Systematic Strategy for Obtaining a Dividing-Wall Column Applied to

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Systematic Strategy for Obtaining Dividing Wall Column Applied to Extractive Distillation Process Gardênia Marinho Cordeiro, Marcella Feitosa de Figueirêdo, Wagner Brandão Ramos, Fabrícia Araújo Sales, Karoline Dantas Brito, and Romildo Pereira Brito Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b05047 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 30, 2017

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Industrial & Engineering Chemistry Research

Systematic Strategy for Obtaining Dividing Wall Column Applied to Extractive Distillation Process Gardênia M. Cordeiro, Marcella F. de Figueirêdo, Wagner B. Ramos*, Fabrícia A. Sales, Karoline D. Brito and Romildo P. Brito Federal University of Campina Grande, Department of Chemical Engineering, Campina Grande, PB, 58109-970, Brazil

Abstract Currently, the process intensification through dividing wall columns (DWC) is one of the most promising alternatives to reduce costs of the distillation process. However, for the extractive distillation, there are still questions whether DWC is a more economical option than the conventional configuration (CS). Normally, the extractive DWCs are simulated with two thermally coupled columns (TCS), and their designs are usually obtained by setting the TCSs in different ways that not necessarily constitute an optimum design. Thus, this work proposes a systematic procedure in terms of stage equilibrium to obtain an optimized DWC configuration, in terms of operability and design. A strict comparison between DWC and CS (also optimized) was performed and the best results of total annual cost (TAC) were obtained for columns with distinct number of stages in each section of the wall, however, these columns did not overcome the optimized conventional systems. Keywords: extractive distillation, optimization, design, thermal coupling, DWC.

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1. Introduction and Problem Definition Due to the efficiency in separation of various mixtures, distillation is the most used separation processes. However, it is a process that demands high energy consumption and it is thermodynamically inefficient; results that come from the vaporization and condensation processes throughout the stages. Currently, the process intensification through dividing wall columns (DWC) is one of the most promising alternatives to reduce energy consumption of the distillation process. According to the literature, DWC compared with conventional configuration (CS) obtain energy reductions between 10 and 40%, depending on each case. Besides, the reduced number of columns, heat exchangers, pipes control systems, etc. could produce also a reduction in investment that could also reach values around 30%.1-3 Regarding specifically the extractive distillation process, several authors have suggested the use of DWC to replace CS.4-8 However, despite being a promising technology, some authors question whether DWC is the best economic option for this specific distillation process. According to Wu et al.5 and Sun et al.9, the use of DWC for separations using solvent with high boiling point relative to the components of the mixture (typical situation in the extractive distillation), presents a limited reduction in the Total Annual Cost (TAC) compared to CS configuration. To overcome this problem and enable the use of DWC, Yu et al.10 proposed the use of an intermediary heating. According to the literature, the DWC's for extractive distillation are usually simulated with two thermally coupled columns (TCS), which are connected through liquid and vapor streams; either with only one reboiler in the extractive column (TCS-I) or in the recovery column (TCS-II).3-8 In fact, one of the major obstacles faced in studies on DWC is the indistinct use of the terms DWC and TCS. Some authors simulate the TCS-I or TCS-II, claiming that they are thermodynamically equivalent to the DWC configurations, which is, apparently, considered ACS Paragon Plus Environment

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sufficient for the random choice of one of these two arrangements to represent a DWC.4-6 The first question about the choice of equivalence is: are these settings, considered thermodynamically equivalent, necessarily also equivalent in terms of number of equilibrium stages? It is necessary to equate a TCS (TCS-I and CST-II) with a DWC under the scope of equilibrium stages, so that the equivalence between these is rigorously obtained. This question on equivalence in terms of equilibrium stages will be taken into consideration in this work. Another point to be questioned is related to designs of a DWC, in which, commonly, are obtained by setting the TCS's in two ways: i) by specifying the number of stages of each TCS column so that, in the conversion to a DWC (called in this work usual DWC), the sections at each side of the wall have approximately the same number of stages;4-6 ii) by specifying the number of stages of each column of the TCS so that the number of stages in the sections next to the wall of a DWC (called in this work unusual DWC) are independent of each other.9,11,12 However, these strategies to obtain DWC does not necessarily constitute an optimum design, besides the use of optimization procedures that do not always guarantee the determination of the global optimum point for this configuration. It similarly occurs with CS's configuration, significantly influencing the results, and consequently, providing inconsistent and biased conclusions, favoring certain configurations. Given these facts, considering the equivalence between TCS and DWC configurations, in terms of equilibrium stages, so that equivalence is obtained accurately, this paper aims to propose the development of a systematic strategy for obtaining an optimized DWC configuration and, therefore, a strict comparison between optimized CS and DWC configurations, both in operational and design terms, for the extractive distillation process. It is worth mentioning that the TCS and DWC configurations, after the systematic strategy of obtaining DWC proposed here, will differentiate only under the practical point of view of


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construction. Thus, the PFDs with optimum results for TCS-I are equivalent to the results of a DWC. The evaluated chemical systems involve separations which use high boiling point solvents relative to the components of the azeotropic mixture; more specifically, the separation of the mixtures ethanol/water and benzene/cyclohexane were evaluated, using ethylene glycol and furfural, respectively, as solvent.

2. Modeling, Simulation and Economic Optimization The CS and TCS configurations were simulated with Aspen Plus platform using the Radfrac routine, counting the stages from top to bottom. The operating pressures of the columns of both configurations were specified so that cooling water could be used in the condensers. The phase equilibrium (VLE) was represented using NRTL equation to calculate the activity coefficient (γ). It is important to mention that these equations were also used by the authors cited in the comparisons presented in this work. The optimization procedure used was the one proposed by Figueirêdo et al.13 The use of this optimization procedure is of fundamental importance, since a rigorous comparison between optimized arrangements should be made. In this approach, the composition of solvent in the liquid phase throughout the extractive section, called solvent content in the extractive section, is used as the main analysis variable of the extractive distillation process, making it possible to simultaneously evaluate the effect of key decision variables in separation: reflux ratio and solvent flow rate. According to the authors, the prerequisite for the use of this procedure is the presence of a plateau in the solvent composition profile, in the liquid phase, specifically in the extractive section. This characteristic is evident in all composition profiles in the liquid phase of solvents


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with high boiling points. The location of the global optimum for CS and TCS is guaranteed, provided that the number of stages of each column is specified. The first stage of optimization to obtain the solvent content and products with desired specifications. For the extractive column (C1), the constraints considered were the purity of the ஽ଵ ஽ଵ ), its recovered fraction ൫ܴ݁ܿ௖௢௠௣ଵ ൯ more volatile component in the top of the column (‫ݔ‬௖௢௠௣ଵ ேிௌை௅ and the solvent content in the solvent feed stage (‫ݔ‬௦௢௟ ). For the recovery column (C2), the ஽ଶ constraint was the purity of the intermediate component at the top of the column (‫ݔ‬௖௢௠௣ଶ ) and its ஽ଶ ൯. To achieve the constraints of column C1, the manipulated recovered fraction ൫ܴ݁ܿ௖௢௠௣ଶ

variables were the extractive column reflux ratio (R1), the solvent flow rate (S) and the distillate flow rate (D1). In the case of column C2, the manipulated variables were the reflux ratio (R2) and the distillate flow rate (D2) for the conventional configuration. In the case of the coupled sequence, due to the absence of the reboiler, the degree of freedom of column C2 decreases and ஽ଶ the only constraint is the purity of the intermediate component at the top of the column (‫ݔ‬௖௢௠௣ଶ ),

at which reflux ratio (R2) is the manipulated variable. In the second step, a sensitivity analysis was used to scan the following variables: the solvent ேிௌை௅ content in C1 (‫ݔ‬ௌை௅ ), azeotrope feed stage position (NFAZE), feed stage position of column

C2 (NF2) for the CS and additionally, in the case of the TCS, the side stream vapor flow rate (FV) and the interconnecting stages of the streams (NFL). The range for sensitivity analysis of these variables was defined based on the convergence of the simulations, which essentially depends on the obedience of the constraints established for each column. Figure 1 illustrates the steps of the optimization procedure used, which generates a matrix whose size depends on the amount of information desired and the number of cases evaluated. ேிௌை௅ Each row of the matrix is a combination of the variables that were analyzed (‫ݔ‬ௌை௅ , NFAZE,

NF2, FV, NFL) and each column of this matrix gives the value of the desired response variables


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(R1, R2, S, D1, D2, SEC, TAC, etc.), allowing to identify the global optimum point of the process which results in lower TAC.

Figure 1. Steps of the optimization procedure.

The economic impact of the considered configurations was evaluated in terms of steam cost, total operating cost (TOC), total investment cost (TIC) and total annual cost (TAC); with the calculations done as described in Luyben14 for a payback period of 3 years. The equations used to calculate the TAC were implemented using Fortran in Aspen Plus. To calculate the TIC, it was considered the main parts of a distillation system: the distillation columns shells, the trays of the columns and heat exchangers (condensers and reboilers). To determine the length of the column, the spacing of 0.6096 meters between the sieve-trays was considered.9 The diameter was obtained directly using the tray sizing tool from Aspen Plus. Due to the high added value compared to other utilities, such as electricity and cooling water, it was considered only the steam used in the reboilers for the TOC calculation. Costs with solvent was also not included, due to the low value of the make-up stream flow rate. ACS Paragon Plus Environment

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The decision on which steam is used in the reboiler is based on the difference between the steam and process fluid temperatures, so that the temperature at the bottom of the column is the determining factor for the type of steam required. The price of steam depends primarily on the location of the plant and varies with time. However, the price of high-pressure steam can reasonably be assumed to be always higher than the medium and low pressure steam. Table 1 shows the pressure and temperature specifications of utilities provided by the simulator.

Table 1. Specifications of pressure and temperature of the steam. Utility

Specification (P,T)

High pressure steam

577 psia; 523.15K

Medium pressure steam

129 psia; 447.15K

Low pressure steam

34 psia; 398.15K

Source: Aspen Plus V.8.8 (2015).

The utilities tool from Aspen Plus was used to calculate the cost of operation of each reboiler, assuming an operating time equal to 8766 hours/year. According to the references used for the system ethanol/water/ethylene glycol4,6, the price of medium and high pressure steam was 8.22 and 9.88 $/GJ, respectively; for the system benzene/cyclohexane/furfural9, the price of low and medium pressure steam was 6.6 and 10.5 $/ton, respectively.

3. Equivalence Between TCS and DWC Usually, the representativeness of a DWC shall be performed by means of a configuration that has a corresponding arrangement. However, in the studies on process intensification using dividing wall columns, different arrangements are used to simulate a DWC, without proving the equivalence between them. ACS Paragon Plus Environment

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Given a TCS and a DWC configurations, they are thermodynamically equivalent if we can go from one to the other by moving column sections from one to another configuration, in such a way that the flowrates, temperatures, and compositions of all internal streams are the same in both configurations. Implicit in that definition is that we are neglecting the different pressure drops that depend on the actual configuration and that the total number of stages is kept constant. Thus, this topic shows the equivalence in terms of equilibrium stages between thermally coupled and DWC configurations. This equivalence is ensured by proceeding so that liquid and vapor flow through the same area and same amount of stages in both configurations. For the TCS-I configuration, the equivalence with the DWC is obtained by performing the following steps: 1. From the two columns of the TCS-I define three regions ((I), (II) and (III)), according to Figure 2a. Note that the interconnection stage of the streams FV and FL divides the column C1 into regions (I) and (III) and that region (II) consists of the entire column C2. 2. Obtain the DWC by initially locating (III) as bottom region, then place the wall to define the regions (I) and (II), as shown in Figure 2b. Note that region (III) is directly connected to the single reboiler of the system and that the vapor from this region is divided between V and V', for regions (I) and (II), respectively.

Applying the same reasoning, the equivalence can also be obtained between the TCS-II (Figure 2c) and DWC configurations, noting that in this case, regions (II) and (III) and the reboiler belong to column C2; again, vapor from region (III) is divided for regions (I) and (II).


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Figure 2. Equivalence between TCS-I (a), DWC (b) and TCS-II (c).

The reverse path is also possible, i.e., starting from Figure 2c it is possible to reach Figure 2a. Thus, both TCS-I and TCS-II are arrangements that can be used in DWC simulations; however, in order to standardize, the TCS-I configuration was used in this work.

4. Usual Comparison Between CS and DWC The usual comparison between CS and DWC was performed using the following literature references: Kiss and Suszwalak,4 Tututi-Avila et al.6 and Sun et al.9 In all three cases, the number of stages of the columns was maintained unchanged, according to each of the respective references. Tututi-Avila et al.,6 referred to as Case I, and Kiss and Suszwalak,4 referred to as


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Case II, evaluate the separation of ethanol-water mixture, using ethylene glycol as solvent, while Sun et al.,9 referred to as Case III, refers to the separation of the benzene/cyclohexane mixture using furfural as solvent. Due to the lack of some important information concerning the PFDs of these works, it was decided to standardize, for each chemical system, the operating conditions in terms of the same input data (temperature, pressure, flow rate, composition) and product specifications (purity and recovered fraction). And, to make possible the reproduction of the results obtained in this work, all the necessary data are shown in the PFDs presented in the following figures were used. Case I is illustrated in Figures 3 and 4 for the CS and TCS-I configurations, respectively, optimized according to Figueirêdo et al.13 It is worth noting that this optimization differs from the method (Genetic Algorithm) used by Tututi-Avila et al.,6 in which the number of stages of the columns was included as a decision variable. The authors referenced in case I used the term "thermodynamically equivalent", ensuring that the simulation results of a TCS-I are equivalent to the results of a DWC. However, in this work, to obtain a DWC in a rigorous way, illustrated in Figure 5, the equivalence procedure was considered, as previously described. In addition to the equivalence, the use of Figueirêdo et al.13 procedure is decisive in the comparison between CS and DWC, since this optimization guarantees the achievement of the global optimum.


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Figure 3. PFD of the CS optimized by Figueirêdo et al.13 procedure, with numbers of stages of the columns proposed by Tututi-Avila et al.6

Figure 4. PFD of the TCS-I optimized by Figueirêdo et al.13 procedure, with numbers of stages of the columns proposed by Tututi-Avila et al.6

It is interesting to note that the supposedly optimized number of stages of each column of the TCS-I in Figure 4 is not much different. This makes it possible to obtain a DWC with the same ACS Paragon Plus Environment

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number of stages in regions (I) and (II), as shown in Figure 5, which generates two rectification sections to separate the two components involved in the binary azeotropic mixture. In practice, this is a typical example, referred to as usual DWC, in which the wall extends to the top of the column, with each condenser disposed on each side of the column, serving to provide the liquid flow into the two sections along the wall.

Figure 5. Usual DWC equivalent to TCS-I of Figure 4.

Case II is illustrated in Figures 6 and 7 for the CS and TCS-I configurations, respectively, and also optimized according to Figueirêdo et al.13 In contrast, the Sequential Quadratic Programming (SQP) optimization method together with a sensitivity analysis to optimize the number of stages was used by Kiss and Suszwalak.4


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Figure 6. PFD of the CS optimized by Figueirêdo et al.13 procedure, with numbers of stages of the columns proposed by Kiss and Suszwalak.4

Figure 7. PFD of the TCS-I equivalent to TCS-II optimized by Figueirêdo et al.13 procedure, with numbers of stages of the columns proposed by Kiss and Suszwalak.4

The TCS proposed by Kiss and Suszwalak4 is TCS-II type, so that, in order to maintain the systematics of the comparison using only TCS-I, initially it was obtained the DWC equivalent to ACS Paragon Plus Environment

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TCS-II and, from the resulting DWC, the TCS-I was obtained using the procedure described above. The obtained PFD is shown in Figure 7. Figure 8 illustrates the DWC equivalent to the TCS-I of Figure 7 and in this case, slightly increasing the spacing between the trays of region (II), the wall of the DWC could extend near the top of the column (usual DWC), since the number of stages in regions (I) and (II) is almost the same.

Figure 8. Usual DWC equivalent to TCS-I of Figure 7.

Case III is illustrated in the PFDs of Figures 9 and 10 for the CS and TCS-I configurations, respectively, optimized according to the procedure of Figueirêdo et al.,13 while Sun et al.9 used Genetic Algorithm as an optimization method with the number of stages of the columns included as decision variable.


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Figure 9. PFD of the CS optimized by Figueirêdo et al.13 procedure, with numbers of stages of the columns proposed by Sun et al.9

Figure 10. PFD of the TCS-I optimized by Figueirêdo et al.13 procedure, with numbers of stages of the columns proposed by Sun et al.9

Sun et al.9 also support the consideration of "thermodynamic equivalence" and use a system with three thermally coupled columns to simulate a DWC. By making an analogy of this system ACS Paragon Plus Environment

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with the thermodynamic equivalence, it is possible to simulate the DWC of the system proposed by these authors as a TCS-I. Figure 11 shows the DWC equivalent to TCS-I of Figure 10 and, as can be seen, the number of stages in regions (I) and (II) is quite different; in this case called unusual DWC. According to Bravo-Bravo et al.,11 the symmetric division of stages in the sections on both sides of the wall may not correspond to the best arrangement with minimum energy requirements.

Figure 11. Unusual DWC equivalent to the TCS-I of Figure 10.

Table 2 presents the summary of the comparison between the economic (TIC, TOC and TAC) and energy results of all PFDs for the three cases analyzed. The lowest TAC results are highlighted in gray and, as can be seen for Cases I and II, unlike the references used as a basis, the TAC of CS was lower than that of DWC. On the other hand, for Case III, the TAC of DWC is lower than that of the CS, and is a result that is in agreement with the reference used.


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Table 2. Comparative results between CS and DWC for the three cases.

Economic and Energetic Performance TIC (US$/year) (% difference) TOC (US$/year) (% difference) TAC (US$/year) (% difference) Qr1 (kW) (% difference)

CS 17 and 19 stages Figure 3

Case I Usual DWC 21 and 17 stages Figure 5


Case II Usual DWC 22 and 14 stages Figure 8


Case III Unusual DWC 47 and 10 stages Figure 11

152,355 (0%)

156,160 (+2.50%)

151,122 (0%)

162,600 (+7.60%)

1,434.041 (0%)

1,419.151 (-1.04%)

634,256 (0%)

694,508 (+9.50%)

645,238 (0%)

724,823 (+12.33%)

4,677.198 (0%)

3,923.924 (-16.11%)

786,611 (0%)

850,668 (+8.14%)

796,360 (0%)

887,423 (+11.43%)

6,111.241 (0%)

5,343.072 (-12.60%)

1,946.9 (0%)

2,227.4 (+14.41%)

1,992.6 (0%)

2,234.7 (+12.14%)

17,463.9 (0%)

23,615.9 (+35.23%)

Qr2 (kW)

414.4

-

411.6

-

17,837.0

-

Qr1 + Qr2 (kW)

2,361.3

2,227.4

2,404.3

2,234.7

35,300.9

23,615.9

(% difference)

(0%)

(-5.67%)

(0%)

(-6.93%)

(0%)

(-33.10%)

SEC (kW/kmol)

27.642

26.075

28.144

27.213

140.86

94.280

(0%)

(-5.67%)

(0%)

(-3.30%)

(0%)

(33.07%)

Steam

MPS/HPS

HPS

MPS/HPS

HPS

LPS/MPS

MPS

The values in Table 2 show that although the DWC configurations are all accompanied by the reduction of energy consumption (QR and SEC), this reduction does not always have a repercussion on the decrease of the TAC, as observed for cases I and II. This result is in agreement with the work of Wu et al.,5 one of the first to state that the economic potential of DWC in relation to CS for the extractive distillation process is limited. They attribute this limitation to the boiling point of the solvent used, which defines the quality of the steam used in the DWC's reboiler. Wu et al.5 concluded that the substitution of a CS for a DWC will only succeed (lower TAC) when the CS uses the same type of steam in the extractive and recovery columns. However, the results for case III, in which low and medium pressure steam was used in the columns of the CS,


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the DWC using medium pressure steam presented lower TAC, showing that one situation or another is case dependent.

5. Systematic Strategy for Obtaining Optimized DWC Although in the results of Table 2 the determination of the global optimum, with the use of the optimization procedure based on the solvent content, has been obtained, the usual and unusual DWC configurations may not be configured in their global optimum relative to the number of stages. The number of stages in each column, and consequently the number of stages in the regions of a DWC, is another determining factor in the search for optimized configurations. It is noticed that the arbitrariness in the design of DWC configurations can lead to inconsistent comparisons and, consequently, favoring one or the other configuration. In addition, the usual and unusual DWC does not guarantee operational similarity with TCS. Thus, considering the following procedures: equilibrium equivalence procedure; optimization procedure of Figueirêdo et al.,13 and a heuristic procedure for the optimization of the number of stages of the columns for the TCS, a new systematic strategy is followed in order to obtain an optimized DWC, equivalent to an optimized TCS-I. The items to be followed are: 1. Optimize the TCS-I using the procedure of Figueirêdo et al.;13 2. Optimize the TCS-I in terms of number of stages of columns C1 and C2, in a heuristic way, according to the following variations in the number of stages of columns C1 and C2: i) with the number of stages proposed by the base cases I, II and III of the previous item; ii) with the increase in the number of stages in column C1; iii) with the increase in the number of stages of column C2; iv) with increase in the number of stages of columns C1 and C2; and v) with the displacement of the stripping section of the recovery column to the extractive column (procedure proposed by Gutiérrez-Guerra1). 3. Obtain the DWC using the equivalence described in item 3; ACS Paragon Plus Environment

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4. Close both ends of region (II), horizontally, to ensure that V and V' and L and L', in both TCS-I and DWC are the same, and obtaining the optimized DWC as shown in Figure 12.

Figure 12. Optimized DWC.

6. Rigorous Comparison Between DWC and CS By obtaining an optimized DWC from the previously described strategy, it is also necessary to obtain an optimized CS, in terms of design and operation, in order to make a rigorous comparison between the DWC and CS configurations. In the same way for the TCS-I configuration, the optimized CS configuration is obtained using the procedure of Figueirêdo et al.13 and optimizing the number of stages of columns C1 and C2, in a heuristic way, similar to the procedure for TCS-I. This item shows the results obtained by the rigorous comparison between the CS and DWC configurations optimized for the two chemical systems considered. Using the specifications used in cases I and II of this work, for the ethanol/water/ethylene glycol system, the lowest TAC for CS was obtained for a sequence consisting of 24 and 15 stages for C1 and C2, respectively, as shown in Figure 13. Medium pressure (8.82 $/GJ) and high pressure (9.88 $/GJ) steam were used in the reboilers of C1 and C2, respectively. ACS Paragon Plus Environment

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Figure 13. PFD of the optimized CS for ethanol/water/ethylene glycol system.

Figure 14 shows the TCS-I obtained from the PFD of Figure 13, whose coupling was performed by transferring the stripping section from C2 to C1, as proposed by Gutierrez-Guerra et al.1 The distillation sequence shows C1 and C2 with 34 and 5 stages, respectively. Other coupling attempts resulted in higher values of TAC. Due to the temperature of the base of C1 from Figure 13, high pressure steam was used in the reboiler of this column, as expected.


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Figure 14. PFD of the optimized TCS-I for ethanol/water/ethylene glycol system.

It is important to emphasize that for the comparison between CS and TCS-I, the composition specifications (ethanol at the top of C1 and water at the top of C2) and recovered fraction (ethanol at C1 top stream) were the same. According to the procedure described in the previous item, the optimal DWC equivalent to the TCS-I of the PFD of Figure 14 is shown in Figure 15, where it is observed that the regions (I), (II) and (III) of the DWC have, respectively, 31, 5 and 3 stages. The procedure to obtain the optimized DWC was also applied to the mixture of case III of the previous item. Similarly, simulations with different numbers of stages for C1 and C2, applying the procedure of Figueirêdo et al.,13 were performed in order to obtain the optimal CS and the optimal TCS-I.


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Figure 15. Optimized DWC equivalent to optimized TCS-I for ethanol/water/ethylene glycol system.

Figure 16 shows the PFD with the best performance in terms of TAC for CS, where it is observed that C1 and C2 have 45 and 28 stages, respectively. Low pressure steam (6.6$/t) and medium pressure steam (10.5$/t) were used to supply the energy demand of columns C1 and C2 respectively. Figure 17 shows the TCS-I that presented lower TAC, which is composed of 57 and 10 stages in columns C1 and C2, respectively. In the distribution of stages, in relation to CS, part of the stripping section of C2 is transferred to C1, which follows in the same line of the procedure proposed by Gutiérrez-Guerra.1 Medium pressure steam was used to supply the energy demand of the system. The DWC equivalent to the TCS-I of the PFD of Figure 17 is shown in Figure 18, where it is observed that the regions (I), (II) and (III) of the DWC have, respectively, 47, 10 and 10 stages; that is, again, even though there was a great disparity between the number of stages in regions (I) and (II), it was the DWC that presented best economic performance.


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Figure 16. PFD of the optimized CS for benzene/cyclohexane/furfural system.

Figure 17. PFD of the optimized TCS-I for benzene/cyclohexane/furfural system.

Table 3 presents the main results for performance evaluation (energy and economic) of the optimized PFDs. For the ethanol/water/ethylene glycol system, the DWC from Table 3 shows lower TAC (742,363 US$ /year) compared to the CS's of Table 2 (786,611 and 796,360 US$/year). ACS Paragon Plus Environment

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Figure 18. Optimized DWC equivalent to optimized TCS-I for benzene/cyclohexane/furfural system.

Table 3. Comparative between optimized CS and optimized DWC for the systems studied.

Economic and Energetic Performance TIC (US$/year) (% difference) TOC (US$/year) (% difference) TAC (US$/year) (% difference) Qr1 (kW) (% difference) Qr2 (kW) Qr1 + Qr2 (kW) (% difference)

System ethanol/water/ethylene glycol optimized CS optimized DWC 24 and 15 stages 34 and 5 stages Figure 13 Figure 15

System benzene/cyclohexane/furfural optimized CS optimized DWC 45 and 28 stages 57 and 10 stages Figure 16 Figure 18

194,886 (0%)

154,619 (-20.66%)

1,532.884 (0%)

1,565.833 (+2.15%)

527,954 (0%)

587,743 (+11.32)

3,332.452 (0%)

3,741.967 (+12.29%)

722,840 (0%)

742,363 (+2.70%)

4,865.336 (0%)

5,307.800 (+9.09%)

1,599.3 (0%) 362.8 1,962.1 (0%)

1,885.0 (+17.88%) 1,885.0 (-3,93%)

14,467.9 (0%) 11,512.8 25,980.8 (0%)

22.967

22.067

103.682

89.922

(0%)

(-3.92%)

(0%)

(-13.27%)

MPS/HPS

HPS

LPS/MPS

MPS

22,520.7 (+55.66%) 22,520.7 (-13,32%)

SEC (kW/kmol) Steam


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In the case of the benzene/cyclohexane/furfural system, the result is otherwise: the CS from Table 3 presents lower TAC (4702.283 US$/year) compared to the DWC (5343.072 US$/year) from Table 2. This comparison between the values of tables 2 and 3 demonstrates the importance of the number of stages of the columns in the comparison of CS and DWC performance. The best TAC results were obtained for columns with the very distinct number of stages in regions (I) and (II), however, these columns did not outperform conventional optimized systems.

7. Concluding Remarks Two examples of chemical systems separated via extractive distillation process were considered to evaluate the feasibility of using alternative configurations (TCS and DWC). The use of an optimization procedure that guarantees global optimization, together with the verification of the influence of the number of stages on the process performance, has led to the conclusion that mistaken comparisons can be made by benefiting from the choice of one or another configuration. To perform a rigorous comparison between the configurations (CS and DWC), a strategy to obtain an optimized DWC, in terms of operation and design, was developed, which was simulated by means of a TCS-I, from the point of view equivalence, which ensures the proper use of both configurations with thermal coupling (TCS-I and TCS-II) for the simulation of a DWC. Under this scope, the optimal DWC obtained had a better energy performance, but not a better economic performance (lower TAC). In fact, the DWC reduces energy costs, but quantitatively, this reduction depends on which CS was taken as a reference.

*

Corresponding author: Tel: +55 83 2101-1872. Fax +55 83 2101-1114. E-mail:

[email protected]


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Acknowledgments The authors thank the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for financial support for this work.

References (1) Gutiérrez-Guerra, R.; Segovia-Hernández, J. G.; Hernández, S. Reducing energy consumption and CO2 emissions in extractive distillation. Chem. Eng. Res. Des. 2009, 87, 145152. (2) Asprion, N.; Kaibel, G.; Dividing wall columns: Fundamentals and recent advances. Chem. Eng. Process. 2010, 49, 139-146. (3) Delgado-Delgado, R. D.; Barroso-Muñoz, F. O.; Segovia-Hernández, J. G.; HernándezEscoto, H.; Castro-Montoya, A. J.; Rico-Ramírez, V. Multiple Steady States in Thermally Coupled Distillation Sequences: Revisiting the Design, Energy Optimization, and Control. Ind. Eng. Chem. Res. 2014, 53, 17515-17521. (4) Kiss, A.; Suszwalak, A. D. Enhanced Bioethanol Dehydration by Extractive and Azeotropic Distillation in Dividing-Wall Columns. Sep. Purif. Technol. 2012, 86, 70-78. (5) 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, 5384-5399. (6) Tututi-Avila, S.; Gutiérrez, A. J.; Hahn, J. Control analysis of an extractive dividing-wall column used for ethanol dehydration. Chem. Eng. Process. 2014, 82, 88-100. (7) Long, N. V. D.; Lee, M. Review of Retrofitting Distillation Columns Using Thermally Coupled Distillation Sequences and Dividing Wall Columns to Improve Energy Efficiency. J. Chem. Eng. Jpn. 2014, 47, 87-108.


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(8) Huang, S.; Li, W.; Li, Y.; Ma, J.; Shen, C.; Xu, C. Process Assessment of Distillation Using Intermediate Entrainer: Conventional Sequences to the Corresponding Dividing-Wall Columns. Ind. Eng. Chem. Res. 2016, 55, 1655-1666. (9) Sun, L.; Wang, Q.; Li, L.; Zhai, J.; Liu, Y. Design and Control of Extractive Dividing Wall Column for Separating Benzene/Cyclohexane Mixtures. Ind. Eng. Chem. Res. 2014, 53, 81208131. (10) Yu, J.; Wang, S.; Huang, K.; Yuan, Y.; Chen, H.; Shi, L. Improving the Performance of Extractive Dividing-Wall Columns with Intermediate Heating. Ind. Eng. Chem. Res. 2015, 54, 2709−2723. (11) Bravo-Bravo, C.; Segovia-Hernández, J. G.; Gutiérrez-Antonio, C.; Durán, A. L.; BonillaPetriciolet, A.; Briones-Ramírez, A. Extractive Dividing Wall Column: Design and Optimization. Ind. Eng. Chem. Res. 2010, 49, 3672-3688. (12) Xia, M.; Yu, B.; Wang, Q.; Jiao, H.; Xu, C. Design and Control of Extractive DividingWall Column for Separating Methylal-Methanol Mixture. Ind. Eng. Chem. Res. 2012, 51, 1601616033. (13) Figueirêdo, M. F.; Brito, K. D.; Ramos, W. B.; Vasconcelos, L. G. S.; Brito, R. P. Optimization of the Design and Operation of Extractive Distillation Process. Sep. Sci. Technol. 2015, 50, 2238-2247. (14) Luyben W. L. Distillation Design and Control Using Aspen Simulation (2nd edition). AIChE and John Wiley & Sons, Inc.: New York, 2013.


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For Table of Contents Only


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Systematic Strategy for Obtaining a Dividing-Wall Column Applied to

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