Process Assessment of Distillation Using Intermediate Entrainer

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Process Assessment of Distillation Using Intermediate Entrainer: Conventional Sequences to the Corresponding Dividing-Wall Columns Shanyuan Huang, Weisong Li, Ye Li, Jian Ma, Changlin Shen, and Chunjian Xu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03597 • Publication Date (Web): 06 Jan 2016 Downloaded from http://pubs.acs.org on January 13, 2016

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Industrial & Engineering Chemistry Research

Process Assessment of Distillation Using Intermediate Entrainer: Conventional Sequences to the Corresponding Dividing-Wall Columns Shanyuan Huang, Weisong Li, Ye Li, Jian Ma, Changlin Shen, Chunjian Xu*

State Key Laboratory of Chemical Engineering, Collaborative Innovation Center of Chemical Science and Engineering, Chemical Engineering Research Center, and School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China

Tel: +86 022-27404440. Fax: +86 022-27404440. E-mail: [email protected].

ABSTRACT: In extractive distillation, an intermediate entrainer that differently interacts with the azeotropic components can be introduced. In this work, the separation system of methanol-toluene has been investigated to evaluate the performance of conventional extractive distillation (CED) and the corresponding dividing wall column (DWC) by using an intermediate entrainer. First, the conventional extractive distillation separation sequences including direct sequence (DS), indirect sequence (IS) are investigated. It is indicated that DS provides much more energy saving and investment reduction, while the DS exhibits an obvious remixing effect and IS has a problem of twice vaporizations of light product. Then, in order to minish the remixing effect, two types of DWC, dividing wall column with the wall in the top (DWC-T) and that in the bottom (DWC-B), are further explored and investigated. It is found that, compared to DWC-T, DWC-B offers more energy savings

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and reduces the remixing effect. Moreover, a basic temperature control scheme for DWC-B is investigated when feed flow rate and composition disturbances are introduced. From the standpoint on both the optimum designs and the fairly good dynamic performance, DWC-B is considered as a suitable selection for the methanol-toluene azeotrope separation. Key words: intermediate entrainer, dividing wall column, direct sequence, indirect sequence 1.

Introduction Distillation is still one of the most widely applied separation technologies in chemical industry.

However, the azeotropic or close boiling point mixtures cannot be completely separated by a simple distillation process. Therefore, several special distillation technologies, such as pressure-swing distillation, heterogeneous azeotropic distillation and extractive distillation, have been proposed. However, pressure-swing process is actually feasible only when the azeotrope mixture is pressure sensitive enough. And the limitation of entrainer selection and multiple steady states lead to a challenge to the design and the control structure of heterogeneous azeotropic distillation.1,2 Because of low energy consumption and flexible selection of entrainers, extractive distillation is used more widely than the other two distillation processes.3-6 Heavy entrainers are commonly used in the extractive distillation. For example, the dehydration of isopropyl alcohol using ethylene glycol or dimethyl sulfoxide as the heavy entrainer.7-9 Actually, separation using intermediate entrainer is also worthy of considering. For the separation of the extractive distillation, the selection of entrainer needs more consideration because different entrainers have a great influence on TAC, especially if the separation in the entrainer recovery column is difficult. Several papers have illustrated the potential feasibility of mixtures using intermediate entrainer for the batch extractive distillation,10-14 but only a few mixtures have been


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applied in continuous extractive distillation.15-17 In our previous work, it has been illustrated that the process for the separation of methanol-toluene mixture using intermediate entrainer has advantages on energy consumption and control performance compared with the process using heavy entrainer.15 In addition, the intermediate entrainer has an another advantage: it will not increase the bottom temperature and can make full use of low grade steam. In spite of the widespread use of extractive distillation, some researches still need to be made to explore the energy saving potential. Nowadays, DWC is considered as an intensification process to obtain energy saving for separating ternary mixtures in one column.18-27 However, studies about extractive dividing wall column mainly focus on the process using heavy entrainer.28-35 The process of DWC using heavy entrainer could only use DWC-T, while both DWC-T and DWC-B can be used for the separation using intermediate entrainer. To the best of our knowledge, works about DWC using intermediate entrainer are rarely reported. In this work, two conventional extractive distillation sequences and two types of dividing wall column are designed, investigated and compared. In the study, the conventional extractive distillation sequences will be respectively integrated into one column to obtain a corresponding dividing wall column.33,36 The selected mixture is the methanol-toluene azeotropic system. Both methanol and toluene are important solvents and chemicals. Meanwhile, the mixture can be obtained in the process of synthesizing xylene by alkylation of benzene and methanol. Triethylamine is selected as the intermediate entrainer which is recommended by Modla16 and Gerbaud14. At last, a basic control structure of DWC-B which is the optimal process will be proposed to investigate the dynamic performance. And it will be from the standpoint on both total cost and dynamic performance to select the most suitable process for the separation of methanol-toluene system using Et3N.



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2.

Analysis for the Conventional Extractive Sequences

2.1. Process Statement In this section, the conventional extractive distillation including direct distillation sequence and indirect distillation sequence are optimally designed and compared for the separation system. As for CED, the process using intermediate entrainer is a little different from that using heavy entrainer. In DS, as the intermediate boiling point component, the entrainer can be recycled from the bottom of extractive distillation column with the heavy component. Similarly, in IS, the entrainer can be taken from the top of the extractive column with the light component. The flowsheets of DS and IS are shown in Figure 1 (a) and Figure 1 (b), respectively. 2.2. Steady State Design and Economical Optimization The methanol-toluene mixture has a minimum azeotrope with a composition of 88.70/11.30 mol% methanol/toluene at 101.3 kPa and an azeotropic temperature of 63.87 °C. The normal boiling points of toluene, methanol and triethylamine (Et3N) are 110.68 °C, 64.53 °C and 88.63 °C, respectively. The steady state simulation is implemented by commercial software (Aspen Plus 7.2). The feed conditions are as follows: a flow rate of 100 kmol/h with an annual working time of 8000 h, a composition of 50/50 mol% methanol/toluene. The product specifications of methanol and toluene are set to be 99.9 mol%. The “RadFrac” distillation routine is used for the rigorous simulations. The condenser pressures of both extractive distillation column and entrainer recovery column are set at 100 kPa with a pressure drop of 0.68 kPa for each tray. In each simulation, the “Design Spec/Vary” feature is used to achieve desired product quality. Necessary skills on how to use the software program about the implementation are covered in Lyuben’s book37 in details. The NRTL activity model is chosen as the property package in all the


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following simulations, using the built-in binary interaction parameters in the simulator, which is recommended by Modla16 and Gerbaud14. After the base case design is finished, the next step is to optimize the process. Total annual cost (TAC) including annual capital costs and operating costs is used as the objective function to optimize the process design parameters. For more details of the cost estimation, please refer to the Supporting Information section (2). The design variables which need to be optimized include entrainer flow rate (S), total stages of the extractive distillation column (NT1), entrainer and fresh streams feeding locations (NFE and NF1), total stages of the entrainer recovery column (NT2), and feeding location of the entrainer recovery column (NF2). Figure 2 shows the optimization procedure for DS and IS. Figure 3 and Figure 4 give the two final optimum flowsheets for DS and IS with detailed equipment sizes, stream information, heat duties, and operating conditions at the steady state design conditions. Details of the comparison between the optimum designs of DS and IS are listed in Table 1. It is observed that, compared to DS, IS has a 32.42% increase in energy consumption and a 24.27% increase in TAC. As for the reboiler duty of C1, IS has a 7.52% energy savings than DS. However, energy consumption of IS in C2 is 149.07% more than that of DS and it leads to a remarkable increase in energy consumption and TAC of IS. Considering to this result of IS, the problem of twice vaporizations of methanol in C1 and C2 has the main contribution to the additional energy consumption. Figure 5 (a), (b) and Figure 6 (a), (b) illustrate the liquid and vapor composition profiles of DS and IS in C1. After examining the liquid composition profiles, it is observed that a remixing effect exists below NFE in both of DS and IS. And yet, the remixing effect of IS appears weaker than that of DS.



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Thus, DS has advantages on lower energy consumption and TAC, while IS has a less remixing effect. If some efforts could be made to solve the problem of twice vaporizations of methanol, IS may have a better performance. 3.

Design and Analysis for Corresponding Dividing Wall Columns

3.1. Process Statement Note from the above discussion, both two conventional extractive distillation sequences have drawbacks for the separation. DS has the problem of remixing effect. And the way to weaken or eliminate the remixing effect is to make a thermal coupling modification. IS has the problem of twice vaporizations of methanol and this problem is the inherent drawback of its own process. It is worth trying to solve this drawback of IS by changing the design flowsheet. Such as CED using heavy entrainer, a satisfied result can be frequently obtained by modifying the process of CED to the corresponding DWC.33 Hence, for CED using intermediate entrainer, the corresponding DWC may solve the problem as well. As for DWC using intermediate entrainer, the dividing wall can be located in the top or in the bottom. The detailed designs of the two DWCs including DWC-T and DWC-B will be studied in this section. As shown in Figure 7 (a), DWC-T has only one reboiler and two condensers. Figure 7 (b) gives the equivalent flowsheet for DWC-T which includes a main column (MC) and a rectifying column (RC) and this flowsheet is the scheme used in this case study. As shown in Figure 7 (a), the fresh feed and entrainer are fed into the left part of DWC. And we can obtain the methanol product at the left top of DWC and the toluene product at the bottom of DWC. The entrainer is distilled out from the right top of DWC and recycled back to the left part. As shown in Figure 8 (a), DWC-B has only one condenser and two reboilers. Figure 8 (b) gives the


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equivalent flowsheet for DWC-B which includes a main column (MC) and a stripping column (SC). This flowsheet is also the scheme used in this study. As shown in Figure 8 (a), the fresh feed and entrainer are fed into the left part of DWC. And we can get the methanol product at the top of DWC and the toluene product at the left bottom of DWC. The entrainer is flowed out from the right bottom of DWC and recycled back to the left part. It can be observed that in the two types of DWC, methanol is vaporized only once from fresh feed stream to product. Thus, because of the difference between IS and the two types of DWC, the problem of the twice vaporizations of methanol can be avoided. 3.2. Design and Economical Optimization of DWC-T and DWC-B 3.2.1. Scheme of DWC-T Steady State Design: In this case, the feed conditions are the same as that of the conventional extractive distillation. The condenser pressures of both the main column and the rectifying column are set at 100 kPa with the tray pressure drop of 0.68 kPa. The two product specifications are both 99.9 mol%. First, we can modify the flowsheet of DS to obtain the initial values of those design variables.33 By adding stripping section of entrainer recovery column (22 stages) under the bottom of extractive column (76 stages), MC is obtained with a stage of 98. Meanwhile, the location of vapor sidestream (NV) can be determined at stage 77 and RC correspondingly has a total stage of 77. Then, some initial value should be determined. Since the Et3N recycle flow rate of DS and IS are both optimized at 8 kmol/h, the initial value of the recycle entrainer (S) is assumed at 8 kmol/h. For the main column, the distillate rate is set at 50.05 kmol/h. The reflux ratio (RR1) is varied to obtain a 99.9 mol% specification of methanol with an assumed value of 2. The vapor sidestream flow



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rate (V) is varied to obtain the 99.9 mol% specification of toluene. As for the vapor sidestream flow rate, the initial value also needs to be estimated. In the vapor sidestream, the flow rate of methanol must be controlled lower than 0.05 kmol/h to keep the recovery of methanol no less than 99.9 mol%. In this case, it is assumed that the vapor stream at stage 76 of DS in C1 and the liquid stream at stage 25 of DS in C2 are a pair of streams to estimate the vapor sidestream flow rate and the liquid backflow sidestream. Figure 5 (a), (b) and (c) illustrate the liquid and vapor composition profiles of DS in C1 and C2. In Figure 5 (b), the toluene vapor composition at stage 76 in C1 is 0.733 mol%. The vapor sidestream composition of methanol and Et3N are 0.05/V and 1-0.733-0.05/V, respectively. For the liquid sidestream, because most of the methanol will move up to the top of RC, it is considered that the liquid backflow sidestream has a 0 mol% composition of methanol. In Figure 5 (c), the Et3N liquid composition at stage 25 is 0.125 mol%. Thus, the initial value of vapor sidestream flow rate can be calculated by the following equation which is based on material balance of Et3N.

(1 - 0.733 - 0.05/V) ∗ V = 0.125 ∗ (V - 8) + 8

(1)

In equation 1, the left part equation represents the Et3N flow rate of the vapor sidestream; the meaning of the right part equation is the sum of Et3N flow rate of the back liquid sidestream and flow rate of recycle entrainer. According to equation 1, the initial value of vapor sidestream flow rate is calculated at 49.648 koml/h. For the rectifying column, because of the lack of a degree of freedom, only one specification can be set. The specification of RC is the flow rate of Et3N (SE). The specification value equals to 8-X and X equals to the sum of the Et3N flow rate in methanol product and toluene product. The initial value of SE is assumed at 7.5 kmol/h. The distillate flow rate of RC is varied to achieve the


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specification of SE. Thus, the initial design variables including the recycle entrainer flow rate (S), the total stages of MC (NT1), reflux ratio (RR1), the total stages of RC (NT2), the location of vapor sidestream (NV) and the vapor sidestream flow rate (V) have been all determined. Economical Optimization: When the base case design is finished, the next step is to optimize the process. Total annual cost including annual capital costs and operating costs is used as the objective function to optimize the process design parameters. Because the construction and installation of DWC are more complex than that of the conventional distillation column, a penalty of roughly 0.2 is added to the column shell cost and the column tray cost in the economic optimization function. For more details of the cost estimation of DWC, please refer to the Supporting Information section (3). The design variables which need to be optimized include total stages of MC (NT1), entrainer and fresh streams feeding locations (NFE and NF1) and vapor sidestream flow rate (V). Figure 9 gives the optimization procedure for DWC-T. The optimum steady design of DWC-T is obtained after iterative optimization procedure finished. It is shown that DWC-T with NT1 = 90, NT2 = 74, NV =75, S = 7.966 kmol/h is the optimum design. Figure 10 gives the final optimum flowsheet for DWC-T with detailed equipment sizes, stream information, heat duties, and operating conditions at the steady state design conditions. 3.2.2. Scheme of DWC-B In this section, the feed conditions, the two product specifications and the operating condition are the same as DWC-T. Alike DWC-T, DWC-B can also be obtained by modifying the flowsheet of IS. By adding rectifying section of entrainer recovery column (59 stages) on the top of the extractive column (46



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stages), MC is obtained with a stage of 105. Meanwhile, the location of liquid sidestream (NL) can be determined at the stage 60 and SC is also determined from stage 60 to stage 105. For the main column, the bottom stream flow rate is fixed at 50.045 kmol/h. The reflux ratio (RR1) is varied to obtain a 99.9 mol% specification of methanol with an assumed value of 2. The liquid sidestream flow rate is varied to obtain the 99.9 mol% specification of toluene. As for the liquid sidestream (L), the initial value also needs to be estimated. In the liquid sidestream, the flow rate of toluene must be controlled lower than 0.05 kmol/h to keep the recovery of methanol no less than 99.9 mol%. In this case, it is assumed that the liquid stream at stage 2 of IS in C1 and the vapor stream at stage 60 of IS in C2 are a pair of streams to estimate the liquid sidestream flow rate and the vapor backflow sidestream. Figure 6 (a), (b) and (c) illustrate the liquid and vapor composition profiles of IS in C1 and C2. In Figure 6 (a), the methanol liquid composition at stage 2 is 0.772 mol%. The liquid sidestream composition of toluene and Et3N are 0.05/L and 1-0.772-0.05/L, respectively. For the vapor sidestream, we consider that all the toluene moves down to the bottom of SC, leading to a 0 mol% composition of toluene in the vapor backflow sidestream. In Figure 6 (c), the Et3N vapor composition is 0.104 mol% at stage 60. Thus, the initial value of liquid sidestream flow rate can be calculated by the following equation that is based on material balance of Et3N.

(1 - 0.772 - 0.05/L) ∗ L = 0.104 ∗ (L - 8) + 8

(2)

In equation 2, the left part equation represents the Et3N flow rate of the liquid sidestream; the right part equation is the sum of the Et3N flow rate of the back vapor sidestream and the flow rate of recycle entrainer. According to equation 2, the initial value of liquid sidestream flow rate is calculated at 58.210 koml/h.


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For the stripping column, the specification of SC is also the flow rate of Et3N (SE) with an assumed initial value of 7.5 kmol/h. And the bottom stream is varied to achieve this specification. Thus, the initial design variables including the recycle entrainer flow rate (S), the total stages of MC (NT1), reflux ratio (RR1), the total stages of SC (NT2), flow rate of entrainer (S), the location of liquid sidestream (NL) and the liquid sidestream flow rate (L) have been determined. The optimization procedure is the same as DWC-T except using V and NV instead of L and NL, respectively. The optimum results are as followings: NT1 = 74, NT2 = 29 (from stage 46 to stage 74), NL =45, S = 9.192 kmol/h. The final optimum flowsheet for DWC-B with detailed equipment sizes, stream information, heat duties, and operating conditions is shown in Figure 11. 3.3. Process Comparison and Analysis Details of the comparison between the optimum designs of DS, IS, DWC-T and DWC-B are listed in Table 2. And all the processes are compared based on DS. It is observed that DWC-T has an 11.30% reduction in total energy consumption and a 5.93% savings in TAC. The decrease extent of energy consumption is not that great as DWC-B. The reason may be that DWC-T still has remixing effect which can be observed in Figure 12 (a). Note from previous discussion, we estimate that IS may have a better performance if the problem of twice vaporizations can be solved. It can be found in Table 2 that DWC-B has a 20.07% reduction in TAC and a 19.24% savings in TAC. And it is observed in Figure 12 (b) that DWC-B has nearly no remixing effect. It is proved that DWC-B inherits the advantage on low remixing effect of IS and at the same time, DWC-B solves the defect of twice vaporizations of methanol in IS. That’s one major advantage of DWC-B in decreasing remixing effect and saving energy. Thus, compared to DWC-T,



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DWC-B has a better performance in reducing energy consumption and decreasing TAC. 4.

Control System of DWC-B Note from the above discussions, DWC-B is a considerable process with a significantly lower

(-20.07%) energy requirement than DS. A control degree of freedom of DWC-B is lost due to the inter coupling of two condensers into one. In addition, the dividing wall column is a highly thermally coupled complicated distillation process and as a result, a disturbance in one part of the column can easily influence the other part leading to an interaction effect in the system. Recently, the DWC control structures using heavy entrainer and even about heterogeneous azeotropic distillation have been reported.31-35,38 However, there are rarely works reported about DWC control structures with intermediate entrainers. Thus, it is of significant importance to investigate the dynamic control performance of DWC-B. 4.1. Control Structure Xia34,35, Ling and Luyben39 have investigated the temperature control structures for DWC. Based on their works, a basic temperature control structure for DWC-B using intermediate entrainer is proposed in this section after several control structures have been tested by trial and error. In this control structure, the reboiler duty of MC and SC, and the mass reflux flow rate of MC are used in tray temperature control loop. And three fictitious fittings are set between the two columns: a pump and a valve are inserted into the liquid side stream to provide enough pressure and pressure drop to control the flow rate of the liquid side stream; a valve is inserted into the backflow vapor stream to ensure that the pressure of the vapor stream equals to the pressure in stage 45 of MC. In MC, the temperature of stage 35 is controlled by manipulating the reflux rate and the temperature of stage 67 is controlled by manipulating reboiler duty of MC. In SC, the temperature of stage 28 is controlled


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by manipulating the reboiler duty of SC. Because the temperature control loop may cause internal shock between the reboiler duty control loop and the reflux rate control loop when the disturbance occurs, as the feedforward control, the Q/F and R/F controllers are added to weaken the internal shock in MC. Figure 13 illustrates the basic control structure for DWC-B. For the open loop sensitivity analysis and the selection of sensitive trays, please refer to the Supporting Information section (5). (1) The fresh feed to MC is flow controlled (reverse acting). (2) The base level in MC is held by manipulating the bottom flow rate (direct acting). (3) The base level in SC is held by manipulating the makeup flow rate (reverse acting). (4) The reflux drum level in MC is held by manipulating the distillate flow rate (direct acting). (5) The top pressure in MC is held by manipulating the condenser heat removal rate (reverse acting). (6) The total recycle solvent flow rate is ratioed to the fresh feed flow rate. (7) The reflux rate of MC is ratioed to the fresh feed flow rate. (8) The temperature of stage 35 in MC is controlled by manipulating the ratio of the fresh feed and reflux rate. (9) The reboiler duty of MC is ratioed to the fresh feed flow rate. (10) The temperature of stage 67 in MC is controlled by manipulating the ratio of the fresh feed and reboiler duty. (11) The liquid side stream is flow controlled (reverse acting). (12) The flow rate of liquid side stream is ratioed to the reboiler duty of MC. (13) The temperature of stage 28 in SC is controlled by manipulating the reboiler duty of SC.



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For a DWC-T, Xia34,35 has pointed out that the adjustment and control of the sidestream is of significant importance for the control stability. However, adjusting the vapor split αV is necessary but impractical through the current industry technology. Considering DWC-B, control of the sidestream can be achieved through the liquid redistributor to adjust the liquid split βL. When using the liquid redistributor, the adjustment of sidestream split can be realized through the industry technology at present. That’s another advantage on control structure about DWC-B. Most of the loops discussed above are standard distillation control strategies. All the flow controllers are PI with KC= 0.5 and τI = 0.3 min. All the level loops are P-only with KC = 2. The pressure controllers are PI with Aspen Dynamic default values: KC= 20 and τI = 12 min. And a 1 min deadtime is inserted in each temperature control loop for the columns. Tyreus−Luyben tuning is applied to determine the gain KC and integral time constant τI of TCR, TCQ1 and TCQ2. Details of tuning and setting are covered in Luyben’s book.40 4.2. Dynamic Performance The dynamic performance of the temperature control structure is tested by two types of disturbances including feed flow rate disturbance and composition disturbance. For each case, the disturbance is introduced at t=1h. And the dynamic responses to these disturbances for the control structure are shown in Figure 14. Figure 14 (a) and (b) illustrate the composition responses of the methanol and the toluene products after introducing ±10% step disturbances in feed flow rate. The purities of methanol and toluene become stable after once oscillation and back close to the desired value quickly around 3h. As for +10% F disturbance, the stable values of methanol and toluene are 99.905% and 99.880%, respectively; for -10% F disturbance, the stable values of methanol and toluene are 99.894% and


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99.916%, respectively. Figure 14 (c) and (d) illustrate the composition responses of the methanol and the toluene products after introducing the methanol composition disturbance. In Figure 14 (c), the control structure makes a good performance under the -10 mol% methanol composition disturbance and becomes stable at 3h. But when +10 mol% methanol composition disturbance is introduced, the methanol product has a little big offset compared with -10 mol% methanol disturbance and becomes stable slower at about 5h. Figure 14 (d) shows that the toluene product becomes stable at 4h after twice oscillations. For +10 mol% methanol disturbance, the stable values of methanol and toluene are 99.821% and 99.909%, respectively; for -10 mol% methanol disturbance, the stable values of methanol and toluene are 99.941% and 99.890%, respectively. Thus, the proposed control structure can handle with both the feed flow rate disturbance and the methanol composition disturbance and give a fairly good performance. The purity of the two products can return back to their desired values in 2 to 4 hours with small offsets (less than 0.1%). 5.

Conclusion In this work, two conventional extractive distillation sequences and two types of dividing wall

column are compared which are designed and optimized for the separation of methanol-toluene system with triethylamine as the entrainer. As for the two conventional extractive distillation sequences, DS has advantages on saving energy and TAC but it has an obvious remixing effect. However, IS has a lower remixing effect but exists a problem of twice vaporizations of methanol. The design of DWC-T which is based on DS is to weaken the remixing effect, while the result is not that satisfied (-11.30% in energy and -5.93% in TAC). DWC-B which is based on IS leads to a 20.07% reduction in energy consumption and a 19.24% savings in TAC. The process of DWC-B does not



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only solve the problem of twice vaporizations in IS, but also inherits the advantage on low remixing effect of IS. In terms of DWC, DWC-B which has nearly no remixing effect and more potential on saving energy is more economical and applicable than DWC-T. A basic control structure is proposed for DWC-B. It is illustrated that the control structure can handle with both the feed flow rate disturbance and the composition disturbance with 10% step changes and the control structure can also enable the adjustment of liquid sidestream split. The purity of the two products returns back quickly to the desired values with tiny offsets in about 2 to 4 hours. From the standpoint of the optimum designs and the fairly good dynamic performance, DWC-B is proved to be a priority selection for the separation of methanol-toluene azeotrope using an intermediate entrainer. Supporting Information (1) Binary Interaction Parameters for the Aspen Plus NRTL Model; (2) Economical Optimization of CED; (3) Economical Optimization of DWC; (4) Temperature Controller Tuning Parameters; (5) the Selection of Sensitive Tray.

ACKNOWLEDGMENTS The authors appreciate the support of the Programme of Introducing Talents of Discipline to Universities (No. B06006) and assistance from the staff at the State Key Laboratory of Chemical Engineering (Tianjin University).

NOMENCLATURE CED= conventional extractive distillation DS= direct sequence


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DWC= dividing wall column DWC-B =dividing wall column with the wall in the bottom DWC-T= dividing wall column with the wall in the top Et3N= triethylamine IS= indirect sequence KC = controller gain L= liquid sidestream flow rate MC = main column NF1= feeding location of fresh feed NF2= feeding location of the entrainer recovery column NFE=feeding location of entrainer stream NL = location of liquid sidestream NT1= number of stages in extractive distillation column or main column NT2= number of stages in entrainer recovery column or rectifying column or stripping column NV = location of vapor sidestream QR = total reboiler duty input QR1 = reboiler duty of extractive distillation column QR2= reboiler duty of entrainer recovery column RC = rectifying column RR1 = reflux ratio of extractive distillation column or the main column RR2= reflux ratio of entrainer recovery column or the rectifying column or the stripping column S= recycle entrainer flow rate



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SC = stripping column SE = flow rate of triethylamine TAC= total annual cost V= vapor sidestream flow rate αV = vapor split ratio βL = liquid split ratio τI = controller integral time constant

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internally heat-integrated distillation column (HIDiC). Appl. Therm. Eng. 2006, 26, 1362−1368. (19) Jana, A. K. Heat integrated distillation operation. Appl. Energy. 2010, 87, 1477−1494. (20) Suphanit, B. Design of internally heat-integrated distillation column (HIDiC): uniform heat transfer area versus uniform heat distribution. Energy. 2010, 35, 1505−1514. (21) Suphanit, B. Optimal heat distribution in the internally heat-integrated distillation column (HIDiC). Energy. 2011, 36, 4171−4181. (22) Nakaiwa, M.; Huang, K.; Owa, M. Potential energy savings in ideal heat-integrated distillation column. Appl. Therm. Eng. 1998, 18, 1077−1087. (23) Nakaiwa, M.; Huang, K.; Endo, A. Internally heat-integrated distillation columns: a review. Chem. Eng. Res. Des. 2003, 81, 162−177. (24) Matsuda, K.; Kawazuishi, K.; Kansha, Y. Advanced energy saving in distillation process with self-heat recuperation technology. Energy. 2011, 36, 4640−4645. (25) Nguyen, N.; Demirel, Y. Using thermally coupled reactive distillation columns in biodiesel production. Energy. 2011, 36, 4838−4847. (26) Babu, G. U. B.; Aditya, R.; Jana, A. K. Economic feasibility of a novel energy efficient middle vessel batch distillation to reduce energy use. Energy. 2012, 45, 626−633. (27) Kiran, B.; Jana, A. K.; Samanta, A. N. A novel intensified heat integration in multicomponent distillation. Energy. 2012, 41, 443−453. (28) Zhang, H.; Ye, Q.; Qin, J. Design and control of extractive dividing-wall column for separating ethyl acetate–isopropyl alcohol mixture. Ind. Eng. Chem. Res. 2013, 53, 1189−1205. (29) Bravo-Bravo, C.; Segovia-Hernández, J. G.; Gutiérrez-Antonio, C.; Duran, A. L.; BonillaPetriciolet, A.; Briones-Ramírez, A. Extractive dividing wall column: design and optimization.


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Res. 2009, 49, 189−203. (40) Luyben, W. L. Plantwide Dynamic Simulators in Chemical Processing and Control; Marcel Dekker: New York, 2002.

Table Captions Table 1. The optimization results of DS and IS Table 2. The optimization results of DS, IS, DWC-T and DWC-B Figure Captions Figure 1. (a) Flowsheet of DS. (b) Flowsheet of IS. Figure 2. Sequential iterative optimization procedure for DS and IS. Figure 3. Optimal process flowsheet for DS. Figure 4. Optimal process flowsheet for IS. Figure 5. (a) Liquid composition profiles of DS in C1. (b) Vapor composition profiles of DS in C1. (c) Liquid composition profiles of DS in C2. Figure 6. (a) Liquid composition profiles of IS in C1. (b) Vapor composition profiles of IS in C1. (c) Vapor composition profiles of IS in C2. Figure 7. (a) Scheme of DWC-T. (b) Equivalent flowsheet for DWC-T. Figure 8. (a) Scheme of DWC-B. (b) Equivalent flowsheet for DWC-B. Figure 9. Sequential iterative optimization procedure for DWC-T. Figure 10. Optimal process flowsheet for DWC-T. Figure 11. Optimal process flowsheet for DWC-B


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 12. (a) Liquid composition profiles of MC in DWC-T. (b) Liquid composition profiles of MC in DWC-B. Figure 13. Basic control structure for DWC-B. Figure 14. Dynamic responses for the control structure: (a), (b) ±10% in feed flow rate; (c), (d) ± 10 mol% in methanol composition.

(a)

(b) A

A A+E

Entrainer F

C1

C2

F

C1

C2

B

B E+B

Figure 1


Entrainer


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2


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Page 25 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MAKEUP 0.06788 kmol/h Et3N

-1356.718 kW 64.195 °C 100 kPa

2

-510.533 kW 88.180 °C 100 kPa

2

D2

D1 49.998 kmol/h 0.999 Methanol 6.43E-04 Toluene 3.57E-04 Et3N

Entrainer

61

F

73

100 kmol/h 0.5 Methanol 0.5 Toluene

75

8.040 kmol/h 8.94E-04 Methanol 0.006 Toluene 0.993 Et3N

24

RR1=6.425 D2=0.725m

RR1=1.774 D1=1.079 m

45

B1


1440.650 kW 118.851 °C

50.018 kmol/h 1.29E-30 Methanol 0.999 Toluene B2 0.001 Et3N 493.355 kW 119.790 °C

Figure 3 MAKEUP 0.06477 kmol/h Et3N

-1282.621 kW 65.788 °C 100 kPa

2

-1211.7833kW 64.194 °C 100 kPa

2

D1


58.071 kmol/h 0.862 Methanol 8.61E-04 Toluene 0.137 Et3N

Entrainer

23

59 F

44 100 kmol/h 0.5 Methanol 0.5 Toluene

1332.275 kW 119.744 °C

RR2=1.478 D2=1.065m

RR1=1.283 D1=1.044 m

67

45 50.017 kmol/h 4.4E-05 Methanol 0.999 Toluene B1 9.56E-04 Et3N

8.034 kmol/h 0.006 Methanol 0.002 Toluene B2 0.992 Et3N 1228.791 kW 100.479 °C

Figure 4



(a) 1.0

(b) 1.0

Methanol Toluene Et3N

0.9 0.8


0.9 0.8 0.7

0.6

0.6

Y (mol%)

0.7

0.5 0.4

0.5 0.4

0.3

0.3

0.2

0.2

0.1

0.1

0.0

0.0 0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Stages

(c)

Stages

1.0

0.8

X(mol%)

X(mol%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34


0.6

0.4

0.2

0.0 0

5

10

15

20 25 Stages

30

35

40

Figure 5


45

Page 27 of 34

(a) 1.0

(b) 1.0


0.9 0.8


0.9 0.8 0.7

0.6

0.6

Y (mol%)

0.7

0.5 0.4

0.5 0.4

0.3

0.3

0.2

0.2

0.1

0.1 0.0

0.0 0

5

10

15

20

25

30

35

40

0

45

5

10

15

20

(c)

25

30

35

40

Stages

Stages 1.0 Methanol Toluene Et3N

0.8

Y (mol%)

X (mol%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

0.2

0.0 0

5

10 15 20 25 30 35 40 45 50 55 60 65

Stages

Figure 6

(a) A

(b) A

Entrainer

Entrainer F

F

RC

MC V

L B

B

Figure 7


45


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 28 of 34

(b) A

A L

F

B

F

Entrainer

MC

V

SC

B Entrainer

Figure 8


Page 29 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 34

-268.647 kW 87.752 °C 100 kPa

-1358.227 kW 64.196 °C 100 kPa

2

2

RR1=1.777 D1=1.218 m


D2 RR2=2.912 D2=0.522m

Entrainer

V 29.835 kmol/h 0.001 Methanol 0.358 Toluene 0.641 Et3N

63

7.966 kmol/h 0.004 Methanol 4E-06 Toluene 0.996 Et3N

74

MAKEUP 0.0694 kmol/h Et3N

F

71


L 21.869 kmol/h 1.72E-04 Methanol 0.489 Toluene 0.511 Et3N

75 89

1715.477 kW 127.626 °C


B

Figure 10 -1464.756 kW 64.209 °C 100 kPa

2 V 10.242 kmol/h 0.586 Methanol 0.002 Toluene 0.412 Et3N

D

42 45

50.048 kmol/h 0.999 Methanol 9.8E-05 Toluene 9.02E-04 Et3N

46 L 19.434 kmol/h 0.373 Methanol 0.003 Toluene 0.624 Et3N

Entrainer RR1=1.995 D1=1.066m

MAKEUP 0.0932 kmol/h Et3N F


D2=0.262m

72 73

73

9.192 kmol/h 0.135 Methanol 0.005 Toluene 0.860 Et3N

50.045 kmol/h 3.9E-5 Methanol 0.999 Toluene B1 9.61E-04 Et3N 1445.713 kW 124.879 °C

B2 100.173 kW 92.328 °C

Figure 11


Page 31 of 34

(a) 1.0

(b) 1.0


0.9 0.8


0.9 0.8

0.7

0.7

0.6

0.6

X(mol%)

X(mol%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.5 0.4

0.5 0.4

0.3

0.3

0.2

0.2

0.1

0.1 0.0

0.0

0

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

5 10 15 20 25 30 35 40 45 50 55 60 65 70

Stages

Stages

Figure 12

Figure 13



(a) 0.99950

(b) 0.9995

+10% F -10% F

0.99925

XB1: Toluene (mol%)

XD: Methanol (mol%)

+10% F -10% F

0.99900

0.99875

0.99850

0

2

4

6

8

0.9990

0.9985

0.9980

10

Time (h)

2

4

6

8

10

Time(h) +10% Methanol -10% Methanol

+10% Methanol -10% Methanol

XB1: Toluene (mol%)

0.9995

0.9990

0.9985

0.9980

0.9975

0

(d) 0.99950

(c) 1.0000

XD: Methanol (mol%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

0

2

4

6

8

0.99925

0.99900

0.99875

0.99850

10

0

2

Time (h)

4

6

Time (h)

Figure 14


8

10

Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. The optimization results of DS and IS Parameters

Direct sequence

Indirect sequence

NT1

76

46

NT2

46

68

NFE

61

23

NF1

73

44

NF2

24

59

RR1

1.774

1.283

RR2

6.425

1.478

Et3N (kmol/h)

8

8

QR1 (kW)

1440.65

1332.27 (-7.52%)

QR2 (kW)

493.35

1228.79 (+149.07%)

QR (kW)

1934.00

2561.07 (+32.42%)

TAC ($106s per year)

1.1611

1.4429 (+24.27%)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

Table 2. The optimization results of DS, IS, DWC-T and DWC-B Parameters

DS

IS

DWC-T

DWC-B

NT1

76

46

90

74

NT2

46

68

74

29 (from 46 to 74)

RR1

1.774

1.283

1.777

1.995

RR2

6.425

1.478

2.912

/

D1 (m)

1.079

1.044

1.218

1.066

D2 (m)

0.725

1.065

0.522

0.262

Et3N (kmol/h)

8

8

8

8

QR1 (kW)

1440.65

1332.27

1715.48

1445.71

QR2 (kW)

493.35

1228.79

/

100.17

Q= QR1+ QR2

1934.00

2561.07

1715.48

1545.89

(kW)

(0.00%)

(+32.42%)

(-11.30%)

(-20.07%)

TAC ($106s per

1.1611

1.4429

1.0923

0.9377

year)

(0.00%)

(+24.27%)

(-5.93%)

(-19.24%)


Process Assessment of Distillation Using Intermediate Entrainer

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