Simulation and Optimization of Distillation Processes for Separating a

Article pubs.acs.org/IECR

Simulation and Optimization of Distillation Processes for Separating a Close-Boiling Mixture of n‑Butanol and Isobutanol Xiaoxin Gao,†,‡,* Jun Chen,† Zhengfei Ma,‡ and Limin Yang† †

School of Petrochemical Engineering, Changzhou University, Changzhou 213164, P. R. China College of Chemistry and Chemical Engineering, Nanjing Technology University, Nanjing 210009, P. R. China

‡

S Supporting Information *

ABSTRACT: Separation of close-boiling mixtures by conventional distillation consumes a large amount of energy because of the very high reflux ratio required. Mechanical vapor recompression heat pumps (MVRHPs) can recycle the energy of the vapor and can thus be used in such distillation processes to save energy. Three different distillation schemes, namely, conventional distillation, top MVRHP distillation, and bottom-flashing MVRHP distillation, were simulated for the separation of the closeboiling mixture of n-butanol and isobutanol using Aspen Plus to determine the economically best option. The research results indicate that, compared to conventional distillation, the energy savings for bottom-flashing MVRHP distillation and top MVRHP distillation can reach 67.92% and 72.92%, respectively, and the TACs correspondingly decrease by 71.74% and 75.57%.

1. INTRODUCTION Distillation is the most important separation technology for separating mixtures and is extensively used in the chemical industry because it can separate mixtures effectively. However, it also consumes a large amount of energy. It is estimated that, among the process technologies in the chemical industry that utilize energy as the separating agent, 40−70% of the energy is consumed in separation units.1 Of this energy, 95% is consumed by distillation processes.2 Currently, with the sharp increase in global energy consumption and rapid growth in energy prices, technologies that can significantly save energy have been sought. Therefore, great research efforts have been devoted to energy savings on distillation. To reduce energy consumption and improve the thermal efficiency of distillation, various improved technologies have been suggested, such as use of a new transport facility, namely, high-gravity rotating bed, and the application of energy-saving technologies, namely, dividing-wall columns (DWCs), multipleeffect distillation, heat-pump distillation, and composite heat integration.3−5 Among energy-saving technologies, mechanical vapor recompression heat pumps (MVRHPs) are the most efficient technique for minimizing energy consumption, especially for vapor-involving systems.6−9 As the vapor compression technique has been fully perfected, it is widely applied in the chemical industry and other industries mainly related to vapor fields, such as evaporation, the desalination of seawater, and the drying of solids.10−15 However, the application of MVRHPs in distillation has rarely been reported in the literature. A flowchart of an MVRHP system is shown in Figure 1. The entire cycle includes an evaporator, a compressor, a condenser, and an expansion valve or their equivalent parts. The evaporator operates under low pressure, and the corresponding saturation temperature of the cycling fluid under this pressure is TL. Qi is the heat absorbed at the low-temperature end by the working fluid. The fluid completely vaporizes to saturated vapor S1, which then enters into the compressor to increase its © XXXX American Chemical Society

Figure 1. MVRHP system flowchart.

pressure and temperature. The compressed vapor becomes superheated vapor S2, and its saturation temperature is correspondingly increased to TH. Subsequently, vapor S2 as a heating agent enters the condenser to release its heat Qo and turn back into saturated liquid S3 under a higher pressure. The saturated liquid then goes through the expansion valve as an isenthalpic process to reduce its temperature and pressure. The working fluid after the expansion valve becomes moist vapor S4, which enters into the evaporator to vaporize fully. These steps form a full working cycle. In such a cycle, the heat Qi is absorbed at the low temperature TL, and the heat Qo is released Received: April 21, 2014 Revised: August 7, 2014 Accepted: August 24, 2014

A

dx.doi.org/10.1021/ie502695x | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

at the high temperature TH. Theoretically, Qo = Qi + W; therefore, the heat released must be greater than the work imposed by the compressor. For an evaporation process using the above principles of MVRHPs, a dilute solution at low temperature is preheated by a heat exchanger and then concentrated by an evaporator. The secondary vapor thus produced is compressed by a compressor, resulting in a stream with a higher pressure, temperature, and enthalpy. The compressed vapor of high quality is sent back to the heating chamber of the evaporator and acts as the heating agent to heat the feed solution. The solution absorbs the latent heat of the vapor and continues to generate the secondary vapor. Within the cycle, the latent heat of the secondary vapor is recycled with the small addition of electrical energy or mechanical work of the compressor. Therefore, reusing the latent heat of the secondary vapor saves a large amount of steam or energy compared to conventional evaporation and reduces the operating costs. Such a cycle provides huge benefits for energy savings. To the authors’ knowledge, the separation of an n-butanol/ isobutanol mixture by these improved distillation techniques has not yet been reported. In the present study, the distillation separation of n-butanol/isobutanol mixtures was performed by conventional distillation, top MVRHP distillation, and bottomflashing MVRHP distillation. The energy consumptions and operating costs for these three schemes were investigated. The results showed that the top MVRHP distillation process can save more energy with a much lower total annual cost (TAC) than for conventional distillation and bottom-flashing MVRHP distillation. Thus, the optimal operating conditions, including operating pressure, reflux ratio, and compression ratio, were determined.

Figure 2. T−x(y) phase equilibrium diagram for the n-butanol/ isobutanol system.

3. SIMULATION OF THE DISTILLATION PROCESS 3.1. Conventional Distillation Process. A representative binary distillation process for finishing the separation task is shown Figure 3. The task of the distillation column is to

2. PROCESS DESIGN AND OBJECTIVE For the conventional distillation process, a binary mixture with 55 wt % n-butanol and 45 wt % isobutanol was fed to the distillation column as a saturated liquid at a rate of 5000 kg/h. The required purities for the isobutanol product at the column top and the n-butanol product at the column bottom were not less than 99 wt %. A plate distillation column with sieve trays was employed. The top condenser was cooled with cooling water, and the bottom reboiler was heated with steam. The phase equilibrium data for the n-butanol/isobutanol mixture were calculated by Wilson’s equation-of-state model, which was previously demonstrated to suit the case of this work.16 Figure 2 shows the resulting phase diagram in terms of the pseudobinary temperature versus composition [T−x(y)] (i.e., the equilibrium relationship between temperature and compositions of liquid and vapor) for the n-butanol/isobutanol system at 100 kPa. For the system selected, the boiling points for isobutanol and n-butanol at 100 kPa are 107.3 and 117.3 °C, respectively, and the relative volatility is 1.39. Using conventional distillation to separate this system requires a very high reflux ratio and, thus, a large amount of energy. For energy savings, the present study aimed to simulate the separation of the n-butanol/isobutanol mixture using three different distillation schemes, namely, conventional, top MVRHP, and bottom-flashing MVRHP distillations, and to optimize the processes with the goal of the lowest TAC. All three of these distillation schemes were simulated with Aspen Plus computer software.

Figure 3. Aspen process flow diagram for conventional distillation.

separate the feed into a liquid distillate with 99 wt % isobutanol and a liquid bottom product with 99 wt % n-butanol. The number of theoretical stages and the reflux ratio required for the column was estimated by using the “DSTWU” block in Aspen Plus. Then, the column was simulated with the “Radfrac” block design to fulfill the separation task for varying reflux ratios. To obtain the best conditions, the operating pressure was varied from 100 to 50 kPa. A plate distillation column with sieve trays was employed. The minimum values of TAC for the different operating pressures were examined. Here, the TAC includes the operating costs and annual capital investment, according to the expression TAC =

capital cost + operating cost payback period

(1)

The costs of fixed capital investment and utility consumption were estimated by the equations listed in the Appendix. The energy consumption (QCons) is mainly attributed to the reboiler duty (QR) and the compressor duty (QC) B



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Table 1. Simulated Results for Conventional Distillation at Different Pressures operating pressure (kPa) parameter

100

90

80

70

60

50

number of stages feed stage reflux ratio diameter (m) height (m) column-shell cost (×103$) reboiler duty (kW) condenser duty (kW) reboiler heat-transfer area (m2) condenser heat-transfer area (m2) total heat-exchanger cost (×103$) operating cost (×103$) capital investment (×103$) total energy consumption (kW) TAC (×103$/year)

61 34 6.55 1.74 43.18 652.36 2604.25 2665.27 2127.65 147.34 1,249.51 151.37 1,901.87 2604.25 1,701.56

61 35 6.25 1.73 43.18 636.38 2509.55 2582.83 1115.36 148.66 886.45 1,458.86 1,534.81 2509.55 1,612.34

61 36 5.83 1.71 43.18 64.037 2396.97 2457.47 710.21 147.92 708.33 1,391.81 1,348.71 2396.97 1,526.68

61 38 5.99 1.77 43.18 664.35 2439.74 2540.35 553.22 161.36 641.24 1,423.26 1,305.6 2439.74 1,553.82

62 40 6.02 1.83 43.18 688.67 2463.71 2579.82 430.42 174.58 585.05 1,439.70 1,273.44 2463.71 1,567.04

62 42 6.17 1.90 43.18 728.56 2531.58 2664.85 359.24 194.85 558.85 1,481.65 1,275.35 2531.58 1,609.18

Q Cons = Q R + 3.29Q C

compressed in compressor C-100 to a higher pressure and a higher temperature. When the pressure at the column top is at 100 kPa and the compression ratio is 2.7, the pressure of the compressed vapor increases from 100 to 270 kPa, and its temperature increases from 107.4 to 132 °C. The latent heat of the top vapor is used to generate the vapor boilup in heat exchanger H-100, and the vapor is condensed to saturated liquid. The temperature difference across the heat-exchanger wall is about 10 °C. This is an efficient value for a heat-transfer process, and it is a compromise between capital and operating costs. The heat balances for the bottom and top equipment are as follows

(2)

For comparison, we assumed that the compressor work was completely converted into the equivalent thermal energy. The factor of 3.29 in eq 2 was used as the ratio of energy costs to electricity costs in China. The relationship between the TAC and the operating pressure for conventional distillation is presented in Table 1. It is noted that, as the operating pressure decreases, the capital investment decreases on the whole, and the operating cost increases. The optimal operating pressure was determined from the minimum TAC. According to Table 1, the optimal pressure for this case is 80 kPa. With this value, the reboiler duty is 2396.97 kW, and the reflux ratio is 5.83. The main parameters for the distillation column are listed in Table S1 (Supporting Information), and the mass and heat balances are provided in Table S2 (Supporting Information). 3.2. Top MVRHP Distillation. Figure 4 shows the process for the top MVRHP distillation scheme. The bottom outlet stream is divided into two parts: one for the bottom product and the other for generating the vapor boilup. The top vapor is

Q C ≈ Q R ≈ Q H‐100

where QC is the duty for the conventional column condenser and QR is the duty of the conventional column reboiler. The saturated liquid then passes through valve VLV-100 to reduce its temperature to 107.4 °C and pressure to 100 kPa, which are the same as the conditions at the reflux position. The saturated liquid is divided in SPLIT-100 into the top product and the reflux stream back to the column. For top MVRHP distillation, the pressure at the column top was varied from 100 to 60 kPa. A plate distillation column with sieve trays was again employed. A maximum thermal efficiency can be obtained by direct heat exchange between the top and bottom streams. Table 2 reports the simulated results for the top MVRHP case under different operating pressures. According to Table 2, the optimal operating pressure at the minimum TAC is 90 kPa. At this pressure , the compression ratio is 2.7, and the compressor work is 198.57 kW. The mass and heat balances for this case are reported in Table S3 (Supporting Information). 3.3. Bottom-Flashing MVRHP Distillation. Figure 5 shows the process of the bottom-flashing MVRHP distillation scheme. The outlet from the column bottom is divided into two streams: One is for the bottom product, and the remainder enters valve VLV-110 to reduce its temperature and pressure as the recycled stream. This stream gains latent heat from the top vapor through heat exchanger H-110 and vaporizes fully into the recycled vapor. When the pressure at the column top is 100 kPa, the temperature of the recycled stream after VLV-110 is reduced from 127.1 to 97.4 °C, and the corresponding pressure

Figure 4. Aspen process flow diagram for top MVRHP distillation. C



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Table 2. Simulated Results for Top MVRHP Distillation at Different Pressures operating pressure (kPa) parameter

100

90

80

70

60

T-100 top temperature (°C) compressor outlet temperature (°C) column-shell cost (×103$) reboiler duty (kW) reboiler area (m2) compressor inlet steam (kg/h) compression ratio compressor work (kW) compressor cost (×103$) total heat-exchanger cost (×103$) electricity cost (×103$) capital investment (×103$) total energy consumption (kW) TAC (×103$/year)

107.4 132 652.36 2482.52 551.67 14372.8 2.7 208.12 179.3 511.67 262.55 1,273.41 684.71 389.89

104.6 129.1 636.38 2390.05 531.12 15609.7 2.7 198.57 172.7 431.24 250.50 1,240.06 653.3 374.54

101.5 126.8 640.37 2336.3 535.23 15577 2.8 199.33 173.2 433.15 251.46 1,246.72 655.80 376.13

98 125.9 664.35 2310.5 513.45 15348.4 3.1 214.56 179.1 421.61 270.68 1,265.06 705.90 397.18

94.2 123.5 622.42 2404 688.67 15600.9 3.3 228.26 193 427.80 287.96 1,309.47 750.98 418.91

Figure 5. Aspen process flow diagram for bottom-flashing MVRHP distillation.

Table 3. Simulated Results for Bottom-Flashing MVRHP Distillation at Different Pressures operating pressure (kPa) parameter

100

90

80

70

60

50

T-110 top temperature (°C) compressor outlet temperature (°C) column-shell cost (×103$) condenser duty (kW) compressor inlet steam (kg/h) relief-valve outlet pressure (kPa) relief-valve outlet temperature (°C) compression ratio compressor work (kW) heat-transfer area (m2) compressor cost (×103$) total heat-exchanger cost (×103$) electricity cost (×103$) capital investment (×103$) total energy consumption (kW) TAC (×103$/year)

107.4 128.2 652.36 2673.12 16594.9 47 97.4 3.5 257.42 1069.25 212.5 679.18 324.75 1,544.94 846.91 479.15

104.5 125.1 636.38 2583.2 15883.6 41.5 94.4 3.5 244.45 1023.05 203.9 659.96 308.38 1,500.24 804.24 458.41

101.4 122.5 640.37 2421 15072.6 36 91.1 3.6 235.29 940.19 197.8 624.71 293.83 1,462.88 774.10 443.12

98 120.6 664.35 2607.93 15217.4 31.5 88 3.8 245.98 1043.17 204.9 668.37 310.31 1,537.62 809.27 464.08

94.2 117 688.67 2550.71 15244.7 27 84.5 3.8 244.12 990.57 203.7 646.26 307.97 1,538.63 803.15 461.83

89.7 114.7 728.56 2729.82 15531.3 21.7 79.7 4.2 264.77 1091.93 217.3 688.51 334.02 1,634.37 871.09 497.45

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MVRHP distillation scheme provides advantages in terms of higher energy savings and a lower TAC.

is reduced from 140 to 47 kPa. The recycled vapor then goes through compressor C-110 to increase its temperature to 127.1 °C as the vapor boilup. For the bottom-flashing MVRHP case, the pressure at the column top was varied from 100 to 50 kPa, and a plate column with sieve trays was again employed. Table 3 reports the simulated results under different pressures. It can be seen that the optimal pressure for this case is 80 kPa. At this pressure, the compression ratio is 3.6, and the compressor work is 235.29 kW. The mass and heat balances for this case are listed in Table S4 (Supporting Information). 3.4. Simulation Results. The simulated results for the three different distillation schemes are reported in Table 4. It

■

Sizing and Economic Basis of the Various Process Schemes

The height of a distillation column was calculated by the equation H = 2(N − 2)

operating pressure (kPa) total energy consumption (kW) operating cost (×103$) capital investment (×103$) TAC (×103$/year)

top MVRHP distillation

SC =

bottom-flashing MVRHP distillation

80

90

80

2412.99

653.3

774.1

1,397.34

250.5

293.83

1,361.7

1,240.06

1,464.55

1,533.51

374.54

443.28

1.2 3.281

The heat-transfer areas of the condenser (SC) and reboiler (SR) were calculated according to the equations

Table 4. Simulation Results of Three Different Optimal Schemes conventional distillation

APPENDIX

SR =

QC UCΔTC QR UR ΔTR

Finally, in terms of the above size estimations, the capital and energy costs of a distillation column were estimated as follows column‐shell cost = 17640D1.066H 0.802 total heat‐exchanger cost = 7296SC 0.65 + 7296SR 0.65 Nomenclature

can be seen that the capital investment for the conventional distillation scheme is not the lowest, even though it does not require a compressor. In comparison with that for conventional distillation, the capital investment for top MVRHP distillation is decreased by 8.05%, whereas that for bottom-flashing MVRHP distillation is increased by 8.61%. Regarding operating aspects, conventional distillation requires both a bottom reboiler and a top condenser, whereas top MVRHP distillation omits the top condenser and bottom-flashing MVRHP distillation omits the bottom reboiler. Hence, the operating costs for the last two cases are relatively low. From high to low, the operating costs fall in the order conventional distillation > bottom-flashing MVRHP distillation > top MVRHP distillation. Correspondingly, compared to the conventional distillation, the TAC for top MVRHP distillation is decreased by 75.57%, and the TAC for bottom-flashing MVRHP distillation is decreased by 71.74%. According to the above analysis, it is obvious that top MVRHP distillation is economically the best option and can save the most energy among these three cases for separating a mixture of n-butanol and isobutanol.

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D = diameter (m) H = height (m) MVRHP = mechanical vapor recompression heat pump N = number of stages QC = heat duty of the condenser (kW) QR = heat duty of the reboiler (kW) SC = heat-transfer area of the condenser (m2) SR = heat-transfer area of the reboiler (m2) ΔT = temperature difference (°C) TAC = total annual cost (103$/year) U = overall heat-transfer coefficient (kW K−1 m−2)

ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. 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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4. CONCLUSIONS The present study investigated the simulation of three different distillation alternatives for separating an n-butanol/isobutanol mixture, including conventional distillation, top MVRHP distillation, and bottom-flashing MVRHP distillation. The simulation results obtained using Aspen Plus were used to evaluate the energy savings and TACs. The results show that substantial energy savings can be achieved by using MVRHPs for separating close-boiling mixtures. For the cases studied, based on conventional distillation, the energy savings for the top MVRHP distillation scheme reach 72.92%, and the TAC is decreased by 75.57%. The energy savings for bottom-flashing MVRHP distillation reach 67.92%, and its TAC is decreased by 71.74%. Obviously, the top

ACKNOWLEDGMENTS We are thankful for support from a project funded by the Priority Academic Program Development of the Jiangsu Higher Education Institution.

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Simulation and Optimization of Distillation Processes for Separating a

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