Performance Enhancement of Reactive Dividing-Wall Column via

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Performance enhancement of reactive dividingwall column via vapor recompression heat pump Shenyao Feng, Xinyu Lyu, Qing Ye, Hui Xia, Rui Li, and Xiaomeng Suo Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02824 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 14, 2016

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Industrial & Engineering Chemistry Research

Performance enhancement of reactive dividing-wall column via vapor recompression heat pump Shenyao Feng, Xinyu Lyu, Qing Ye*, Hui Xia, Rui Li and Xiaomeng Suo

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China

Author Information * Corresponding author. Tel.: +86 519 86330355. Fax: +86 519 86330355. E-mail address: [email protected].

ACS Paragon Plus Environment


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Abstract: In this work, a novel energy-efficient distillation technology combining reactive dividing-wall column (RDWC) with vapor recompression heat pump (VRHP) is proposed. The integrated RDWC with VRHP models with intermediate reboiler or bottom reboiler are designed for reaching better energy-saving and economic performance. The results showed that the reactive dividing-wall column with vapor recompression heat pump and intermediate reboiler is more attractive than the reactive dividing-wall column with vapor recompression heat pump and bottom reboiler in this study. Adding a preheater to the reactive dividing-wall column with vapor recompression heat pump and intermediate reboiler (RDWC-VRHP-IR2) can further reduce the compression ratio and reach better performance. The RDWC-VRHP-IR2 saves 57.06% of total utility consumption, 31.31% of total annual cost with the payback period of 5 years and 61.52% of CO2 emissions compared with the conventional two-column reactive distillation system.


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1. Introduction Distillation is widely used in the chemical industry. However, this separation method is quite energy-intensive that it accounts for more than half of the energy consumption in the chemical industry1,2. As the energy is obtained by burning fossil fuels, there are strong correlations between the energy consumption of distillation and the CO2 emissions3. Due to the increasing public concern of energy and environment, the novel distillation processes with less energy consumption has received researchers’ attention4. Low thermodynamic efficiency is the major defect of the conventional distillation column5,6. In literature and industry, the dividing-wall column (DWC) and the heat pump are two popular approaches to improving the thermodynamic efficiency of the distillation process7,8. The DWC is thermodynamically equivalent to the Petlyuk column. A vertical wall is introduced into the distillation column, thus the pre-fractionator and the main column of the Petlyuk column are merged into a single column in the DWC9. Because the DWC can avoid unnecessary mixing effects, the improvement of the thermodynamic efficiency can result in considerable energy savings compared with the conventional column10. Kaibel11 investigated the energy consumption of the DWC, which is 20-35% lower than that in the conventional fractionation scheme. Dejanović et al.12 developed a packed dividing-wall column, which led to 50.7% and 43.3% savings in total annual costs and energy consumption respectively compared with the conventional two-column-in-series configuration. The DWC can be used in the reactive distillation (RD), thus it can take advantages of both RD and DWC13,14, which is called the reactive dividing-wall column (RDWC). This novel intensification scheme can reduce energy consumption and equipment size of the process. Kiss et al.15 used the RDWC for the synthesis of dimethyl ether. Compared with the traditional reaction and dividing-wall separation process, the RDWC could save up to 58% of energy consumption. In the conventional distillation column, the reboiler needs high-quality energy, but the condenser can only provide low-grade heat16. Heat pump can upgrade the



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low-grade heat to the high-quality energy giving heat to the reboiler2,17. It includes three main types: the vapor recompression heat pump (VRHP), the bottom flash heat pump and the closed cycle heat pump18. The VRHP attracted most researchers’ attention among the aforementioned types because of its significant benefits18–21. The top vapor stream is compressed to a higher pressure in the VRHP, until its dew point temperature is higher than the temperature of the liquid stream, making it possible to give heat to the liquid stream22. The energy consumption of the distillation column can be significantly reduced by the addition of the VRHP, due to its high thermodynamic efficiency and less requirement of external heat source or condenser23,24. Danziger19 integrated the VRHP with the distillation process for the separation of close boiling components, as a result, the energy consumption of the integrated process saved by over 80% compared with the conventional distillation process. Although the VRHP and the DWC are well-known energy-efficient approaches to the distillation column, a few studies have been reported integrating the DWC and the VRHP. Van Duc Long et al.2 studied the energy efficiency of the VRHP and the DWC, indicating a mutual promoting relationship between them. With the addition of the VRHP, the condenser duty and reboiler duty was reduced by 33.07% and 35.76% respectively. Li et al.25 combined the VRHP and the DWC together. Compared with the corresponding DWC, the total annual cost was saved by 14.76% with a payback period of 8 years. The CO2 emission of the new process was reduced by 47.21%. The thermodynamic efficiency raised from 9.56% of the DWC to 22.64% of the new process. The combination of the VRHP and the DWC is beneficial to the improvement of the energy efficiency of the process, result in the reduction of the energy consumption. The RDWC can cut down more than one half of the energy consumption compared with the reaction and dividing-wall separation process15,26, thus the integration of the VRHP and the RDWC can further reduce the energy consumption of the process. However, the integration of the VRHP and the RDWC have not been fully explored yet. In this work, three novel reactive dividing-wall column with vapor


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recompression heat pump (RDWC-VRHP) models are put forward to the synthesis of n-propyl propionate (PROPRO), aim to further improve energy-saving and economic performance of the process. The reactive dividing-wall column with vapor recompression heat pump and intermediate reboiler (RDWC-VRHP-IR), the RDWC-VRHP-IR with preheater and the reactive dividing-wall column with vapor recompression heat pump and bottom reboiler (RDWC-VRHP-BR) are proposed in this paper. The optimal model is chosen by calculating and comparing the total annual cost (TAC), the total utility consumption (TUC), the thermodynamic efficiencies and the CO2 emissions of all the models. 2. Reaction statement PROPRO is synthetized by esterification reaction of 1-propanol (PROOH) and propanoic acid (PROAC)27, which is expressed by equation (1): O

OH OH +

+ O

H2O

O

(1)

Detailed kinetic data of the reaction can be found in Keller et al.’s paper27. Figure 1 presents the residue curve map of the components under normal pressure. The Hayden-O’Connell equation28 is used to describe the vapor phase. The universal quasichemical (UNIQUAC) activity coefficient model29 is used to depict the liquid phase. The UNIQUAC interaction parameters for this model are given in the supporting information.



0.1

0.9

0.2

0.8 0.7

0.4

0.6

2

H

O

0.3

0.5

0.5

0.6 87. 70 °C 0.7

H OO PR

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

0.4 0.3

86. 84 °C

0.8

0.2

0.9

0.1 89. 26 °C

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

PROPRO

Figure 1. Residual curve map for the ternary system H2O/PROPRO/PROOH. 3. Modification 3.1 Two-column reactive distillation system and RDWC Xu et al.30 investigated a two-column reactive distillation system for the synthesis of PROPRO. The whole system included a reactive distillation (RD) column and a conventional distillation column. PROAC and PROOH was fed from the top tray and the bottom tray of the reactive zone respectively. Aqueous phase of the top stream from the RD column was removed, while organic phase of the top stream went back to the RD column. Bottom stream of the RD column was fed to the conventional distillation column. Light components from the distillation column went back to the RD column along with the PROOH feed flow, and PROPRO was obtained at the bottom of the distillation column. A reactive dividing-wall column (RDWC) was studied based on Xu et al.’s work. PROAC and PROOH flows were fed to the reactive distillation column (RDC). The top stream of the RDC was cooled in the condenser, then went into the decanter of the RDC (DEC), where the aqueous phase was removed, and the organic phase went back to the RDC. Top stream of the recovery column (RC) recycled to the RDC with the PROOH feed flow, and PROPRO was obtained at the bottom of the PROPRO column (PPC). Detailed optimization has been studied in our early work31, and the optimal


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process is presented in Figure 2. The results showed that the RDWC saves 28.87% of the total reboiler duty in comparison with the two-column reactive distillation system. Detailed data are listed in Table 1.

Figure 2. Optimal process of the RDWC. 3.2 Reactive dividing-wall column with vapor recompression heat pump and intermediate reboiler (RDWC-VRHP-IR) Comparing with the two-column reactive distillation system, the RDWC shows good performance in economic and energy-saving areas. However, overhead vapor of the RDC is condensed in the condenser of the RDC (CON1) directly, resulting in unnecessary energy loss. With the addition of an intermediate reboiler (IR), the operating and capital costs of the distillation process could be reduced compared with the conventional column32. An energy-saving method is to transfer heat from the top stream of the RDC to the IR, but the temperature of the top stream of the RDC is not high enough to transfer heat. Using VRHP is a good solution to this problem, as



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VRHP can compress the overhead vapor stream into higher pressure and temperature. The VRHP models should be used when the temperature of the bottom is closed to the temperature of the top, in other words, it is suitable for close boiling mixtures. The reflux ratio of the distillation should be larger, so that the top steam can provide enough heat to the bottom stream. The RDWC configuration can meet all the requirements above. In this section, VRHP with IR is added to the RDWC. For convenience, the RDWC-VRHP-IR1 is used to cite this process. Figure 3 shows the flow diagram of the RDWC-VRHP-IR1. PROAC and PROOH flows are fed to the RDC. Top stream of the RDC is compressed in the compressor (HPUMP), thus the top stream can reach higher pressure and temperature. Then the compressed top stream can give heat to the side stream of the PPC in the IR. The outlet stream comes to the CON1 and recycles to the DEC, where aqueous phase of the stream is removed and organic phase return to the RDC. Drawn out from a certain stage of the PPC, the side stream is heated in the IR, and recycles to the next stage of the PPC. After being condensed in the condenser of the RC (CON2), the top stream of the RC is split into two streams in the reflux tank (RT). One stream recycles to the PROOH feed flow; the other one is refluxed back to the RC. PROPRO is obtained at the bottom of the PPC. The pressure drop between stages of all columns is 0.0068 atm. The top stage pressure of the RC and the RDC is set as 1 atm. The feed flow rate of PROAC and PROOH into the RDC is 50 and 60 kmol/h respectively. The PROPRO product purity is set above 99.5 mol%. The RDWC is simulated by the rigorous distillation module RadFrac with the help of Aspen Plus 8.4.


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Figure 3. Flow diagram of the RDWC-VRHP-IR1. For convenience of calculating the total energy consumption of the process, the TUC is defined as: ܷܶ‫ܳ∑ = ܥ‬ோா஻ + ∑ܳ௉ோா + 3∑ܳ஼ைெ௉ (5)

Where ܳோா஻ (kW) represents the heat duty of the reboiler, ܳ௉ோா (kW) denotes the heat duty of the preheater, and ܳ஼ைெ௉ (kW) is the compression duty. The factor of 3 for the compression duty is assumed to convert the work of the compressor into thermal energy with same effort of the electrical work33,34. Economic performance is an important measurement to the whole process, where the capital cost and the operating cost should be considered together. Here, the economic evaluation is carried out by calculating the TAC, and detailed calculation process is given in the Supporting Information. Equations to calculate the thermodynamic efficiency and CO2 emissions are presented in the Supporting



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Information for quick reference. There are several operating conditions that can influence the performance of the process. Undoubtedly, the number of stages of the RDC (ܰோ஽஼ ), the number of stages of the RC (ܰோ஼ ) and the number of stages of the PPC (ܰ௉௉஼ ) are fundamental operating conditions of the process. Besides, the location of the side stream of the PPC (ܰௌ஼ ) and the flow rate of the side stream of the PPC (‫ܨ‬ௌ஼ ) are two important operating conditions of the process. Furthermore, the compression ratio of the HPUMP (ܴ௖ ) is the determinant operating condition of the process. It has effects on the temperature and pressure of the compressed top stream of the RDC, further influences the duty of the IR. However, these operating conditions influence the TUC in a single direction, thus set the TAC as the objective of the optimization is a better choice. A systematic procedure for the optimization has developed, as showed in Figure 4. The SQP optimization method and the sensitivity analysis tool from Aspen Plus 8.4 are adopted in the optimization procedure.

Figure 4. Systematic procedure for the optimization. A better initial design can facilitate the optimization process. Referring to our


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early work31, the main distillation column, including the RDC and the PPC, has 44 stages, and the condenser is set as the first stage. The RDC and the RC has 31 and 15 stages respectively. The side stream of the PPC is 144 kmol/h at the 33rd stage of the main distillation column, the compression ratio is set as 1.4. The differential temperature driving force of the IR is maintained at 5°C. Figure 5 shows the effects of stage number in the RDC, RC and PPC on the TAC and the TUC. As the stage number raises, the TUC decreases. The minimum TAC exists as there are 29, 5 and 12 stages in the RDC, RC and PPC respectively. It can be concluded that the addition of the VRHP with IR cut down the separation difficulty of the mixture in the RC and the rectifying section of the RDC, thus the stage number of these sections can be reduced in comparison with the RDWC. 687.5

1052

TAC (103$/y) TUC (kW)

1060


1050 1048

700

687.0

1058 1056

1046

3

1042 1040 1038

698

1036

TUC (kW)

699

TAC (103$/y)

1054

1044

1052 1050

686.0

1048 1046

685.5

1034 697

686.5

1044 1042

1032 685.0

1030 27

28

29

30

TUC (kW)

701

TAC (10 $/y)

31

1040 1

2

RDC Stage Number

3

4

5

6

7

RC Stage Number

692

1056 1054


691

1052

690

1050 1048

689 1046 688

1044 1042

687 1040

TUC (kW)

TAC (103$/y)

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1038

686

1036 685 1034 684 10

11

12

13

14

15

1032 16

PPC Stage Number

Figure 5. Effects of stage number in the (a) RDC, (b) RC and (c) PPC on the TAC and the TUC. Figure 6 demonstrates how the TAC and the TUC are affected by the side stream of the PPC. As the stage number of the side stream increases, the TAC and the TUC increases. On the contrary, the TAC and the TUC decreases as the flow rate of the side



stream increases. Considering the potential constraints of the stage drying to the flow rate of the side stream, 144 kmol/h at the 31st stage of the main distillation column is chosen as the optimal value of the side stream. 1130

TAC (10 $/y) TUC (kW)

705

1054

1120

3


689

1110 1100

1070 1060

690

1050

1050

3

1080 695

688

TAC (10 $/y)

1090

TUC (kW)

700

1052

1048 687 1046

TUC (kW)

710

TAC (103$/y)

686 1044

1040 685 30

31

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34

35

36

1030 37

685

110

Side Stream Stage Number

115

120

125

130

135

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145

1042

Side Stream Flow Rate (kmol/h)

Figure 6. Effects of the side stream of the PPC on the TAC and the TUC. Figure 7 indicates the effects of the compression ratio on the TAC and the TUC. Undoubtedly, as the compression ratio increases, the TAC increases more quickly, while the TUC increases in line. However, if the compression ratio is lower than 1.4, the compressed top stream of the RDC will not provide enough heat to the side stream of the PPC, limiting the possibility of further optimization in some extent. 696


694

1100

690 1060

TUC (kW)

1080

692

3

TAC (10 $/y)

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688

686

1040

684

1.4

1.5

1.6

1.7

Compression ratio

Figure 7. Effects of the compression ratio on the TAC and the TUC, in the RDWC-VRHP-IR1. Figure 3 shows the optimal condition of the RDWC-VRHP-IR1. Because of the addition of vapor recompression heat pump with IR, the duty of the IR is 821.81 kW, and the output work of the HPUMP is 35.87 kW. The duty of the REB is reduced from 1718.10 kW of the RDWC to 915.03 kW, and the duty of the CON1 is reduced from 1277.85 kW to 510.48 kW.


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Adding a preheater is an auxiliary way in the heat integrated distillation columns3,35. Seeking for a better energy-saving solution, a preheater (PHE) is added to the

RDWC-VRHP-IR1,

as

showed

in

Figure

8.

For

convenience,

the

RDWC-VRHP-IR2 is used to cite this process. Different from the RDWC-VRHP-IR1, the top stream of the RDC is heated in the PHE before compressed in the HPUMP, so that it can cut down the compression ratio of the HPUMP.

Figure 8. Process flow diagram of the RDWC-VRHP-IR2. The optimization process is similar with that of RDWC-VRHP-IR1. Figure 9 indicates the effects of the duty of the PHE and compression ratio on the TAC and the TUC. As a result, the TUC increases in line as the duty of the PHE increases. The minimum TAC exists as the duty of the PHE and the compression ratio is 93.04 kW and 1.3 respectively.



700 1120 690

Rc=1.3

Rc=1.6

Rc=1.4

Rc=1.7

Rc=1.5 1100

680

TUC (kW)

670

3

TAC (10 $/y)

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660

Rc=1.3

1060

Rc=1.4

650

Rc=1.5 640 630

1080

1040

Rc=1.6 Rc=1.7 0

20

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Duty of the PHE (kW)

1020

0

20

40

60

80

100

120


Figure 9. Effects of the duty of the PHE (ܳ௉ுா ) and the compression ratio (ܴ௖ ) on (a) the TAC and (b) the TUC, in the RDWC-VRHP-IR2. Figure 8 shows the optimal condition of the RDWC-VRHP-IR2. Because of the addition of the PHE, the inlet temperature of the HPUMP is increased from 102.1°C to 129.1°C, the output work of the HPUMP is decreased to 35.87 kW, the duty of the PHE is 93.04 kW, and the duty of the IR is increased to 900.84 kW. Furthermore, the duty of the REB is reduced to 836.56 kW. Compared with the RDWC-VRHP-IR1, the inlet stream temperature of the HPUMP is increased due to the addition of the PHE. Therefore, the outlet stream temperature of the HPUMP is increased as well. Accordingly, the duty of the IR is increased and the duty of the REB is increased. Meanwhile, the capital cost of the REB is reduced, thus the TAC is reduced. 3.3 Reactive dividing-wall column with vapor recompression heat pump and bottom reboiler (RDWC-VRHP-BR) In this section, vapor recompression heat pump with bottom reboiler (BR) is added to the RDWC. Figure 10 shows the flow diagram of the RDWC-VRHP-BR. Different from the RDWC-VRHP-IR2, the compressed top stream of the RDC gives heat in the BR. Boilup stream of the PPC is split into two streams. One stream is boiled up in the REB. The other stream is boiled up in the BR. Top stage pressure, pressure drop between stages, feed streams flow rate and the PROPRO product purity are kept the same as those of the RDWC-VRHP-IR1. The optimization process is similar to that of the RDWC-VRHP-IR2.


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Figure 10. Flow diagram of RDWC-VRHP-BR. In convenience of comparison, all stage numbers are same as those of the RDWC-VRHP-IR1. Figure 11 shows effects of the preheater duty and the compression ratio on the TAC and the TUC in the RDWC-VRHP-BR. The TUC increases as the duty of the PHE increases. The minimum TAC exists as the compression ratio and the duty of the PHE is 1.8 and 93.04 kW respectively.



780

1320

1310

770

1300

3

TUC (kW)

760

TAC (10 $/y)

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

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750

Rc=1.8 Rc=1.9

740

Rc=1.8

Rc=2.1

0

20

Rc=1.9

1260

Rc=2.2 720

1280

1270

Rc=2.0 730

1290

Rc=2.1

Rc=2.0 40

60

80


100

120

1250

0

20

Rc=2.2 40

60

80

100

120


Figure 11. Effects of the duty of the PHE (ܳ௉ுா ) and the compression ratio (ܴ௖ ) on (a) the TAC and (b) the TUC, in the RDWC-VRHP-BR. Figure 10 indicates the optimal case of the RDWC-VRHP-BR. The duty of the PHE is 93.04 kW, the output work of the HPUMP is 81.02 kW, the duty of the BR is 808.42 kW, the reboiler duty of the PPC is reduced from 1718.10 kW to 919.33 kW, and the condenser duty of the RDC is reduced from 1277.85 kW to 648.12 kW. Compared with the RDWC-VRHP-IR2, the temperature of the outlet stream of the HPUMP should be higher to keep the certain differential temperature driving force because the temperature of the boilup stream of the PPC is higher than that of the side stream of the PPC. Therefore, the compression ratio of the HPUMP is higher, the compressor cost and electricity cost are increased, thus the TAC is increased. Furthermore, the duty of the BR in the RDWC-VRHP-BR can’t reach that of the IR in the RDWC-VRHP-IR2, thus the RDWC-VRHP-BR needs more energy compared with the RDWC-VRHP-IR2. 3.4 Comparison Detailed comparisons of all the optimal cases and the conventional two-column reactive distillation system are listed in Table 1. The TUC of the RDWC, RDWC-VRHP-IR1, RDWC-VRHP-IR2 and RDWC-VRHP-BR decreases by 28.87%, 56.77%, 57.06% and 48.01% respectively in comparison with the two-column reactive distillation system. It can be concluded that the RDWC costs less energy compared with the two-column reactive distillation system. The integration of the VRHP and the RDWC can further cut down energy consumption, as the


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RDWC-VRHP-IR2 needs less demands of the utilities than the RDWC-VRHP-BR. Comparing with the two-column reactive distillation system, a 9.59% saving has reached in the total capital cost (TCC) of the RDWC. However, the TCC of RDWC-VRHP-IR1, RDWC-VRHP-IR2 and RDWC-VRHP-BR increases by 17.23%, 3.96% and 9.03% respectively. The total operating cost (TOC) of the RDWC, RDWC-VRHP-IR1, RDWC-VRHP-IR2 and RDWC-VRHP-BR decreases by 26.62%, 52.15%, 52.96% and 40.45% respectively in comparison with the two-column reactive distillation system. Although the TCC increases due to the integration of the VRHP and the RDWC, the TOC of the models decrease a lot, where the reduction of the energy consumption plays a leading role, thus the decrement of the TOC can compensate the increment of the TCC. Furthermore, in a longer period, economic advantages of the models become more obvious. The TAC of the RDWC, RDWC-VRHP-IR1, RDWC-VRHP-IR2 and RDWC-VRHP-BR decreases by 20.14%, 25.76%, 31.31% and 21.63% respectively with the payback period of 5 years in comparison with the two-column reactive distillation system. The TAC of those models reduced further when the payback period is set as 8 years, as showed in Table 1. The thermodynamic efficiency of RDWC-VRHP-IR1, RDWC-VRHP-IR2 and RDWC-VRHP-BR is 22.50%, 22.95% and 17.96% respectively. It is significantly higher than that of the RDWC, which is 16.93%. As the utility cost of these RDWC-VRHP models are smaller and can be used more efficiently than the RDWC, exergy loss of these models are reduced in comparison with the RDWC. These RDWC-VRHP models also show good environmental performance. The CO2 emissions in the RDWC, RDWC-VRHP-IR1, RDWC-VRHP-IR2 and RDWC-VRHP-BR reduces 28.87%, 62.12%, 61.52% and 58.07% respectively in comparison with the two-column reactive distillation system. Among all the cases listed in Table 1, the RDWC-VRHP-IR2 is the most attractive process for its best energy-saving and economic performance. Compared with the RDWC-VRHP-BR, a smaller temperature difference is more beneficial to the RDWC-VRHP system. It indicates that the RDWC-VRHP models have same



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applicable conditions with the VRHP configurations.


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Table 1. Comparison of all the optimal cases. Parameter

Two-column

RDWC

RDWC-VRHP-IR1

RDWC-VRHP-IR2

RDWC-VRHP-BR

Preheater duty (kW) Compression ratio Compressor duty (kW) Heat exchanger duty (kW) Column vessel cost (103$) Plate cost (103$) Heat exchangers cost (103$) Compressor cost (103$) Steam cost (103$/year) Cooling water cost (103$/year) Electricity cost (103$/year) TUC (kW) TCC (103$) TOC (103$/year) TAC with the payback period of 5 years (103$/year) TAC with the payback period of 8 years (103$/year) Thermodynamic efficiency (%) CO2 emissions (kg/h)

884.44 111.42 759.44 537.06 34.74 2415.53 1755.30 571.80 922.86 791.21 12.98 1233.04

907.96 117.74 561.33 381.99 37.57 1718.10 1587.04 419.57 736.97 617.95 16.93 877.03

1.4 43.05 821.81 788.76 100.29 975.47 193.15 203.44 21.92 48.21 1044.17 2057.67 273.58 685.11 530.79 22.50 467.09

93.04 1.3 35.87 900.83 792.76 101.01 764.70 166.33 206.68 22.08 40.18 1037.23 1824.79 268.95 633.91 497.05 22.95 474.53

93.04 1.8 81.02 808.42 826.22 107.69 657.51 324.45 225.18 24.68 90.78 1255.94 1913.77 340.50 723.26 579.72 17.96 516.99



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4. Conclusion The novel RDWC-VRHP models are proposed in this paper. Energy-saving, economic and environmental performances are compared before and after the addition of the VRHP. The performance of the RDWC is better than that of the conventional two-column reactive distillation system. After the integration of the RDWC and the VRHP,

TUC,

TAC

and

CO2

emissions

can

be

further

reduced.

The

RDWC-VRHP-IR1 is more attractive than the RDWC-VRHP-BR. Adding a preheater to the RDWC-VRHP-IR1 can reduce the compression ratio and achieve better performance. Therefore, the RDWC-VRHP-IR2 shows the best performance among models proposed in this paper. Comparing with the conventional two-column reactive distillation system, the RDWC-VRHP-IR2 saves 57.06% of the TUC, 31.31% of the TAC with the payback period of 5 years and 61.52% of the CO2 emissions respectively. In this paper, the RDWC-VRHP-IR shows better performance than the RDWC-VRHP-BR. However, it doesn’t mean that the RDWC-VRHP-IR is better than the RDWC-VRHP-BR. Furthermore, if the temperature of the bottom is not closed to the temperature of the top, using the RDWC-VRHP models may not be a wise choice. Although the combination of the RDWC and the VRHP can greatly enhance the steady-state performance of the process, dynamics and control of the RDWC-VRHP process is still an important issue to be addressed in the future.


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Nomenclature Abbreviations BR = bottom reboiler CON1 = condenser of the RDC CON2 = condenser of the RC DEC = decanter of the RDC DWC = dividing-wall column HPUMP = heat pump IR = intermediate reboiler PHE = preheater PP = payback period PPC = PROPRO column PROAC = propanoic acid PROOH = 1-propanol PROPRO = n-propyl propionate RC = recovery column RD = reactive distillation RDC = reactive distillation column RDWC = reactive dividing-wall column RDWC-VRHP = reactive dividing-wall column with vapor recompression heat pump RDWC-VRHP-BR = reactive dividing-wall column with vapor recompression heat pump and bottom reboiler RDWC-VRHP-IR = reactive dividing-wall column with vapor recompression heat pump and intermediate reboiler REB = reboiler of the PPC RT = reflux tank of the RC TAC = total annual cost TCC = total capital cost TOC = total operating cost TUC = total utility consumption



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VRHP = vapor recompression heat pump Variables ‫ = ܣ‬heat transfer area, m2 ‫ܥ‬௖௢௠௣ = compressor cost, $ ‫ܥ‬௖௩ = column vessel cost, $ ‫ܥ‬௖௪ = cooling water cost, $ ‫ܥ‬௘௖ = electricity cost, $ ‫ܥ‬௛௘ = heat exchanger cost, $ ‫ܥ‬௣௟ = plate cost, $ ‫ܥ‬௦௧ = steam cost, $ ሾ‫ܱܥ‬ଶ ሿா௠௜௦௦ = CO2 emissions, kg/h ‫ܨ‬ௌ஼ = flow rate of the side stream of the PPC, kmol/h ܰ௉௉஼ = number of stages of the PPC ܰோ஼ = number of stages of the RC ܰோ஽஼ = number of stages of the RDC ܰௌ஼ = location of the side stream of the PPC ܳ௉ுா = duty of the PHE, kW ܴ௖ = compression ratio


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Acknowledgements We are thankful for the assistance from the staff at the Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology from the School of Petrochemical Engineering (Changzhou University).



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Supporting Information Binary parameters for the UNIQUAC activity coefficient model (Table S1), Calculation method of the TAC, thermodynamic efficiency and CO2 emissions (Appendix S1). This information is available free of charge via the Internet at http://pubs.acs.org/.


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Table of Contents Figure 1. Residual curve map for the ternary system H2O/PROPRO/PROOH. Figure 2. Optimal process of the RDWC. Figure 3. Flow diagram of the RDWC-VRHP-IR1. Figure 4. Systematic procedure for the optimization. Figure 5. Effects of stage number in the (a) RDC, (b) RC and (c) PPC on the TAC and the TUC. Figure 6. Effects of the side stream of the PPC on the TAC and the TUC. Figure 7. Effects of the compression ratio on the TAC and the TUC, in the RDWC-VRHP-IR1. Figure 8. Process flow diagram of the RDWC-VRHP-IR2. Figure 9. Effects of the duty of the PHE (ܳ௉ுா ) and the compression ratio (ܴ௖ ) on (a) the TAC and (b) the TUC, in the RDWC-VRHP-IR2. Figure 10. Flow diagram of RDWC-VRHP-BR. Figure 11. Effects of the duty of the PHE (ܳ௉ுா ) and the compression ratio (ܴ௖ ) on (a) the TAC and (b) the TUC, in the RDWC-VRHP-BR Table 1. Comparison of all the optimal cases.


Performance Enhancement of Reactive Dividing-Wall Column via

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