Application of Mechanical Vapor Recompression Heat Pump to

Mar 5, 2015 - College of Chemistry and Chemical Engineering, Nanjing ... the TACs for double-effect distillation with double MVRHPs, top MVRHP distill...
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Application of Mechanical Vapor Recompression Heat Pump to Double-Effect Distillation for Separating N,N‑Dimethylacetamide/ Water Mixture Xiaoxin Gao,*,†,‡ Jun Chen,† Jiankan Tan,† Ying Wang,† Zhengfei Ma,‡ and Limin Yang† †

Jiangsu Province Key Laboratory of Fine Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu Province 213164, People’s Republic of China ‡ College of Chemistry and Chemical Engineering, Nanjing Technology University, Nanjing 210009, People’s Republic of China S Supporting Information *

ABSTRACT: A coupled separation system involving a mechanical vapor recompression heat pump (MVRHP) and doubleeffect distillation (MED) was proposed. In a case of separating N,N-dimethylacetamide (DMAC)/water mixture, four distillation schemes (i.e., conventional distillation, top MVRHP distillation, double-effect distillation, and double-effect distillation with double MVRHP) were conceptually constructed and simulated by using Aspen Plus with the binary interaction parameters of the NRTL thermodynamic model. We evaluated these four schemes to choose the best one with the goal of the minimum total annual cost (TAC). Compared to the conventional distillation, the TACs for double-effect distillation with double MVRHPs, top MVRHP distillation, and double-effect distillation were decreased by 47.76, 32.23, and 5.45%, respectively. It indicates that the double-effect distillation with double MVRHPs has obvious advantages over the other three distillation schemes in terms of economic efficiency. generating flow-up vapor. The advantages of MVRHP include recycling the vapor produced, simpler process, lower operating cost, and easier operation. The disadvantage of MVRHP is high investment cost. When MVRHP is applied to MED, the top vapor of the second column is recompressed to raise its temperature and pressure and then used to heat the first reboiler for generating the flow-up vapor of the first column, and so on. In the literature, separation of the DMAC/water mixture has been reported by using conventional distillation, top MVRHP distillation, and double-effect distillation. However, to our knowledge, the application of combined MVRHP and MED technique to separate this mixture has not been reported. In the present study, a coupled technique of MVRHP and MED to separate DMAC and water mixture was proposed. To compare advantages and disadvantages among different techniques, we conceptually constructed four distillation schemes, namely, conventional distillation, top MVRHP distillation, double-effect distillation, and double-effect distillation with double MVRHP, and we simulated them by using Aspen Plus with the binary interaction parameters of the NRTL thermodynamic model. The operating parameters were investigated for these four distillation processes. Moreover, the energy consumption and total annual cost (TAC) for the four schemes were discussed comprehensively. It is expected through this study that a new technique for separating such mixtures could be constructed,

1. INTRODUCTION A multieffect distillation (MED) scheme has been developed and applied in the industry due to its energy saving effect. MEDs for seawater desalination1−5 and separation6−10 of liquid mixtures have made remarkable advances, especially with advantages of easy operation and saving large amount of energy. In an MED system, for example, a double-effect distillation with two columns, the first column top vapor generated from the first column reboiler was employed to heat the second column reboiler. So, the consumption of heating agent for reboilers can be reduced and corresponding energy consumption can be reduced to approximately 50%. For a multieffect distillation with three or more columns, the energy consumption can be reduced further. For example, for a threeeffect distillation with three columns, the energy consumption can be reduced to around 33%. In an MED system, the total temperature difference between the reboiler of the first column and the reboiler of the last column is distributed in each column. It implies that the temperature difference for each column decreases as the column number increases. To meet the specified evaporation rate of reboilers, their heat transfer areas have to be increased. Therefore, costs of reboilers increase, while the energy consumption decreases as the column number increases. A mechanical vapor recompression heat pump (MVRHP) has been employed to separate close-boiling mixtures and desalinate seawater for reducing energy consumption.11−15 In an MVRHP system, the vapor at the column top with lower pressure and temperature passes to a compressor, which raises the vapor temperature and pressure by applying compressor work. The resulting vapor with higher temperature and pressure gives up its latent heat to heat the reboiler for © 2015 American Chemical Society

Received: Revised: Accepted: Published: 3200

November 27, 2014 February 15, 2015 March 5, 2015 March 5, 2015 DOI: 10.1021/ie504664h Ind. Eng. Chem. Res. 2015, 54, 3200−3204

Article

Industrial & Engineering Chemistry Research and the advantages and disadvantages of the technique could be fully explored.

2. PROCESS OBJECTIVE AND DESIGN A mixture of DMAC and water (30:70, w/w) was fed to the column at a rate of 5000 kg/h as a saturated liquid at atmosphere. The product purity of DMAC reached 99.9 wt % or above at the column bottom. The water purity reached 99.9 wt % or above at the column top. The column pressure drop was 40 kPa. Figure 1 shows the temperature versus composition

Figure 2. Aspen process flow diagram for conventional distillation.

to be 5 years, and the operating time is 8000 h every year. The expression is as follows: TAC =

capital costs + annual operating costs payback period

The annual operating costs comprise the costs of compressor electricity and the cost of steam and cooling water. The capital costs comprise costs of distillation columns, heat exchangers, compressor(s), and other equipment. The TAC for the different operating pressures for the conventional distillation is shown in Table 1. It is noted that the operating cost decreased as the operating pressure decreased, but the capital investment increased. According to Table 1, when the operating pressure is 20 kPa, the TAC is lowest. With this value, the reflux ratio is 0.134, the column diameter is 1.64 m, the condenser duty is 2605.91 kW, the reboiler duty is 2869.51 kW. 3.2. Top MVRHP Distillation. The conditions for the optimal conventional distillation were also used for the top MVRHP distillation column. The Aspen Plus diagram for the top MVRHP distillation is shown in Figure 3. The column top vapor flows into the compressor (C1), its temperature and pressure are increased to a higher temperature and pressure after C1. When the column top pressure is 20 kPa and the compression ratio is 28, the top vapor temperature is increased from 60 to 156 °C, the top vapor pressure is increased from 20 to 560 kPa. The outlet stream at the bottom splits into two parts: one is the requirement product DMAC, the other (B2) flows into heat exchanger (H1) and is heated by the compressed top vapor (V2) for generating flow-up vapor (B3). The compressed top vapor (V2) is changed into saturated liquid (L1) after H1. The L1 stream passes through a relief valve to reduce its pressure to 20 kPa and temperature to 60 °C, then this stream is divided into two streams: one is the requirement product water, and the other is as the recycled stream. The simulation results for the top MVRHP distillation at 20 kPa are presented in Table 2. The TAC for the top MVRHP distillation is $1,829,910 (Section S1, Supporting Information). 3.3. Double-Effect Distillation. The Aspen Plus diagram for the double-effect distillation is shown in Figure 4. The double-effect distillation divides a conventional distillation column into two columns and the two columns are operated at different pressures. For this case, the first column (T1) pressure is at 600 kPa, the second column (T2) is at 20 kPa.

Figure 1. Phase equilibrium (t−xy) diagram for the DMAC/water system.

(t − xy) phase diagram of the DMAC-water mixture at 101.3 kPa with the binary interaction parameters of the NRTL thermodynamic model (Appendix). The boiling point for DMAC was 166 °C at 101.3 kPa, and the saturated steam (1.0 MPa) was used to heat the reboiler. The cooling water was used to cool the top condenser. The phase equilibrium data for the DMAC/water mixture were used by NRTL thermodynamic model,16 which was formerly proved to fit the case of the separation of DMAC/water mixture.

3. SIMULATION OF THE DISTILLATION PROCESS 3.1. Conventional Distillation Process. The Aspen Plus diagram for the conventional distillation is shown in Figure 2. The feed was entered into the column and progressed vapor− liquid mass transfer in a distillation column to reach the separation task. The different operating pressures were screened to obtain the optimal conditions. The number of theoretical stages, the feed stage, and the reflux ratio required for the column were initially calculated by using the DSTWU block, which was based on Fenske−Underwood−Gilliland in Aspen Plus. Then, the column was simulated with the Radfrac block design to achieve the separation task. The minimum TAC for different operating pressures was calculated. Here, TAC includes the annual capital investment and the annual operating costs, the payback period is assumed 3201

DOI: 10.1021/ie504664h Ind. Eng. Chem. Res. 2015, 54, 3200−3204

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Industrial & Engineering Chemistry Research Table 1. TAC for Different Operating Pressures operating pressure reflux ratio height (m) number of stages feed stage diameter (m) column shell cost (103$) reboiler duty (kW) condenser duty (kW) heat transfer area of reboiler (m2) heat transfer area of condenser (m2) total heat exchanger cost (103$) operating cost (103$) capital investment (103$) TAC (103$)

90 0.167 24.16 35 20 1.51 351.34 3029.31 2583.62 1140.82 167.55 911.99 1767.15 1263.33 2019.81

80 0.163 24.16 35 21 1.52 354.25 3008.61 2580.11 743.12 176.50 746.74 1757.47 1100.98 1977.67

70 0.159 24.16 35 21 1.53 357.85 2991.54 2581.84 540.2 188.07 655.23 1750.27 1013.08 1952.89

60 0.155 24.16 35 21 1.55 361.93 2972.79 2583.93 416.38 203.06 598.62 1742.4 960.55 1934.51

50 0.15 24.16 35 20 1.57 366.53 2952.05 2586.69 332.6 223.57 563.52 1733.76 930.05 1919.77

40 0.146 24.16 35 19 1.59 371.86 2928.62 2590.43 271.81 253.93 545.62 1724.12 917.48 1907.61

30 0.141 24.16 35 19 1.61 378.07 2901.65 2596.11 225.36 305.22 547.53 1713.24 925.6 1898.36

20 0.134 24.16 35 18 1.64 385.45 2869.51 2605.91. 188.34 417.84 588.44 1700.8 973.89 1895.58

10 0.128 24.16 35 18 1.68 394.39 2828.02 2627.84 157.68 998.93 845.53 1686.76 1239.92 1934.75

Figure 4. Aspen Plus diagram for the double-effect distillation.

the second column is required for heat transfer. For the doubleeffect distillation, heat is only added in the bottom reboiler of the first column and removed in the top condenser of the second column. The simulation results for the double-effect distillation are presented in Table 3. 3.4. Double-Effect Distillation with Double MVRHPs. The Aspen Plus diagram for double-effect distillation process with double MVRHPs involving two columns and two MVRHPs is shown in Figure 5.

Figure 3. Aspen Plus diagram for the top MVRHP distillation.

Table 2. Simulation Results for the Top MVRHP Distillation at 20 kPa parameters

value

T1 top temperature (°C) compressor outlet temperature (°C) number of stages feed stage column shell cost (103$) reboiler duty (kW) reboiler area (m2) inlet steam of compressor (kg/h) compression ratio compressor work (kW) compressor cost (103$) total heat exchanger cost (103$) electricity cost (103$) capital investment (103$) total energy consumption (kW) COP TAC (103$)

60 156 35 18 385.45 2870 637.78 4043.71 28 1222.38 568.3 485.42 1542.08 1439.17 1222.38 2.35 1829.91

Table 3. Simulation Results for the Double-Effect Distillation at 20 kPa

The column pressures are arranged such that latent heat of the top vapor (energy removal) in first column can be used to heat the reboiler (energy input) of the second column. In addition, a certain temperature difference between the top temperature of the first column and the bottom temperature of 3202

parameter

T1

T2

top temperature (°C) bottom temperature (°C) reflux ratio number of stages feed stage top steam (kg/h) reboiler duty (kW) DMAC product purity at the bottom (wt %) heat exchanger duty (kW) operating pressure (kPa) total column shell cost (103$) total heat-exchanger costs (103$) total energy consumption (kW) capital investment (103$) operating costs (103$) TAC (103$)

158.9 166.5 0.26 16 13 2074.6 1937.29 44.7 1204.57 600 309.06 803.45 1937.29 1112.58 1089.1 1311.62

60 147 0.16 35 18 2048.9 99.9 20 247.83

DOI: 10.1021/ie504664h Ind. Eng. Chem. Res. 2015, 54, 3200−3204

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Figure 5. Aspen Plus diagram for double-effect with double MVRHPs distillation.

The mixture (V3) of the column (T1) top vapor and column (T2) top vapor flows into compressor (C1) for raising its temperature and pressure by the compressor work. The vapor (V4) after C1 is divided into two parts: part of the vapor (V5) is used to heat the T1 reboiler; the other (V6) is passed into compressor (C2), where it is compressed by the C2 work to a higher temperature and pressure. The vapor (V7) after C2 is used to heat the T2 reboiler. These two vapors release their latent heat, change to saturated liquid, and flow into flash with lower pressure. The generating vapor in flash is used to preheat the fresh feed and the liquid is splitted into three: one is for water product, the other two are as the reflux flows of the T1 and the T2, respectively. The bottom product of the T2 reached the requirement of DMAC. The simulation results for the double-effect distillation with double MVRHPs are shown in Table 4. 3.5. Simulation Results. To analyze four distillation processes, we show the main data with the energy-savings and economic effects in Table 5. The capital investment for the MVRHP distillation is the highest. Because the temperature difference between the column top and column bottom is large, in order to meet the requirement of heat transfer, the compressor work is large, and thus, the capital investment for the compressor is expensive. Compared to conventional distillation, the energy consumption for top MVRHP distillation, double-effect distillation and double-effect distillation with double MVRHPs is decreased by 57.4, 32.48, and 78.4%, respectively. Correspondingly, the TAC for top MVRHP distillation is decreased by 3.47%, double-effect distillation by 30.81%, and double-effect distillation with double MVRHPs by 47.76%, based on conventional distillation. For two MVRHP distillation processes, the COP for doubleeffect distillation with double MVRHPs has advantages over top MVRHP distillation. According to the above analysis, for the energy-saving and economic effects, the double-effect distillation with double MVRHPs has advantages over other distillation schemes for separating DMAC/water mixture.

Table 4. Simulation Results for the Double-Effect Distillation with Double MVRHPs parameter

value

T1 operating pressure (kPa) C1 compressor outlet pressure (kPa) C1 compression ratio C1 inlet steam of the compressor (kg/h) C1 compressor work (kW) condensing temperature of low pressure steam (°C) T1 bottom temperature (°C) T1 reboiler duty (kW) DMF product purity at the bottom of T1 (wt %) T1 COP T2 operating pressure (kPa) C2 compressor outlet pressure (kPa) C2 compression ratio C2 inlet steam of the compressor (kg/h) C2 compressor work (kW) condensing temperature of high pressure steam (°C) T2 bottom temperature (°C) T2 reboiler duty (kW) T2 COP total energy consumption (kW) capital investment (103$) operating costs (103$) TAC (103$)

80 200 2.5 4087.43 273.48 120 108.8 1608 44.7 5.88 80 1060 5.3 1967.43 345.43 182.5 171 1432 4.15 618.91 1046.69 780.78 990.12

double-effect distillation, and double-effect distillation with double MVRHPs. Among them, double-effect distillation with double MVRHPs has been recommended to separate the DMAC and water mixture because it can mostly reduce energy consumption and save TAC. For the cases studied, based on conventional distillation, the TAC and the energy consumption for top MVRHP distillation are decreased by 3.47 and 57.4% respectively; the TAC and the energy consumption for double-effect distillation are decreased by 30.81 and 32.48%, respectively; and the TAC and the energy consumption for the double-effect distillation with double MVRHPs are decreased by 47.76 and 78.4% respectively. It is obvious that the double-effect distillation with double MVRHPs has a great advantages in terms of both energy and TAC savings.

4. CONCLUSIONS In this study, we investigated the simulation of four distillation processes for separating a mixture of DMAC and water, including conventional distillation, top MVRHP distillation, 3203

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Table 5. Comparison of Results of Different Distillation Processes conventional distillation

top MVRHP distillation

doubleeffect distillation

double-effect with double MVRHP distillation

2869.51

1222.38

1937.29

618.91

1700.8

1542.08

1089.1

780.78

973.89

1439.17

1112.58

1046.69

1895.59

2.35 1829.91 65.68

1311.62 583.97

5.02 990.12 905.47

total energy consumption (kW) annual operating cost (103$) capital investment (103$) COP TAC (103$) saved TAC (103$)

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APPENDIX

Binary Interaction Parameters (°C) for NRTL Model of DMAC/Water

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Component i DMAC Component j water Aij 0.981 Aji 0.5288 Bij −236.058 Bji −300.008 Cij 0.3

ASSOCIATED CONTENT

S Supporting Information *

Calculation example for the top MVRHP distillation. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-519-86330255. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for support from A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institution.

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REFERENCES

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DOI: 10.1021/ie504664h Ind. Eng. Chem. Res. 2015, 54, 3200−3204