Design and Control of Extractive Dividing-Wall Column for Separating

Article pubs.acs.org/IECR

Design and Control of Extractive Dividing-Wall Column for Separating Ethyl Acetate−Isopropyl Alcohol Mixture Hao Zhang,† Qing Ye,*,† Jiwei Qin,† Hong Xu,† and Ning Li‡ †

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Institute of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China ‡ Institute of Petrochemical Technology, China University of Petroleum, Beijing 102200, People’s Republic of China ABSTRACT: An extractive dividing-wall column (EDWC) is simulated as a three-column system in this work. The design and control of the EDWC to separate ethyl acetate and isopropyl alcohol using ethylene glycol as the entrainer is studied in Aspen Plus and Aspen Dynamics. The optimum EDWC design with minimal total annual cost is screened first; then sensitivity analysis is conducted to investigate whether the entrainer flow rate and vapor split ratio can be used as control variables to hold the purity specifications; finally, three control structures for the EDWC are established, and the dynamic results of these control structures are compared. It is found that two improved control structures achieve satisfactory performance with much smaller deviation and shorter settling time by comparison to the basic control structure. Dynamic results also revealed that it is useful to adjust the entrainer flow rate or vapor split ratio to hold the purity specifications.

1. INTRODUCTION Ethyl acetate (EA) and isopropyl alcohol (IPA) are very important raw materials that are manufactured on a large scale for use as intermediates and solvents. In the process of the production of ampicillin sodium, a large amount of waste EA− IPA mixture is produced. The recycling of waste materials will reduce the production cost and environmental pollution. However, EA and IPA form an azeotrope with 74.0 wt % EA at atmosphere pressure. It cannot be separated into pure components via ordinary distillation. Extractive distillation (ED) is a popular and usual method to separate azeotropic systems and other mixtures with a relative volatility of the key components below 1.1.1,2 Figure 1 shows a conventional ED

Figure 2. Common scheme of an EDWC.

Figure 1. Process flow sheet of ED.

and B move to the bottom. The bottom product of ED (B + entrainer) is fed into the entrainer recovery column (RC) to make B and the entrainer separate. Then entrainer is recycled back to the ED column. In order to save energy and capital investment, an extractive dividing-wall column (EDWC; Figure 2), known as one kind of thermally coupled distillation sequence, is developed.3−6 Several cases studied by Bravo-Bravo et al.7 have shown the feasibility of performing extractive separations in dividing-wall distillation columns. Gutiérrez-Guerra et al.8 studied three separation systems, and they found that the reboiler duty can be saved in

arrangement/column (CEDC). The fresh feed (F: A + B) from the lower part and the entrainer from the upper stage are fed into the ED column. The entrainer alters the relative volatility between A and B to make A move to the top of the ED column

Received: Revised: Accepted: Published:

© 2013 American Chemical Society

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October 26, 2013 December 9, 2013 December 19, 2013 December 19, 2013 dx.doi.org/10.1021/ie403618f | Ind. Eng. Chem. Res. 2014, 53, 1189−1205

Industrial & Engineering Chemistry Research

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composition control strategy, in which it is also possible to obtain the same control effectiveness. In addition, his model is more reasonable if αv can be configured directly. Wu et al.22 studied the control structure of the EDWC simulated as two columns also. Their control structure featured three temperature control loops on the columns. All of the disturbances are well rejected with small offsets in product purities. However, large deviation is not effectively suppressed when a feed composition disturbance is introduced. This defect can be solved if the entrainer flow rate or vapor split ratio is manipulated. The objective of this work is to investigate the design and control of a separation EA−IPA mixture using a new EDWC model simulated as three columns. To solve the defect that αv cannot be configured in the EDWC model simulated as two columns, we decide to simulate EDWC as a three-column system. Consequently, the effects of αv on other parameters can be investigated more easily. Control structures featuring temperature control or temperature + composition control are established to hold purity specifications when feed disturbances are introduced. However, little research has been done on the new model. Therefore, it is necessary to study control of the EDWC simulated as a new model based on the pioneers’ contributions.

the range between 20 and 30% compared with the CEDC. Wang et al.9 studied EDWC for separating dimethyl carbonate (DMC) and methanol using phenol as the entrainer, and they found that 17.6% reboiler duty can be saved compared with the CEDC. Kiss and Suszwalak10 studied the EDWC for the purpose of bioethanol dehydration, and they found that 9.4% reboiler duty can be saved compared with the CEDC. Arifin and Chien11 studied the EDWC for IPA dehydration using dimethyl sulfoxide (DMSO) as the entrainer. They found that the total annual cost (TAC) is reduced by as much as 32.7% and the energy requirement is also cut by 30.3% compared with heterogeneous azeotropic distillation. Therefore, the EDWC is a promising configuration for saving energy and capital investment. Dynamic control is also an important aspect for ED. Control of the EDWC is much more difficult than that of the CEDC because of its inner structures and interactions among control loops in the EDWC.12 Control of the CEDC and dividing-wall column (DWC) has been studied by many researchers, but control of the EDWC has received less attention, and little research has been done on it. However, the EDWC could be considered as a combination of the DWC and CEDC,13,14 so it may be useful to investigate control of the DWC first in order to study that of the EDWC. Wolff and Skogestad15 pointed out that the DWC has four control degrees of freedom: reflux ratio (R), vapor boilup (V), side steam (S), and liquid spilt ratio (βL). The four control degrees could be used as four control variables during operation to maintain the purities of three products and minimize the energy cost. Wolff and Skogestad,15 Mutalib and Smith,16 and Serraet al.17,18 proposed their own control structures featuring three composition control loops. One common point of these control structures is to control the purities of three products by manipulating the reflux, boilup vapor, and side stream. Their control structures proved to be useful by introducing feed composition disturbances. Ling and Luyben19 proposed their control structure featuring four composition control loops. Their control structure provided a good dynamic response when feed disturbances were introduced, and the reboiler duty is minimized at the same time. However, online composition analyzers are expensive, require high maintenance, and introduce long time delays. Therefore, a temperature control strategy is considered because of its low price, good reliability, and rapid response. However, temperature measurements only give an estimation of the composition. As we all know, in a binary system at constant pressure, there exists one-to-one correspondence between the temperature and composition. However, in a ternary system, this relationship became much more complex. So, control structures using only simple temperature control loops do not necessary provide a good dynamic response in the face of feed disturbance. In view of this, they20 proposed a new control structure using temperature differences to estimate the composition, and this structure provided a good dynamic response. Unfortunately, the relationships between the temperature differences and composition in the EDWC are much more complicated than that of the DWC. So, this control structure does not suit the EDWC. Recently, Xia et al.21 put forward the model of the EDWC simulated as two columns and established a control structure for the system. In the control structure, he used four composition control loops to maintain the purities of two products and obtained quite effective control results. However, he did not consider a temperature control strategy or a temperature +

2. STEADY-STATE DESIGN 2.1. Flow Sheet of the EDWC. Figure 2 shows the common scheme of an EDWC. This column is separated by a vertical wall into left and right parts. The fresh feed and entrainer are fed into the left part. A component is withdrawn from the top of the left part. B and the entrainer are separated in the right part with the B component withdrawn in the top. The entrainer of high purity is obtained in the bottom and recycled back to the left part. It should be noted that only one reboiler is used to provide heat and the vapor from the bottom is split by the wall into left and right parts in proportion to the cross-sectional area of each side. In this work, a new EDWC model (Figure 3) is used to simulate the EDWC. The vapor split ratio can be configured, and

Figure 3. EDWC model simulated as three columns. 1190

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Figure 4. Process flow diagram of the EDWC drawn by Aspen Plus.

Figure 5. Process flow diagram of the CEDC.

the influence of this parameter is investigated directly. So, this model is closer to the EDWC concept. The results of steady-state design and dynamic control may be more accurate compared to that of the EDWC simulated as a two-column system. This new EDWC model consists of three columns: the ED and IPA product (IPAC) columns are linked to the RC via four interconnected vapor and liquid streams. The EA−IPA mixture and entrainer are fed into the ED column at two suitable locations, respectively. A high-purity EA product is obtained from the top of the ED column, a IPA product is obtained from the top of the IPAC, and the bottom stream of the RC is ethylene glycol (EG) of high purity, which is recycled back to the ED column. Figure 4 shows the process flow diagram drawn in Aspen Plus. In this work, EG is selected as the entrainer to separate the EA−IPA mixture because of its good selectivity and dissolubility

Figure 6. Calculation of the EDWC diameter.

to this system. For this overall three-component system, the NRTL model is used to describe the nonideality of the vapor− liquid equilibrium. The complete NRTL model binary 1191



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Figure 7. Process of TAC calculation.

parameters are taken from the Aspen Plus database. The azeotropic point and composition calculated by the NRTL model are quite close to the experimental measurements.23 The ED column is assumed to have a total of 34 stages; the IPAC column has a total of 8 stages. Here, Aspen notation of numbering stages is used, with stage 1 as the condenser and the last stage as the reboiler. Both ED and IPAC columns use stage 1 as the condenser, but neither of the two columns has a reboiler. The RC has a total of six stages, with stage 6 as the reboiler, but the RC has no condenser. The tray pressure drop of the three columns is set at 0.0068 atm. The condenser pressure of the ED and IPAC columns is configured at 1 atm. At this pressure, the reflux-drum temperatures are 350 K (ED) and 355 K (IPAC), which ensures the use of cooling water in the overhead of the condensers. In addition, it is convenient to operate and control the EDWC at atmospheric pressure. The conditions of the feed are set as follows: flow rate of 1000 kg/h, composition of 70/30 wt % EA−IPA, and temperature at 355.4 K, which permits gas-phase feeding. The conditions of the recycle entrainer are set at a flow rate of 1000 kg/h and a

temperature of 340.4 K. Both of the product specifications are set above 99.8 wt %. The specification for the bottom of the RC is 99.999 wt % EG. For the ED column, the reflux ratio is fixed at 2.37, and the flow rate at the top is set at 700.4 kg/h. For the IPAC, the reflux ratio and flow rate are set at 0.133 and 299.6 kg/h. 2.2. Global Economical Optimization. For the design of CEDC, a few parameters should be fixed: total stages (ED NT and RC NT) of ED and RC, reflux ratio (RR1 and RR2), entrainer flow rate (EF), temperature of the entrainer fed into the ED column, and inlet position of the fresh feed and entrainer. Compared with the CEDC, the design of the EDWC involves two more parameters to be fixed: the total stages of the strip section (RC) and vapor split ratio (αv). In view of the complexity of the design for the EDWC, the steady-state design of the CEDC to separate the same system is presented first for the purpose of fixing some parameters of the EDWC at a certain range, and the energy consumption and capital investment of the EDWC can be evaluated by comparison to those of the CEDC. Figure 5 shows the process flow diagram of the CEDC for separating the same system. 1192



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2.3. Economical Evaluation Criteria. A number of parameters of the three columns are optimized based on the TAC suggested by Douglas.24 This evaluation criterion is widely used in screening optimal chemical process designs. The economics of the EDWC are considered in terms of energy cost and capital investment. TAC is defined as the sum of the annual operation cost and capital investment divided by a 3-year payback period. For an EDWC, capital investment includes only one column vessel and three heat exchangers (two condensers and only one reboiler), and the sizes of the three columns with sieve plates are calculated by the Aspen tray sizing section. Because only one shell is required for EDWC configuration, the equivalent diameter22 of the upper part of the EDWC is calculated as illustrated in Figure 6. The diameter is backcalculated so that the total cross-sectional area is provided. However, in view of the complexity of the EDWC inner structure, it is more difficult to construct and install than conventional distillation columns. So, for this reason, we assume that 20% extra capital investment of the column vessel is needed. Other items including reflux drums, pumps, pipes, valves, and electricity are not considered because their expense is much smaller than the cost of the vessel and heat exchangers. The relationships between the sizing, cost, and price of the MP and HP streams are taken from Lyuben’s paper.25 2.4. Global Optimization Sequence. The systematic global optimization sequence for the CEDC has been studied by many researchers, and the TAC of the EDWC simulated as two columns has been researched by Xia et al. On the basis of their previous work, the sequence for the EDWC simulated as three columns is illustrated in Figure 7. The process of calculation can be simplified as Figure 8. The feed stages of the fresh feed and entrainer (NF and NEF) are

Figure 9. Effect of ED NT on the capital investment and operation cost.

on the operation cost and capital investment. With increasing ED NT, the cost of the operation cost decreases while the capital investment increases. From an economic point of view, a tradeoff between the capital investment and operation cost is considered. Figure 10 shows the effect of EF and ED NT on

Figure 10. Effect of ED NT and EF on the TAC.

the TAC. Note that there exists a minimum TAC when EF is 1000 kg/h and ED NT is 34. 2.4.2. αv. This parameter should be considered carefully because the vapor steam and heat from the top of the RC is split into the left and right parts according to its value. When the total stages of ED increase, the heat input that the ED column requires becomes less, and the optimal vapor split value should be fixed at a smaller value to prevent more heat from moving to ED and being wasted. However, at the same time, the capital investment of ED increases. That is, once ED NT is given, there should exist an optimal value of αv that minimizes the energy cost and gives the minimal TAC. Figure 11 shows the effect of αv on the TAC, and the value of αv is optimal when ED NT is fixed. 2.4.3. RC NT and IPAC NT. The function of the RC is to provide heat to ED and IPAC and to prevent light components (EA and IPA) from going to the bottom. If the total stage number of the RC is too small, the reboiler has to provide much more heat to ensure high purity of the entrainer. The function of the IPAC is to withdraw IPA product and to prevent EG from going to the top. Figure 12 shows the effect of RC NT and IPAC NT on the TAC. Therefore, RC NT and IPAC NT are fixed at 6 and 8, respectively. Table 1 gives the design and economic results obtained by TAC calculation introduced above so that the EDWC can be

Figure 8. Simplified representation of TAC calculation.

used for the inner iterative loop to minimize energy cost, αv for the middle loop to minimize energy cost, NT of ED and EF for the outer loop to minimize the TAC. After that, RC NT and IPAC NT are varied to minimize the TAC. To make the circulation process easier and faster, the initial values of RR1, RR2, and NT of RC are speculated by referring to the CEDC of minimal TAC. 2.4.1. EF, RR1, and ED NT. For a given EF value, there exists an optimal RR1 that gives maximum purities of the two products. For a given RR1 value, the purities of the two products increase with the rising EF value. In other words, the optimal RR1 is fixed when the EF value is given. Figure 9 shows the effect of ED NT 1193



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Figure 11. Effect of αv on the TAC.

Figure 13. Flow sheet of the EDWC with details.

2.5.1. Recycle EF. For an EDWC fixed with definite stages, there is an optimal EF to satisfy two-product specifications and minimize the reboiler duty. The entrainer temperature is set at 340.3 K, which is thought to be appropriate when it is 10−15 K lower than the reflux-drum temperature of the ED column.26,27 The results are obtained using the design/spec section of Aspen Plus based on the EDWC of the optimal parameters. The purity of the entrainer is held at 99.999% by adjusting the reboiler duty, with the flow rate of the EA product (D1) being fixed at 700.4 kg/h and the flow rate of the IPA product (D2) being fixed at 299.6 kg/h. Figure 14 shows that two-product specifications can be achieved when the EF exceeds 1000 kg/h. With rising EF, the purities of both products increase because a higher EF benefits the separation of EA and IPA. This phenomenon indicates that increasing EF helps to maintain the purity of the two products, and it may be useful to apply this strategy in dynamic control. However, the control range of EF should be considered carefully because the reboiler duty is also influenced by EF. 2.5.2. αv. αv is defined as V1/V, as shown in Figure 2. Generally, it is thought to be fixed during the operation; however, some researchers16,18 try to use it as an additional control variable because the EDWC is more difficult to control than the CEDC, and adjustment for αv might be helpful to maintain the product purity when a feed disturbance is introduced. Figure 15 illustrates the effect of αv on XD1, XD2, RR1, and energy consumption. The results are obtained using the design/ spec section of Aspen Plus based on optimized configurations of the EDWC. The purity of the entrainer is held at 99.999% by adjusting the reboiler duty, with the flow rate of the EA product (D1) being fixed at 700 kg/h and the flow rate of the IPA product (D2) being fixed at 300 kg/h. With an increase in αv in the range between 0.5 and 0.75, the purities of both products increase. However, the reboiler duty increases too. Therefore, there exists an optimal αv value to satisfy the specifications of both products and minimize energy consumption. This phenomenon can be explained by when αv increases, more IPA is brought to the ED column than to IPAC, and as a result, RR1 must be increased to hold D1 constant. With rising RR1, the purity of D1 increases. At the same time, less EA is brought to the IPAC, and as a result, the purity of D2 increases. It should be noted that once αv exceeds 0.75 (the red point in the upper two pictures), XD1 begins to fall. At this time, the reflux

Figure 12. Effect of RC NT and IPAC NT on the TAC.

Table 1. Economic Results of the EDWC and CEDC parameter

EDWC

CEDC

ED NT IPAC NT RC NT RR1 RR2 αv EF/kg/h column diameter/m reflux drum for ED (L/H)/m reflux drum for RC (L/H)/m base (L/H)/m rotal reboiler duty/MW capital investment/106$ operation cost/106$ TAC/106$

34 8 6 2.37 0.132 0.67 1000 0.436 0.671/1.34, 0.671/1.34 0.368/0.736, 0.368/0.736 0.482/0.965, 0.482/0.965 0.265 0.170a 0.0401 0.111

40 \ 8 2.00 0.138 \ 1000 0.411/0.216

0.279 0.210 0.0539 0.124

a

The capital investment of the EDWC increases about 20% if the manufacturing difficulty of the EDWC is considered.

compared with the CEDC in terms of capital investment and energy cost. The flow sheet of the EDWC with details is shown in Figure 13. 2.5. Sensitivity Analysis. The steady-state design of the EDWC with minimal TAC is established. On this basis, the relationships between some important variables that we are interested in are studied by the design/spec section of Aspen Plus, which may be useful to establish dynamic control schemes for the EDWC. 1194



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Figure 14. Effect of EF on XD1, XD2, and the reboiler duty.

ratio becomes very large, and consequently the entrainer is highly diluted. So, the purity of D1 begins to decrease. This phenomenon also indicates that the purities of the D1 and D2 products can be controlled by manipulating αv in the range between 0.65 and 0.75. This idea, of course, should be tested in dynamic response to evaluate its control effectiveness.

3. CONTROL STRATEGY FOR THE EDWC In this part, three control structures are proposed for the EDWC, the parameters of which are configured based on the steady-state design of minimal TAC. A basic control structure is put forward first, and then two improved structures are presented on the basis of the basic control structure so that the purities of products can be held when feed disturbances are introduced. It is necessary to determine the size of the EDWC before steady-state simulation is converted to the dynamic one. Thus,

Figure 15. Effect of αv on XD1, XD2, RR1, and energy consumption.

the tray sizing section in Aspen Plus is employed to define the sizes of equipment such as reflux drums, column base, and so on (shown in Table 1). The reflux drums and column bases are sized to provide 5 min of holdup when at the 50% liquid level. Pumps and valves are inserted to provide enough pressure drops so that the flow sheet is fully pressure-driven and the changes in the flow rates are handled with good range ability. 1195



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Figure 16. Temperature profiles of the three columns and temperature differences of every stage.

Then, the flow sheet is pressure checked and converted to a dynamic simulation. 3.1. Selecting Temperature Control Trays. Dynamic control of the CEDC has been studied by quite a lot of researchers. For the ED column and RC, optimal temperature control trays are selected to control the columns by regulating

the reflux ratio or reboiler duty. It is often possible to achieve very effective control. Therefore, tray temperature control is considered first for the EDWC. Figure 16 shows the temperature profile of three columns. Because the tray temperature is used, we should find out the best temperature control locations on which the temperature is held 1196



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Figure 17. CS1 and its control panel.

sensitivity criterion is used to find the tray in which there is the largest change in temperature for a change in the manipulated variable. For the ED column, a small change (±1% of RR1) is

constant. A few methods are well summarized in Luyben’s book,28 and these methods only need steady-state information. Therefore, analysis can be performed on Aspen Plus. Here, a 1197



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Figure 18. Dynamic responses (CS1) to flow rate disturbances.

(1) The fresh feed flow rate is held constant by flow control (reserve acting). (2) EF is ratioed to the feed flow rate by manipulating the bottom flow rate (reserve acting). The temperature of the entrainer is held constant by controlling the cooler duty of the heat exchanger. (3) The top pressures of ED and IPAC are controlled by manipulating the two condenser duties (reserve acting). (4) The levels of two reflux drums are controlled by manipulating the flow of the two top products (direct acting). (5) The level of the RC base is controlled by manipulating the makeup flow rate (reserve acting), which is suggested by Grassi29 and Lyuben.30 Once a positive 20% step change in the feed flow rate occurs, there is an immediate increase in the total flow rate of the entrainer. As a result, the base level starts to drop at the same time. However, as more entainer is fed into the column, the base level will be brought back eventually. This structure proves to be useful in a few papers.27,31,32 (6) The temperature on the 13th tray of ED and the temperature of the 4th tray of IPAC are controlled by manipulating the corresponding reflux rates (reserve acting), which are cascaded with the feed flow rate by R/F ratio control.

Table 2. Parameters of All Temperature Controllers parameter

TC1

TC2

TC3

TC-HX

ultimate gain ultimate period/min gain, Kc integral time/min

5.800 7.200 1.817 15.84

0.4390 6.960 0.1372 15.31

10.00 12.67 3.123 12.67

0.1811 3.96 0.5795 1.8

made in RR1. The resulting change in the temperature of all trays is compared to find which tray has the largest change in temperature. The procedure is repeated for the other manipulated variables. For the IPAC, a small change is made in RR2. Also for the RC, a small change is made in the reboiler duty. The tray with the largest temperature change is considered to be the most “sensitive” and will be selected. The results are shown in Figure 16, and each column has a point of the largest temperature difference. Therefore, the 13th tray of ED, 4th tray of the IPAC, and 3rd tray of the RC are selected as control stages. 3.2. Control Structures for the EDWC. 3.2.1. Basic Control Structure 1 (CS1). The basic control structure established for the three-column system and the controller face plate are presented in Figure 17. The control strategy is outlined as follows: 1198



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Figure 19. Dynamic responses of CS1 to composition disturbance.

Figure 20. Control structure of CS2. 1199



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Figure 21. Dynamic response (CS2) to flow rate disturbances.

Figure 22. Dynamic responses (CS2) to composition disturbances. 1200



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Figure 23. Control structure of CS3.

Figure 24. Dynamic responses (CS3) to flow rate disturbances. 1201



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Figure 25. Dynamic responses (CS3) to composition disturbances.

(7) The temperature on the 3rd tray of the RC is controlled by the reboiler heat input (reserve acting). (8) 1 min of dead time is inserted into all temperature control loops to fit the practical operation. Common proportional and integral (PI) settings are utilized for the control loops. All level loops are only proportion controllers with Kc (gain) = 2. The pressure controllers of two condensers are PI with Kc = 20 and τ1 (integral time) = 12 min. The flow controllers are PI with Kc = 0.5 and integral time = 0.3 min. However, for all of the control loops with dead time, relayfeedback tests are run and Tyreus−Luben turning is utilized to get the ultimate gains and periods of these controllers. The PI parameters of the temperature controllers are listed in Table 2. The basic control structure has been established, and all controllers have been configured. Then this control structure is tested by introducing disturbances to investigate its control effectiveness. Figure 18 shows the results of dynamic response upon going through the ±20% step changes in the feed flow rate at 0.5 h. As can be seen from the temperature plots, three controlled tray temperatures and the recycle entrainer temperature are brought back to their set values and product purities are held close to the desired set values when this system reaches a

new steady state at 3 h. However, deviation of the two product purities is quite large when the feed flow rate is changed. Figure 19 shows dynamic responses to disturbances of ±10% composition change of EA at 0.5 h. To fit real industrial practice, the composition of EA is reduced to 0.63 (blue lines) and increased to 0.77 (red lines), and the component of IPA is changed correspondingly as well. As can be seen from the pictures, two product purities are held close to the set points; however, the purity of EA is a bit lower than the set value when the composition of EA in the fresh feed is changed to 63 wt %. As a basic control structure, CS1 is established to investigate the control effectiveness of a tray temperature control strategy. Dynamic response shows that three temperature control structures work well when a disturbance is introduced. In addition, the purities of the two products are held close to the set values by this control structure. However, we also note that large deviation of the two product purities occurs when feed disturbances are introduced. Therefore, this control structure should be improved to obtain a better dynamic performance. 3.2.2. Improved Control Structure (CS2) with a QR/F Ratio and IPA Composition Controller Cascaded with an E/F Ratio. To reduce large deviation, the basic control structure must be 1202



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Figure 26. Dynamic responses (CS1, CS2, and CS3) of purities to feed disturbances.

3.2.3. Improved Control Structure (CS3) with QR/F and αv Control. In CS2, EF is adjusted to meet the specifications of the two products. Once the purity of IPA is lower than the set value of 99.8%, the EF is increased to bring the purities of both products back. Because adjustment of αv has an effect on XD1 and XD2, we try to use it as a control variable to hold the purities. Here, a new control structure is put forward based on CS1. As shown in Figure 23, the volume flow of the vapor stream into ED from the bottom is manipulated by αv, which is controlled by a IPA composition controller (Kc = 4.173 and τ1 = 64.68). The reason why the purity of IPA can be controlled by αv has been discussed above in sensitivity analysis. Once the purity of D2 is

improved. As shown in Figure 20, the third tray temperature of the RC is controlled by the reboiler heat input, which is cascaded with the feed flow rate by a QR/F ratio, so the reboiler duty can be changed immediately when a ±20% step change in the feed flow rate occurs. In addition, the composition of IPA is controlled by adjusting the mass ratio of entrainer to feed (EF/F) via a composition controller (Kc = 95.155 and τ1 = 138.6 min). The effect of EF on XD1 and XD2 has been introduced in sensitivity analysis. Figures 21 and 22 give dynamic responses to flow rate disturbances and composition disturbances. The purities of both products are brought close to the set points, and deviations are much smaller than the results of CS1. 1203



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lower than the set value, the value of αv is increased to improve the purity. In CS3, the control range of αv is set between 0.65 and 0.75. Dynamic responses for flow rate disturbances are shown in Figure 24, while responses for composition disturbances are shown in Figure 25. The purities of the two products are brought back to the set points, and deviations as well as the results of CS2 are quite small. 3.3. Comparisons of Three Control Structures. The control effectiveness of three control structures should be compared with each other in the terms of transient deviation and settling time. Figure 26 shows the dynamic results of the two product purities to flow rate change. As shown in these pictures, CS1 cannot overcome the large transient when feed flow rate disturbances occur, while CS2 and CS3 give small peak transient deviation. In addition, the settling times of CS2 and CS3 are shorter than that of CS1. Figure 26 also demonstrates responses of the two product purities to feed composition change. As can be seen from these pictures, the dynamic performance of CS3 is much better than that of CS1. However, CS2 does not work well in rejecting the composition disturbance of EA 63 wt % because the IPA purity a bit lower than the set value in a new steady state. This problem can be solved when the composition disturbance is limited to a bit smaller range. In general, CS2 and CS3 show much better responses than CS1 with the same disturbances. CS3 performs a little better than CS2. However, from the perspective of practical application, the device to realize control of αv will make the inner structure of the EDWC more complicated. Therefore, CS2 is more likely to be used in industrial practice.

ACKNOWLEDGMENTS

We are thankful for support from the project fund of the China Petroleum & Chemical Corp. (411024) and assistance from the staff at the Institute of Petrochemical Technology (Changzhou University and China University of Petroleum Beijing).

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4. CONCLUSIONS In this work, the design and control of an EDWC simulated as three columns to separate a EA−IPA mixture using EG as the entrainer are studied. The steady state of the EDWC of minimal TAC is established first, and then sensitivity analysis is conducted to investigate the influence of EF and αv on the purities of the two products. We find that it might be useful to select EF and αv as manipulating variables to reject feed disturbances. Then, the best temperature control locations are found according to sensitivity criterion, and a basic control structure is put forward. After that, the control effectiveness of the tray temperature loops is tested by introducing feed disturbances. According to the dynamic results, the tray temperatures are brought back to the set values; however, large deviations of product purities occur and the purity of EA does not reach the set point when one component disturbance is introduced. To solve these problems, two improved control structures are put forward based on the basic control structure and sensitivity analysis. It is found that two improved control structures achieve satisfactory performance with much smaller deviation and shorter settling time by comparison to the basic control structure. Dynamic results also revealed that it is useful to adjust EF or αv to hold the purity specifications.

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NOMENCLATURE CEDC = conventional extractive distillation arrangement/ column CS = control structure/scheme DWC = dividing-wall columns EA = ethyl acetate ED = the extractive distillation column ED NT = total stages of ED EDWC = extractive dividing-wall column EF = the entrainer feed/entrainer flow/entrainer flow rate EF/F = the mass ratio of entrainer to feed EG = ethylene glycol F = the fresh feed of the extractive distillation column IPA = isopropanol IPAC = IPA product column NF = feed stage of the fresh feed NEF = feed stage of the entrainer RR1 = the reflux ratio of ED RR2 = the reflux ratio of RC for CEDC or the reflux ratio of IPAC for EDWC RC = the entrainer recovery column RC NT = total stages of RC R/F = the mass ratio of reflux flow rate to flow rate of F TAC = total annual cost TC = temperature controller XD1 = the purity of top product in ED XD2 = the purity of top product in RC QR = reboiler duty QR/F = reboiler duty of ED/mass flow rate of F PI = proportional and integral settings Kc = gain αv = vapor split ratio τ1 = integral time REFERENCES

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Design and Control of Extractive Dividing-Wall Column for Separating

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