Design and Control of Dividing-Wall Column for tert-Butanol

Article pubs.acs.org/IECR

Design and Control of Dividing-Wall Column for tert-Butanol Dehydration System via Heterogeneous Azeotropic Distillation Hao Yu, Qing Ye,* Hong Xu, Hao Zhang, and Xin Dai Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Institute of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China ABSTRACT: In this Article, a three-column model of azeotropic dividing-wall column (A-DWC) for heterogeneous azeotropic distillation (HAD) is investigated by demonstrating the design and control of tert-butanol (t-butanol) dehydration using cyclohexane as entrainer. Significant energy consumption savings of 23.80% and a minimal total annual cost (TAC) reduction of 19.93% between the optimum A-DWC design and the original two-column design can be obtained, respectively. Sensitivity analysis of whether the liquid split ratio (βL) can be a manipulated variable to maintain the product purities is conducted before the control structure for the A-DWC is established. Meanwhile, with two reboiler duties (QRs) still preserved in the A-DWC, the proposed control structure can solve disturbance issues and maintain the product purities very close to the set points with small deviation and short settling time.

1. INTRODUCTION tert-Butanol is widely used in the fields of chemicals and pharmaceuticals, and is usually obtained by hydration or the hydrolysis method. The demand for the product anhydrous t-butanol is large. However, t-butanol and water can form a homogeneous azeotrope at atmospheric pressure. It cannot be separated into pure components via the ordinary distillation method. Azeotropic distillation is a popular and usual method to separate azeotropic systems using an entrainer to introduce heterogeneous low boiling azeotropes with one or more of the original components and to generate two immiscible liquid phases that can be conveniently separated.1,2 In this situation, the phase separation of the condensed vapor on top of the column is necessary. The advantage of this separation is to utilize a natural liquid−liquid separation in a decanter. Both liquid phases have different concentrations of entrainer. The organic phase has more entrainer, and the aqueous phase has little. Each phase is separated into a different column to obtain a pure product and recycle of the entrainer at the same time. Although the azeotropic distillation technique is widely used, it has lost public acceptance due to its high energy consumption. In the open literature, dividing-wall column (DWC) is one of the best examples of proven process intensification technology for the separation of mixtures with three or even more components in distillation, as it allows significant savings in energy consumption. Varieties of case studies of utilizing this technology with significant energy savings could be found. Annakou and Mizsey3 compared the conventional distillation sequences, heatintegrated schemes, and the fully thermally coupled distillation column known as DWC. The result showed that the energy savings of DWC were considerable as compared to the conventional systems. Emtir and Fonyo4 studied an ethanol/ n-propanol/n-butanol ternary mixture at three different feed compositions; the result showed that the reductions in TAC of DWC are uniformly about 28−33%. Hernández et al.5 studied DWC with two cases of different ternary mixture for three different feed compositions, respectively. The result showed that © 2015 American Chemical Society

each case for different feed composition resulted in energy savings of 15−34%. Gómez-Castro et al.6 studied DWC with six cases of different feed components. They found that each case obtained a different degree of energy saving about 3.99−32.74%. Long and Lee7 tested the proposed design of DWC for a natural gas liquid (NGL) recovery system; DWC reduced steam cost and TAC by 28.23% and 25.49%, respectively. Recently, Kiss and Suszwalak8 demonstrated the utility of an A-DWC for the bioethanol dehydration system using n-pentane as entrainer. The result showed that A-DWC can lead to 20.2% of reduction in energy saving. Wu et al.9 put forward an A-DWC model simulated as two thermally couple columns and proposed a control structure by demonstrating the pyridine dehydration system using toluene as entrainer. The result showed 29.48% and 31.41% reductions in energy consumption and TAC, respectively. Meanwhile, the proposed control structure achieved satisfactory performance. Wu et al.10 also investigated the utility of an A-DWC for the 1,4-dioxane dehydration system. The result showed that significant savings of 49.09% in operating costs and 43.43% in TAC can be achieved. Another important aspect for azeotropic distillation is dynamic control. However, the control of DWC is much more complex than that of the conventional column system as its inner structures and interactions among control loops in it. Various authors have selected different ternary component systems to be separated and have proposed different kinds of control structures including conventional control strategies as well as advanced ones. Yildirim11 pointed out that DWC has seven degrees-offreedom. Six of them are usual for a distillation column with side stream; they are the condenser duty (QC), the product rates including distillate (D), side stream (S), and bottoms (B), the Received: Revised: Accepted: Published: 3384

November 1, 2014 March 12, 2015 March 17, 2015 March 17, 2015 DOI: 10.1021/ie504325g Ind. Eng. Chem. Res. 2015, 54, 3384−3397

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Industrial & Engineering Chemistry Research

Figure 1. RCMs of t-butanol/water/cyclohexane system.

Figure 3. Process of the conventional two-column system TAC calculation.

Figure 2. Process flow sheet of t-butanol dehydration system.

reflux (L) and the QR, or, equivalently, the vapor boilup (V). The additional one is βL. Van Zile12 and Kaibel13 proposed that there was a method to manipulate βL in DWC for practical application. In a two-column model, only one split liquid stream can be manipulated, while the two split liquid streams can both be manipulated in the three-column model, which is more in agreement with practical application. Ling and Luyben14 manipulated the βL to achieve some control objective during operation. They proposed a new control structure by controlling the heavy component composition in the prefractionator section to maintain the product purities and also minimize the energy consumption. Van Diggelen et al.15 made a comparison of various control strategies based on proportional integral derivative (PID) loops, within a multiloop framework, versus more advanced controllers such as linear quadratic Gaussian (LQG)/linear quadratic regulation (LQR), generic model control (GMC), and high order controllers obtained by H∞ controller synthesis and μ-synthesis. Ling et al.16 also proposed a new control structure by controlling the remixing of the intermediate component at the top trays in the prefractionator section for the separation of a benzene/toluene/ xylene (BTX) mixture. Kiss and Bildea17 gave an overview of the available control strategies for DWC, varying from the classic three-point control structure and PID controllers in a multiloop framework to model predictive controllers (MPC) and other

Figure 4. Effect of NT1 on the TAC of the conventional two-column system.

advanced control strategies. Kiss and Rewagad18 studied a separation of a ternary mixture BTX in a DWC; several conventional control structures based on PID control loops were used as a control basis, which were enhanced by adding an extra loop controlling the heavy component composition in the top of the prefractionator, by using the liquid split as an additional manipulated variable. With the same system, they19 also advanced control strategies based on MPC, coupled or not with PID, which were enhanced by adding an extra loop controlling the heavy component in the top of the feed side of the column, using the liquid split as manipulated variable, thus implicitly achieving energy minimization. 3385

DOI: 10.1021/ie504325g Ind. Eng. Chem. Res. 2015, 54, 3384−3397

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Industrial & Engineering Chemistry Research The objective of this work is to investigate the design and control of the t-butanol dehydration system using an A-DWC model simulated as three-column. Furthermore, there is no paper

Figure 8. Common scheme of an A-DWC. Figure 5. Effect of NT2 on the TAC of the conventional two-column system.

Figure 6. Process flow sheet of conventional two-column system with details.

Figure 9. Process flow sheet of the A-DWC.

Figure 7. Liquid composition profiles of the conventional two-column system. 3386


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azeotrope with t-butanol and water in industry separation. The t-butanol/water mixture has an azeotrope with composition of 62.1 mol % t-butanol and an azeotropic temperature of 79.9 °C at atmospheric pressure. By adding cyclohexane into the system, three additional azeotropes are formed. One desirable azeotrope (t-butanol/water/cyclohexane) is heterogeneous, with an azeotropic temperature of 65.2 °C, which is the minimum temperature for the entire ternary system. Another two azeotropes between t-butanol, water, and cyclohexane are also formed with azeotropic temperatures of 71.9 and 69.5 °C, respectively. As shown in Figure 1, the distillation boundaries divide the residue curve maps (RCMs) of the ternary system into three distillation regions.

in the open literature that has studied the detailed control of this three-column model of A-DWC for HAD system with one column serving as a preconcentrator column and another column serving as a HAD column with an undivided rectifying section. The energy saving potential of the A-DWC will be investigated as compared to two conventional two-column systems. In the A-DWC, βL can be configured, and the effect of this parameter will be investigated more directly and easily. Except the energy saving potential, a control structure featuring tray-temperature control is established to closely examine the dynamic control ability of the A-DWC.

2. CONVENTIONAL TWO-COLUMN DESIGN For the t-butanol dehydration system, cyclohexane is commonly selected as the entrainer to form a ternary heterogeneous

Figure 10. Calculation of the A-DWC diameter.

Figure 12. Effect of NT1 on the TAC of the A-DWC.

Figure 11. Process of TAC calculation of the A-DWC. 3387


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which serves as the HAD column. High-purity t-butanol product is obtained at the bottom of the HAD column; meanwhile, the minimum boiling t-butanol/water/cyclohexane ternary azeotrope is formed on the top of the HAD column, which is heterogeneous, and the top vapor stream is then condensed to form two liquid phases in the decanter. The organic phase containing mainly cyclohexane is refluxed back to the HAD column, and the aqueous phase is drawn out from the decanter to be brought into the preconcentrator column. For this overall ternary system, the NRTL model is used to describe the nonideality of the vapor−liquid equilibrium. The azeotropic point and composition calculated by the NRTL model are quite close to the experimental measurements.20 The tray pressure drop of the columns is set at 0.0068 atm. The conditions of the feed are set as follows: flow rate of 100 kmol/h, composition of 50/50 mol % t-butanol/water, and temperature at 30 °C. The entrainer makeup flow rate is set at 0.004 kmol/h to balance out the tiny entrainer loss through the two product streams. For the design of the conventional two-column system, a few parameters should be investigated: total number of trays (NT) of the preconcentrator column and the HAD column, and feed locations of each feed. The specifications of the preconcentrator column and the HAD column are to have the bottom purity to be 99.8 mol % of water and t-butanol, respectively. By varying the NT and the feed locations simultaneously in the rigorous simulation, optimum tray number as well as the optimum feed locations for the columns can be obtained through exhaustive comparison of the various TAC results. A number of parameters of the columns are optimized on the basis of the TAC suggested by Douglas.21 This evaluation criterion is widely used in screening optimal chemical process designs. The economics of the process are considered in terms of energy cost and capital investment. The capital investment is considered in terms of the investment in the major pieces of equipment including vessels and heat exchangers (HXs). Other items including decanter, pumps, valves, and pipes can be neglected as their expense is much smaller than the cost of the vessels and HXs. TAC is defined as the sum of the annual operation cost and capital investment divided by a 3-year payback period. The relationships between the sizing, cost, and price of the low-pressure steam are taken from Lyuben’s paper.22 The tray number is counting from top to bottom with the condenser/reflux drum as the first tray and the reboiler as the last tray. Each TAC in the figures is selected as the minimum by varying feed locations with a NT. The systematic global optimization sequence for the conventional two-column system is illustrated in Figure 3. Figures 4 and 5 show the effect of the preconcentrator column NT and the HAD column NT on the TAC of the conventional two-column system, respectively. The optimum NT of the preconcentrator column is 8 with feed location at the third and sixth tray. The optimum NT of the HAD column is 32 with feed location at the sixth tray. The flow sheet of the conventional twocolumn system with details is shown in Figure 6. The liquid composition profiles for the conventional two-column system with minimal TAC are shown in Figure 7.

Figure 2 shows the process flow sheet of a conventional azeotropic distillation for the t-butanol dehydration system with two columns and one top decanter. The fresh feed of the t-butanol/water mixture is fed into the column C-1, which serves as the preconcentrator column where at the bottom they obtain high-purity water product. The top product of the binary azeotrope of t-butanol/water is brought into the column C-2,

3. DIVIDING-WALL COLUMN DESIGN The common scheme of an A-DWC is shown in Figure 8. This column is separated by a vertical wall into left and right parts. The fresh feed (F: A + B) is fed into the left part, and entrainer is made up into the decanter. B component is withdrawn from the bottom of the left part. A/B binary homogeneous azeotrope is separated into the right part with the aid of the entrainer and A

Figure 13. Effect of βL on the TAC of the A-DWC.


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Figure 16. Process flow sheet of the A-DWC with details.

Figure 17. Liquid composition profiles of the A-DWC. 3389


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Figure 9 shows the process flow sheet of the A-DWC for separating the same system, which consists of three columns. The lower two stripping columns are linked to the upper undivided rectifying section via four interconnected vapor and liquid streams. Column C-1 serves as the preconcentrator column and column C-2 serves as the HAD column, while column C-3 serves as the undivided rectifying section. In this A-DWC, βL is defined as βL = L2/L. In this A-DWC, the conditions of fresh feed and the specifications of columns are the same with the conventional two-column system. As compared to the conventional twocolumn system, the design of this A-DWC involves two more parameters to be investigated: the undivided rectifying section NT and βL. For the A-DWC, the capital investment includes only one column vessel and three HXs, and the sizes of the three columns with sieve plates are calculated by the Aspen tray sizing section. As only one shell is required for the A-DWC configuration, the equivalent diameter of the lower part of the A-DWC is calculated as illustrated in Figure 10. The value will be used to calculate the capital cost for the column shell. The diameter is back-calculated, so that the total cross-sectional area from both sides of the dividing wall can be provided. However, in view of the complexity of the A-DWC inner structure, it is more difficult to construct and install than conventional distillation columns, so that 20% extra capital investment of the column vessel is needed. The systematic global optimization sequence for the A-DWC is illustrated in Figure 11. The feed locations of fresh feed and aqueous phase are used for the inner iterative loop to minimize TAC, the βL is used for the middle loop to minimize TAC, and the preconcentrator column NT is for the outer loop to minimize the TAC. After that, the HAD column NT and the undivided rectifying section NT are varied to minimize the TAC.

component withdrawn in the bottom. It should be noted that the liquid stream from the top is split by βL. In this work, a three-column model is used to simulate the ADWC. Two stripping columns are introduced into the system with the advantages in energy conservation that the energy carried by the overhead vapor is conserved instead of being removed in a condenser.23 A fictitious valve is placed in each of the two vapor lines of the two divided bottom sections with a very small design pressure drop. βL can be configured, and the influence of this parameter is investigated directly. So, this model is close to the A-DWC concept. Table 1. Economic Results of the Conventional Two-Column and A-DWC parameter NT1 NT2 NT3 βL column diameter/m reflux drum (L/H)/m base (L/H)/m decanter (L/H)/m reboiler duty/GJ/h consender duty/GJ/h heat exchanger duty/GJ/h capital investment/106$ operation cost/106$ TAC/106$

conventional twocolumn 8 32

0.98/1.74 0.47/0.94 1.01/2.02, 0.82/1.64 2.00/4.00 6.99/14.61 5.82 14.69 0.688 1.210 1.465

A-DWC 6 31 2 0.90 1.62 0.47/0.94, 0.83/1.66 1.90/3.80 6.44/10.03 15.37 0.752a 0.922 1.173

a

Capital investment of the A-DWC increases about 20% if the manufacturing difficulty of the A-DWC is considered.

Figure 18. Effect of βL on the purity of water, t-butanol, and the two QRs. 3390


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Figure 19. Open-loop temperature sensitivity analyses of A-DWC.

Figure 20. Overall control strategy of A-DWC.

optimal value of βL that minimizes the energy consumption and gives the minimal TAC. Figure 13 shows the effect of βL on the TAC of the A-DWC, and the optimal value of βL is 0.90. Figure 14 shows the effect of the HAD column NT on the TAC of the A-DWC. The optimal NT of the HAD column is 31, which can meet the minimal TAC and separate the t-butanol more efficiently. The minimal NT of the undivided rectifying section can reduce both the preconcentrator column and HAD column energy

Figure 12 shows the effect of the preconcentrator column NT and feed locations on the TAC of the A-DWC. The optimum NT of the preconcentrator column is 6 with the fresh feed location at the third tray and the aqueous phase feed location at the fourth tray. The parameter βL should be considered carefully because the liquid stream is split into the preconcentrator column and HAD column according to its value. When the βL is changed, the liquid streams split into the two columns are also changed with the result that the two QRs changed. That is, there should exist an 3391


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Figure 21. Control structure in a P&I drawing.

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 A-DWC. As the A-DWC is more difficult to control than the conventional two-column, the adjustment for βL might be helpful to maintain the products purities. Figure 18 illustrates the effect of βL on the purity of t-butanol, water, and the two QRs. The results are obtained using the design/spec section of Aspen Plus based on optimized configurations of the A-DWC. With an increase in βL in the range between 0.80 and 0.90, the purities of both products increase. When the value of βL is in the range of 0.86−0.95, the two products maintain at high purity, and it shows that βL can be held in a certain range to meet the two products specifications. However, the QRs have different performance. With the value of βL increasing, the QR of the preconcentrator column reduces and that of the HAD column shows opposite results. Therefore, there exists an optimal βL value to satisfy the specifications of both products and minimize energy consumption. This phenomenon indicates that the purities of water and t-butanol can be controlled by manipulating βL. This result would be examined in dynamic response to evaluate its control effectiveness.

Table 2. Parameters of All Temperature Controllers parameters

TC1

TC2

TCDEC

controlled variable manipulated variable KU (%/%) PU (min) KC (%/%) τ1 (min)

T5 QR1 20.128 4.80 6.290 10.56

T17 QR2 60.132 3.00 18.791 6.60

TDEC QHX 8.758 6.60 2.740 14.52

consumption as well as the TAC of the A-DWC. Figure 15 shows the effect of the undivided rectifying section NT on the TAC of the A-DWC. Note that the TAC is reducing with decrease of the undivided rectifying section NT. Using two trays of undivided rectifying section, one can obtain the minimal TAC of the A-DWC. The flow sheet of the A-DWC with details is shown in Figure 16. The liquid composition profiles for the A-DWC with minimal TAC are shown in Figure 17. It is shown that the design serves the purpose of carrying large amounts of water to the preconcentrator column bottom and a large amount of t-butanol to the HAD column bottom. Table 1 gives the design and economic results obtained by TAC calculation introduced above so that the A-DWC can be compared to the conventional two-column system in terms of capital investment and energy consumption. It can be found that significant energy consumption savings of 23.80% and TAC reduction of 19.93% between the optimum A-DWC design and the conventional two-column design can be obtained, respectively. The steady-state design of the A-DWC with minimal TAC is established. On this basis, the relationships between some

4. CONTROL STRATEGY FOR THE A-DWC In this section, an efficient control structure is proposed for the A-DWC, the parameters of which are configured on the basis of the steady-state design of minimal TAC. Dynamics and control of the proposed design flow sheet will be examined to determine if any sacrifice needs to be made. It is necessary to determine the size of the A-DWC before steady-state simulation is converted to the dynamic one. Thus, the tray sizing section in Aspen Plus is employed to define the sizes of equipment such as decanter, 3392


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Figure 22. Dynamic responses to feed flow rate disturbances.

column base, and so on (shown in Table 1). The 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. The flow sheet then is pressure checked and converted to a dynamic simulation. 4.1. Selecting Temperature Control Trays. In the control study, control structures by using multiloop tray temperature

control are usually implemented for wider industrial applications. Wu et al.24 studied the extractive dividing-wall column system; they pointed out that the control performance was hampered as losing one important degree-of-freedom, which is a QR. It this case, it does not happen as both QRs are still preserved in the A-DWC. Therefore, tray temperature control is considered for the A-DWC. Because the tray temperature is used, we should find out the best temperature control locations on which the 3393


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Figure 23. Dynamic responses to feed composition disturbances.

is compared to find which tray has the largest change in temperature. For the HAD column, a small change is also made in QR2. The tray with the largest temperature change is considered to be the most “sensitive” and will be selected. The results are shown in Figure 19, and each column has a point of the largest temperature difference. Therefore, the fifth tray of the preconcentrator column and the 17th tray of the HAD column are selected as control locations.

temperature is held constant. A few methods are well summarized in Luyben’s book,25 and these methods only need steady-state information. Therefore, analysis can be performed on Aspen Plus. In this case, a 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 preconcentrator column, a small change (±0.1%) is made in QR1. The resulting change in the temperature of all trays 3394


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Figure 24. Dynamic responses to βL manipulated variable.

4.2. Establishing Control Structure for the A-DWC. The overall control structure established for the three-column system is shown in Figure 20, and Figure 21 shows the control structure in a P&I drawing. The inventory control loops are designed as follows:

(2) The top pressure of the undivided rectifying section is controlled by manipulating the top vapor flow (reserve acting). (3) The levels of the bases of the preconcentrator column and HAD column are controlled by manipulating the flow rate of the two bottom products. For the undivided rectifying section, the level is controlled by manipulating the flow rate of the last tray (directing acting).

(1) The fresh feed flow rate is held constant by flow control (reserve acting). 3395


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5. CONCLUSIONS In this Article, an A-DWC design for t-butanol dehydration system via HAD is proposed. By comparing with the conventional two-column system, significant savings of energy consumption (23.80%) and TAC (19.93%) can be obtained by the proposed A-DWC design with the benefit of having only one column shell. Before establishing the control structure for the ADWC simulated as three-column, sensitivity analysis of whether βL can be a manipulated variable to maintain the products at high purity is conducted. As the two important degrees-of-freedom of two QRs are preserved in the A-DWC, a tray temperature control strategy still be implemented to select two control points to pair with these two QRs; the proposed control structure can solve disturbance issues and maintain the product purities very close to the set points with small deviation and short settling time, despite whether feed flow rate and feed composition disturbances or βL are the manipulated variables.

(4) For the decanter, the organic phase level is controlled by manipulating the makeup flow, and the aqueous phase level is controlled by manipulating the aqueous outlet flow (directing acting). (5) The flow rate of the liquid stream from the undivided rectifying section is ratioed to the liquid stream split into the HAD column that is held constant by flow control (reserve acting). (6) The flow rate of the organic reflux flow is controlled by the two QRs, which is cascaded with the sum of the two QRs. (7) The fifth tray temperature of the preconcentrator column is controlled by the QR1, which is cascaded with the feed flow rate by a QR1/F ratio (reserve acting). For the HAD column, the 17th tray temperature is controlled by the QR2, which is cascaded with the feed flow rate by a QR2/F ratio (reserve acting). The temperature of the decanter is controlled by manipulating the condenser heat removal duty of HX (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 gain (Kc) = 2. The pressure controllers of the three columns are PI with Kc = 20 and integral time (τ1) = 12 min. The flow controllers are PI with Kc = 0.5 and τ1 = 0.3 min. For all of the control loops with dead time, relay-feedback 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. 4.3. Dynamic Results of the A-DWC. The control structure has been established, and all controllers have been configured. The control structure is examined by introducing disturbances to investigate its control effectiveness. The results of the closed-loop dynamic simulations with ±20% changes in the feed flow rate at 0.5 h are shown in Figure 22. As can be seen from the temperature plots, both temperature control points are able to quickly return back to their set points. Both products are maintained at high purity, despite these unmeasured disturbances. The flow rates of both two column bottoms are changed in ratio with these unmeasured disturbances as well as the two QRs, respectively. The organic reflux flow and the aqueous outlet flow are correspondingly increased/decreased with the feed flow rate changes. The closed-loop dynamic simulations with ±20% changes in the feed composition at 0.5 h are shown in Figure 23. To fit real industrial practice, the composition of t-butanol is reduced to 40 mol % (black lines) and increased to 60 mol % (red lines), and the component of water is changed correspondingly as well. As can be seen from the figure, two product purities are held close to the set points, despite these unmeasured disturbances. The four flow rates are correspondingly performed with the feed composition changes. The closed-loop dynamic simulations with ±10% changes in βL at 0.5 h are shown in Figure 24. The reason the purities of the two products can be controlled by βL has been discussed above in the steady design of the A-DWC; unlike the products that would maintain a high purity in a narrow range in the steady design, the purities of the products can quickly return back to their set points in the face of a large range of βL manipulated variables due to the proposed control structure of the A-DWC. Again, the control performances are all very satisfactory with both products still maintained at high purity.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 519 86330355. Fax: +86 519 86330355. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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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).

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NOMENCLATURE A-DWC = azeotropic dividing-wall column HAD = heterogeneous azeotropic distillation t-butanol = tert-butanol QR = reboiler duty TAC = total annual cost βL = liquid split ratio DWC = dividing-wall column NGL = natural gas liquid QC = condenser duty D = distillate S = side stream B = bottoms L = reflux V = vapor boilup PID = proportional integral derivative LQG = linear quadratic Gaussian LQR = linear quadratic regulation GMC = generic model control BTX = benzene/toluene/xylene MPC = model predictive controllers RCM = residual curve map NF = the feed stage of the fresh feed NT = the total number of trays HX = heater exchanger F = the fresh feed XB1 = the purity of bottom product water XB2 = the purity of bottom product t-butanol QR/F = reboiler duty/mol flow rate of F PI = proportional and integral Kc = gain DOI: 10.1021/ie504325g Ind. Eng. Chem. Res. 2015, 54, 3384−3397

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Industrial & Engineering Chemistry Research τ1 = integral time TC = temperature controller KU = ultimate gains PU = ultimate periods

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(24) Wu, Y. C.; Hsu, P. H. C.; Chien, I. L. Critical Assessment of the Energy-Saving Potential of an Extractive Dividing-Wall Column. Ind. Eng. Chem. Res. 2013, 52, 5384. (25) Luyben, W. L.; Chien, I. L. Design and Control of Distillation Systems for Separating Azeotropes; Wiley: New York, 2011.

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