy solvents from the aspects of economics and controllability [2–8]. Compared to heavy solvents, the intermediateboiling solvent is more attractive because it does not cause an increase in the temperature of the system to avoid the using of expensive highpressure steam [9]. However, only a limited number of studies on the extractive distillation process with intermediate-boiling entrainers have been reported and most of the literature focused on the separation of methanol/toluene binary azeotrope [10–14]. The methanol and toluene azeotropic mixture are often encountered in the alkylation of toluene and methanol to manufacture paraxylene process [15]. The effluent must be effectively treated to avoid environmental pollution and waste of resources; however, the presence of azeotrope poses a challenge for the separation effectively. Modla [11]



compared the economics of three different extractive distillation separation configurations with triethylamine (Et3N) as intermediateboiling solvent, namely conventional extractive distillation (CED), thermally integrated extractive distillation (TIED) and extractive dividing-wall column (EDWC). Luyben [12] investigated both the design and control for conventional direct and indirect extractive distillation sequences. Ma et al. [13] compared the economics and controllability of CED with aniline as heavy solvent and Et3N as intermediate-boiling solvent, respectively. Huang et al. [14] proposed two type of CED sequences including direct sequence and indirect sequence and two corresponding EDWC sequences including dividing wall column with the wall in the top (DWC-T) and that of in the bottom (DWC-B) and further evaluated the economy and controllability. However, the EDWC configuration proposed is more complicated and more suitable for heavy solvent rather than intermediate-boiling solvent compared to the EDWC configuration proposed by Modla [11]. Nonetheless, the boiling point of intermediate-boiling solvent belongs to the operating temperature range of the separation system compared to that with heavy solvent, which provides the energy-efficient EDWC configuration with additional degrees of freedom (such as liquid split ratio (βL), vapor split ratio (βV) and solvent side withdrawn) and a prefractionator (PFT)

Corresponding author. E-mail address: [email protected] (J. Du).

https://doi.org/10.1016/j.cep.2020.107862 Received 9 December 2019; Received in revised form 6 February 2020; Accepted 14 February 2020 Available online 15 February 2020 0255-2701/ © 2020 Elsevier B.V. All rights reserved.

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Nomenclature ASVM CC CED DWC-B DWC-T EDC EDWC Et3N ISE KC MC M/F PFT

QR1/F1 QR1/FM RR S/F1 S/FM S/FT SRC SVD TC TIED βL βV τI ΔT

Averaged absolute variation magnitudes Composition controller Conventional extractive distillation Dividing wall column with the wall in the bottom Dividing wall column with the wall in the top Extractive distillation column Extractive dividing-wall column Triethylamine Integral squared error Gains Main column Makeup flowrates to fresh feed flowrates Prefractionator

Reboiler duty to fresh feed flowrates Reboiler duty to fresh methanol feed flowrates Reflux ratio Solvet flowrates to fresh feed flowrates Solvent flowrates to fresh methanol feed flowrates Solvent flowrates to fresh toluene feed flowrates Solvent recovery column Singular value decomposition method Temperature controller Thermally integrated extractive distillation Lquid split ratio Vapor split ratio Integral time Temperature differences [K]

squared error (ISE) and energy consumption after stabilization for the feasible control structures of CED, TIED and EDWC in the face of the same feed disturbances are completed.

without condenser and reboiler. In addition, the controllability for another energy-efficient TIED configuration is also difficult due to the fact that heat integration gives rise to the absence of reboiler. These factors add great challenges to the controllability of the energy-efficient extractive distillation configurations with intermediate-boiling solvent, to our best knowledge, which has been not studied. To overcome this limitation, it is worthwhile to develop their control structures and evaluate dynamics performances. The purpose of this paper is to construct robust control structures for three different extractive distillation configurations including CED, TIED and EDWC used in the separation of the methanol/toluene azeotrope with intermediate-boiling Et3N as solvent, which is the extended work of Modla10 and provide some guidance for developing control structures of extractive distillation process with intermediate-boiling solvent. The control structures are developed and established and the dynamic performances are tested and analyzed via introducing feed flowrates and feed compositions disturbances for three different extractive distillation configurations, especially for TIED and EDWC, respectively. What’s more, the comparisons of products purities, integral

2. Steady-state design of different extractive distillation configurations The premise of exploring dynamic control is to establish the feasible steady-state process, thus, three different extractive distillation separation configurations are established and simulated, namely CED, TIED, and EDWC on the basis of the Modla and Luyben’s work [11,12]. In these references, the total fresh feed flowrate is 100 kmol/h with the composition of 50 mol% methanol and 50 mol% toluene, the NRTL thermodynamic equation are chosen as feasible physical properties method to predict the vapor-liquid equilibrium performances in the simulation process. The products purities of methanol, toluene, and Et3N are specified as 99 mol%, respectively. Simultaneously, the same optimization design parameters as the references including the number of stages, feed locations, side withdrawn location, and operating

Fig. 1. CED separation configuration with detailed material and energy information. 2

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enter the MC from the 13th and 40th trays, respectively, while the side withdrawn liquid and vapor streams of MC enter the PFT from the top and bottom, respectively. The methanol product, Et3N product, and toluene product can be produced from the distillate, side withdrawn, and bottom, respectively. Simultaneously, the solvent Et3N is cooled to 323 K and recycled to the PFT with solvent makeup stream. Table 1 shows the comparison of important parameters with the work of Modla [10] and Luyben [11]. It can be seen that three simulation results show a slightly difference caused by different simulation softwares (ChemCad and Aspen Plus), software version, convergence methods, and pressure drop not explicitly given. As far as our simulation results are concerned, TIED and EDWC processes can be reduced by 21.28 % and 52.70 % in terms of energy costs due to the use of heat integration and thermally coupled technology, respectively compared to CED configuration. However, the improvement in energy efficiency will inevitably lead to more complex and difficult dynamic control problems due to the lack or increase in degrees of freedom. Thus, the simple and feasible control structures of three proposed alternative designs (CED, TIED, and EDWC) will be further developed and established to deal with feed flowrates and feed compositions disturbances.

pressure are given directly, and other design parameters missing in the reference including reflux ratio (RR) and distillate flowrates are recalculated via the “Design Spec/Vary” function to meet the products purities and recovery mentioned in the references. 2.1. Conventional extractive distillation (CED) Fig. 1 shows the CED separation flowsheet consisting of one extractive distillation column (EDC) and one solvent recovery column (SRC). The fresh feed and solvent flow into the EDC from the 22th tray. Methanol product with a purity of 99 mol% is produced at the top of the EDC, while the bottom stream contains 25.6 mol% toluene and 74.4 mol % Et3N. Toluene and Et3N products with purities of 99 mol% are obtained at the bottom and distillate of the SRC, respectively. The solvent Et3N is cooled to 323 K and recycled to the EDC with a small amount of solvent makeup stream. 2.2. Thermally integrated extractive distillation (TIED) Fig. 2 shows the partially thermally integrated extractive distillation process (TIED) with detailed material and energy information. In the work of Modla [11], the operating pressure of the SRC is increased to 5 atm to meet the temperature difference (55 K) required for heat exchange between the overhead condenser (426 K) of the SRC and the reboiler (371 K) of the EDC. The recovered energy between EDC and SRC is 2499.95 kW, and SRC needs an auxiliary condenser to provide the rest of the cooling duty (2090.50 kW). Compared to CED, the reboiler duty of SRC is increased by 1028.26 kW since the pressure is increased from 1 atm to 5 atm. Thus, the net energy reduction of overall process is 1478.37 kW compared to CED (6945.98 kW).

3. Dynamic control The investigation of operability and dynamic controllability for the separation configurations with low costs and high energy efficiency is more important than steady-state design. In the section, the control structures of the mentioned three alternative separation schemes are constructed and tested via introducing same magnitude feed flowrates and compositions disturbances. The suitable heights and diameters of reflux drums and column bases are calculated via the heuristic rule, which means reflux drums and column bases are filled during 10 min. In addition, the reflux drums and column bases are approximately cylindrical and the height is assumed to be twice the diameter during calculation. Simultaneously, the enough and reasonable pressure drops are specified to ensure that steady-state file can be exported to Aspen Dynamics after pressure-checking as pressure-driven model [16–18].

2.3. Extractive dividing-wall column (EDWC) Fig. 3 presents the extractive dividing-wall column (EDWC) schematic and its thermodynamically equivalent model consisting of a main column (MC) and a prefractionator (PFT) without condenser and reboiler. The methanol and toluene azeotropic mixture and solvent Et3N are fed to the PFT. The top vapor and bottom liquid steams of the PFT

Fig. 2. TIED separation configuration with detailed material and energy information. 3

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Fig. 3. EDWC separation configuration (a) EDWC schematic (b) thermodynamically equivalent model with detailed material and energy information.

singular value decomposition (SVD) method, the slope criterion and sensitivity criterion [19,20]. Slope criterion is to choose the tray where there are large changes in temperature from tray to tray; Sensitivity criterion is to identify the tray where there is the largest change in temperature for a change in the manipulated variables (i.e. reflux ratio, reboiler duty), and SVD criterion is to choose temperature sensitive trays via decomposing the matrixes obtained via sensitivity criterion using standard SVD programs. Compared to sensitivity criterion and SVD criterion, the slope criterion is the preferred option due to its simplicity for the temperature profiles with the significant temperature breaks. Compared to slope criterion, the sensitivity criterion and SVD criterion should be adopted to determine temperature sensitive trays for the flat temperature profiles without large temperature changes from tray to tray. In the article, the slope criterion is used as the unique way to select temperature sensitive trays since all temperature profiles exist significant temperature breaks. Fig. 4 shows the temperature and temperature differences (ΔT) between adjacent trays profiles for EDC and SRC. As can be seen clearly from Fig. 4, the temperature sensitive

Table 1 Comparison of important parameters with the references for three alternative schemes. CED

TIED

EDWC

Items

Modla

Luyben

Wang

Modla

Wang

Modla

Wang

RR1 RR2 QR1(kW) QR2(kW) Total

2.83 1.95 2000 3831 5831

2.89 2.44 2492 4406 6898

2.88 2.46 2506 4439 6945

2.83 2.37 0 4572 4572

2.89 3.21 0 5467 5467

– 5.29 – 3215 3215

– 4.78 – 3285 3285

3.1. Control of CED configuration The determination of temperature sensitivity tray is the significant step for the construction of the control structures. Up to now, many methods are applied to choose temperature sensitivity trays, such as the

Fig. 4. The temperature and ΔT profiles for EDC and SRC (a) temperature profile for EDC, (b) ΔT profiles for EDC, (c) temperature profile for SRC, and (d) ΔT profiles for SRC. 4

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tray is 23th tray for EDC, and that of is 29th tray for SRC. Fig. 5 gives the control structure without any composition controller of the CED configuration. As for the setting of controller parameters, all level controllers are proportional-only controllers with integral time (τI = 9999 min) and gains (KC = 2). All flow controllers are proportional-integral controller with KC = 0.5 and τ1 = 0.3 min, while pressure controllers are tuned with KC = 20 and τ1 = 12 min. For temperature controllers, the relayfeedback testing and Tyreus-Luyben tuning rules are used to get the ultimate KC and τI, while 1 min dead time element is inserted in temperature control loop [21–24]. The detailed control loops with controlled variables and operating variables are enumerated below:

3.2. Control of TIED configuration The temperature sensitivity stage should be re-determined due to the increase in the column pressure and the change in the number of stages for the SRC. Fig. 7 presents the temperature and ΔT between adjacent trays profiles. Thus, the new temperature sensitive tray is 44th tray for high pressure SRC. Fig. 8 shows the overall control structure for the TIED configuration. There are several different control loops compared to the control structure of the CED due to the emergence of thermal integration with auxiliary condenser, as following: (1) The operating pressure of high pressure SRC is maintained via the heat removal rate in the auxiliary condenser. (2) The temperature of sensivity tray (19th tray) in the EDC is controlled by manipulating RR1 rather than integrated reboiler duty. (3) A feedforward ratio control loop of reboiler duty to fresh feed flowrates (QR1/F1) is inserted. (4) The temperate of 44th stage in the high pressure SRC is kept constant via manipulating reboiler duty. (5) The flowsheet equation is compiled in the Aspen Dynamics, as following:

(1) Fresh feed is flow-controlled. (2) The reflux-dump level of the EDC is controlled via manipulating the distillate flowrates, however, the reflux-drum level of the SRC is controlled via manipulating makeup solvent flowrates rather than distillate flowrates since the composition of distillate is mainly solvent. (3) The base levels of the EDC and SRC are maintained via manipulating their bottom flowrates. (4) The pressures of the EDC and SRC are controlled by manipulating their condenser heat removal. (5) The constant reflux ratios (RR1 and RR2) of the EDC and SRC are employed as proportional controllers. (6) The temperatures of sensivity trays (23th tray and 29th tray) in the EDC and SRC are controlled by manipulating their reboiler duty. (7) The ratio of solvet flowrates to fresh feed flowrates (S/F) is kept constrant.

“Blocks("SRC").condenser(1).Q=Blocks("PC2″).OP ("EDC").QReb”

-

Blocks

Where Blocks("SRC").condenser(1).Q is the condenser duty of the SRC, the Blocks("PC2″).OP is the output sign of pressure controller in the SCR, and Blocks("EDC"). QReb” is the reboiler duty of the EDC. To verify the effectiveness of the proposed control structure in resisting feed disturbances, the dynamic performances are tested via introducing specified feed flowrates and compositions disturbances, as shown in Fig. 9. The toluene purity can be stabilized after 10 h and the deviation from the initial set value is within the acceptable range via the regulation of control structure in the face of specified feed disturbances. However, an unexpected phenomenon can be observed that the methanol product purity can reach stabilized after 10 h with the deviation of far more than 0.2 % from the initial set value at feed flowrates of 90 kmol/h (98.62 mol%) and methanol composition of 60 mol% (97.07 mol%). Thus, the proposed control structure is useless in dealing with specified feed disturbances, and the control structure should be further modified. In the basic control structure, the solvent flowrates and reboiler

To verify and evaluate the effectiveness of the control structure, the dynamic performances are tested via introducing ± 10 % feed flowrates and feed compositions disturbances from 50 mol%(methanol)/50 mol %(toluene) to 40 mol%(methanol)/60 mol%(toluene) and 60 mol%(methanol)/ 40 mol%(toluene) at time =2 h, as shown in Fig. 6. It can be discovered that the product purities of methanol and toluene can be stabilized after a period and has a deviation of less than 0.2 % from the initial set value. Overall, the control structure proposed shows the effectiveness in deal with specified feed disturbances.

Fig. 5. Overall control strategy for CED process. 5

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Fig. 6. Dynamic responses for feed disturbances (a) feed flowrates (b) feed composition.

Fig. 7. The temperature and ΔT profiles for high pressure SRC (a) temperature profile (b) ΔT profile.

Fig. 8. Overall basic control strategy for TIED process.

6

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Fig. 9. Dynamic responses for feed disturbances (a) feed flowrates (b) feed composition.

Fig. 10. Dynamic responses of three modified control structures for feed disturbances (a) Improved QR1/FM control structure (b) Improved S/FM control structure and (c) Improved QR1/FM and S/FT and control structure..

is revised from 97.07 mol% to 99.23 mol% and decreased from 99.12 mol% to 98.31 mol% in the face of two feed compositions disturbances, respectively. It can be concluded that the methanol product purity after smoothing has the same increase or decrease tendency as the provided QR1, thus, the QR1/FM control loop cannot improve control structure effectiveness compared to the QR1/F1 in dealing with specified feed disturbances. Thus, another new ratio control loop of solvent

duty of the EDC do not vary with the changes of feed composition when fixing the ratio control loops of QR1/F1 and S/F1, which is the main factor leading to substandard methanol product. Therefore, the QR1/F1 is firstly replaced by the new ratio control loop of reboiler duty to fresh methanol feed flowrates (QR1/FM), and the dynamic performances for improved QR1/FM control structure is shown in Fig. 10(a). Compared to basic control structure, the methanol product purity after stabilization 7

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Fig. 10. (continued)

toluene flowrates) is proposed to further improve control performance via considering the effects of the basic and the first two improved control structures. Fig. 10(c) demonstrates the dynamic responses in the face of the specified feed disturbances after 2 h. All product purities can be maintained near the initial set points after stabilization in resisting feed compositions disturbances, however, the methanol product purity can only be maintained at 98.62 mol% for the -10 % feed throughput disturbances. This is because the solvent flowrates and reboiler duty are same for the basic and three improved control structures with different

flowrates to fresh methanol flowrates (S/FM) is considered to improve the ability of the control structure to handle feed disturbances. Fig. 10(b) presents the dynamic responses of improved S/FM control structure. The dynamic performance has been obviously improved compared to the basic and improved QR1/FM control structures, however, the methanol product purity is only maintained at 98.40 % after smoothing when the feed methanol composition is 60 mol%, which still has a large deviation from the initial set value. The third improved control structure with QR1/FM and S/FT (solvent flowrates to fresh

Fig. 11. Modified control structure with ΔT for the TIED process. 8

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for the actual EDWC configuration and no valves and pumps are required between the PFT and MC in the steady model. However, valves with some pressure drops must be inserted between the PFT and MC, otherwise, the steady-state file cannot be exported into Aspen Dynamics as pressure-driven model. To solve this problem, Li and Luyben [28] suggested that some “fictitious” compressors are installed on the vapor lines coming out the top of the stripper and the tops of the two sides of the wall to compensate for this additional pressure drops, and some pressure controllers are installed to hold the back pressure in each of these three vessels by manipulating the work of the compressors. However, there is complex phenomena for the mixtures involved in the paper that superheating of the compressor inlet is necessary to avoid condensation on compression [29], which inevitably causes the difficulty of control and is more inconsistent with the actual EDWC configuration. Therefore, the operation pressures of the PFT and MC are designed to be slightly different via different pressure drops and the pressure on the first stage in the distillation column to ensure the steady-state file without “fictitious” compressors can be exported into Aspen Dynamics as pressure-driven model. Simultaneously, the introduction of the pumps is also for this purpose. What’s more, the core parts of the control structures for the EDWC configuration are βL control loop, βV control loop, other ratio control loops, temperature control loops and composition control loops. Therefore, the pressure problem caused by the difference between the thermodynamically equivalent model and the actual EDWC configuration can be simplified in the published literatures [30–34]. The temperature and ΔT between adjacent trays profiles are presented in Fig. 13. It can be seen that the stage with largest slope is 7th for the FPT, and the stages with larger slope are 20th and 54th for the MC. Fig. 14 shows the basic control structure for the EDWC. There are several different control loops compared to the CED and TIED control structures due to more complex degrees of freedom, as following:

ratio control loops in the face of feed throughput disturbances. In fact, the corresponding solvent flowrates and reboiler duty of the EDC will decrease proportionally in the face of the situation of feed flow rate (90 kmol/h) in the basic and three improved control structures with different ratio control loops. However, the relationship between the feed flowrates, the required solvent flowrates and reboiler duty is complex rather than simple linear proportional relationship, which cause a reduction in the methanol product purity due to the insufficient solvent flowrates and reboiler duty calculated via the ratio control loops. Thus, more effective control structures with temperature difference (ΔT) should be proposed to address this challenge via obtaining a more reasonable RR1. The three improved control structures are shown in Fig. S1 of Supporting Information. Based on the four control structures, the fifth control structure with temperature difference (ΔT) is proposed to improve the controllability in the face of feed disturbances, especially for the -10 % feed flowrates disturbances, as shown in Fig. 11. Compared with single temperature control schemes, temperature difference control scheme can better keep the temperature gradient in the distillation column constant in the face of feed disturbances due to the large stage span, which can further obtain a more reasonable RR1. Some heuristic rules for determining the stages of the temperature difference control loop have been applied to different control structures, for example, between the reboiler and the stage with large slope [25], between two sensitive stages [26] and averaged absolute variation magnitudes (ASVM) methods [27]. In the paper, the stages (23th and 19th) with large peaks are selected as temperature-sensitive stages of the temperature difference control loop and the ΔT is controlled via manipulating RR1. The dynamic performances of the fourth improved control structure are tested via introducing specified feed disturbances, as shown in Fig. 12. It can be seen that all products purities can be quickly stabilized, and the deviations are controlled within acceptable limits after smoothing. Thus, the control structure can be used as a feasible solution to control the TIED process.

(1) The feedforward ratio control loop of makeup flowrates to fresh feed flowrates (M/F) is inserted into the control structure. (2) The pressure of the PFT is controlled via manipulating its overhead vapor flowrates. (3) The βL is kept constant. (4) The temperature on 7th stage of the PFT is controlled via manipulating the βV. (5) The RR and the ratio of side withdrawn liquid flowrates and reflux flowrates (S/R) of the MC is kept constant.

3.3. Control of EDWC configuration It should be emphasized that the thermodynamically equivalent model should be applied during the simulation of dynamic control due to the limitation of Aspen Plus and Aspen Dynamics software rather than the actual EDWC configuration. In addition, the operation pressures of the PFT and MC are the same since they are in the same column

Fig. 12. Dynamic responses of control structure with ΔT for feed disturbances (a) feed flowrates (b) feed composition. 9

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Fig. 13. The temperature and ΔT profiles for PFT and MC (a) temperature profile for PFT, (b) ΔT profiles for PFT, (c) temperature profile for MC, and (d) ΔT profiles for MC.

(6) The temperature on 54th stage of the MC is controlled via manipulating it reboiler duty. The reason why the 20th stage is not used as the temperature control point is that the reboiler duty should generally control the stage in the stripping section.

maintained close to initial set values in resisting feed compositions disturbances. However, an unexpected phenomenon can be seen that the control structure is ineffective for the control of the toluene product purities. Thus, a more effective control structure should be proposed, as shown in Fig. 16. Fig. 16 shows the improved control structure based on the basic control structure. In the improved control structure, the temperature controller (TC2) is replaced by an expensive but advanced composition controller (CC) to maintain toluene product purity. In the composition control loop, the toluene composition is controlled via manipulating reboiler duty and a dead time element with 3 min is inserted. Fig. 17

The control ability of the control structure is verified via introducing the specified feed disturbances at the time =2 h and the dynamic responses are shown in Fig. 15. It can be clearly seen that the basic control structure can maintain products purities within acceptable deviations after smoothing in the face of feed flowrates disturbances. Simultaneously, the methanol product purity after smoothing can be

Fig. 14. The basic control structure of EDWC. 10

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Fig. 15. Dynamic responses of the basic control structure for feed disturbances (a) feed flowrates (b) feed composition.

Fig. 16. The improved control structure with CC of EDWC.

the number of controllers. The comparisons of products purities and energy consumption after smoothing for the optimal control structures of the CED, TIED and EDWC in dealing with the same feed disturbances are shown in Fig. 18. The controllability of two energy-efficient separation configurations (TIED and EDWC) under the influence of control structure proposed is not weakened in spite of the increase of energy integration and the change of degrees of freedom compared to CED, and even appears some advantages from the perspective of product purities. In addition, the EDWC still presents energy advantageous compared to CED and TIED in the face of feed disturbances. However, an interesting but unexpected phenomenon can be observed that the energy consumption of the TIED after smoothing is slightly higher than that of the CED when the methanol composition is 40 mol% in the fresh feed. Overall, the reboiler duty of the EDWC after smoothing can save 51.86 %/39.11 % (+10 %), 51.79 %/38.56 % (-10 %) than that of the CED and TIED in the face of feed flowrates disturbances, respectively, while can save 57.78 %/57.89 % (40 mol% M), 46.46 %/10.27 % (60 mol% M) in the face of feed compositions disturbances. What’s more, integral squared error (ISE) is used to further compare the

shows the dynamic responses of the improved control structure with CC in dealing with specified feed disturbances. It can be seen that all products purities are acceptable after smoothing and the control performances have been greatly improved under the action of the CC, especially for the toluene product purity. The advanced but expensive composition controllers can more effectively control product purity, but inevitably increase economic costs and design difficulties compared to simple temperature controllers. In addition, the composition controllers also have applications in the actual operation, for example, the separation process for the systems with relative volatility close to 1 via the superfractionator [35]. Overall, the improved control structure is a suitable alternative for the EDWC. 4. Controllability and energy consumption comparison Table 2 summarizes the number and types of controllers for the optimal control structures of individual extractive distillation configuration. It can be seen that many complex but advance controllers are used in two energy-saving configurations, which cause an increase in 11

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Fig. 17. Dynamic responses of the improved control structure with CC controller for feed disturbances (a) feed flowrates (b) feed composition.

control structure with QR/FM and S/FT proportional control loops and ΔT control loop is determined to be the optimal control structure among the five control structures. For the EDWC, two different control structures are successively established to resist specified feed disturbances. The second control structure can handle feed disturbances more efficiently than the first control structure due to the introduction of expensive but advanced CC. It can be seen that EDWC with highest energy efficiency and more degree of freedoms can still be effectively controlled via comparing and analyzing the products purities and ISE index after stabilization of the three separation configurations. What’s more, the energy consumption of the EDWC still demonstrates energy efficiency, which can save up to 57.89 % compared to CED and TIED configurations in dealing with specified feed flowrates and compositions disturbances. Overall, three separation configurations are controllable alternatives for separating binary azeotrope with intermediate boiling solvent, at least for the azeotrope studied in the paper. Since the intermediate-boiling solvents have the advantage of not causing a rise in system temperature, a more energy-saving heat pump assisted extractive distillation process should be further investigated in terms of steady-state simulation and dynamic control.

Table 2 Comparison of the number and types of controllers for the optimal control structures of individual extractive distillation configurations. Controllers

pressure controllers level controllers flow controllers proportional controllers temperature controllers temperature difference composition controllers flowsheet equation Total

Number CED

TIED

EDWC

2 4 4 3 2 0 0 0 15

2 4 4 4 2 1 0 1 18

2 3 6 5 1 0 1 0 18

dynamic performances of three configurations, except for products purities and energy consumption, which is calculated via set value of purity, the actual value of purity, initial time, and terminal time [36,37], as shown in Table 3.

ISE=

∫t

t

( y− y sp)2dt

0

(1) CRediT authorship contribution statement

The results indicated that the average ISE of the EDWC is lowest in the face of feed flowrates disturbances, but is highest in the face of feed compositions disturbances compared to TIED and CED, which is caused by large fluctuations and slow recovery time.

Chao Wang: Conceptualization, Methodology, Software, Data curation, Writing - Original draft. Yu Zhuang: Methodology, Writing review & editing, Visualization, Supervision. Linlin Liu: Writing - review & editing, Supervision. Lei Zhang: Writing - review & editing, Supervision. Jian Du: Conceptualization, Supervision.

5. Conclusions In the article, the steady state simulation and dynamic control of three different extractive distillation separation schemes, namely CED, TIED and EDWC are investigated via taking the separation of methanol and toluene with intermediate boiling Et3N as solvent as an example. The energy consumption of the latter two separation configurations can cause a reducing of 21.28 % and 52.70 % compared with the first one, respectively. The different control structures for three separation processes are established and assessed via introducing the same feed flowrates and compositions disturbances, respectively. For the CED, a conventional control structure with simple control loops can hold products purities within acceptable deviations. For the TIED, a variety of control structures with different proportional control loops are developed and constructed to address the specified feed disturbances. The

Declaration of Competing Interest The authors have approved this manuscript and declared no competing financial interests.

Acknowledgments The authors would like to gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (No. 21776035) and China Postdoctoral Science Foundation (No. 2019TQ0045). 12

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Fig. 18. Comparison of controllability and energy consumption for CED, TIED and EDWC. Table 3 Comparison of the ISE index for the optimal control structures of three extractive distillation configurations.

Feed flowrates (+10 %) Feed flowrates (-10 %)

M:T = 4:6

M:T = 6:4

−5

methanol (×10 ) toluene(×10−5) average(×10−5) methanol(×10−5) toluene(×10−5) average(×10−5) methanol(×10−5) toluene(×10−5) average(×10−5) methanol(×10−5) toluene(×10−5) average(×10−5)

CED ISE

TIED ISE

EDWC ISE

0.04003 8.22693 4.15350 6.08107 6.41925 6.25016 12.87794 1.67475 7.27635 6.36888 1.61217 3.99052

7.98934 1.74537 8.86203 20.28610 1.35522 10.82066 4.71649 10.79526 7.75588 0.08770 5.00252 2.54511

1.14599 1.46921 1.88060 0.51403 1.64085 1.07744 174.55896 6.41455 90.48675 203.24911 5.47375 104.36143

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