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Control of an energy-saving side-stream extractive distillation process with different disturbance conditions
Separation and Purification Technology 210 (2019) 195–208
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Control of an energy-saving side-stream extractive distillation process with different disturbance conditions Kang Maa, Mengxiao Yua, Yao Daia, Yixin Mab, Jun Gaob, Peizhe Cuia, Yinglong Wanga, a b
T
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College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Side-stream extractive distillation Control structure Dynamic controllabilities Separation
It is important to study the dynamic controllability for the side-stream extractive distillation due to its superiority of energy saving. Control structures of the side-stream extractive distillation process are special and complex due to the instability of side-stream flow rate. In this work, the dynamic control of the side-stream extractive distillation was explored for separating azeotropic mixture of acetone and methanol. The detailed control structures were used to investigate the control strategies of side-stream extractive distillation. During the whole design process, the control of flow rate on side stream is a key factor for achieving this process efficient control. A new control structure combining a component controller and a side-stream throughput valve was proposed to achieve good dynamic performance for the side-stream extractive distillation process when ± 10% disturbances were introduced, but it is difficult to control the ± 20% feed disturbances. In addition, the sidestream extractive distillation takes about a longer time to reach a steady state while maintaining the purity of products, compared with conventional process. Research on control performance is of great significance to the development of energy saving technology for side-stream extractive distillation.
1. Introduction For chemical production purposes, distillation is by far the most commonly used separation technology due to its advantages in operation and control [1–3]. It separates mixtures by heating streams to vapor-liquid phases based on differences in relative volatility or boiling points of the components. In addition, some special distillation methods were developed to separate azeotropic mixtures effectively, including pressure-swing distillation [4–7], azeotropic distillation [8,9], extractive distillation [10–15], catalytic distillation [16,17], etc. Extractive distillation is one of the effective methods to separate binary azeotropic mixtures or close boiling point mixtures. Although extractive distillation technology is widely used, one major obstacle is the huge energy requirements [2]. There is a great deal of papers on energy saving technology of extractive distillation as well as the corresponding design, and many achievements have been made to date [18–29]. Chien et al. [9] investigated a heterogeneous azeotropic dividing-wall column for separating water and pyridine using toluene as solvent and the dividing-wall column provided great energy savings. Li et al. [12] provided three energy saving extractive distillation processes for separating toluene and 2-methoxyethanol to achieve energy saving. The results show that
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extractive dividing-wall column has better performance in terms of economy. Zhao et al. [19] proposed a thermally coupled ternary extractive distillation process for separating tetrahydrofuran/ethanol/ water using a mixed solvent (ethylene glycol and dimethyl sulfoxide) as entrainer to achieve energy saving and reduce annual total cost. An et al. [30] developed a new two-column extractive distillation system based on a three-column conventional process. The new alternative process is energy saving by combining extractive distillation column and preconcentration column. Li et al. [31] explored an energy saving technology of extractive distillation process by combining intermediate heating and heat-integrated technology. They designed a novelty extractive distillation process using a side reboiler to reduce the energy requirement. Aniya et al. [32] designed a extractive distillation process with different class of entrainers for tert butyl alcohol dehydration. The results show that the energy saving effect of extractive distillation using solvent and salt as mixed solvent is the most significant. Although some process intensification technologies reduce the energy consumption of the process and improve the economy, it makes the process more complicated than conventional process and makes the operability and control more difficult. In addition to studying the energy saving technologies of extractive distillation, dynamic control strategy is an important factor that must be considered in the process of chemical
Corresponding author. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.seppur.2018.08.004 Received 20 June 2018; Received in revised form 29 July 2018; Accepted 5 August 2018 Available online 07 August 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Flowsheet of side-stream extractive distillation system.
Fig. 2. Temperature and temperature slope profiles of side-stream extractive distillation.
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Fig. 3. Basic control structure with fixed ratio of side-stream extractive distillation.
conventional process, the results showed that dynamic response of conventional process to disturbances was better. A recent paper by Salvador et al.[45] presented an improved process with a side-stream extractive distillation system for energy savings. The proposed method avoids remixing problem in the first column. Compared to conventional process and extractive distillation with thermal coupling, the proposed system can reduce more energy consumption. However, control performance of the proposed energy saving process was not explored in that work. Studying the controllability of energyefficient side-stream extractive distillation is of great significance for facing feed composition disturbance and feed flowrate disturbance to ensure the purity and stable operation of production. Accurate control structures for the side-stream distillation process are necessary. The aim of this work is to explore the dynamic controllability of side-stream extractive distillation process. In this work, we explore the detailed control strategies of this energy saving system for separating methanol and acetone using water as solvent. The ± 10% and ± 20% feed and composition disturbances were introduced to test the stability and robustness of the proposed control strategies.
Table 1 Tuning parameters of basic control structure for side-stream extractive distillation process. Controller
TC1
TC2
TC3
Controller action Controlled variable Manipulated variable Transmitter range (°C) Controller output range Gain KC Integral time τI (min)
Reverse T1,75 QC1 0–200.46 0–70.41 1.64 10.56
Reverse T2,33 QC2 0–192.14 0–41.39 1.74 10.56
Reverse T1 QCOOLER 0–93.70 −10.71 to 0 0.28 5.28
production [33–40]. Therefore, it is of great significance to develop dynamic control strategies of energy saving extractive distillation technologies. Dynamic control strategies regarding energy saving extractive distillation technologies have been studied and analyzed by many researchers in recent years [4,5,36,38,39,41]. Wang et al. [20] explored an efficient control strategy of extractive distillation using decanter for separating heterogeneous ternary azeotropic mixture. In this control process, the improvement is that the proportional controller replaced the temperature controller of first column. Luyben [33] studied the dynamic controllability of a pressure-swing distillation with vapor recompression. The novelty of control scheme is that the flowrate of the distillate recycle is used as the throughput manipulator. Salvador et al. [34] investigated two control strategies of extractive dividing-wall column for separating ethanol and water. The results show that dynamic performance of extractive dividing-wall column is comparable to conventional process. Zheng et al. [42] explored an effective control strategy of highly heat-integrated extractive distillation. Three temperature controllers were proposed and installed in the column C1. The proposed control strategy can achieve good dynamic performance. Salvador et al. [43] proposed dynamic control structures of different dividing-wall distillation column for separating BTX. The satellite column is the most energy-efficient process and corresponding dynamic structures were explored. Luyben [44] explored control strategies of thermally coupled ternary extractive distillation processes. Compared with the dynamics of thermally coupled processes and the dynamics of
2. Steady-state design and selection of temperature sensitive trays In this work, the steady state and dynamics of this process were simulated using Aspen Plus and Aspen Dynamics software. The steady state design of the side-stream extractive distillation for separating methanol and acetone was from Salvador’s paper [45]. Fig. 1 shows the flowsheet of side-stream extractive distillation with detail informations. Feed rate is 540 kmol/h with composition 50 mol% methanol and 50 mol% acetone. Product purities both are 99.5 mol%. The simulation results obtained in this paper are slightly different from Salvador’s paper. The main difference is that the purity of acetone is 99.5 mol%, not 99.4 mol%. It does not affect the control design of side-stream extractive distillation process. Before exporting from Aspen Plus software to the Aspen dynamic mode, the main parameter should be set as follow. The tray pressure drop of the two processes is set to be 0.0068 atm. When the vessel is half full, there is a 5 min liquid holdup. It is very necessary that valid phases of all valves are set to Liquid-Only. The temperature-sensitive trays of two columns were selected through the “slope criterion” proposed by Luyben [46]. Fig. 2 shows the 197
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Fig. 4. Dynamic responses of the basic control structure for the side-stream extractive distillation: (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 5. Control structure of side-stream extractive distillation with a side-stream composition/temperature cascade connection.
levels in two columns (direct acting). (3) The makeup water flow rate is manipulated to hold the sump level of the extractive distillation column C1 (reverse acting). (4) The bottom rate of solvent recovery distillation column is manipulated to hold the sump level of column C2 (direct acting). (5) The operating pressures in two columns are controlled by manipulating the condenser duties of two columns (reverse acting). (6) The cooler heat duty is manipulated to control the temperature of recycling solvent (reverse acting). (7) The solvent flow rate at bottom of extractive distillation column C1 is in proportion to the total feed flow rate. (8) The temperature in stage 75 of C1 column and the temperature in stage 33 of C2 column are controlled by operating the corresponding reboiler duty input (reverse acting).
Table 2 Tuning parameters of side-stream composition/temperature cascade control structures for side-stream extractive distillation process. Controller
TC1
TC2
TC3
CC1
Controller action Controlled variable Manipulated variable Transmitter range (°C) Controller output range Gain KC Integral time τI (min)
Reverse T1,75 QC1 0–200.46 0–70.41 1.64 10.56
Reverse T2,33 QC2 0–192.14 0–41.39 1.74 10.56
Reverse T1 QCOOLER 0–93.70 −10.71 to 0 0.28 5.28
Direct X(acetone) RR1 0–0.0035 0–200.46 0.049 9.24
temperature curves and temperature slope profiles of two columns in side-stream extractive distillation process. The feed is fed on stage 55 and the solvent is fed on stage 33. The side stream outlet location is stage 73. It can be seen from the temperature curves of C1 and C2 columns that there are obvious fluctuations in the fresh feed stage and solvent feed stage. The temperature fluctuation of feed stage will affect the stability of the control process, so, these feed locations are not selected as the temperature control points. Therefore, stage 75 of C1 column and stage 33 of C2 column are selected for temperature control.
As for basic side-stream extractive distillation controller tuning parameters, level control is set with large integral time (9999 min) and Kc = 2 to implement proportion-only control. For pressure controllers in both columns, the tuning parameters are set at Kc = 20 and τI = 12 min. The total feed flow controller settings are set at Kc = 0.5 and τI = 0.3 min. Three deadtime elements (1 min) are inserted into three corresponding temperature control loops [44]. Then relay-feedback tests are run, and the Tyreus–Luyben tuning rule is used to determine tuning parameters of three temperature controllers. The tuning parameters of three controllers are listed in Table 1. The dynamic performances of basic control structures are tested when the flow rate disturbances and feed composition disturbances are introduced. As shown in Fig. 4(a) and (b), the flow rate disturbance and the feed composition disturbance are introduced at 0.5 h, and the process is paused at 20 h. It is noticed that product purities of two columns are not able to return to the desired values at the new steady state when a ± 10% feed disturbances are introduced. Through the analysis of basic control in dynamic process, the component of acetone was delivered into the C2 column by side stream. It is impossible for methanol to be purified in C2 column. Therefore, some improved control schemes should be further explored to achieve efficient control.
3. Dynamic control for side-stream extractive distillation 3.1. Basic dynamic control of side-stream extractive distillation It is of great significance to study the dynamic controllability of side-stream extractive distillation for promoting the development of energy-efficient distillation process. As shown in Fig. 3, the original control structure of the side-stream extractive distillation process was proposed, based on the conventional control structure of the extractive distillation system for separating binary mixture of acetone and methanol [47]. The detailed control structures and the related settings are listed below: (1) The fresh feed flow is controled by throughput valve (reverse acting). (2) The distillate flow rates are manipulated to control the reflux tank 199
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Fig. 6. Dynamic responses of the composition/temperature cascade control structure for the side-stream extractive distillation: (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 7. Control structure of side-stream extractive distillation with side stream flow rate/feed flow rate ratio (S/F) and composition controller connection.
methanol mole fraction signal is fed as the process variable into a composition controller that output signal changed the S/F ratio. The control structure is shown in Fig. 7. The tuning parameters of the three temperature controllers and one composition controller are listed in Table 3. To test the control performance of S/F control structure and composition controller connection for side-stream extractive distillation, ± 10% feed disturbances are introduced. The dynamic performance results are shown in Fig. 8 and revealed that the control structure can effectively control both processes disturbances. Meanwhile, the control structure can be kept constant after 8 h under the ± 10% flow rate disturbance, and the system takes about 16 h to return to the desired value under the ± 10% feed composition disturbances. The ± 20% feed flow rate and feed composition disturbances are introduced and shown in Fig. 9. The results indicated the system could not reach a new steady state within 20 h after ± 20% feed composition disturbances.
Table 3 Tuning parameters of S/F with composition controller connection control structure for side-stream extractive distillation process. Controller
TC1
TC2
TC3
CC1
Controller action Controlled variable Manipulated variable Transmitter range (°C) Controller output range Gain KC Integral time τI (min)
Reverse T1,75 QC1 0–200.46 0–70.41 1.64 10.56
Reverse T2,33 QC2 0–192.14 0–41.39 1.74 10.56
Reverse T1 QCOOLER 0–93.70 −10.71 to 0 0.28 5.28
Direct X(methanol) S/F 0–1.99 0–2.78 16.06 205.92
3.2. Control structure of side-stream extractive distillation with a sidestream composition/temperature cascade connection On the basis of basic control structure, a cascade side-stream composition/temperature control structure was introduced. The control structure is shown in Fig. 5. The acetone purity in the side stream is detected (3 min deadtime) and controlled by manipulating the set point of the temperature controller which is on “cascade”. This control structure is explored to control the acetone purity in the side stream. The tuning parameters of the three temperature controllers and the side stream composition controller are listed in Table 2. ± 10% disturbances are introduced and the control performances are shown in Fig. 6. As can be seen, this structure can be kept constant after 4 h and the acetone purity is well held under the ± 10% feed disturbances. However, the methanol purity can reach 99.41 mol% instead of 99.5 mol% after ± 10% feed disturbances. This control structure can be selected when the purity requirement of methanol is not high.
3.4. Improved dynamic control of side-stream extractive distillation Through the investigated of the above three structures, the results indicate that a suitable side stream flow rate control structure in the side-stream extractive distillation is crucial to obtain effective control of side-stream extractive distillation. A new controller structure is proposed to control side stream flowrate and composition in this process. The difference with the basic side-stream extractive distillation controllers is that product purity of solvent recovery column C2 is controlled by manipulating side stream flow rate (direct acting). The improved control structures of side-stream extractive distillation are shown in Fig. 10. As for controller tuning parameters, parameter setting is the same as that of basic dynamic process. The tuning parameters including three temperature controllers and one composition controllers are listed in Table 4. Fig. 11(a) and (b) shows the dynamic performances of the improved control strategies under ± 10% flow rate disturbances and ± 10% feed composition disturbances. It is noticed that product purities can come very close to the initial values at the new steady state about 8 h when ± 10% flow rate disturbance is introduced. This structure can also be kept constant and desired values after 12 h for a ± 10% feed composition disturbance. Therefore, the whole control structures can achieve good controllability for side-stream extractive distillation.
3.3. Control structure of side-stream extractive distillation with side stream flow rate/feed flow rate ratio (S/F) and composition controller connection To achieve better dynamic controllability, finding the appropriate controller to manipulate side stream is the key factor. Professor William L. Luyben reviewed and proposed the control structure of side-stream extractive distillation with side stream flow rate/feed flow rate ratio (S/ F) and composition controller connection [48]. S/F control structure and composition controller connection is added to control the flow rate of the side stream based on the basic control structure. This is that the 201
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Fig. 8. Dynamic responses of control structure of side-stream extractive distillation with sidestream flowrate /feed flowrate ratio (S/F) and composition cascade connection: (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 9. Dynamic responses of control structure of side-stream extractive distillation with sidestream flowrate /feed flowrate ratio (S/F) and composition cascade connection: (a) ± 20% feed flow rate disturbance; (b) ± 20% feed composition disturbance.
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Fig. 10. Improved control structure of side-stream extractive distillation.
throughput valve was proposed to control side stream for the sidestream extractive distillation dynamic control. Its structures can achieve the required purity of solvent recovery column and obtain good dynamic responses, by directly controlling the valve of side stream and reducing the flow rate of the side stream when the product purity of methanol is not up to the purity requirement. The new dynamic structures take about 8 h to reach steady state when encounter a ± 10% feed rate disturbance and take about 12 h to reach steady state when encounter a ± 10% feed composition disturbance. ± 20% disturbances are also introduced to test the effect of the proposed control structures, as shown in Fig. 12. This control structures can also achieve controllability when introducing ± 20% feed rate disturbance. It is note that the stable time delayed from 8 h to 14 h after introducing disturbances, compared with the controllability of ± 10% feed rate disturbances. However, this control structures cannot achieve controllability when introducing −20% feed composition disturbance. As shown in Fig. 12(b), there is an obvious fluctuation when introducing −20% feed composition disturbances. The product purities cannot return to their desired values in a short time. For ± 20% feed disturbances, it is necessary to further develop its control structure. Therefore, the study of dynamic control for special distillation is still a challenging work. In addition, the dynamic controllabilities of the conventional extractive distillation process and the side-stream extractive distillation process were compared and analyzed in this section. The dynamic controllabilities of conventional extractive distillation for separating acetone and methanol have been studied by Luyben [47]. The basic control structures proposed by Luyben can effectively control flow rate disturbance and feed composition disturbance in conventional extractive distillation. The conventional process requires about 1.5 h to arrive steady state, while the side-stream extractive distillation take about 8 h to arrive steady state when encounter a ± 10% feed rate disturbance and take about 12 h to arrive steady state when encounter a ± 10% feed composition disturbance. Conventional process achieves better dynamic performances than side-stream extractive distillation process. Although the economy of side-stream extractive distillation is improved, the stable time is delayed from 1.5 h to 12 h after introducing disturbances. Conventional extractive distillation has greater operability and controllability, due to its simple process. Side-stream
Table 4 Tuning parameters of improved control structure for side-stream extractive distillation. Controller
TC1
TC2
TC3
CC1
Controller action Controlled variable Manipulated variable Transmitter range (°C) Controller output range Gain KC Integral time τI (min)
Reverse T1,75 QC1 0–200.46 0–70.41 1.64 10.56
Reverse T2,33 QC2 0–192.14 0–41.39 1.74 10.56
Reverse T1 QCOOLER 0–93.70 −10.71 to 0 0.28 5.28
Direct X(methanol) RR2 0–1.99 0–100 21.70 129.36
4. Discussion and comparison During the whole design process, controling composition and flowrate of side stream is the key factor to the control effect. After testing, the basic structure cannot effectively control the side-stream extractive distillation when introducing ± 10% feed disturbances. The purities of the two columns are not returned to the specifications. The main reason for the poor dynamic control performance is that these control structures cannot effectively control composition and flow rate of side stream. Therefore, a side-stream composition/temperature cascade control structure is explored to control the acetone purity in the side stream. The results show the acetone in C1 column can keep almost the desired purity after 4 h and the purity of methanol in C2 column has a little deviation from the initial value when introducing ± 10% feed disturbances. The initial value of methanol purity is 99.5 mol% instead of 99.41 mol%. The control structure can be selected when the purity requirement of methanol is not high. If the purity requirements are relatively high, new control structures need to be developed. To solve this purity problem, the control structure of S/F with composition controller connection is proposed for side-stream extractive distillation and can achieve the required purity of two columns. Note that the disturbance can be kept constant after 16 h when introducing ± 10% feed composition disturbances. As shown in Fig. 9, the dynamic responses showed that the system could not reach a new steady state within 20 h after adding ± 20% feed composition disturbances. A new control structure combining a composition controller and a side-stream 204
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Fig. 11. Dynamic responses of the improved control structure for the side-stream extractive distillation: (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 12. Dynamic responses of the improved control structure for the side-stream extractive distillation: (a) ± 20% feed flow rate disturbance; (b) ± 20% feed composition disturbance.
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extractive distillation can be more energy efficient, due to its improved structure. Therefore, for the extractive distillation of high control requirements, conventional process should be selected. Otherwise the side-stream extractive distillation can be adopted in the future conceptual design of extractive distillation. This work provided an improved control strategy to achieve the stable control of energy-saving side-stream extractive distillation. These studies will contribute to develop the dynamic control for separating binary azeotropic mixtures by side stream extractive distillation.
[9]
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5. Conclusion [13]
In the previous work, Salvador et al. proposed an energy-saving side-stream extractive distillation process. Several control structures to achieve stable control of side-stream extractive distillation using Aspen Plus Dynamics software were explored. Due to the complexity of sidestream extractive distillation process, the conventional basic control structure cannot achieve effective dynamic control. A side-stream composition/temperature cascade control structure was explored and the acetone in C1 column can keep almost the desired purity and the purity of methanol in C2 column has a little deviation from the initial value when introducing ± 10% feed disturbances. The control structure of S/F with composition controller connection can keep almost the desired purity except for a long recovery time under ± 10% feed composition disturbances. The improved control structure combining a component controller and a side-stream throughput valve was proposed. It was observed that both disturbances processes can be effectively controlled under ± 10% flow rate and feed composition disturbances. For ± 20% flow rate and feed composition disturbances, more control structure strategies need to be further explored to achieve its controllability for side-stream extractive distillation. These studies will promote the application of energy-efficient extractive distillation with side stream in the chemical process industry.
[14]
[15]
[16]
[17]
[18] [19]
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6. Notes [24]
The authors declare no competing financial interest. [25]
Acknowledgments
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The manuscript was critically reviewed by the onymous reviewer Professor William L. Luyben. Professor William L. Luyben suggested us to use the control structures with side stream flow rate/feed flow rate ratio(S/F) and composition controller connection which improve the quality of this paper to a large extent. We express our sincere gratitude to the Professor William L. Luyben. This work is supported by the National Natural Science Foundation of China (No. 21776145).
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Control of an energy-saving side-stream extractive distillation process with different disturbance conditions Kang Maa, Mengxiao Yua, Yao Daia, Yixin Mab, Jun Gaob, Peizhe Cuia, Yinglong Wanga, a b
T
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College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Side-stream extractive distillation Control structure Dynamic controllabilities Separation
It is important to study the dynamic controllability for the side-stream extractive distillation due to its superiority of energy saving. Control structures of the side-stream extractive distillation process are special and complex due to the instability of side-stream flow rate. In this work, the dynamic control of the side-stream extractive distillation was explored for separating azeotropic mixture of acetone and methanol. The detailed control structures were used to investigate the control strategies of side-stream extractive distillation. During the whole design process, the control of flow rate on side stream is a key factor for achieving this process efficient control. A new control structure combining a component controller and a side-stream throughput valve was proposed to achieve good dynamic performance for the side-stream extractive distillation process when ± 10% disturbances were introduced, but it is difficult to control the ± 20% feed disturbances. In addition, the sidestream extractive distillation takes about a longer time to reach a steady state while maintaining the purity of products, compared with conventional process. Research on control performance is of great significance to the development of energy saving technology for side-stream extractive distillation.
1. Introduction For chemical production purposes, distillation is by far the most commonly used separation technology due to its advantages in operation and control [1–3]. It separates mixtures by heating streams to vapor-liquid phases based on differences in relative volatility or boiling points of the components. In addition, some special distillation methods were developed to separate azeotropic mixtures effectively, including pressure-swing distillation [4–7], azeotropic distillation [8,9], extractive distillation [10–15], catalytic distillation [16,17], etc. Extractive distillation is one of the effective methods to separate binary azeotropic mixtures or close boiling point mixtures. Although extractive distillation technology is widely used, one major obstacle is the huge energy requirements [2]. There is a great deal of papers on energy saving technology of extractive distillation as well as the corresponding design, and many achievements have been made to date [18–29]. Chien et al. [9] investigated a heterogeneous azeotropic dividing-wall column for separating water and pyridine using toluene as solvent and the dividing-wall column provided great energy savings. Li et al. [12] provided three energy saving extractive distillation processes for separating toluene and 2-methoxyethanol to achieve energy saving. The results show that
⁎
extractive dividing-wall column has better performance in terms of economy. Zhao et al. [19] proposed a thermally coupled ternary extractive distillation process for separating tetrahydrofuran/ethanol/ water using a mixed solvent (ethylene glycol and dimethyl sulfoxide) as entrainer to achieve energy saving and reduce annual total cost. An et al. [30] developed a new two-column extractive distillation system based on a three-column conventional process. The new alternative process is energy saving by combining extractive distillation column and preconcentration column. Li et al. [31] explored an energy saving technology of extractive distillation process by combining intermediate heating and heat-integrated technology. They designed a novelty extractive distillation process using a side reboiler to reduce the energy requirement. Aniya et al. [32] designed a extractive distillation process with different class of entrainers for tert butyl alcohol dehydration. The results show that the energy saving effect of extractive distillation using solvent and salt as mixed solvent is the most significant. Although some process intensification technologies reduce the energy consumption of the process and improve the economy, it makes the process more complicated than conventional process and makes the operability and control more difficult. In addition to studying the energy saving technologies of extractive distillation, dynamic control strategy is an important factor that must be considered in the process of chemical
Corresponding author. E-mail address: [email protected] (Y. Wang).
https://doi.org/10.1016/j.seppur.2018.08.004 Received 20 June 2018; Received in revised form 29 July 2018; Accepted 5 August 2018 Available online 07 August 2018 1383-5866/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Flowsheet of side-stream extractive distillation system.
Fig. 2. Temperature and temperature slope profiles of side-stream extractive distillation.
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Fig. 3. Basic control structure with fixed ratio of side-stream extractive distillation.
conventional process, the results showed that dynamic response of conventional process to disturbances was better. A recent paper by Salvador et al.[45] presented an improved process with a side-stream extractive distillation system for energy savings. The proposed method avoids remixing problem in the first column. Compared to conventional process and extractive distillation with thermal coupling, the proposed system can reduce more energy consumption. However, control performance of the proposed energy saving process was not explored in that work. Studying the controllability of energyefficient side-stream extractive distillation is of great significance for facing feed composition disturbance and feed flowrate disturbance to ensure the purity and stable operation of production. Accurate control structures for the side-stream distillation process are necessary. The aim of this work is to explore the dynamic controllability of side-stream extractive distillation process. In this work, we explore the detailed control strategies of this energy saving system for separating methanol and acetone using water as solvent. The ± 10% and ± 20% feed and composition disturbances were introduced to test the stability and robustness of the proposed control strategies.
Table 1 Tuning parameters of basic control structure for side-stream extractive distillation process. Controller
TC1
TC2
TC3
Controller action Controlled variable Manipulated variable Transmitter range (°C) Controller output range Gain KC Integral time τI (min)
Reverse T1,75 QC1 0–200.46 0–70.41 1.64 10.56
Reverse T2,33 QC2 0–192.14 0–41.39 1.74 10.56
Reverse T1 QCOOLER 0–93.70 −10.71 to 0 0.28 5.28
production [33–40]. Therefore, it is of great significance to develop dynamic control strategies of energy saving extractive distillation technologies. Dynamic control strategies regarding energy saving extractive distillation technologies have been studied and analyzed by many researchers in recent years [4,5,36,38,39,41]. Wang et al. [20] explored an efficient control strategy of extractive distillation using decanter for separating heterogeneous ternary azeotropic mixture. In this control process, the improvement is that the proportional controller replaced the temperature controller of first column. Luyben [33] studied the dynamic controllability of a pressure-swing distillation with vapor recompression. The novelty of control scheme is that the flowrate of the distillate recycle is used as the throughput manipulator. Salvador et al. [34] investigated two control strategies of extractive dividing-wall column for separating ethanol and water. The results show that dynamic performance of extractive dividing-wall column is comparable to conventional process. Zheng et al. [42] explored an effective control strategy of highly heat-integrated extractive distillation. Three temperature controllers were proposed and installed in the column C1. The proposed control strategy can achieve good dynamic performance. Salvador et al. [43] proposed dynamic control structures of different dividing-wall distillation column for separating BTX. The satellite column is the most energy-efficient process and corresponding dynamic structures were explored. Luyben [44] explored control strategies of thermally coupled ternary extractive distillation processes. Compared with the dynamics of thermally coupled processes and the dynamics of
2. Steady-state design and selection of temperature sensitive trays In this work, the steady state and dynamics of this process were simulated using Aspen Plus and Aspen Dynamics software. The steady state design of the side-stream extractive distillation for separating methanol and acetone was from Salvador’s paper [45]. Fig. 1 shows the flowsheet of side-stream extractive distillation with detail informations. Feed rate is 540 kmol/h with composition 50 mol% methanol and 50 mol% acetone. Product purities both are 99.5 mol%. The simulation results obtained in this paper are slightly different from Salvador’s paper. The main difference is that the purity of acetone is 99.5 mol%, not 99.4 mol%. It does not affect the control design of side-stream extractive distillation process. Before exporting from Aspen Plus software to the Aspen dynamic mode, the main parameter should be set as follow. The tray pressure drop of the two processes is set to be 0.0068 atm. When the vessel is half full, there is a 5 min liquid holdup. It is very necessary that valid phases of all valves are set to Liquid-Only. The temperature-sensitive trays of two columns were selected through the “slope criterion” proposed by Luyben [46]. Fig. 2 shows the 197
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Fig. 4. Dynamic responses of the basic control structure for the side-stream extractive distillation: (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 5. Control structure of side-stream extractive distillation with a side-stream composition/temperature cascade connection.
levels in two columns (direct acting). (3) The makeup water flow rate is manipulated to hold the sump level of the extractive distillation column C1 (reverse acting). (4) The bottom rate of solvent recovery distillation column is manipulated to hold the sump level of column C2 (direct acting). (5) The operating pressures in two columns are controlled by manipulating the condenser duties of two columns (reverse acting). (6) The cooler heat duty is manipulated to control the temperature of recycling solvent (reverse acting). (7) The solvent flow rate at bottom of extractive distillation column C1 is in proportion to the total feed flow rate. (8) The temperature in stage 75 of C1 column and the temperature in stage 33 of C2 column are controlled by operating the corresponding reboiler duty input (reverse acting).
Table 2 Tuning parameters of side-stream composition/temperature cascade control structures for side-stream extractive distillation process. Controller
TC1
TC2
TC3
CC1
Controller action Controlled variable Manipulated variable Transmitter range (°C) Controller output range Gain KC Integral time τI (min)
Reverse T1,75 QC1 0–200.46 0–70.41 1.64 10.56
Reverse T2,33 QC2 0–192.14 0–41.39 1.74 10.56
Reverse T1 QCOOLER 0–93.70 −10.71 to 0 0.28 5.28
Direct X(acetone) RR1 0–0.0035 0–200.46 0.049 9.24
temperature curves and temperature slope profiles of two columns in side-stream extractive distillation process. The feed is fed on stage 55 and the solvent is fed on stage 33. The side stream outlet location is stage 73. It can be seen from the temperature curves of C1 and C2 columns that there are obvious fluctuations in the fresh feed stage and solvent feed stage. The temperature fluctuation of feed stage will affect the stability of the control process, so, these feed locations are not selected as the temperature control points. Therefore, stage 75 of C1 column and stage 33 of C2 column are selected for temperature control.
As for basic side-stream extractive distillation controller tuning parameters, level control is set with large integral time (9999 min) and Kc = 2 to implement proportion-only control. For pressure controllers in both columns, the tuning parameters are set at Kc = 20 and τI = 12 min. The total feed flow controller settings are set at Kc = 0.5 and τI = 0.3 min. Three deadtime elements (1 min) are inserted into three corresponding temperature control loops [44]. Then relay-feedback tests are run, and the Tyreus–Luyben tuning rule is used to determine tuning parameters of three temperature controllers. The tuning parameters of three controllers are listed in Table 1. The dynamic performances of basic control structures are tested when the flow rate disturbances and feed composition disturbances are introduced. As shown in Fig. 4(a) and (b), the flow rate disturbance and the feed composition disturbance are introduced at 0.5 h, and the process is paused at 20 h. It is noticed that product purities of two columns are not able to return to the desired values at the new steady state when a ± 10% feed disturbances are introduced. Through the analysis of basic control in dynamic process, the component of acetone was delivered into the C2 column by side stream. It is impossible for methanol to be purified in C2 column. Therefore, some improved control schemes should be further explored to achieve efficient control.
3. Dynamic control for side-stream extractive distillation 3.1. Basic dynamic control of side-stream extractive distillation It is of great significance to study the dynamic controllability of side-stream extractive distillation for promoting the development of energy-efficient distillation process. As shown in Fig. 3, the original control structure of the side-stream extractive distillation process was proposed, based on the conventional control structure of the extractive distillation system for separating binary mixture of acetone and methanol [47]. The detailed control structures and the related settings are listed below: (1) The fresh feed flow is controled by throughput valve (reverse acting). (2) The distillate flow rates are manipulated to control the reflux tank 199
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Fig. 6. Dynamic responses of the composition/temperature cascade control structure for the side-stream extractive distillation: (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 7. Control structure of side-stream extractive distillation with side stream flow rate/feed flow rate ratio (S/F) and composition controller connection.
methanol mole fraction signal is fed as the process variable into a composition controller that output signal changed the S/F ratio. The control structure is shown in Fig. 7. The tuning parameters of the three temperature controllers and one composition controller are listed in Table 3. To test the control performance of S/F control structure and composition controller connection for side-stream extractive distillation, ± 10% feed disturbances are introduced. The dynamic performance results are shown in Fig. 8 and revealed that the control structure can effectively control both processes disturbances. Meanwhile, the control structure can be kept constant after 8 h under the ± 10% flow rate disturbance, and the system takes about 16 h to return to the desired value under the ± 10% feed composition disturbances. The ± 20% feed flow rate and feed composition disturbances are introduced and shown in Fig. 9. The results indicated the system could not reach a new steady state within 20 h after ± 20% feed composition disturbances.
Table 3 Tuning parameters of S/F with composition controller connection control structure for side-stream extractive distillation process. Controller
TC1
TC2
TC3
CC1
Controller action Controlled variable Manipulated variable Transmitter range (°C) Controller output range Gain KC Integral time τI (min)
Reverse T1,75 QC1 0–200.46 0–70.41 1.64 10.56
Reverse T2,33 QC2 0–192.14 0–41.39 1.74 10.56
Reverse T1 QCOOLER 0–93.70 −10.71 to 0 0.28 5.28
Direct X(methanol) S/F 0–1.99 0–2.78 16.06 205.92
3.2. Control structure of side-stream extractive distillation with a sidestream composition/temperature cascade connection On the basis of basic control structure, a cascade side-stream composition/temperature control structure was introduced. The control structure is shown in Fig. 5. The acetone purity in the side stream is detected (3 min deadtime) and controlled by manipulating the set point of the temperature controller which is on “cascade”. This control structure is explored to control the acetone purity in the side stream. The tuning parameters of the three temperature controllers and the side stream composition controller are listed in Table 2. ± 10% disturbances are introduced and the control performances are shown in Fig. 6. As can be seen, this structure can be kept constant after 4 h and the acetone purity is well held under the ± 10% feed disturbances. However, the methanol purity can reach 99.41 mol% instead of 99.5 mol% after ± 10% feed disturbances. This control structure can be selected when the purity requirement of methanol is not high.
3.4. Improved dynamic control of side-stream extractive distillation Through the investigated of the above three structures, the results indicate that a suitable side stream flow rate control structure in the side-stream extractive distillation is crucial to obtain effective control of side-stream extractive distillation. A new controller structure is proposed to control side stream flowrate and composition in this process. The difference with the basic side-stream extractive distillation controllers is that product purity of solvent recovery column C2 is controlled by manipulating side stream flow rate (direct acting). The improved control structures of side-stream extractive distillation are shown in Fig. 10. As for controller tuning parameters, parameter setting is the same as that of basic dynamic process. The tuning parameters including three temperature controllers and one composition controllers are listed in Table 4. Fig. 11(a) and (b) shows the dynamic performances of the improved control strategies under ± 10% flow rate disturbances and ± 10% feed composition disturbances. It is noticed that product purities can come very close to the initial values at the new steady state about 8 h when ± 10% flow rate disturbance is introduced. This structure can also be kept constant and desired values after 12 h for a ± 10% feed composition disturbance. Therefore, the whole control structures can achieve good controllability for side-stream extractive distillation.
3.3. Control structure of side-stream extractive distillation with side stream flow rate/feed flow rate ratio (S/F) and composition controller connection To achieve better dynamic controllability, finding the appropriate controller to manipulate side stream is the key factor. Professor William L. Luyben reviewed and proposed the control structure of side-stream extractive distillation with side stream flow rate/feed flow rate ratio (S/ F) and composition controller connection [48]. S/F control structure and composition controller connection is added to control the flow rate of the side stream based on the basic control structure. This is that the 201
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Fig. 8. Dynamic responses of control structure of side-stream extractive distillation with sidestream flowrate /feed flowrate ratio (S/F) and composition cascade connection: (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 9. Dynamic responses of control structure of side-stream extractive distillation with sidestream flowrate /feed flowrate ratio (S/F) and composition cascade connection: (a) ± 20% feed flow rate disturbance; (b) ± 20% feed composition disturbance.
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Fig. 10. Improved control structure of side-stream extractive distillation.
throughput valve was proposed to control side stream for the sidestream extractive distillation dynamic control. Its structures can achieve the required purity of solvent recovery column and obtain good dynamic responses, by directly controlling the valve of side stream and reducing the flow rate of the side stream when the product purity of methanol is not up to the purity requirement. The new dynamic structures take about 8 h to reach steady state when encounter a ± 10% feed rate disturbance and take about 12 h to reach steady state when encounter a ± 10% feed composition disturbance. ± 20% disturbances are also introduced to test the effect of the proposed control structures, as shown in Fig. 12. This control structures can also achieve controllability when introducing ± 20% feed rate disturbance. It is note that the stable time delayed from 8 h to 14 h after introducing disturbances, compared with the controllability of ± 10% feed rate disturbances. However, this control structures cannot achieve controllability when introducing −20% feed composition disturbance. As shown in Fig. 12(b), there is an obvious fluctuation when introducing −20% feed composition disturbances. The product purities cannot return to their desired values in a short time. For ± 20% feed disturbances, it is necessary to further develop its control structure. Therefore, the study of dynamic control for special distillation is still a challenging work. In addition, the dynamic controllabilities of the conventional extractive distillation process and the side-stream extractive distillation process were compared and analyzed in this section. The dynamic controllabilities of conventional extractive distillation for separating acetone and methanol have been studied by Luyben [47]. The basic control structures proposed by Luyben can effectively control flow rate disturbance and feed composition disturbance in conventional extractive distillation. The conventional process requires about 1.5 h to arrive steady state, while the side-stream extractive distillation take about 8 h to arrive steady state when encounter a ± 10% feed rate disturbance and take about 12 h to arrive steady state when encounter a ± 10% feed composition disturbance. Conventional process achieves better dynamic performances than side-stream extractive distillation process. Although the economy of side-stream extractive distillation is improved, the stable time is delayed from 1.5 h to 12 h after introducing disturbances. Conventional extractive distillation has greater operability and controllability, due to its simple process. Side-stream
Table 4 Tuning parameters of improved control structure for side-stream extractive distillation. Controller
TC1
TC2
TC3
CC1
Controller action Controlled variable Manipulated variable Transmitter range (°C) Controller output range Gain KC Integral time τI (min)
Reverse T1,75 QC1 0–200.46 0–70.41 1.64 10.56
Reverse T2,33 QC2 0–192.14 0–41.39 1.74 10.56
Reverse T1 QCOOLER 0–93.70 −10.71 to 0 0.28 5.28
Direct X(methanol) RR2 0–1.99 0–100 21.70 129.36
4. Discussion and comparison During the whole design process, controling composition and flowrate of side stream is the key factor to the control effect. After testing, the basic structure cannot effectively control the side-stream extractive distillation when introducing ± 10% feed disturbances. The purities of the two columns are not returned to the specifications. The main reason for the poor dynamic control performance is that these control structures cannot effectively control composition and flow rate of side stream. Therefore, a side-stream composition/temperature cascade control structure is explored to control the acetone purity in the side stream. The results show the acetone in C1 column can keep almost the desired purity after 4 h and the purity of methanol in C2 column has a little deviation from the initial value when introducing ± 10% feed disturbances. The initial value of methanol purity is 99.5 mol% instead of 99.41 mol%. The control structure can be selected when the purity requirement of methanol is not high. If the purity requirements are relatively high, new control structures need to be developed. To solve this purity problem, the control structure of S/F with composition controller connection is proposed for side-stream extractive distillation and can achieve the required purity of two columns. Note that the disturbance can be kept constant after 16 h when introducing ± 10% feed composition disturbances. As shown in Fig. 9, the dynamic responses showed that the system could not reach a new steady state within 20 h after adding ± 20% feed composition disturbances. A new control structure combining a composition controller and a side-stream 204
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Fig. 11. Dynamic responses of the improved control structure for the side-stream extractive distillation: (a) ± 10% feed flow rate disturbance; (b) ± 10% feed composition disturbance.
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Fig. 12. Dynamic responses of the improved control structure for the side-stream extractive distillation: (a) ± 20% feed flow rate disturbance; (b) ± 20% feed composition disturbance.
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extractive distillation can be more energy efficient, due to its improved structure. Therefore, for the extractive distillation of high control requirements, conventional process should be selected. Otherwise the side-stream extractive distillation can be adopted in the future conceptual design of extractive distillation. This work provided an improved control strategy to achieve the stable control of energy-saving side-stream extractive distillation. These studies will contribute to develop the dynamic control for separating binary azeotropic mixtures by side stream extractive distillation.
[9]
[10]
[11]
[12]
5. Conclusion [13]
In the previous work, Salvador et al. proposed an energy-saving side-stream extractive distillation process. Several control structures to achieve stable control of side-stream extractive distillation using Aspen Plus Dynamics software were explored. Due to the complexity of sidestream extractive distillation process, the conventional basic control structure cannot achieve effective dynamic control. A side-stream composition/temperature cascade control structure was explored and the acetone in C1 column can keep almost the desired purity and the purity of methanol in C2 column has a little deviation from the initial value when introducing ± 10% feed disturbances. The control structure of S/F with composition controller connection can keep almost the desired purity except for a long recovery time under ± 10% feed composition disturbances. The improved control structure combining a component controller and a side-stream throughput valve was proposed. It was observed that both disturbances processes can be effectively controlled under ± 10% flow rate and feed composition disturbances. For ± 20% flow rate and feed composition disturbances, more control structure strategies need to be further explored to achieve its controllability for side-stream extractive distillation. These studies will promote the application of energy-efficient extractive distillation with side stream in the chemical process industry.
[14]
[15]
[16]
[17]
[18] [19]
[20]
[21]
[22]
[23]
6. Notes [24]
The authors declare no competing financial interest. [25]
Acknowledgments
[26]
The manuscript was critically reviewed by the onymous reviewer Professor William L. Luyben. Professor William L. Luyben suggested us to use the control structures with side stream flow rate/feed flow rate ratio(S/F) and composition controller connection which improve the quality of this paper to a large extent. We express our sincere gratitude to the Professor William L. Luyben. This work is supported by the National Natural Science Foundation of China (No. 21776145).
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