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 Benzene/Cyclohexane Mixtures Lanyi Sun,* Qiuyuan Wang, Lumin Li, Jian Zhai, and Yuliang Liu State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China S Supporting Information *

ABSTRACT: This paper considers the design and control of the separation of benzene and cyclohexane process via extractive distillation in a dividing wall column. To aid the separation, furfural is used as the heavy boiling entrainer. The optimal flow sheet with minimum energy requirements has been established using the multiobjective genetic algorithm with constrains. Three control strategies are proposed: the basic control strategy uses four composition controllers, and two improved control strategies with and without vapor split ratio use temperature controllers that are more practical in application than the basic control strategy. The dynamic simulations reveal that the three control strategies can object to the feed disturbances well. The dynamic responses of two improved control structures show that vapor split ratio as a manipulated variable can maintain the product purities at their set points when a feed disturbance is introduced.

1. INTRODUCTION Petrochemical industries often require high-purity cyclohexane, which is widely used not only in the production of nylon (adipic acid, caprolactam), paints, and varnishes but also in the synthesis of pharmaceutical intermediates. Moreover, cyclohexane is an excellent solvent for resins, fats, paraffin oils, butyl rubber, etc. As an important commodity chemical, cyclohexane is mainly produced by the addition of hydrogen to benzene over a Ni or Pt catalyst. The reaction is typically carried out at 573 K and in the pressure range 20−25 atm with a large amount of heat released. The unreacted benzene, which is present in the reactor’s effluent stream, must be removed to obtain high purity cyclohexane. Separation process of benzene and cyclohexane is expensive and cumbersome by a conventional distillation process, considering the only 0.6 K difference in the boiling points of benzene and cyclohexane and the small interaction parameter between them.1 The most commonly used methods for separation of aromatic/aliphatic mixtures are given in Table 1,2 which

distillation is more available for the isolation of aromatics from pyrolysis gasoline (approximately 65−90% aromatics); thus, it is the main separation technique in industry to separate benzene and cyclohexane.3−5 High energy requirements, accompanied by high capital and operating costs and process complexity, will be needed to meet such a demand for high purity cyclohexane (99.0% minimum purity). Process intensification has received considerable attention all over the world in recent decades, due to the increasing awareness of the limited energy resources.6−8 In order to obtain further energy saving, a novel configuration, which integrated two columns into one shell, is identified as a dividing wall column (DWC).9 Compared with conventional configurations, DWCs often provide more than 30% reductions in capital costs and roughly 30% savings in energy costs. Since two columns are integrated into one shell, DWCs can save more space required for installation as well as a higher purity of side product compared with conventional sequences.10−13 Two-column extractive distillation sequence integrated into one shell forms an extractive dividing wall column (EDWC). The rest of the paper is organized as follows. First, process flow sheet and steady state are investigated by Aspen Plus. Next, the optimal flow sheet is determined by minimizing heat duty of the overall system with the multiobjective genetic algorithm with constrains. Then, three effective control strategies for this system are provided. Finally, the disturbance rejection capability of this process is evaluated for flow and composition variations by Aspen Dynamics, and Conclusions are discussed in the last section.

Table 1. Methods for Aromatic Recovery process liquid−liquid extraction extractive distillation azeotropic distillation crystallization

adsorption on solids

separation problem BTX separation from reformate gasoline BTX separation from pyrolysis gasoline BTX separation from pyrolysis gasoline isolation of p-xylene from m/p-mixtures

requirements for basic or economical operation lower aromatic content (20−65%) medium aromatic content (65−90%) high aromatic content (>90%)

distillative preseparation of o-xylene and ethylbenzene from C8 aromatic fractions isolation of p-xylene from continuous, reversible, and selective adsorption C8 aromatic fractions

2. STEADY-STATE DESIGN AND OPTIMIZATION OF EXTRACTIVE DISTILLATION PROCESS 2.1. Entrainer Screening and Selection. Choosing an effective solvent, which is a challenging issue to a successful

shows that the major technologies presently available to separate aromatic/aliphatic mixtures are azeotropic distillation and extractive distillation. If the aromatic content is greater than 90%, such as with the pyrolysis of gasoline or crude benzene from coal coking, azeotropic distillation is economical. Extractive © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8120

October 1, 2013 March 25, 2014 April 12, 2014 April 13, 2014 dx.doi.org/10.1021/ie500291a | Ind. Eng. Chem. Res. 2014, 53, 8120−8131

Industrial & Engineering Chemistry Research

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separation, has a profound effect on the design of an extractive distillation process. Several demanding rules should be followed to get a suitable solvent for any close boiling mixture,14,15 e.g., the selectivity, the volatility of solvent, its environmental impact, or even the toxicity of solvent.16 Four entrainers for the benzene−cyclohexane system have been studied in the same conditions, and furfural is found to be the best candidate. The main physical properties and the simulation results with four different entrainers are given in Figure 1 and Table 2, respectively. The results of calculating the

Figure 2. Residual curve map (RCM) of the benzene−cyclohexane− furfural system at 101.3 kPa.

the problem of high energy requirements, the EDWC technique is applied to the benzene−cyclohexane system. Since there is no off-shelf DWC unit in Aspen Plus, PRO/II, and other currently available process simulators, the steady design of an EDWC always uses a thermodynamically equivalent process for simulation, which is shown in Figure 3b. The main column (column I) is equivalent

Figure 1. Effects of solvent on VLE of cyclohexane(1)−benzene(2) system n(cyclohexane):n(benzene):n(entrainer) = a:(0.5 − a):0.5.

Table 2. Comparison of the Main Physical Properties for Different Entrainers solvent

molecular formula

molecular weight, g/mol

boiling point, K

density at 293 K, kg/m3

furfural DMSO DMF sulfolane

C5H4O2 C2H6OS C3H7NO C4H8O2S

96.08 78.13 73.09 120.17

435 462 425 558

1160 1100.4 948 1261

relative volatility are comparable based on the same thermodynamics model.17 Figure 1 shows that the dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and sulfolane provide high selectivity, while DMSO and DMF are not taken into consideration for their high prices and great toxicities. Although a novel extractive distillation operating second recovery column under vacuum pressure18 has been proposed to avoid the high bottom temperature caused by sulfolane, the low temperature in the condenser would result in a consequent increase in the operation cost. In this paper, furfural will be used in the separation of benzene and cyclohexane azeotropic system. 2.2. Residue Curve Map (RCM) for Benzene−Cyclohexane System. For distinguishing between feasible and infeasible sequences, a residue curve map (RCM) technique is introduced as a simple method in the benzene−cyclohexane−furfural system. Figure 2 shows the RCM curves for this ternary system. As can be seen in Figure 2, the benzene−cyclohexane azeotrope is the unstable node, furfural is the stable node, and both benzene and cyclohexane are the saddles. Therefore, the products cannot be obtained simultaneously. There are no distillation boundaries in the RCM, which indicates that the benzene/cyclohexane mixtures can be separated easily in the presence of furfural. 2.3. Process Design and Sensitivity Analysis. 2.3.1. Process Flow Sheet and Steady-State Design. In order to solve

Figure 3. (a) Optimum process flow diagram for the conventional configurations. (b) Process flow diagram for the EDWC.

to the first column in Figure 3a. The rectifying column (column II) and the stripping column (column III) are equivalent to the rectifying section and stripping section of the entrainer recovery column, respectively, as shown in Figure 3. Highly pure cyclohexane is distilled at the top of the main column, the benzene is collected from the top of the side column, and the furfural of the stripping column is recycled back to the main column at an appropriate feeding location. Figure 3b (right) shows the process 8121

dx.doi.org/10.1021/ie500291a | Ind. Eng. Chem. Res. 2014, 53, 8120−8131


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flow diagram implemented in Aspen Plus, which also presents the design parameters of this EDWC configuration. Assume that the main column has a total of 30 stages, and the first stage is condenser. The stripping column consists of 10 stages, and the side column comprises 7 stages. The condenser pressure of the main column is set at 101.325 kPa and the side column is 106.325 kPa, which give refluxed drum temperatures 353.8 and 355.2 K, respectively. Thus, it is high enough to permit the use of cooling water in the overhead condensers. At stage 19 (NF = 19) of the main column, a feed rate of 1000 kmol/h at 353.15 K is fed in. The recycle solvent subcooled to a temperature of 343.15 K in the following simulations returns back to the stage of 8, which is also a process design variable. The initial value of the reflux ratio for the main column is 4.2. The split fraction is assumed to be 0.68 to achieve the desired products. 0.77 is chosen as the reflux ratio of the side column. Under these specifications, the purities are 99.3 and 99.8 wt % for cyclohexane and benzene, respectively, and 99.9 wt % furfural can be obtained in the EDWC (stripping section) bottom. NRTL is used in the simulations since it is capable of describing the physical properties of this system.5 2.3.2. Sensitivity Analysis. Because the reflux ratio of the main column (RR1) and vapor split ratio (αV) have significant effects on the composition profiles and reboiler duty for an EDWC, sensitivity analysis has to be carried out to seek the appropriate range of vapor split ratio and the optimum RR1. Figure 4 shows the influence of the reflux ratio of the main column and vapor split ratio on the cyclohexane composition on

Figure 5. Effect of the reflux ratio of the main column RR1 and vapor split ratio αV on the reboiler heat duty (QR).

2.4. Optimization of Extractive Distillation Process. Multiobjective genetic algorithms applied to optimize the dividing wall distillation columns have been published in recent papers.19,20 However, there are only a few papers that applied multiobjective genetic algorithm to optimize an EDWC process. Evaluating of the objective function using a multiobjective genetic algorithm with constrains, coupled with Aspen Plus21 as a design tool, can obtain the rigorous results. Instead of obtaining one optimal design, a set of optimal designs is obtained throughout this procedure, which integrates the Pareto front. In this way, the engineer can choose a trade-off by picking some points along the Pareto front. In the optimization of multivariable functions, the stochastic optimization methods present a reasonable computational effort, and they just need calculations of the objective function without problem reformulation. As the thermodynamically equivalent of the EDWC, three coupled RADFRAC units are used in Aspen Plus. The manipulated variables include heat duty, reflux ratio, total number of stages, locations of the feed, extracting agent flow, and vapor flow to the side column. In terms of multiobjective optimization, there are three objectives to minimize: the number of stages of the total columns, the heat duty of the sequence, and the extracting agent flow, which are in competition and constrained by the desired purities and recoveries in each product stream, and therefore the objectives must be optimized simultaneously. This problem can be expressed as follows:

Figure 4. Effect of the reflux ratio of the main column RR1 and vapor split ratio αV on the cyclohexane composition XD1 in the distillate of the EDWC.

min(Ni , Q R , FEA ) = f (Q R , RR i , Ni , NF, i , FEA , V2) s.t.

the top of the main column (XD1). It can be seen that the cyclohexane composition is higher with the increase of vapor split ratio at a higher reflux ratio. Nevertheless, the value of the vapor split ratio must be between 0.55 to 0.65 in case the excessive vapor flow back into the main column with benzene may cause a lower cyclohexane composition and energy waste. In addition, the vapor split ratio also has a great effect on reboiler duty. A drop of the vapor split ratio decreases reboiler heat duty as presented in Figure 5. This could be owed to the lower vapor flow returning into the main column that causes less liquid in the bottom to be vaporized. Therefore, the EDWC may be operated at a reflux ratio above 3.5 with vapor split ratio of 0.6 or higher to satisfy the 99.3% mass specification for cyclohexane.

yk⃗ ≥ xk⃗

where Ni is the number of stages of the column i, QR is the reboiler heat duty, FEA is the extracting agent flow, RRi is the reflux ratio, NF,i is the feed stage number of column i, and V2 is the value of the vapor stream flowed into the side column. yk⃗ and xk⃗ are the vectors of obtained and required purities and recoveries for the k components, respectively. For the EDWC, 2000 individuals and 40 generations are chosen as parameters of the genetic algorithm, with 0.80 and 0.05 of crossover and mutation fraction. Here are the processes. First, a feasible initial design of the EDWC is given as the original solution to the algorithm of 8122



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each run. The algorithm generates N individuals (i.e., new designs) based on the initial solution to make up the first population. The manipulated variables of each of the N individuals are sent to Aspen Plus to perform the simulation, and then Aspen Plus gives the values of objective functions and constraints for each individual to the algorithm. The population is divided into subpopulations in terms of the number of satisfied constraints with the retrieved information; at this time, the best individuals satisfy the c constraints, followed by those individuals that reach c − 1 constraints, etc. Inside each subpopulation, the individuals are ranked according to the value of the fitness function. The original objective functions can be optimized through the classification of the population, which can also minimize the difference between the required and obtained constraints (recoveries and purities). Finally, a set of optimal designs of the EDWC is obtained. More detailed information about this algorithm and its link to Aspen Plus can be found in the original work.21 Figure 6 shows the Pareto front for the benzene/cyclohexane mixtures, which includes the objectives to minimize: heat duty

algorithm, and the objectives of conventional configuration to minimize are heat duties of the sequence, extracting agent flow, and the total number of stages. According to the recent paper by Wu et al.,23 since a heavy entrainer is introduced in the extractive distillation system, often cases show that the reboiler duty is reduced but with adversely increasing total steam cost that has a strong effect on the total annual cost (TAC). Hence, the TAC of the conventional two-column system and the EDWC are compared in Table 3. Table 3. Comparison between the Conventional TwoColumn Design and the EDWC Design conventional two-column configurations reboiler duty (Gcal/h) condenser duty (Gcal/h) required steam type

total reboiler duty (Gcal/h) (% difference) total steam cost (1000 $/y) (% difference) total operating cost (1000 $/y) (% difference) total capital cost (1000 $/y) (% difference) total annual cost (1000 $/y) (% difference)

C1

C2

16.8 17.5 8.5 15.1 low-pressure mediumpressure (50 psig, (150 psig, 421 K) 458 K) 34.3 (0%)

EDWC 26.9 18.2 medium-pressure (150 psig, 458 K) 26.9 (−21.6%)

4839.3 (0%)

4750.9 (−1.8%)

4914.0 (0%)

4797.1 (−2.4%)

1607.2 (0%)

1411.0 (−12.2%)

6521.1 (0%)

6208.1 (−4.8%)

The results show that the TAC of EDWC is 4.8% less than that of the conventional design, and the steam cost is also reduced by 1.8%. Needing only one column is the additional advantage of this EDWC design configuration, which reduces the space requirement. To a certain extent, the separation of benzene/ cyclohexane mixtures can benefit from the EDWC configuration. Figures 7−9 show the temperature and composition profiles of the three columns for the flow sheet, respectively. There is a rapid rise in the temperature for stage 8 and a rapid fall for stage 19

Figure 6. Pareto front of the EDWC for benzene/cyclohexane mixtures.

of the sequence, extracting agent flow, and the total number of stages. At the end, 20 optimal designs are observed that make up the Pareto front, which indicated that a dividing wall distillation column can perform an extractive separation. These optimal designs satisfy the specified purities and recoveries with the lowest energy possible. In our work, the energy requirements is the criterion for choosing our particular needs; thus the design with the minimum reboiler duty is chosen as the final design. The number of stages for the main column, the side column, and the stripping column are 32, 10, and 15, respectively. Bravo-Bravo et al.22 reported that the classical dividing wall column with symmetry on both sides of the wall may not correspond to the best scheme with lower energy requirements; thus, the results indicate that the use of sizing on both sides of the wall can be an optimization variable for the optimal design of EDWC. The recycle solvent returns back to stage 9 (NFS = 9), and the feed flows to stage 19 (NF = 19). 3.80 is chosen as the reflux ratio of the main column. For the side column, the value of the reflux ratio is set to 0.74. In addition, the V2 is 1046.9 kmol/h, and the extracting agent flow is 2610 kmol/h. Under the above conditions, the reboiler heat duty is 26.9 Gcal/h, which presents approximately 22% savings of the total reboiler duty compared with the heat duty of the conventional configuration shown in Figure 3a, which is also optimized using the multiobjective genetic

Figure 7. Main column for (a) temperature profile and (b) composition profile.

Figure 8. Side column for (a) temperature profile and (b) composition profile. 8123



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Table 4. Controller Tuning Parameters

CC1 CC2 CC3 CC4

Figure 9. Stripping column for (a) temperature profile and (b) composition profile.

controlled variable

manipulated variable

controller gain KC (%)

controller integral time τI (min)

y9(Ben) x15(Fur) y32(Cyc) x1(Ben)

S QR αv L

0.68 48 0.47 28

27.0 22.0 42.5 49.6

3. CONTROL OF EXTRACTIVE DIVIDING WALL COLUMN 3.1. Exporting to Aspen Dynamics. To test the control strategies of this system, pressure-driven simulation in Aspen Dynamics is applied. Before exporting the Aspen Plus steadystate simulation to Aspen Dynamics, the tray-sizing option in Aspen Plus is used to determine the needed size of equipment, such as diameters of the three columns, weir height, base, and reflux drum of each column. The base and reflux drum volumes of each column are all sized to give 10 min holdup with 50% liquid level. Pumps and valves are almost specified to have pressure drops of about 3 atm with the valve half open to handle changes in flow rates. At this point, all equipment has been sized. There are two items remaining to consider. First, the pressure of the feed stream leaving valve must be exactly equal to the pressure in the stage where it is fed in. Second, there is not any real valve or compressor in an EDWC inside, and the fictitious valves and pumps are to produce the pressure drop for the requirement of Aspen Dynamics. Then it is ready to export the developed steady-state simulation to Aspen Dynamics, 3.2. Basic Control Strategy (CS1). 3.2.1. State for Basic Control Structure (CS1). The basic control structure for this system is proposed in a series of works.24−26 The basic control structure CS1 and the corresponding controller faceplates are shown in Figure 10. The basic control schemes are given as below. (1) The fresh feed to the main column is controlled by manipulating the flow rate (reverse acting). (2) The base level in the stripping column is held through controlling the makeup flow rate of furfural (reverse acting).

Figure 10. (a) Basic control structure CS1 for the EDWC. (b) Controller faceplates.

in Figure 7a, at which the entrainer and fresh feed are fed in, respectively. It is obvious that stage 16 displays a fairly steep slope for the temperature. In Figures 8a and 9a, two steep slopes are found near the 9th and 3rd stages, respectively, and the profile distinguishing features indicate that these stages are proper temperature control points for the corresponding columns.

Figure 11. Dynamic responses to feed disturbances for the CS1: ±20% in feed flow rate. 8124



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Figure 12. Dynamic responses to feed disturbances for the CS1: ±10% in benzene composition.

(3) The base levels in the main column and side column are held by manipulating the respective bottom flow rate (direct acting). (4) The reflux drum levels in the main column and side column are held by manipulating the corresponding distillates flow rate (direct acting). (5) The total recycle solvent flow rate to the fresh feed flow rate ratio is added. (6) The top pressures of the main column and side column are controlled by adjusting the corresponding condenser heat removal rate (reverse acting). (7) The reflux ratio in the main column is fixed. (8) The feed temperature of recycle entrainer is controlled by manipulating the cooler heat removal (reverse acting). (9) The solvent composition in the bottom of the stripping column is controlled by the reboiler duty, which is ratioed to feed flow rate forming a feed-forward controller. (10) The composition of benzene in the distillate of the side column is controlled by manipulating the corresponding reflux flow rate (reverse acting). (11) The vapor composition of benzene on the feed stage is controlled using the CC1 controller by adjusting the S/F ratio. The S/F ratio is on cascade since it obtains the set point signal from the CC1 controller. (12) The vapor sidestream (V2) molar flow rate is controlled through adjusting the valve V7 (reverse acting). (13) The vapor sidestream molar flow rate is ratioed to the reboiler heat input. The FC3 controller is on cascade for its set point signal received from the VR/QR ratio. (14) The cyclohexane composition in the vapor sidestream is controlled using the CC3 controller by adjusting the VR/QR ratio (reverse acting). Since the set point signal of the VR/QR ratio comes from the CC3 controller, the VR/QR ratio is on cascade. (15) Reboiler heat duty to feed flow (QR/F) ratio is added to this system to change reboiler heat input immediately when feed fluctuates. Many control loops described above are typical distillation control structures. The bottom level of the stripping column is held by the entrainer makeup flow suggested by Grassi27 and Luyben.28 The CC3 controller is set to adjust the vapor split ratio when the cyclohexane composition in the vapor sidestream changes

Figure 13. (a) Improved control structure CS2 for the EDWC; (b) controller faceplates.

during the operation of the EDWC. It should be noticed that adjusting the vapor split ratio is impractical in the chemical industry now for the EDWCs. The normal settings of the proportional−integral (PI) type power-flow controller are KC = 0.5 and τI = 0.3 min. All level loops are P-only and the KC is 2, and the pressure controller is PI with its default values. Considering measurement and actuator lags in any real physical system, a 3 min dead time is inserted into the corresponding composition control loops. To determine the ultimate gains and integral time constant for the four composition controllers, the Tyreus− Luyben tuning is used, and the tuning results are listed in Table 4. For the detailed process of tuning and setting the reader is referred to Luyben’s book.29 3.2.2. Performance of Basic Control Structure CS1. Two types of disturbances are used to test the proposed control structure performance, namely, ±20% changes in fresh feed flow rate and ±10% changes in fresh feed benzene composition. 8125



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Figure 14. Dynamic responses to feed disturbances for the CS2: ± 20% in feed flow rate.

relatively long time to settle is not good for the controllability and will lead to economic losses. 3.3. Improved Control Strategy (CS2). Taking into account the applicability and cost, the usage of basic control structure CS1 with only composition controllers will be limited. Comparing with composition control, temperature control brings in shorter drag time and costs less in both equipment and maintenance, and few articles on temperature control of EDWC could be found in the open literature. Thus, the use of temperature control instead of direct composition will be explored in this section.

The simple PI composition control actually works quite well for the benzene and cyclohexane system. As the feed-forward controller QR/F ratio is added for the control system, it can offer an immediate adjustment in reboiler heat input when subjected to 20% changes in feed flow rate. Results of the basic control structure for disturbances in both feed flow rate and feed composition are illustrated in Figures 11 and 12. Although the system reaches a new steady state at last, the basic control structure presents relatively large overshooting and settles after a long time for the two kinds of disturbances, especially for benzene composition decreased by 10%. The 8126



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Figure 15. Dynamic responses to feed disturbances for the CS2: ± 10% in benzene composition.

After an amount of structures are evaluated by trial and error, an improved control structure is proposed. Figure 13 shows the improved control structure CS2 for the EDWC and its controller faceplates. The difference from the CS1 is shown as follows: (1) The stage 16 temperature of the main column and the stage 9 temperature of the side column are controlled using the TC16 and TC9 controllers by adjusting the RR1 and RR2, respectively. (2) The temperature of stage 3 in the stripping column is controlled using the TC3 controller by adjusting the VR/QR ratio (reverse acting). (3) The vapor sidestream molar flow rate V2 is ratioed to the reboiler heat input. Because the set-point signal of the

Figure 16. Improved control structure CS3 for the EDWC. 8127



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Figure 17. Dynamic responses to feed disturbances for the CS3: ± 20% in feed flow rate.

3.4. Improved Control Strategy without Vapor Split Ratio (CS3). The control structure with or without vapor split has become the hot topic.30−33 On the one hand, the vapor split ratio can keep the products on high purities when feed disturbances are introduced. On the other hand, the control of vapor split ratio would be difficult to manipulate in a real process. Thus, we study the improved control structure for the case when the vapor split ratio is not available as a degree of freedom. Then a new control structure without vapor split ratio is put forward based on the CS2, and the corresponding flow sheet is present in Figure 16. The main differences are that the QR/F ratio controls

FC3 controller is received from the VR/QR ratio, the FC3 controller is on cascade. (4) The total recycle solvent flow rate to the fresh feed flow rate ratio is added to control the total recycle solvent flow rate, and the FC2 is on cascade. Figure 14 shows the dynamic responses for ±20% changes in fresh feed rate at the time of 1 h. The product composition settles in less than 5 h for this system, and steady-state offsets are quite small for the products. For ±10 mol % benzene feed composition changes, product compositions settle in about 5 h as shown in Figure 15. 8128



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Figure 18. Dynamic responses to feed disturbances for the CS3: ± 10% in benzene composition.

the stage 3 temperature instead of the furfural composition of the stripping column and the vapor split ratio is not the manipulate variable compared with the CS2. Figures 17 and 18 give the responses to feed disturbances for the CS3, which are similar to those of the CS2, and the purities of both products are brought close to the set points. It is interesting to compare the responses of the three alternative control structures for the EDWC to feed flow rate and feed composition disturbances. The improved control structures CS2 and CS3 have lower peak transients and shorter settle times than the basic control structure CS1. The short settle time is due to temperature controllers, which are characterized by low price, good reliability, and rapid response compared with the composition controllers. Moreover, the dynamic response of control structures with vapor split ratio show better performance

in products purity than control structure CS3, which indicate that the control of vapor split ratio is good for maintaining the product purities at their set points when a feed disturbance occurs. However, considering the impossibility of adjusting vapor split ratio in the chemical industry at present, the control structure CS3 may be the preferred choice to separate benzene and cyclohexane via EDWC.

4. CONCLUSIONS The design and control of the benzene and cyclohexane separation process implemented in the EDWC are thoroughly investigated in this paper. To optimize the flow sheet, the multiobjective genetic algorithm with constrains is used, which minimizes the overall heat duty with desired product compositions. From the optimal results, it is found that the optimum flow 8129



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sheet of EDWC can achieve up to 22% savings of the total reboiler duty; meanwhile, the steam cost and the TAC are reduced by 1.8% and 4.8%, respectively. After the investigation of the benzene−cyclohexane system thoroughly, we highly approve the view reported by Wu et al.,23 namely, that the dividing-wall column is a promising technology to save energy and the space required for separating ternary azeotrope, while the energysaving potential is limited. Thus, the total reboiler duty, the steam cost, and the TAC should be checked carefully in the design of an EDWC configuration. As for the overall control strategy of the process, it is found that the basic control strategy can work well during 20% in the feed flow rate and 10% in the benzene composition changes. However, the relatively large overshooting and long time to settle are not good for the controllability. In addition, there is a limit in the application of using four composition controllers in industry. Thus, the improved control structures CS2 and CS3 are recommended, which can effectively handle the disturbances and keep the two products on high purities. And the CS3 using temperature control without vapor split ratio is more practical in industry than the CS2 and CS1. All the performances of the control structures reveal that robust control can be achieved for the extractive distillation in a dividing wall column, and the CS3 is worth considering in the separating of benzene/cyclohexane mixtures.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Detailed calculation procedures of TAC. This material is available free of charge via the Internet at http://pubs.acs.org.

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NF,i = feed stage number of column i Ni = the number of stages of column i PI = proportional−integral QR = reboiler heat duty RRi = the reflux ratio of column i S = recycle solvent flow rate VR = vapor sidestream flow rate V2 = vapor stream flowed into the side column XD1 = cyclohexane composition on the top of the main column x1 = mole fraction of cyclohexane in liquid phase x′1 = benzene composition in the side column of stage 1 x15 = furfural composition in the stripping column of stage 15 y1 = mole fraction of cyclohexane in vapor phase y9 = benzene composition in the main column of stage 9 y32 = cyclohexane composition in the main column of stage 32 αV = vapor split ratio λV = latent heat of the steam τI = controller integral time constant

AUTHOR INFORMATION

Corresponding Author

*L.S. Tel.: +86 13854208340. Fax: +86 0532 86981787. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21276279) and the Development of key technologies project of Qingdao Economic and Technological Development Zone under Grant 2013-1-57. Further the authors are grateful to the editor and the anonymous reviewers for their helpful comments and constructive suggestions with regard to the revision of the paper.

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NOMENCLATURE Ben = benzene Cyc = cyclohexane Cs = the saturated steam price DMF = N,N-dimethylformamide DMSO = dimethyl sulfoxide DWC = dividing wall column EDWC = extractive dividing wall column F = fresh feed flow rate FEA = the extracting agent flow Fur = furfural KC = controller gain L = reflux flow rate in the rectifying column NF = feeding location of the fresh feed NFS = feeding location of the recycle solvent stream 8130



Article

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Design and Control of Extractive Dividing Wall Column for Separating

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