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Control comparison of extractive distillation configurations for separating ethyl acetate-ethanol-water ternary mixture using ionic liquids as entrainer
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Control comparison of extractive distillation configurations for separating ethyl acetate-ethanol-water ternary mixture using ionic liquids as entrainer Qi Pan, Xianyong Shang, Shoutao Ma, Jie Li, Yunfei Song, Mengying Sun, Jiyan Liu, Lanyi Sun
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State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, China
ARTICLE INFO
ABSTRACT
Keywords: Extractive distillation Dynamic simulation Ionic liquids Control structure Exergy analysis
The new extractive distillation configurations, which use ionic liquids (ILs) as entrainer to develop green processes, have attracted widespread attention among scholars. Design and comparison of different process control schemes can facilitate the application of new solvents in future continuous production. In this paper, three extractive distillation configurations, including conventional extractive distillation (CED), multistage solvent recovery (MSR), and thermally coupled extractive distillation (TCED), are selected from previous work. The separation system of ethyl acetate-ethanol-water ternary mixture is studied, and 1-butyl-3-methylimidazolium acetate is used as an efficient entrainer. In order to compare the dynamic characteristics, six proportionalintegral control schemes are established to evaluate the plant-wide control performance of those arrangements. Then, the improved control schemes, which correspond to the three extractive distillation configurations, are selected to test the dynamic behavior under complex disturbance conditions. Integral absolute error and destroyed exergy are calculated to examine the robustness and energy efficiency of the system. The results show that the plant-wide dynamic responses of MSR process are satisfactory in terms of product purity, and the total destroyed exergy of CED process is relatively small during the operation period. There is an effective trade-off between controllability and energy efficiency, and the optimal control scheme should be determined based on actual conditions.
1. Introduction
by using Aspen Plus software. Compared with traditional solvent (ethylene glycol), ILs had obvious advantages in terms of solvent ratios and overall heat duties. Díaz et al. [18] applied a multiscale methodology to integrating molecular modeling and process simulation, which could help to screen suitable solvents for extractive distillation. Based on this research framework, the ionic liquid of 1-ethyl-3-methyl imidazolium dicyanamide was selected as an entrainer to separate aromatic–aliphatic hydrocarbon mixtures. To illustrate the economics of using ILs as entrainer, Chen et al. [19] developed two separation systems in an extractive distillation column, respectively, which corresponded to acetone-methanol and isopropyl alcohol-water azeotropic mixture. The results indicated that the trade-off between ILs and industrial entrainers required detailed economic evaluation. Hu et al. [20] proposed a systematic approach to screen suitable ILs for separating specific mixtures. The tert-butanol dehydration process was selected as a research case to test the separation performance of different solvents by extractive distillation. Zhu et al. [21] explored the separation of carbon dioxide-ethane system, and three ILs were used as entrainers for extractive distillation process to evaluate thermodynamic efficiency. The properties of ILs were obtained from the experimental
As a special thermal separation technique, extractive distillation has become an effective separation method for azeotropic or close-boiling compounds. The two main factors, which include process design and solvent (or entrainer) selection, can significantly affect separation efficiency and energy consumption of the extractive distillation process [1]. Before determining the separation strategy for the specified mixture, the feasibility of various intensification extractive distillation techniques should be examined to further maximize economic and environmental benefits, such as side-stream extractive distillation [2–4], pressure-swing extractive distillation [5,6], extractive heterogeneous azeotropic distillation [7–9], thermally coupled extractive distillation [10,11], and extractive dividing wall column configuration [12–15]. The new extractive distillation configurations, which use ionic liquids (ILs) as entrainer to develop green processes, have attracted widespread attention among scholars [16]. Li et al. [17] studied the effects of different ILs for separating acetonitrile and water azeotrope mixture, and a conventional extractive distillation process was designed
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Corresponding author. E-mail address: [email protected] (L. Sun).
https://doi.org/10.1016/j.seppur.2019.116290 Received 12 May 2019; Received in revised form 28 October 2019; Accepted 3 November 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Qi Pan, et al., Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116290
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Fig. 1. The flowsheet of CED process.
Fig. 2. The flowsheet of MSR process.
data, which could help to define components and conduct the simulation. Pereiro et al. [22] adopted 1-butyl-3-methylimidazolium methyl sulfate as an efficient ionic liquid entrainer to break the azeotrope system that consisted of heptane and ethanol. A laboratory-scale extractive distillation column was used to examine the utility of new solvent in the petrochemical process, and the comparison between experimental data and simulated data further illustrated the separation performance of ILs. The above researches suggest that the excellent properties of ILs lay the foundation for its application as solvents for extractive distillation process. However, there are very few reports on the design of control systems for extractive distillation processes using
ILs as entrainers. The supplementary work of this field will be the focus of this paper, which can help to promote the application of new solvents in future continuous production. The exploration of dynamic controllability for extractive distillation process is an essential part for ensuring optimal and safe operation. In particular, the investigation of a new control strategy for the complex extractive distillation system, such as multi-azeotrope distillation configuration, thermally coupled extractive distillation, and extractive dividing wall column, poses greater challenges for engineers due to the strong coupling and nonlinear characteristics of the system. Zhang et al. [23] used a kind of mixed entrainer to separate tetrahydrofuran-
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Fig. 3. The flowsheet of TCED process.
Fig. 4. The basic control scheme CS1 for CED process.
ethanol-water ternary mixture in an extractive distillation column, and different control structures were proposed to evaluate the anti-interference ability of the system. Through the simulation results, they found that the dynamic behavior of extractive distillation for separating ternary azeotropes was complex because there were many operational parameters and interactions in the system. Luyben [24] demonstrated
the controllability of the extractive distillation process for separating the ternary mixture and presented the control comparison of conventional and thermally coupled configuration in terms of dynamic performance. The results showed that there was an effective trade-off between steady-state economics and dynamic controllability. Three extractive distillation configurations, including conventional extractive
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Fig. 5. Dynamic responses of the CS1 control structure. (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 5. (continued)
Fig. 6. The improved control scheme CS2 for CED process.
distillation, extractive dividing wall column (EDWC), and side-stream extractive distillation column, were developed by Wang et al. [4]. The control schemes that correspond to the three arrangements were further studied in the dynamic environment. The response results indicated that economic benefits could be realized by developing complex processes without deterioration of control behavior. Tututi-Avila et al. [25]
explored the design and control of a novel EDWC arrangement, and the total annual cost (TAC) was selected as the objective function to optimize design parameters. The control structure with adjustable vapor split exhibited good dynamic responses for facing feed disturbances. Yang et al. [9] proposed a systematic approach for separating methanol-toluene-water heterogeneous mixtures by using EDWC. Aspen
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Fig. 7. Dynamic responses of the CS2 control structure. (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 7. (continued)
Fig. 8. The basic control scheme CS3 for MSR process.
Dynamics simulator was used to investigate the dynamic performance of the system, and the control structure with feed-forward ratio played a positive role in maintaining product concentration. In summary, the study of the dynamic behavior is a unique and complex issue when the new solvents, such as ILs, are selected to separate the ternary mixture in
an extractive distillation column. Some criteria of plant-wide control need to be considered to reduce the interactions among different operating units, and the exergy loss generated during operation should also be discussed to test the energy efficiency of the control system. Ethyl acetate is a multifunctional solvent that can be used for the
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Fig. 9. Dynamic responses of the CS3 control structure. (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 9. (continued)
production of coating material, synthetic fibers, and other chemical products. Ethyl acetate can be obtained from esterification reaction by using acetic acid and ethanol as raw materials, but it is difficult to produce high purity compounds because of the azeotrope of ethyl acetate-ethanol-water. Some researchers have explored the effects of different conventional solvents on the separation process [26,27]. However, traditional solvents for extractive distillation process generally have the disadvantages of low separation efficiency and poor environmental benefits. A new solvent named ILs, which has advantages of low melting point, low volatilization, easy recovery, good selectivity, and functional design, is developed to overcome this problem [28,29]. In this work, three extractive distillation processes, including conventional extractive distillation (CED), multistage solvent recovery
(MSR), and thermally coupled extractive distillation (TCED), are selected from previous work [30]. And 1-butyl-3-methylimidazolium acetate is selected as an efficient ionic liquid entrainer to separate ethyl acetate-ethanol-water ternary mixture by extractive distillation technology. Six control schemes (including basic and improved control strategies) are developed to evaluate the plant-wide dynamic controllability and flexibility of the proposed extractive distillation processes, and the dynamic performance is tested by adding four different feed conditions as disturbance variables. Then, the improved control schemes, which correspond to the three configurations, are selected to test the dynamic behavior of the system under complex disturbance conditions. The integral absolute error (IAE) and destroyed exergy are used as the evaluation indicators to examine the robustness and energy efficiency of the system. The optimal control scheme can be determined
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Fig. 10. The improved control scheme CS4 for MSR process.
through comprehensive comparison.
2.2. Multistage solvent recovery process
2. Steady-state process description
In the CED process, the operating temperature of EDC-2 is higher than that of EDC-1, and a large amount of entrainer is fed into the EDC2 through the transportation pipeline. The entrainer is not separated from the ethanol-water mixture obtained at the bottom of EDC-1, which leads to an increase in the energy consumption of EDC-2. Therefore, the MSR process, which adds a flash between the two extractive distillation columns to separate the excess entrainer, is shown in Fig. 2. Moreover, the liquid composition and temperature slope profiles are also plotted under steady-state conditions and shown in Figs. S3 and S4, respectively. The 27th stage of EDC-1 and the 32th stage of EDC-2 are selected as the temperature-sensitive trays for the dynamic processes.
2.1. Conventional process The conventional process and optimized parameters, which are shown in Fig. 1, are obtained by using Aspen Plus software for separating ethyl acetate-ethanol-water ternary mixture in an extractive distillation column. It is remarkable that the design parameters are slightly adjusted to get similar product specifications for the three extractive distillation processes, which helps to fairly compare different system performances in dynamic simulations. The CED flowsheet consists of two extractive distillation columns, a flash, and a buffer tank. The fresh feed flow rate is 100 kmol/h, and the temperature is 308.15 K. The feed composition consists of 53.58 mol% EtAc, 18.52 mol% EtOH, and 27.9 mol % H2O [31]. IL as another feed stream is introduced to the top of EDC-1 at a flow rate of 12.73 kmol/h. There is almost no loss of ILs during the separation process, and it can be fully recycled and reused. However, some special situations, including that the entrainer is leaked or contaminated, may occur during operation. The makeup solvent stream should be added to the intermediate buffer tank to compensate for the entrainer loss. The liquid composition profiles for CED process are shown in Fig. S1. The operating pressure of two extractive distillation columns is set at 1 atm, and the flash is operated under vacuum to ensure the stability of the entrainer. The temperature slope profiles of two extractive distillation columns are shown in Fig. S2. It can be seen that there is a very steep temperature gradient at the bottom of two columns respectively, which may cause the system to generate a large process gain. In other words, it may also cause a large change of the tray temperature when there is a slight reboiler heat input. Therefore, it is very important to maintain temperature stability at the bottom of two extractive distillation columns. Based on the slope criterion provided by Luyben [32], the 28th stage of EDC-1 and the 41th stage of EDC-2 can be used as the sensitive trays to indirectly adjust product quality.
2.3. Thermally coupled process The thermally coupled systems of distillation columns are more attractive in terms of operating costs and equipment investment than that of conventional processes, and this conclusion has been demonstrated in many literatures [10,11,24]. Therefore, the TCED process is designed for separating ethyl acetate-ethanol-water mixture, and the optimized flowsheet is shown in Fig. 3. The TCED process consists of a main column (MC), a side column (SC), a flash, and a buffer tank. The feed mixture and entrainer are fed into the main column at the 22th and 6th stages respectively, and side streams are arranged at the 38th stage. The products EtAc and EtOH are distilled from the MC and SC extractive distillation columns, respectively, and the product H2O is taken from the flash. The entrainer, which is obtained from the bottom of flash, is fed into the buffer tank and redistributed to the top of MC and SC. The liquid composition and temperature slope profiles are shown in Figs. S5 and S6. It can be seen that the temperature of MC has a large change at the 32th and 44th stages respectively, which indicates that the two stages can be used as candidate temperature-sensitive trays. There is no suitable one in the SC as a candidate tray used to adjust the temperature. The comparison of the steady-state design for the three processes, as
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Fig. 11. Dynamic responses of the CS4 control structure. (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 11. (continued)
shown in Table S1, indicates that TCED process is more economically attractive than the other two configurations. However, the integrated systems often increase the difficulty for developing the control systems. The comparison about dynamic performance of different processes should be investigated to promote the application of new solvents in the future continuous production.
arrangements. The response curves are drawn by using Aspen Dynamics and used to upgrade the control configuration. The purity of the products during the dynamic simulation process should be no less than 99.5 mol%, which is consistent with the specifications recorded in the literature [35]. 3.1. Control structure for CED
3. Control system design
The steady-state process is exported to Aspen Dynamics with pressure-driven simulation after adding the pumps and valves for the proposed processes. The appropriate size, which is generally set to provide 5 min of liquid holdup when the vessel reaches a half-full, needs to be determined for the column sump, reflux drum, flash, and buffer tank, respectively. In addition, some controlled variables, such as flow rate, level, pressure, concentration, and temperature, need to be maintained by installing the controllers. The conventional controllers with the gain
The dynamic behavioral studies of complex extractive distillation system can reveal the interactions among the different variables and assist us in predicting the risks inherent in the control process, which may help to reduce the operating costs and improve safety and economics of the process [33,34]. In this work, proportional-integral (PI) control schemes are established to evaluate the plant-wide dynamic controllability and flexibility of the three extractive distillation
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Fig. 12. The basic control scheme CS5 for TCED process.
of 2 and the integration time of 9999 min are employed to all level controllers. The parameters of the pressure controllers are specified as initial values, and the PI controllers are also used to control the throughput with the gain of 0.5 and the integration time of 0.3 min. The parameters of concentration and temperature, including ultimate periods and frequencies, are determined by the relay feedback method and Tyreus-Luyben tuning rules [32]. Remarkably, the deadtime blocks of 1 min and 3 min are inserted to each temperature and composition loop to approach the industrial situation. The tuning results of temperature and composition control loops for the six schemes are listed in Table S2. The basic scheme CS1 for CED process is shown in Fig. 4, and the details of the multivariable control system are as follows:
338.63 K (reverse acting). (10) The flow rate of makeup entrainer is manipulated to control the level of intermediate buffer tank (reverse acting). Note that the controllers for the entrainer feed flow rate are set to the initial state of the cascade, and the setpoint is adjusted at any time to resist the disturbance of the feed flow rate. Moreover, the makeup entrainer stream is arranged to deal with some special situations, including that the entrainer is leaked or contaminated. However, when the system is under normal operating conditions, the loss of the entrainer is negligible. The control valve used to compensate for the entrainer loss is set to the initial state of manual adjustment in the dynamic simulation process, which contributes to the integral calculation of the dynamic model. The size of intermediate buffer tank should be larger than the design value in order to resist the complex feed disturbances. The system introduces the disturbance variables after running for 1 h in the closed-loop state, and the dynamics test is completed at 10 h. The dynamic response curves of the control structure CS1 are shown in Fig. 5. When the ± 10% feed flow rate disturbance is added into the multi-variable control system, the purity of the three products changes smoothly with a small oscillation in 5 h. There is a small steady-state error between the new stable value and the setpoint, which does not affect the specification of the product. The overshoots of the three products are within an acceptable range, and the minimum concentration product is greater than 99.6 mol%. Observing the changes of the three controlled temperatures, it can be seen that the temperatures could be restored to the setpoint after 3 h, and there is no steady-state error for each control loop. The temperature of 41th stage in EDC-2, which may be one of the key parameters, has a much higher overshoot than other controlled variables. The reason may be that the entrainer is not separated from the mixture obtained at the bottom of EDC-1, which
(1) The flow control valve is employed to adjust the flow rate of the ethyl acetate-ethanol-water mixture (reverse acting). (2) The total entrainer flow rate of EDC-1 is manipulated by the ratio of solvent to fresh feed flow rate, and the entrainer flow rate of EDC-2 is proportional to the bottom flow rate of EDC-1. (3) The heat duty of EDC-1 and EDC-2 are used as manipulated variables to adjust the temperature of 28th stage and 41th stage (reverse acting), respectively. (4) The condenser duty and operating pressure constitute a pressure control loop (reverse acting). (5) The distillate flow rate is used to control the reflux drum level (direct acting). (6) The sump level is manipulated via the control of the bottom flow rate (direct acting). (7) The reflux ratios in EDC-1 and EDC-2 are fixed. (8) A temperature controller (reverse acting), a liquid level controller (direct acting), and a pressure controller (reverse acting) are added to the flash. (9) The temperatures of recycled entrainer streams are controlled at
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Fig. 13. Dynamic responses of the CS5 control structure. (a) ± 10% feed flow rate disturbance; (b) +10% and −5% feed composition disturbance.
has a large influence on the temperature of sensitive tray in EDC-2 after introducing feed disturbances. When the ± 10% feed composition disturbance is added into the multi-variable control system, the large overshoot and steady-state error are generated with the purity of each product. Especially when the system is disturbed by −10% feed composition disturbance, the difference between the stabilized purity and setpoint is about 0.7 mol%, which makes the purity unable to meet the product specification. Observing the system response curves of the temperature, all these sensitive tray temperatures also return to their setpoints and recover smoothly in about 3 h. The above analysis shows that the CS1 control structure can effectively resist the disturbance of the feed flow rate, but the system lacks direct and effective control loops to resist the feed composition disturbance. Therefore, it is necessary to further improve the control performance of CS1 by using some dynamic intensification methods.
In order to solve the problem that the CS1 scheme cannot resist the disturbance of the feed composition, the improved control scheme CS2 is developed for CED process. Based on the CS1 control structure, two feed-forward control loops are added for CS2 to assist in adjusting the bottom temperatures of the two extractive distillation columns. The reflux ratios are corrected by two concentration controllers, which helps to maintain product purity when introducing feed composition disturbance. The improved control structure CS2 is shown in Fig. 6. After introducing ± 10% feed flow rate and ± 10% feed composition disturbances, the dynamic response curves of the CS2 system are shown in Fig. 7. When facing the flow rate disturbance, all the controlled variables can reach the steady-state again after 4 h, and the new stable values are the same as the setpoints except the purity of the water. The overshoots of three products meet the requirements for purity specifications, which means that the minimum product purity is greater than the value provided in the literature [35]. Due to the
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Fig. 13. (continued)
addition of the feed-forward control structure, the temperature stability of EDC-1 and EDC-2 is significantly improved, and the overshoot of 41th stage temperature in EDC-2 is reduced from the original 6–2 K. When facing the feed composition disturbance, the purity of EtAc product is adjusted for a longer period and returns to the steady-state again after about 10 h. The reason may be that there is a small reflux ratio of EDC-1 used to control product purity, which can only provide the limited improvement in terms of control performance. Even if the concentration controllers are added for the system to correct the value of reflux ratio, the dynamic responses of the multivariable control system are still delayed after inputting disturbance variables. Nevertheless, there is no steady-state error in EtAc purity when the system returns to a new steady-state, which indicates that the improved control structure CS2 plays a positive role in maintaining product concentration. In summary, the CS2 control structure has been improved in resistance to feed composition disturbance and can be selected as the
optimal control strategy for the CED process. 3.2. Control structure for MSR Based on the characteristics of MSR process, the basic control scheme CS3 is developed, as shown in Fig. 8. The CS3 control structure is established by adding some control loops under the framework of the CS1 scheme, and the new details are as follows: (1) The flash-1 is arranged between the two extractive distillation columns to separate the excess entrainer. A temperature controller (reverse acting), a liquid level controller (direct acting), and a pressure controller (reverse acting) are added to the flash-1. (2) The heat duties of EDC-1 and EDC-2 are used as manipulated variables to adjust the temperature of 27th stage and 32th stage (reverse acting), respectively.
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Fig. 14. The improved control scheme CS6 for TCED process.
(3) Two entrainer streams are taken from the bottom of the intermediate buffer tank. One of the streams is mixed with the recovery stream of flash-2 and fed into EDC-1. The total entrainer flow rate of EDC-1 is manipulated by the ratio of solvent to fresh feed flow rate, and the flow rate from the intermediate buffer tank is controlled by the difference between the total entrainer flow rate and the recovery solvent flow rate. Another stream is fed into EDC-2 after passing through the cooling system, and its flow rate is proportional to the top flow rate of flash-1.
performance, and the response curves of CS4 are shown in Fig. 11. When adding the disturbance of ± 10% to the feed flow rate, it can be seen that the purity of EtAc is maintained quite close to the specified value. There is a small steady-state error in the purity of EtOH and H2O respectively, which meets product quality requirements within an acceptable range. By bringing the temperatures back to the setpoints, the control system can handle the disturbances well. When adding the disturbance of ± 10% to the feed composition, the purity of EtAc product is adjusted for a longer period and returns to the steady-state again after about 8 h. However, there is no steady-state error in EtAc purity when the system returns to a new steady-state. The minimum purity meets the design specification, which benefits from the addition of the feed-forward control structures and concentration controllers. In summary, the improved control scheme CS4 for MSR process performs well in terms of robust stability and controllability.
For the dynamic simulation of CS3 shown in Fig. 9, ± 10% feed flow rate and feed composition disturbances are used as input step changes for the dynamic model. When the feed flow rate changes, the dynamic responses of the product purity have significant improvements in terms of overshoot and steady-state error compared with the CS1 control strategy, which may benefit from the addition of flash-1 used to effectively offset feed disturbances. This conclusion can also be obtained from the temperature variation of sensitive tray which is reduced from 6 K to 3 K in EDC-2. However, when the feed composition changes, the responses show poor dynamic performance, especially for that of the product purity. There are extremely large overshoots in the system, and the maximum value is about 0.5 mol%. The purity of EtAc product has a large steady-state error, and the minimum purity is 99.46 mol% which does not meet the product specification. It is necessary to further upgrade the CS3 control scheme. The CS4 control structure, as shown in Fig. 10, is established under the framework of the basic control structure for MSR process. Two feedforward control loops are added to assist in adjusting the bottom temperatures of the two extractive distillation columns. The reflux ratios are corrected by two concentration controllers, which helps to maintain product purity when introducing feed composition disturbance. Different feed disturbances are added to test the dynamic
3.3. Control structure for TCED Compared with the conventional extractive distillation process, the thermally coupled system is more attractive in terms of operating costs and equipment investment. However, due to the strong coupling and nonlinear characteristics, the control scheme for this complex multivariable system is very difficult to design. In this paper, the basic control scheme CS5, as shown in Fig. 12, is developed for the TCED process, and the details of the control structure are as follows: (1) The total entrainer flow rate of MC is manipulated by the ratio of solvent to fresh feed flow rate, and the entrainer flow rate of SC is proportional to side flow rate produced from the 38th stage of MC. (2) The heat duty of MC is used as a manipulated variable to adjust the temperature of 44th stage (reverse acting). (3) The setpoint of side flow rate is adjusted by the temperature of 32th
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Fig. 15. Dynamic responses of the CS6 control structure. (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
stage in MC.
product. The dynamic response of EtAc product performs poorly in terms of overshoot and steady-state error, and the re-stable value of the product purity is less than design specification. Some feasible control structures should be developed to achieve system upgrades. For a thermally coupled distillation configuration, the flow rate of the interconnecting stream is a key variable to ensure system stability. Following the suggestion of Luyben [32], the 32th stage temperature and fresh feed flow rate are used to adjust the side flow rate. The feedforward control loops and the concentration controllers are adopted for the improved control structure CS6 which is shown in Fig. 14. Different kinds of step signals, including the feed flow rate and feed composition disturbances, are introduced to evaluate the dynamic controllability. The dynamic responses, as shown in Fig. 15, indicate that the structure is significantly upgraded in terms of overshoot, settling time, and steady-state error. The control system can effectively handle the ± 10% feed composition disturbance, which shows that the CS6 scheme has better dynamic controllability than CS5, and it should
The dynamic response curves of CS5 are illustrated in Fig. 13. When the ± 10% feed flow rate disturbance is added to the system, the control effect on the quality of EtAc and EtOH products is satisfactory, and the minimum product purity is greater than 99.8 mol%. However, the dynamic response curves for water product do not meet specifications in terms of steady-state error and overshoot. Observing the temperature response curves, it is found that the temperature of the three sensitive trays can be restored to the original values after a short settling time. And the maximum overshoot is less than 4 K, which indicates that the two-point temperature structure plays a positive role in the control process. When the +10% feed composition disturbance is added to the system, there is a considerable overshoot for EtOH product, which correspond to the minimum product purity of 98.2 mol%. After adding a disturbance of −10% to the feed composition disturbance, the dynamic system shows an unacceptable steady-state error for EtAc
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Fig. 15. (continued)
be selected as the preferred control strategy for TCED process.
IAE value generated during the dynamic control process [36–38]. And the dynamic response curves of the multivariable control systems are shown in Fig. 16. When the control systems face complex disturbance conditions, it can be seen that the change of feed composition has a greater impact on the system than that of feed flow rate. After adding +20% composition disturbance, the CS6 control scheme shows a large overshoot in the purity of EtOH, and it is about 9 mol%. Moreover, the CS6 control strategy also exhibits poor dynamic performance in maintaining H2O product specification. When the EtAc composition of feed stream is restored from the +20% step value to the original value, there is a large overshoot of 1.4 mol% in EtAc product by observing the dynamic response curves of CS4, but the settling time is very short. Compared with the other two control strategies, the CS2 scheme exhibits worse dynamic responses in terms of frequency regulation. The calculation results of the total IAE value, as shown in Fig. 17, indicate that the MSR process exhibits the best performance characteristics in
4. Comparison and discussion In order to compare the dynamic characteristics of the three extractive distillation configurations, the improved control schemes, which correspond to the three extractive distillation configurations, are selected to evaluate the plant-wide dynamic behavior of those arrangements under complex disturbance conditions. The IAE and destroyed exergy, as shown in Table S3, are calculated to examine the robustness and energy efficiency of the system. Different input variables, including the ± 20% feed flow rate disturbance and ± 20% feed composition disturbance, are added to the control systems after running for one hour in the dynamic environment, and the type of disturbance is changed every two hours. The purity is taken as investigation variable to calculate the total
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Fig. 18. Comparison of different control strategies in terms of the destroyed exergy.
terms of controllability or operability compared with the other two flowsheets. This may benefit from the addition of flash-1 used to purify entrainers and effectively offset feed disturbances. Exergy can be used as an indicator to evaluate the quality and efficiency of the heating source, and the calculation of the destroyed exergy generated under dynamic operating conditions can help engineers to understand the law of system energy change more comprehensively [39–41]. The destroyed exergy of the three improved control schemes under complex disturbances is shown in Fig. 18. When the control systems face feed flow rate disturbance, the response curves of the destroyed exergy have the same trend, indicating that the feed flow rate has a linear relationship to the exergy of multivariable system. After adding composition disturbance, the destroyed exergy of the CS2 and CS6 control schemes have greater fluctuations, which results in more exergy being consumed during operation. From the perspective of dynamic response results, a good control scheme will play a positive role in the energy efficiency of the system. The total destroyed exergy can be obtained by calculating the area between the response curves and the coordinate axes. The total exergy consumed by the CS2, CS4, and CS6 control schemes during the test period of 10 h are 4016 kW, 4387 kW, and 4748 kW, respectively. It is worth noting that the TCED process is advantageous in terms of TAC, but there is no advantage in terms of exergy loss. The reason is that the TCED configuration has only one reboiler, which may cause the system to consume more high-grade energy sources [42]. Considering the controllability and energy efficiency of dynamic processes, the plant-wide control strategy of MSR process is satisfactory and should be screened as the best scheme for separating ethyl acetate-ethanol-water ternary mixture.
Fig. 16. Dynamic responses of the product purity for the three extractive distillation configurations under complex disturbance conditions.
5. Conclusion In this paper, three extractive distillation configurations are selected to investigate the suitability of the new solvent for continuous production. Six control schemes are established to evaluate the plant-wide dynamic controllability and flexibility of those arrangements, and the dynamic performance is tested by adding four different feed disturbances as input variables. Then, the improved control schemes, which correspond to the three configurations, are screened to assess the dynamic behavior of the system under complex disturbance conditions. The results show that the plant-wide dynamic responses of MSR process are satisfactory with better anti-interference ability and dynamic behavior, and the total destroyed exergy of CED process is relatively small during the operation time. Although the TCED process is satisfactory in terms of TAC results, it performs poorly in terms of control performance and exergy loss under the dynamic environment. The study suggests that there is an effective trade-off between steady-state economy and dynamic controllability.
Fig. 17. Comparison of different control strategies in terms of IAE.
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Declaration of Competing Interest
separation of acetonitrile and water, Ind. Eng. Chem. Res. (2019). [18] I. Díaz, J. Palomar, M. Rodríguez, J. de Riva, V. Ferro, E.J. González, Ionic liquids as entrainers for the separation of aromatic–aliphatic hydrocarbon mixtures by extractive distillation, Chem. Eng. Res. Des. 115 (2016) 382–393. [19] H. Chen, M. Chen, B. Chen, I.L. Chien, Critical assessment of using an ionic liquid as entrainer via extractive distillation, Ind. Eng. Chem. Res. 56 (2017) 7768–7782. [20] Y. Hu, Y. Su, S. Jin, I.L. Chien, W. Shen, Systematic approach for screening organic and ionic liquid solvents in homogeneous extractive distillation exemplified by the tert-butanol dehydration, Sep. Purif. Technol. 211 (2019) 723–737. [21] Z. Zhu, J. Hu, X. Geng, B. Qin, K. Ma, Y. Wang, J. Gao, Process design of carbon dioxide and ethane separation using ionic liquid by extractive distillation, J. Chem. Technol. Biotechnol. 93 (2018) 887–896. [22] A.B. Pereiro, A. Rodríguez, Azeotrope-breaking using [BMIM] [MeSO4] ionic liquid in an extraction column, Sep. Purif. Technol. 62 (2008) 733–738. [23] X. Zhang, Y. Zhao, H. Wang, B. Qin, Z. Zhu, N. Zhang, Y. Wang, Control of a ternary extractive distillation process with recycle splitting using a mixed entrainer, Ind. Eng. Chem. Res. 57 (2018) 339–351. [24] W.L. Luyben, Control comparison of conventional and thermally coupled ternary extractive distillation processes, Chem. Eng. Res. Des. 106 (2016) 253–262. [25] S. Tututi-Avila, A. Jiménez-Gutiérrez, J. Hahn, Control analysis of an extractive dividing-wall column used for ethanol dehydration, Chem. Eng. Process. 82 (2014) 88–100. [26] A. Yang, H. Zou, I.L. Chien, D. Wang, S. Wei, J. Ren, W. Shen, Optimal design and effective control of triple-column extractive distillation for separating ethyl acetate/ ethanol/water with multiazeotrope, Ind. Eng. Chem. Res. 58 (2019) 7265–7283. [27] A.J. Toth, Comprehensive evaluation and comparison of advanced separation methods on the separation of ethyl acetate-ethanol-water highly non-ideal mixture, Sep. Purif. Technol. 224 (2019) 490–508. [28] V. Gerbaud, I. Rodriguez-Donis, L. Hegely, P. Lang, F. Denes, X. You, Review of extractive distillation. Process design, operation, optimization and control, Chem. Eng. Res. Des. 141 (2019) 229–271. [29] W.L. Luyben, Improved design of an extractive distillation system with an intermediate-boiling solvent, Sep. Purif. Technol. 156 (2015) 336–347. [30] S. Ma, X. Shang, L. Li, Y. Song, Q. Pan, L. Sun, Energy-saving thermally coupled ternary extractive distillation process using ionic liquids as entrainer for separating ethyl acetate-ethanol-water ternary mixture, Sep. Purif. Technol. 226 (2019) 337–349. [31] J. Yang, M. Zhou, Y. Wang, X. Zhang, G. Wu, Simulation of pressure-swing distillation for separation of ethyl acetate-ethanol-water, IOP Conf. Series: Mater. Sci. Eng. 274 (2017) 012026. [32] W.L. Luyben, Distillation Design and Control using Aspen Simulation, John Wiley & Sons, New York, 2013. [33] A. Yang, W. Shen, S.A. Wei, L. Dong, J. Li, V. Gerbaud, Design and control of pressure-swing distillation for separating ternary systems with three binary minimum azeotropes, AlChE J. 65 (2019) 1281–1293. [34] Q. Pan, J. Li, X. Shang, S. Ma, J. Liu, M. Sun, L. Sun, Controllability, energy-efficiency, and safety comparisons of different control schemes for producing n-butyl acetate in a reactive dividing wall column, Ind. Eng. Chem. Res. 58 (2019) 9675–9689. [35] M.A. Santaella, A. Orjuela, P.C. Narváez, Comparison of different reactive distillation schemes for ethyl acetate production using sustainability indicators, Chem. Eng. Process. 96 (2015) 1–13. [36] Q. Pan, X. Shang, J. Li, S. Ma, L. Li, L. Sun, Energy-efficient separation process and control scheme for extractive distillation of ethanol-water using deep eutectic solvent, Sep. Purif. Technol. 219 (2019) 113–126. [37] Q. Zhang, M. Liu, C. Li, A. Zeng, Design and control of extractive distillation process for separation of the minimum-boiling azeotrope ethyl-acetate and ethanol, Chem. Eng. Res. Des. 136 (2018) 57–70. [38] A.A. Kiss, R.R. Rewagad, Energy efficient control of a BTX dividing-wall column, Comput. Chem. Eng. 35 (2011) 2896–2904. [39] B. Suphanit, A. Bischert, P. Narataruksa, Exergy loss analysis of heat transfer across the wall of the dividing-wall distillation column, Energy 32 (2007) 2121–2134. [40] M.T. Munir, W. Yu, B.R. Young, Recycle effect on the relative exergy array, Chem. Eng. Res. Des. 90 (2012) 110–118. [41] M.T. Munir, W. Yu, B.P. Young, Eco-efficiency and control loop configuration for recycle systems, Korean J. Chem. Eng. 30 (2013) 997–1007. [42] Y.C. Wu, P.H. Hsu, I.L. Chien, Critical assessment of the energy-saving potential of an extractive dividing-wall column, Ind. Eng. Chem. Res. 52 (2013) 5384–5399.
The authors declared that there is no conflict of interest. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant: 21476261 and Grant: 21676299). The authors are grateful to the editor and the anonymous reviewers. Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.116290. References [1] Z. Lei, C. Li, B. Chen, Extractive distillation: a review, Sep. Purif. Rev. 32 (2003) 121–213. [2] S. Tututi-Avila, N. Medina-Herrera, J. Hahn, A. Jiménez-Gutiérrez, Design of an energy-efficient side-stream extractive distillation system, Comput. Chem. Eng. 102 (2017) 17–25. [3] K. Ma, M. Yu, Y. Dai, Y. Ma, J. Gao, P. Cui, Y. Wang, Control of an energy-saving side-stream extractive distillation process with different disturbance conditions, Sep. Purif. Technol. 210 (2019) 195–208. [4] C. Wang, C. Wang, Y. Cui, C. Guang, Z. Zhang, Economics and controllability of conventional and intensified extractive distillation configurations for acetonitrile/ methanol/benzene, Ind. Eng. Chem. Res. 57 (2018) 10551–10563. [5] W. Li, L. Shi, B. Yu, M. Xia, J. Luo, H. Shi, C. Xu, New pressure-swing distillation for separating pressure-insensitive maximum boiling azeotrope via introducing a heavy entrainer: design and control, Ind. Eng. Chem. Res. 52 (2013) 7836–7853. [6] J.P. Knapp, M.F. Doherty, A new pressure-swing-distillation process for separating homogeneous azeotropic mixtures, Ind. Eng. Chem. Res. 31 (1992) 346–357. [7] A.J. Toth, E. Haaz, T. Nagy, R. Tari, A.J. Tarjani, D. Fozer, A. Szanyi, K.-A. Koczka, L. Racz, G. Ugro, P. Mizsey, Evaluation of the accuracy of modelling the separation of highly non-ideal mixtures: extractive heterogeneous-azeotropic distillation, A. Espuña, M. Graells, L. Puigjaner (Eds.), Computer Aided Chemical Engineering, Elsevier, 2017, pp. 241–246. [8] C. Yi, W. Huang, I.L. Chien, Energy-efficient heterogeneous extractive distillation system for the separation of close-boiling cyclohexane/cyclohexene mixture, J. Taiwan Inst. Chem. Eng. 87 (2018) 26–35. [9] A. Yang, R. Wei, S. Sun, S.a. Wei, W. Shen, I.L. Chien, Energy-saving optimal design and effective control of heat integration-extractive dividing wall column for separating heterogeneous mixture methanol/toluene/water with multiazeotropes, Ind. Eng. Chem. Res. 57 (2018) 8036–8056. [10] Y. Zhao, K. Ma, W. Bai, D. Du, Z. Zhu, Y. Wang, J. Gao, Energy-saving thermally coupled ternary extractive distillation process by combining with mixed entrainer for separating ternary mixture containing bioethanol, Energy 148 (2018) 296–308. [11] C. Wang, C. Guang, Y. Cui, C. Wang, Z. Zhang, Compared novel thermally coupled extractive distillation sequences for separating multi-azeotropic mixture of acetonitrile/benzene/methanol, Chem. Eng. Res. Des. 136 (2018) 513–528. [12] I. Dejanović, L. Matijašević, Ž. Olujić, Dividing wall column—a breakthrough towards sustainable distilling, Chem. Eng. Process. 49 (2010) 559–580. [13] Ö. Yildirim, A.A. Kiss, E.Y. Kenig, Dividing wall columns in chemical process industry: a review on current activities, Sep. Purif. Technol. 80 (2011) 403–417. [14] Z. Feng, W. Shen, G.P. Rangaiah, L. Dong, Proportional-integral control and model predictive control of extractive dividing-wall column based on temperature differences, Ind. Eng. Chem. Res. 57 (2018) 10572–10590. [15] H. Zhang, Q. Ye, J. Qin, H. Xu, N. Li, Design and control of extractive dividing-wall column for separating ethyl acetate–isopropyl alcohol mixture, Ind. Eng. Chem. Res. 53 (2014) 1189–1205. [16] Z. Lei, C. Dai, J. Zhu, B. Chen, Extractive distillation with ionic liquids: a review, AlChE J. 60 (2014) 3312–3329. [17] J. Li, T. Li, C. Peng, H. Liu, Extractive distillation with ionic liquid entrainers for the
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Control comparison of extractive distillation configurations for separating ethyl acetate-ethanol-water ternary mixture using ionic liquids as entrainer Qi Pan, Xianyong Shang, Shoutao Ma, Jie Li, Yunfei Song, Mengying Sun, Jiyan Liu, Lanyi Sun
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State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum (East China), Qingdao, Shandong 266580, China
ARTICLE INFO
ABSTRACT
Keywords: Extractive distillation Dynamic simulation Ionic liquids Control structure Exergy analysis
The new extractive distillation configurations, which use ionic liquids (ILs) as entrainer to develop green processes, have attracted widespread attention among scholars. Design and comparison of different process control schemes can facilitate the application of new solvents in future continuous production. In this paper, three extractive distillation configurations, including conventional extractive distillation (CED), multistage solvent recovery (MSR), and thermally coupled extractive distillation (TCED), are selected from previous work. The separation system of ethyl acetate-ethanol-water ternary mixture is studied, and 1-butyl-3-methylimidazolium acetate is used as an efficient entrainer. In order to compare the dynamic characteristics, six proportionalintegral control schemes are established to evaluate the plant-wide control performance of those arrangements. Then, the improved control schemes, which correspond to the three extractive distillation configurations, are selected to test the dynamic behavior under complex disturbance conditions. Integral absolute error and destroyed exergy are calculated to examine the robustness and energy efficiency of the system. The results show that the plant-wide dynamic responses of MSR process are satisfactory in terms of product purity, and the total destroyed exergy of CED process is relatively small during the operation period. There is an effective trade-off between controllability and energy efficiency, and the optimal control scheme should be determined based on actual conditions.
1. Introduction
by using Aspen Plus software. Compared with traditional solvent (ethylene glycol), ILs had obvious advantages in terms of solvent ratios and overall heat duties. Díaz et al. [18] applied a multiscale methodology to integrating molecular modeling and process simulation, which could help to screen suitable solvents for extractive distillation. Based on this research framework, the ionic liquid of 1-ethyl-3-methyl imidazolium dicyanamide was selected as an entrainer to separate aromatic–aliphatic hydrocarbon mixtures. To illustrate the economics of using ILs as entrainer, Chen et al. [19] developed two separation systems in an extractive distillation column, respectively, which corresponded to acetone-methanol and isopropyl alcohol-water azeotropic mixture. The results indicated that the trade-off between ILs and industrial entrainers required detailed economic evaluation. Hu et al. [20] proposed a systematic approach to screen suitable ILs for separating specific mixtures. The tert-butanol dehydration process was selected as a research case to test the separation performance of different solvents by extractive distillation. Zhu et al. [21] explored the separation of carbon dioxide-ethane system, and three ILs were used as entrainers for extractive distillation process to evaluate thermodynamic efficiency. The properties of ILs were obtained from the experimental
As a special thermal separation technique, extractive distillation has become an effective separation method for azeotropic or close-boiling compounds. The two main factors, which include process design and solvent (or entrainer) selection, can significantly affect separation efficiency and energy consumption of the extractive distillation process [1]. Before determining the separation strategy for the specified mixture, the feasibility of various intensification extractive distillation techniques should be examined to further maximize economic and environmental benefits, such as side-stream extractive distillation [2–4], pressure-swing extractive distillation [5,6], extractive heterogeneous azeotropic distillation [7–9], thermally coupled extractive distillation [10,11], and extractive dividing wall column configuration [12–15]. The new extractive distillation configurations, which use ionic liquids (ILs) as entrainer to develop green processes, have attracted widespread attention among scholars [16]. Li et al. [17] studied the effects of different ILs for separating acetonitrile and water azeotrope mixture, and a conventional extractive distillation process was designed
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Corresponding author. E-mail address: [email protected] (L. Sun).
https://doi.org/10.1016/j.seppur.2019.116290 Received 12 May 2019; Received in revised form 28 October 2019; Accepted 3 November 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Qi Pan, et al., Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116290
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Fig. 1. The flowsheet of CED process.
Fig. 2. The flowsheet of MSR process.
data, which could help to define components and conduct the simulation. Pereiro et al. [22] adopted 1-butyl-3-methylimidazolium methyl sulfate as an efficient ionic liquid entrainer to break the azeotrope system that consisted of heptane and ethanol. A laboratory-scale extractive distillation column was used to examine the utility of new solvent in the petrochemical process, and the comparison between experimental data and simulated data further illustrated the separation performance of ILs. The above researches suggest that the excellent properties of ILs lay the foundation for its application as solvents for extractive distillation process. However, there are very few reports on the design of control systems for extractive distillation processes using
ILs as entrainers. The supplementary work of this field will be the focus of this paper, which can help to promote the application of new solvents in future continuous production. The exploration of dynamic controllability for extractive distillation process is an essential part for ensuring optimal and safe operation. In particular, the investigation of a new control strategy for the complex extractive distillation system, such as multi-azeotrope distillation configuration, thermally coupled extractive distillation, and extractive dividing wall column, poses greater challenges for engineers due to the strong coupling and nonlinear characteristics of the system. Zhang et al. [23] used a kind of mixed entrainer to separate tetrahydrofuran-
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Fig. 3. The flowsheet of TCED process.
Fig. 4. The basic control scheme CS1 for CED process.
ethanol-water ternary mixture in an extractive distillation column, and different control structures were proposed to evaluate the anti-interference ability of the system. Through the simulation results, they found that the dynamic behavior of extractive distillation for separating ternary azeotropes was complex because there were many operational parameters and interactions in the system. Luyben [24] demonstrated
the controllability of the extractive distillation process for separating the ternary mixture and presented the control comparison of conventional and thermally coupled configuration in terms of dynamic performance. The results showed that there was an effective trade-off between steady-state economics and dynamic controllability. Three extractive distillation configurations, including conventional extractive
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Fig. 5. Dynamic responses of the CS1 control structure. (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 5. (continued)
Fig. 6. The improved control scheme CS2 for CED process.
distillation, extractive dividing wall column (EDWC), and side-stream extractive distillation column, were developed by Wang et al. [4]. The control schemes that correspond to the three arrangements were further studied in the dynamic environment. The response results indicated that economic benefits could be realized by developing complex processes without deterioration of control behavior. Tututi-Avila et al. [25]
explored the design and control of a novel EDWC arrangement, and the total annual cost (TAC) was selected as the objective function to optimize design parameters. The control structure with adjustable vapor split exhibited good dynamic responses for facing feed disturbances. Yang et al. [9] proposed a systematic approach for separating methanol-toluene-water heterogeneous mixtures by using EDWC. Aspen
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Fig. 7. Dynamic responses of the CS2 control structure. (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 7. (continued)
Fig. 8. The basic control scheme CS3 for MSR process.
Dynamics simulator was used to investigate the dynamic performance of the system, and the control structure with feed-forward ratio played a positive role in maintaining product concentration. In summary, the study of the dynamic behavior is a unique and complex issue when the new solvents, such as ILs, are selected to separate the ternary mixture in
an extractive distillation column. Some criteria of plant-wide control need to be considered to reduce the interactions among different operating units, and the exergy loss generated during operation should also be discussed to test the energy efficiency of the control system. Ethyl acetate is a multifunctional solvent that can be used for the
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Fig. 9. Dynamic responses of the CS3 control structure. (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 9. (continued)
production of coating material, synthetic fibers, and other chemical products. Ethyl acetate can be obtained from esterification reaction by using acetic acid and ethanol as raw materials, but it is difficult to produce high purity compounds because of the azeotrope of ethyl acetate-ethanol-water. Some researchers have explored the effects of different conventional solvents on the separation process [26,27]. However, traditional solvents for extractive distillation process generally have the disadvantages of low separation efficiency and poor environmental benefits. A new solvent named ILs, which has advantages of low melting point, low volatilization, easy recovery, good selectivity, and functional design, is developed to overcome this problem [28,29]. In this work, three extractive distillation processes, including conventional extractive distillation (CED), multistage solvent recovery
(MSR), and thermally coupled extractive distillation (TCED), are selected from previous work [30]. And 1-butyl-3-methylimidazolium acetate is selected as an efficient ionic liquid entrainer to separate ethyl acetate-ethanol-water ternary mixture by extractive distillation technology. Six control schemes (including basic and improved control strategies) are developed to evaluate the plant-wide dynamic controllability and flexibility of the proposed extractive distillation processes, and the dynamic performance is tested by adding four different feed conditions as disturbance variables. Then, the improved control schemes, which correspond to the three configurations, are selected to test the dynamic behavior of the system under complex disturbance conditions. The integral absolute error (IAE) and destroyed exergy are used as the evaluation indicators to examine the robustness and energy efficiency of the system. The optimal control scheme can be determined
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Fig. 10. The improved control scheme CS4 for MSR process.
through comprehensive comparison.
2.2. Multistage solvent recovery process
2. Steady-state process description
In the CED process, the operating temperature of EDC-2 is higher than that of EDC-1, and a large amount of entrainer is fed into the EDC2 through the transportation pipeline. The entrainer is not separated from the ethanol-water mixture obtained at the bottom of EDC-1, which leads to an increase in the energy consumption of EDC-2. Therefore, the MSR process, which adds a flash between the two extractive distillation columns to separate the excess entrainer, is shown in Fig. 2. Moreover, the liquid composition and temperature slope profiles are also plotted under steady-state conditions and shown in Figs. S3 and S4, respectively. The 27th stage of EDC-1 and the 32th stage of EDC-2 are selected as the temperature-sensitive trays for the dynamic processes.
2.1. Conventional process The conventional process and optimized parameters, which are shown in Fig. 1, are obtained by using Aspen Plus software for separating ethyl acetate-ethanol-water ternary mixture in an extractive distillation column. It is remarkable that the design parameters are slightly adjusted to get similar product specifications for the three extractive distillation processes, which helps to fairly compare different system performances in dynamic simulations. The CED flowsheet consists of two extractive distillation columns, a flash, and a buffer tank. The fresh feed flow rate is 100 kmol/h, and the temperature is 308.15 K. The feed composition consists of 53.58 mol% EtAc, 18.52 mol% EtOH, and 27.9 mol % H2O [31]. IL as another feed stream is introduced to the top of EDC-1 at a flow rate of 12.73 kmol/h. There is almost no loss of ILs during the separation process, and it can be fully recycled and reused. However, some special situations, including that the entrainer is leaked or contaminated, may occur during operation. The makeup solvent stream should be added to the intermediate buffer tank to compensate for the entrainer loss. The liquid composition profiles for CED process are shown in Fig. S1. The operating pressure of two extractive distillation columns is set at 1 atm, and the flash is operated under vacuum to ensure the stability of the entrainer. The temperature slope profiles of two extractive distillation columns are shown in Fig. S2. It can be seen that there is a very steep temperature gradient at the bottom of two columns respectively, which may cause the system to generate a large process gain. In other words, it may also cause a large change of the tray temperature when there is a slight reboiler heat input. Therefore, it is very important to maintain temperature stability at the bottom of two extractive distillation columns. Based on the slope criterion provided by Luyben [32], the 28th stage of EDC-1 and the 41th stage of EDC-2 can be used as the sensitive trays to indirectly adjust product quality.
2.3. Thermally coupled process The thermally coupled systems of distillation columns are more attractive in terms of operating costs and equipment investment than that of conventional processes, and this conclusion has been demonstrated in many literatures [10,11,24]. Therefore, the TCED process is designed for separating ethyl acetate-ethanol-water mixture, and the optimized flowsheet is shown in Fig. 3. The TCED process consists of a main column (MC), a side column (SC), a flash, and a buffer tank. The feed mixture and entrainer are fed into the main column at the 22th and 6th stages respectively, and side streams are arranged at the 38th stage. The products EtAc and EtOH are distilled from the MC and SC extractive distillation columns, respectively, and the product H2O is taken from the flash. The entrainer, which is obtained from the bottom of flash, is fed into the buffer tank and redistributed to the top of MC and SC. The liquid composition and temperature slope profiles are shown in Figs. S5 and S6. It can be seen that the temperature of MC has a large change at the 32th and 44th stages respectively, which indicates that the two stages can be used as candidate temperature-sensitive trays. There is no suitable one in the SC as a candidate tray used to adjust the temperature. The comparison of the steady-state design for the three processes, as
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Fig. 11. Dynamic responses of the CS4 control structure. (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 11. (continued)
shown in Table S1, indicates that TCED process is more economically attractive than the other two configurations. However, the integrated systems often increase the difficulty for developing the control systems. The comparison about dynamic performance of different processes should be investigated to promote the application of new solvents in the future continuous production.
arrangements. The response curves are drawn by using Aspen Dynamics and used to upgrade the control configuration. The purity of the products during the dynamic simulation process should be no less than 99.5 mol%, which is consistent with the specifications recorded in the literature [35]. 3.1. Control structure for CED
3. Control system design
The steady-state process is exported to Aspen Dynamics with pressure-driven simulation after adding the pumps and valves for the proposed processes. The appropriate size, which is generally set to provide 5 min of liquid holdup when the vessel reaches a half-full, needs to be determined for the column sump, reflux drum, flash, and buffer tank, respectively. In addition, some controlled variables, such as flow rate, level, pressure, concentration, and temperature, need to be maintained by installing the controllers. The conventional controllers with the gain
The dynamic behavioral studies of complex extractive distillation system can reveal the interactions among the different variables and assist us in predicting the risks inherent in the control process, which may help to reduce the operating costs and improve safety and economics of the process [33,34]. In this work, proportional-integral (PI) control schemes are established to evaluate the plant-wide dynamic controllability and flexibility of the three extractive distillation
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Fig. 12. The basic control scheme CS5 for TCED process.
of 2 and the integration time of 9999 min are employed to all level controllers. The parameters of the pressure controllers are specified as initial values, and the PI controllers are also used to control the throughput with the gain of 0.5 and the integration time of 0.3 min. The parameters of concentration and temperature, including ultimate periods and frequencies, are determined by the relay feedback method and Tyreus-Luyben tuning rules [32]. Remarkably, the deadtime blocks of 1 min and 3 min are inserted to each temperature and composition loop to approach the industrial situation. The tuning results of temperature and composition control loops for the six schemes are listed in Table S2. The basic scheme CS1 for CED process is shown in Fig. 4, and the details of the multivariable control system are as follows:
338.63 K (reverse acting). (10) The flow rate of makeup entrainer is manipulated to control the level of intermediate buffer tank (reverse acting). Note that the controllers for the entrainer feed flow rate are set to the initial state of the cascade, and the setpoint is adjusted at any time to resist the disturbance of the feed flow rate. Moreover, the makeup entrainer stream is arranged to deal with some special situations, including that the entrainer is leaked or contaminated. However, when the system is under normal operating conditions, the loss of the entrainer is negligible. The control valve used to compensate for the entrainer loss is set to the initial state of manual adjustment in the dynamic simulation process, which contributes to the integral calculation of the dynamic model. The size of intermediate buffer tank should be larger than the design value in order to resist the complex feed disturbances. The system introduces the disturbance variables after running for 1 h in the closed-loop state, and the dynamics test is completed at 10 h. The dynamic response curves of the control structure CS1 are shown in Fig. 5. When the ± 10% feed flow rate disturbance is added into the multi-variable control system, the purity of the three products changes smoothly with a small oscillation in 5 h. There is a small steady-state error between the new stable value and the setpoint, which does not affect the specification of the product. The overshoots of the three products are within an acceptable range, and the minimum concentration product is greater than 99.6 mol%. Observing the changes of the three controlled temperatures, it can be seen that the temperatures could be restored to the setpoint after 3 h, and there is no steady-state error for each control loop. The temperature of 41th stage in EDC-2, which may be one of the key parameters, has a much higher overshoot than other controlled variables. The reason may be that the entrainer is not separated from the mixture obtained at the bottom of EDC-1, which
(1) The flow control valve is employed to adjust the flow rate of the ethyl acetate-ethanol-water mixture (reverse acting). (2) The total entrainer flow rate of EDC-1 is manipulated by the ratio of solvent to fresh feed flow rate, and the entrainer flow rate of EDC-2 is proportional to the bottom flow rate of EDC-1. (3) The heat duty of EDC-1 and EDC-2 are used as manipulated variables to adjust the temperature of 28th stage and 41th stage (reverse acting), respectively. (4) The condenser duty and operating pressure constitute a pressure control loop (reverse acting). (5) The distillate flow rate is used to control the reflux drum level (direct acting). (6) The sump level is manipulated via the control of the bottom flow rate (direct acting). (7) The reflux ratios in EDC-1 and EDC-2 are fixed. (8) A temperature controller (reverse acting), a liquid level controller (direct acting), and a pressure controller (reverse acting) are added to the flash. (9) The temperatures of recycled entrainer streams are controlled at
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Fig. 13. Dynamic responses of the CS5 control structure. (a) ± 10% feed flow rate disturbance; (b) +10% and −5% feed composition disturbance.
has a large influence on the temperature of sensitive tray in EDC-2 after introducing feed disturbances. When the ± 10% feed composition disturbance is added into the multi-variable control system, the large overshoot and steady-state error are generated with the purity of each product. Especially when the system is disturbed by −10% feed composition disturbance, the difference between the stabilized purity and setpoint is about 0.7 mol%, which makes the purity unable to meet the product specification. Observing the system response curves of the temperature, all these sensitive tray temperatures also return to their setpoints and recover smoothly in about 3 h. The above analysis shows that the CS1 control structure can effectively resist the disturbance of the feed flow rate, but the system lacks direct and effective control loops to resist the feed composition disturbance. Therefore, it is necessary to further improve the control performance of CS1 by using some dynamic intensification methods.
In order to solve the problem that the CS1 scheme cannot resist the disturbance of the feed composition, the improved control scheme CS2 is developed for CED process. Based on the CS1 control structure, two feed-forward control loops are added for CS2 to assist in adjusting the bottom temperatures of the two extractive distillation columns. The reflux ratios are corrected by two concentration controllers, which helps to maintain product purity when introducing feed composition disturbance. The improved control structure CS2 is shown in Fig. 6. After introducing ± 10% feed flow rate and ± 10% feed composition disturbances, the dynamic response curves of the CS2 system are shown in Fig. 7. When facing the flow rate disturbance, all the controlled variables can reach the steady-state again after 4 h, and the new stable values are the same as the setpoints except the purity of the water. The overshoots of three products meet the requirements for purity specifications, which means that the minimum product purity is greater than the value provided in the literature [35]. Due to the
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Fig. 13. (continued)
addition of the feed-forward control structure, the temperature stability of EDC-1 and EDC-2 is significantly improved, and the overshoot of 41th stage temperature in EDC-2 is reduced from the original 6–2 K. When facing the feed composition disturbance, the purity of EtAc product is adjusted for a longer period and returns to the steady-state again after about 10 h. The reason may be that there is a small reflux ratio of EDC-1 used to control product purity, which can only provide the limited improvement in terms of control performance. Even if the concentration controllers are added for the system to correct the value of reflux ratio, the dynamic responses of the multivariable control system are still delayed after inputting disturbance variables. Nevertheless, there is no steady-state error in EtAc purity when the system returns to a new steady-state, which indicates that the improved control structure CS2 plays a positive role in maintaining product concentration. In summary, the CS2 control structure has been improved in resistance to feed composition disturbance and can be selected as the
optimal control strategy for the CED process. 3.2. Control structure for MSR Based on the characteristics of MSR process, the basic control scheme CS3 is developed, as shown in Fig. 8. The CS3 control structure is established by adding some control loops under the framework of the CS1 scheme, and the new details are as follows: (1) The flash-1 is arranged between the two extractive distillation columns to separate the excess entrainer. A temperature controller (reverse acting), a liquid level controller (direct acting), and a pressure controller (reverse acting) are added to the flash-1. (2) The heat duties of EDC-1 and EDC-2 are used as manipulated variables to adjust the temperature of 27th stage and 32th stage (reverse acting), respectively.
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Fig. 14. The improved control scheme CS6 for TCED process.
(3) Two entrainer streams are taken from the bottom of the intermediate buffer tank. One of the streams is mixed with the recovery stream of flash-2 and fed into EDC-1. The total entrainer flow rate of EDC-1 is manipulated by the ratio of solvent to fresh feed flow rate, and the flow rate from the intermediate buffer tank is controlled by the difference between the total entrainer flow rate and the recovery solvent flow rate. Another stream is fed into EDC-2 after passing through the cooling system, and its flow rate is proportional to the top flow rate of flash-1.
performance, and the response curves of CS4 are shown in Fig. 11. When adding the disturbance of ± 10% to the feed flow rate, it can be seen that the purity of EtAc is maintained quite close to the specified value. There is a small steady-state error in the purity of EtOH and H2O respectively, which meets product quality requirements within an acceptable range. By bringing the temperatures back to the setpoints, the control system can handle the disturbances well. When adding the disturbance of ± 10% to the feed composition, the purity of EtAc product is adjusted for a longer period and returns to the steady-state again after about 8 h. However, there is no steady-state error in EtAc purity when the system returns to a new steady-state. The minimum purity meets the design specification, which benefits from the addition of the feed-forward control structures and concentration controllers. In summary, the improved control scheme CS4 for MSR process performs well in terms of robust stability and controllability.
For the dynamic simulation of CS3 shown in Fig. 9, ± 10% feed flow rate and feed composition disturbances are used as input step changes for the dynamic model. When the feed flow rate changes, the dynamic responses of the product purity have significant improvements in terms of overshoot and steady-state error compared with the CS1 control strategy, which may benefit from the addition of flash-1 used to effectively offset feed disturbances. This conclusion can also be obtained from the temperature variation of sensitive tray which is reduced from 6 K to 3 K in EDC-2. However, when the feed composition changes, the responses show poor dynamic performance, especially for that of the product purity. There are extremely large overshoots in the system, and the maximum value is about 0.5 mol%. The purity of EtAc product has a large steady-state error, and the minimum purity is 99.46 mol% which does not meet the product specification. It is necessary to further upgrade the CS3 control scheme. The CS4 control structure, as shown in Fig. 10, is established under the framework of the basic control structure for MSR process. Two feedforward control loops are added to assist in adjusting the bottom temperatures of the two extractive distillation columns. The reflux ratios are corrected by two concentration controllers, which helps to maintain product purity when introducing feed composition disturbance. Different feed disturbances are added to test the dynamic
3.3. Control structure for TCED Compared with the conventional extractive distillation process, the thermally coupled system is more attractive in terms of operating costs and equipment investment. However, due to the strong coupling and nonlinear characteristics, the control scheme for this complex multivariable system is very difficult to design. In this paper, the basic control scheme CS5, as shown in Fig. 12, is developed for the TCED process, and the details of the control structure are as follows: (1) The total entrainer flow rate of MC is manipulated by the ratio of solvent to fresh feed flow rate, and the entrainer flow rate of SC is proportional to side flow rate produced from the 38th stage of MC. (2) The heat duty of MC is used as a manipulated variable to adjust the temperature of 44th stage (reverse acting). (3) The setpoint of side flow rate is adjusted by the temperature of 32th
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Fig. 15. Dynamic responses of the CS6 control structure. (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
stage in MC.
product. The dynamic response of EtAc product performs poorly in terms of overshoot and steady-state error, and the re-stable value of the product purity is less than design specification. Some feasible control structures should be developed to achieve system upgrades. For a thermally coupled distillation configuration, the flow rate of the interconnecting stream is a key variable to ensure system stability. Following the suggestion of Luyben [32], the 32th stage temperature and fresh feed flow rate are used to adjust the side flow rate. The feedforward control loops and the concentration controllers are adopted for the improved control structure CS6 which is shown in Fig. 14. Different kinds of step signals, including the feed flow rate and feed composition disturbances, are introduced to evaluate the dynamic controllability. The dynamic responses, as shown in Fig. 15, indicate that the structure is significantly upgraded in terms of overshoot, settling time, and steady-state error. The control system can effectively handle the ± 10% feed composition disturbance, which shows that the CS6 scheme has better dynamic controllability than CS5, and it should
The dynamic response curves of CS5 are illustrated in Fig. 13. When the ± 10% feed flow rate disturbance is added to the system, the control effect on the quality of EtAc and EtOH products is satisfactory, and the minimum product purity is greater than 99.8 mol%. However, the dynamic response curves for water product do not meet specifications in terms of steady-state error and overshoot. Observing the temperature response curves, it is found that the temperature of the three sensitive trays can be restored to the original values after a short settling time. And the maximum overshoot is less than 4 K, which indicates that the two-point temperature structure plays a positive role in the control process. When the +10% feed composition disturbance is added to the system, there is a considerable overshoot for EtOH product, which correspond to the minimum product purity of 98.2 mol%. After adding a disturbance of −10% to the feed composition disturbance, the dynamic system shows an unacceptable steady-state error for EtAc
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Fig. 15. (continued)
be selected as the preferred control strategy for TCED process.
IAE value generated during the dynamic control process [36–38]. And the dynamic response curves of the multivariable control systems are shown in Fig. 16. When the control systems face complex disturbance conditions, it can be seen that the change of feed composition has a greater impact on the system than that of feed flow rate. After adding +20% composition disturbance, the CS6 control scheme shows a large overshoot in the purity of EtOH, and it is about 9 mol%. Moreover, the CS6 control strategy also exhibits poor dynamic performance in maintaining H2O product specification. When the EtAc composition of feed stream is restored from the +20% step value to the original value, there is a large overshoot of 1.4 mol% in EtAc product by observing the dynamic response curves of CS4, but the settling time is very short. Compared with the other two control strategies, the CS2 scheme exhibits worse dynamic responses in terms of frequency regulation. The calculation results of the total IAE value, as shown in Fig. 17, indicate that the MSR process exhibits the best performance characteristics in
4. Comparison and discussion In order to compare the dynamic characteristics of the three extractive distillation configurations, the improved control schemes, which correspond to the three extractive distillation configurations, are selected to evaluate the plant-wide dynamic behavior of those arrangements under complex disturbance conditions. The IAE and destroyed exergy, as shown in Table S3, are calculated to examine the robustness and energy efficiency of the system. Different input variables, including the ± 20% feed flow rate disturbance and ± 20% feed composition disturbance, are added to the control systems after running for one hour in the dynamic environment, and the type of disturbance is changed every two hours. The purity is taken as investigation variable to calculate the total
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Fig. 18. Comparison of different control strategies in terms of the destroyed exergy.
terms of controllability or operability compared with the other two flowsheets. This may benefit from the addition of flash-1 used to purify entrainers and effectively offset feed disturbances. Exergy can be used as an indicator to evaluate the quality and efficiency of the heating source, and the calculation of the destroyed exergy generated under dynamic operating conditions can help engineers to understand the law of system energy change more comprehensively [39–41]. The destroyed exergy of the three improved control schemes under complex disturbances is shown in Fig. 18. When the control systems face feed flow rate disturbance, the response curves of the destroyed exergy have the same trend, indicating that the feed flow rate has a linear relationship to the exergy of multivariable system. After adding composition disturbance, the destroyed exergy of the CS2 and CS6 control schemes have greater fluctuations, which results in more exergy being consumed during operation. From the perspective of dynamic response results, a good control scheme will play a positive role in the energy efficiency of the system. The total destroyed exergy can be obtained by calculating the area between the response curves and the coordinate axes. The total exergy consumed by the CS2, CS4, and CS6 control schemes during the test period of 10 h are 4016 kW, 4387 kW, and 4748 kW, respectively. It is worth noting that the TCED process is advantageous in terms of TAC, but there is no advantage in terms of exergy loss. The reason is that the TCED configuration has only one reboiler, which may cause the system to consume more high-grade energy sources [42]. Considering the controllability and energy efficiency of dynamic processes, the plant-wide control strategy of MSR process is satisfactory and should be screened as the best scheme for separating ethyl acetate-ethanol-water ternary mixture.
Fig. 16. Dynamic responses of the product purity for the three extractive distillation configurations under complex disturbance conditions.
5. Conclusion In this paper, three extractive distillation configurations are selected to investigate the suitability of the new solvent for continuous production. Six control schemes are established to evaluate the plant-wide dynamic controllability and flexibility of those arrangements, and the dynamic performance is tested by adding four different feed disturbances as input variables. Then, the improved control schemes, which correspond to the three configurations, are screened to assess the dynamic behavior of the system under complex disturbance conditions. The results show that the plant-wide dynamic responses of MSR process are satisfactory with better anti-interference ability and dynamic behavior, and the total destroyed exergy of CED process is relatively small during the operation time. Although the TCED process is satisfactory in terms of TAC results, it performs poorly in terms of control performance and exergy loss under the dynamic environment. The study suggests that there is an effective trade-off between steady-state economy and dynamic controllability.
Fig. 17. Comparison of different control strategies in terms of IAE.
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Declaration of Competing Interest
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