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Batch Distillation with Vapour Compression Applying Different Working Fluids
Jiří Jaromír Klemeš, Petar Sabev Varbanov and Peng Yen Liew (Editors) Proceedings of the 24th European Symposium on Computer Aided Process Engineering – ESCAPE 24 June 15-18, 2014, Budapest, Hungary. Copyright © 2014 Elsevier B.V. All rights reserved.
Batch Distillation with Vapour Compression Applying Different Working Fluids Gabor Modla*, Peter Lang Budapest University of Technology and Economics, Department of Building Services and Process Engineering, Muegyetem rkp. 3-5, Budapest, H-1521, Hungary [email protected]
Abstract Application of a heat pump system (HP) is a possibility for decreasing the energy demand of distillation. The separation of a low relative volatility mixture (n-heptane - toluene) by batch distillation with vapour compression (BD-VC) is investigated. The working fluids (WFs) studied are n-pentane, n-hexane and methanol, ethanol, iso-propanol. The column equipped with standard reactor-reboiler is simulated in a rigorous way (including the conditions of heat transfer). The influence of the main operational parameters and of the selection of the WF on the effectiveness (cost saving, COP, payback time) of the process is investigated. Keywords: batch distillation, heat-pump, vapour compression, energy saving.
1. Introduction The energy demand of the distillation is usually very high. The heat duty to be furnished in the reboiler (at higher temperature) nearly equals to that to be withdrawn (at lower temperature) in the condenser therefore great saving of energy can be reached by the thermal coupling of the condenser and reboiler which requires the increase of the temperature and pressure (compression) of the working fluid (WF). The WF can be the top vapour itself (vapour recompression, VRC) or a material (pure substance or mixture) which is independent of the mixture to be separated (vapour compression, VC). The application possibilities of different heat pump (HP) systems for distillation were first studied for the continuous process. Bruinsma and Spoelstra (2010) gave a comprehensive review of the different methods. About the application of VRC for batch distillation (BD) Jana and his team published several papers (e.g. Jana and Maiti, 2013) recently. Modla and Lang (2013) studied the BD separation of a close boiling hydrocarbon mixture (n-heptane – toluene) by dynamic simulation and cost calculations. The following HP methods were investigated: vapour recompression (BD-VRC), vapour recompression with the application of an external heat exchanger (BD-VRC-E) and vapour compression (BD-VC, WF: n-pentane). The most favourable results (shortest payback time of the additional investment) were obtained for the BD-VRC-E and BD-VC systems. In this paper the vapour compression system (Figure 1) will be studied by applying different WF-s. The goals of this paper: -to study the process of BD with vapour compression by dynamic simulation, -to study its economic feasibility by cost calculations, -to compare the effectiveness of different working fluids. For the simulation of the BD-VC system the different modules of the ChemCad professional flow-sheet simulator are used. Standard reactor-reboiler is applied and
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simulated in a rigorous way (including the conditions of heat transfer). The influence of the main operational parameters (e.g. compression ratio) on the effectiveness (cost saving, COP, payback time) of the process is investigated. The mixture to be separated is n-heptane – toluene. The working fluids applied are n-pentane, n-hexane and methanol, ethanol, iso-propanol.
2. Batch distillation with vapour compression In batch distillation with vapour compression (VC, Figure 1) the working fluid (WF) is independent of the mixture to be separated. The basic parts of a VC cycle are as follows (Figure 2). The WF is evaporated at the condenser (between 1 and 2), compressed to a higher pressure with higher saturation temperature (3→4), condensed in the reboiler (5→6), and cooled down by expansion over a throttle valve (7→1) to a (saturation) temperature below the condenser temperature. The optional parts of the cycle depending among others on the thermodynamic properties of the WF are: superheating of the WF (2→3, if necessary in order to prevent the (partial) condensation of WF in the compressor) and in the reboiler cooling down of WF to its dewpoint (4→5, if it leaves the compressor as superheated vapour), subcooling of the condensed WF before expansion (6→7). The compressor (the heat pump system) can be already operated during the heating-up of the column.
3. Working fluids The application of several WF-s was studied. The criteria for the selection were the following ones: 1. The bubble point of the working fluid at 1.01 bar must be less at least by 15 °C than the lowest temperature of the top vapour (in our case the bubble point of the light component at 1.01 bar, 98.4-15=83.4 °C). 2. The critical temperature of the WF must be higher than the maximal temperature at the utility side of the reboiler (in our case the bubble point of the heavy component at Superheater Condenser After-cooler
Compressor Column
Throttle valve
Distillate tank
Compressed vapour
Figure 1. Scheme of batch distillation with vapour compression
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log P
log P
(7)
(6)
(5)
Mixed phase Liquid phase
(7)
Mixed phase
(1)
Vapour phase (2)
(3)
(5)
(6)
Vapour phase
Liquid phase
h
(2)
(1)
a.
(4)
Dry WF
h
b. Wet WF
Figure 2. Thermodynamic cycles of VC for different types of WFs
1.11 bar plus the temperature difference for the heat transfer: 111+15 = 126 °C or the saturation temperature of the heating steam of 4 bar: 143.7 °C). (If this criterion is satisfied the maximal pressure at the utility side of the reboiler remains under the critical pressure of the WF, as well.) Vapour pressure- temperature and pressure- enthalpy curves of the components to be separated and of some potential WFs are shown in Figure 3. The relevant thermodynamic data of these substances are given in Table 1. 16
p [bar] 14 N-heptane
12
logP
water methanol ethanol
Toluene 10
Water N-hexane
8
i-propanol
n-hexane n-pentane n-heptane
Methanol 6 i-propanol 4
n-pentane Ethanol
2
0 60
70
80
90
100
110
120
130
140
150
T [C]
h
Figure 3. Vapour pressure-temperature and pressure-enthalpy curves
Table 1. Thermodynamic data of the substances studied NBP [° C] n-pentane n-hexane methanol ethanol i-propanol water n-heptane toluene
36.07 68.73 64.70 78.29 82.26 100.0 98.43 110.5
p° [bar] 83.4 3.99 1.53 2.01 1.21 1.03 0.53 0.62 0.43
p° [bar] 126 ° C 10.24 4.46 7.54 5.09 4.45 2.39 2.11 1.54
p° [bar] 143.7 14.23 6.47 11.98 8.3 7.28 4.00 3.21 2.38
λ λ λ [kJ/mol [kJ/mol [kJ/mol 83.4 126 ° C 143.7 22.65 18.74 16.70 28.07 24.54 22.84 33.85 30.07 28.25 38.27 34.39 32.51 39.79 34.66 32.2 41.58 39.48 38.56 32.82 29.61 28.11 34.95 32.41 31.26
Tcr [° C] 196.5 234.2 239.5 240.8 235.2 374.2 267.0 320.6
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From the point of view of compression costs it is favourable if -The increase of the vapour pressure to the given increase of the boiling point (Δ T) is the lowest possible (Δ p0rel/Δ T=min.). In this case the compression ratio is minimal resulting in lower operating cost (best WF: n-pentane, worst WF-s: i-propanol and ethanol), -The latent heat of vaporisation at the operating pressures is the highest possible. In this case the flow rate of WF is minimal, smaller size compressor is needed resulting in the decrease of both investment and operating costs (best WFs: i-propanol, ethanol, worst: n-pentane). The WFs can be divided into two different types from the point of view of compression. The saturated vapour side of the pressure-enthalpy curve of the two paraffins strongly inclines to the right (Figure 2a) and the saturated vapour (2) would partially condense in the compressor without superheating (to point 3) before compression. This type of WF-s is called dry fluid by Chen et al. (2010) who studied different Rankine cycles. The right branch of the logP-h curve is almost vertical (Figure 2b) for the alcohols (and water, “wet” fluids). Compressing the saturated vapour we get superheated vapour, which cools down to its dew point in the reboiler (5) before condensing. These WFs do not need superheating before compression. In our case n-pentane, n-hexane are dry and methanol, ethanol, isopropanol are wet fluids, respectively.
4. Calculation results To compare the different working fluids the separation of a mixture of n-heptane (50 mol %)- toluene is considered. This is a mixture of low relative volatility where the heat pump system can be economical. The specified product purity (xD,av) is 0.98. The number of theoretical stages is 50 (excluding the total condenser and the reboiler). The pressure drop of the column is 0.1 bar. The reboiler is of commercial type (DIN AE1000). At the beginning of the process the binary feed (charge) is filled in the reboiler and the column is empty. During the start-up total reflux is applied. The start-up ends when the xD reaches 0.9975. During the next process step (production) the reflux ratio is 12. First the flow rate of the heating steam is determined for the BD without VC which determines the duration of the heating up and that of the production. The highest heat duty must be provided during the start-up otherwise the length of this step (without product withdrawal) would be too high. The flow rates of the WFs are determined so that the duration of heating up is equal to that of the BD. Hence the duration of the production step is practically the same if the dew point of the different heating media (water steam and WFs) equals. The size of the compressor is determined for the heating up (highest heat duty). Polytropic compression with efficiency of 90 % is assumed. 4.1. Case 1 The dew point is minimal (126 °C). The duration of the heating up, from which the molar flow rates of the WFs (VWF) are determined: 305 min. Before compression the WFs are superheated to minimal extent (if necessary at all). The batch operation time (BOT) is 1,402 min in all cases. The cost of compressor is proportional to the power of its motor (MP). The calculation results are shown in Table 2.
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Table 2.Comparison of the different WFs (condensation at 126 °C). Working fluid steam (2.39 bar) n-pentane n-hexane methanol ethanol i-propanol
Pin [bar]
DT [°C]
Pout [bar]
Tout [°C]
VWF [kmol/h]
MP [kW]
Qce [MJ]
SMP [MJ]
COP
PBP [Y]
4 1.53 2.01 1.21 1.03
15 32 0 0 0
10.24 4.46 7.54 5.09 4.45
127.4 126.1 203.6 153.4 131.6
5.55 10 7.6 6.3 6.0 5.7
8.53 7.69 8.58 8.53 8.03
-501 -456 -109 -163 -211
717 646 720 717 675
2.6 2.9 2.6 2.6 2.8
36.9 24.0 37.7 36.8 28.2
By the results we can conclude that -The lowest MP is required for the n-hexane, it is nearly the same for all other WFs. -The flow rate of WF is highest for the n-pentane. -Since the compressor is operating with the same power during the heating up and the production, therefore during this latter when the heat duty of the distillation is lower, the compressor provides too much heat and extra cooling must be applied (the heat to be withdrawn is Qce). -The average value of COP is low, because the compressor is still operated with high power in the production step whilst the heat duty is already low. Hence the payback time (PBT) is very high. 4.2. Case 2 The dew point of WFs is equal to that of heating steam of 4 bar (143.7 °C). The duration of the heating up is 211 min. The WFs are superheated to a minimal extent. BOT is 802 min in all cases. The calculation results are shown in Table 3. By the results we can conclude that: -Because of the higher compression ratio a compressor of higher power is needed. -The lowest MP is required for the isopropanol and ethanol. Table 3. Comparison of the different WFs (condensation at 143.7 °C, without control) Working fluid steam (4.0 bar) n-pentane n-hexane methanol ethanol i-propanol
Pin [bar]
DT [°C]
Pout [bar]
-
-
-
4 1.53 2.01 1.21 1.03
19 24.5 0 0 0
14.23 6.47 11.9 8.3 7.28
Tout [°C]
VWF [kmol/h]
MP [kW]
Qce [MJ]
-
-
143.7 143.8 250.1 178.3 149.6
11 8.2 6.1 5.7 5.8
13.04 11.57 11.79 11.26 11.22
-311 -278 -60 -87 -121
SMP [MJ]
COP
PBP [Y]
627 556 567 542 539
3.1 3.5 3.4 3.6 3.6
15.5 11.6 12.0 10.9 10.9
Table 4. Comparison of the different WFs (condensation at 143.7 °C, with control)
Working fluid n-pentane n-hexane methanol ethanol i-propanol
Pin [bar] 4 1.53 2.01 1.21 1.03
DT [°C] 19 24.5 0 0 0
Pout [bar] 14.23 6.47 11.9 8.3 7.28
Tout [°C] 143.7 143.8 250.09 178.29 149.63
VWF, max [kmol/h] 11 8.2 6.1 5.7 5.8
MP [kW] 13.04 11.7 11.82 11.25 11.4
Qce [MJ] -244 -216 -42 -63 -93
SMP [MJ] 496 437 405 402 420
COP 3.9 4.4 4.8 4.8 4.6
PBP [Y] 12.9 10.1 10.0 9.3 9.6
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G. Modla, P. Lang
0.7
VWF [kmol/h]
Vf 0.6 0.5
12
0.06
VWF [kmol/h]
Vf
12
10
0.05
10
8
0.04
8
6
0.03
6
4
0.02
4
2
0.01
2
0
0
0.4 0.3 0.2 0.1 0 0
200
400
a. without control
600
[min] 800
0 0
200
400
600
[min] 800
b. with control
Figure 4. The evolution of the vapour fraction in the WF leaving the reboiler and flow rate of WF
-The WF flow rates only slightly varied but in different ways (both increase and decrease occur). -The average COP increased, therefore PBT decreased but it is still too high. 4.3. Case 3 Case 2 is so modified that during the production the increase of the vapour fraction (above 0.05) in the WF leaving the reboiler (Figure 4a) is prevented with a controller modifying the flow rate of the WF (Figure 4b). The results are shown in Table 4. Due to the increase of COP the PBP decreased further for each WF.
5. Conclusion The effectiveness of different working fluids (n-pentane, n-hexane and methanol, ethanol, iso-propanol) for vapour compression HP system integrated to real batch distillation columns was compared. The separation of a mixture of low relative volatility (n-heptane - toluene) was simulated in a rigorous way with the dynamic modules of the ChemCad. Cost calculations were also performed. For the minimal condensation temperature of heating media (minimal compression ratio) the payback time was very high for all WFs. By increasing this temperature and by decreasing the flow rate of the WF during the production by the aid of a controller the payback times considerably decreased. The lowest payback time (somewhat less than 10 y) was obtained for ethanol and isopropanol.
Acknowledgement This work was supported by the Hungarian Research Funds (OTKA, No.: K-106268).
References D. Bruinsma, S. Spoelstra, 2010, Heat pumps in distillation, Distillation and Absorption Conference 2010, Eindhoven, 21–28. H. Chen, D. Yogi Goswami, E. K. Stefanakos, 2010, A review of thermodynamic cycles and working fluids, Renewable and Sustainable Energy Reviews, 14, 3059–3067. A. K. Jana, D. Maiti, 2013, Assessment of the implementation of vapour recompression technique in batch distillation, Separation and Purification Technology, 107, 1-10. G. Modla, P. Lang, 2013, HP systems with mechanical compression for batch distillation, Energy, 62, 403-417.
Batch Distillation with Vapour Compression Applying Different Working Fluids Gabor Modla*, Peter Lang Budapest University of Technology and Economics, Department of Building Services and Process Engineering, Muegyetem rkp. 3-5, Budapest, H-1521, Hungary [email protected]
Abstract Application of a heat pump system (HP) is a possibility for decreasing the energy demand of distillation. The separation of a low relative volatility mixture (n-heptane - toluene) by batch distillation with vapour compression (BD-VC) is investigated. The working fluids (WFs) studied are n-pentane, n-hexane and methanol, ethanol, iso-propanol. The column equipped with standard reactor-reboiler is simulated in a rigorous way (including the conditions of heat transfer). The influence of the main operational parameters and of the selection of the WF on the effectiveness (cost saving, COP, payback time) of the process is investigated. Keywords: batch distillation, heat-pump, vapour compression, energy saving.
1. Introduction The energy demand of the distillation is usually very high. The heat duty to be furnished in the reboiler (at higher temperature) nearly equals to that to be withdrawn (at lower temperature) in the condenser therefore great saving of energy can be reached by the thermal coupling of the condenser and reboiler which requires the increase of the temperature and pressure (compression) of the working fluid (WF). The WF can be the top vapour itself (vapour recompression, VRC) or a material (pure substance or mixture) which is independent of the mixture to be separated (vapour compression, VC). The application possibilities of different heat pump (HP) systems for distillation were first studied for the continuous process. Bruinsma and Spoelstra (2010) gave a comprehensive review of the different methods. About the application of VRC for batch distillation (BD) Jana and his team published several papers (e.g. Jana and Maiti, 2013) recently. Modla and Lang (2013) studied the BD separation of a close boiling hydrocarbon mixture (n-heptane – toluene) by dynamic simulation and cost calculations. The following HP methods were investigated: vapour recompression (BD-VRC), vapour recompression with the application of an external heat exchanger (BD-VRC-E) and vapour compression (BD-VC, WF: n-pentane). The most favourable results (shortest payback time of the additional investment) were obtained for the BD-VRC-E and BD-VC systems. In this paper the vapour compression system (Figure 1) will be studied by applying different WF-s. The goals of this paper: -to study the process of BD with vapour compression by dynamic simulation, -to study its economic feasibility by cost calculations, -to compare the effectiveness of different working fluids. For the simulation of the BD-VC system the different modules of the ChemCad professional flow-sheet simulator are used. Standard reactor-reboiler is applied and
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simulated in a rigorous way (including the conditions of heat transfer). The influence of the main operational parameters (e.g. compression ratio) on the effectiveness (cost saving, COP, payback time) of the process is investigated. The mixture to be separated is n-heptane – toluene. The working fluids applied are n-pentane, n-hexane and methanol, ethanol, iso-propanol.
2. Batch distillation with vapour compression In batch distillation with vapour compression (VC, Figure 1) the working fluid (WF) is independent of the mixture to be separated. The basic parts of a VC cycle are as follows (Figure 2). The WF is evaporated at the condenser (between 1 and 2), compressed to a higher pressure with higher saturation temperature (3→4), condensed in the reboiler (5→6), and cooled down by expansion over a throttle valve (7→1) to a (saturation) temperature below the condenser temperature. The optional parts of the cycle depending among others on the thermodynamic properties of the WF are: superheating of the WF (2→3, if necessary in order to prevent the (partial) condensation of WF in the compressor) and in the reboiler cooling down of WF to its dewpoint (4→5, if it leaves the compressor as superheated vapour), subcooling of the condensed WF before expansion (6→7). The compressor (the heat pump system) can be already operated during the heating-up of the column.
3. Working fluids The application of several WF-s was studied. The criteria for the selection were the following ones: 1. The bubble point of the working fluid at 1.01 bar must be less at least by 15 °C than the lowest temperature of the top vapour (in our case the bubble point of the light component at 1.01 bar, 98.4-15=83.4 °C). 2. The critical temperature of the WF must be higher than the maximal temperature at the utility side of the reboiler (in our case the bubble point of the heavy component at Superheater Condenser After-cooler
Compressor Column
Throttle valve
Distillate tank
Compressed vapour
Figure 1. Scheme of batch distillation with vapour compression
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log P
log P
(7)
(6)
(5)
Mixed phase Liquid phase
(7)
Mixed phase
(1)
Vapour phase (2)
(3)
(5)
(6)
Vapour phase
Liquid phase
h
(2)
(1)
a.
(4)
Dry WF
h
b. Wet WF
Figure 2. Thermodynamic cycles of VC for different types of WFs
1.11 bar plus the temperature difference for the heat transfer: 111+15 = 126 °C or the saturation temperature of the heating steam of 4 bar: 143.7 °C). (If this criterion is satisfied the maximal pressure at the utility side of the reboiler remains under the critical pressure of the WF, as well.) Vapour pressure- temperature and pressure- enthalpy curves of the components to be separated and of some potential WFs are shown in Figure 3. The relevant thermodynamic data of these substances are given in Table 1. 16
p [bar] 14 N-heptane
12
logP
water methanol ethanol
Toluene 10
Water N-hexane
8
i-propanol
n-hexane n-pentane n-heptane
Methanol 6 i-propanol 4
n-pentane Ethanol
2
0 60
70
80
90
100
110
120
130
140
150
T [C]
h
Figure 3. Vapour pressure-temperature and pressure-enthalpy curves
Table 1. Thermodynamic data of the substances studied NBP [° C] n-pentane n-hexane methanol ethanol i-propanol water n-heptane toluene
36.07 68.73 64.70 78.29 82.26 100.0 98.43 110.5
p° [bar] 83.4 3.99 1.53 2.01 1.21 1.03 0.53 0.62 0.43
p° [bar] 126 ° C 10.24 4.46 7.54 5.09 4.45 2.39 2.11 1.54
p° [bar] 143.7 14.23 6.47 11.98 8.3 7.28 4.00 3.21 2.38
λ λ λ [kJ/mol [kJ/mol [kJ/mol 83.4 126 ° C 143.7 22.65 18.74 16.70 28.07 24.54 22.84 33.85 30.07 28.25 38.27 34.39 32.51 39.79 34.66 32.2 41.58 39.48 38.56 32.82 29.61 28.11 34.95 32.41 31.26
Tcr [° C] 196.5 234.2 239.5 240.8 235.2 374.2 267.0 320.6
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From the point of view of compression costs it is favourable if -The increase of the vapour pressure to the given increase of the boiling point (Δ T) is the lowest possible (Δ p0rel/Δ T=min.). In this case the compression ratio is minimal resulting in lower operating cost (best WF: n-pentane, worst WF-s: i-propanol and ethanol), -The latent heat of vaporisation at the operating pressures is the highest possible. In this case the flow rate of WF is minimal, smaller size compressor is needed resulting in the decrease of both investment and operating costs (best WFs: i-propanol, ethanol, worst: n-pentane). The WFs can be divided into two different types from the point of view of compression. The saturated vapour side of the pressure-enthalpy curve of the two paraffins strongly inclines to the right (Figure 2a) and the saturated vapour (2) would partially condense in the compressor without superheating (to point 3) before compression. This type of WF-s is called dry fluid by Chen et al. (2010) who studied different Rankine cycles. The right branch of the logP-h curve is almost vertical (Figure 2b) for the alcohols (and water, “wet” fluids). Compressing the saturated vapour we get superheated vapour, which cools down to its dew point in the reboiler (5) before condensing. These WFs do not need superheating before compression. In our case n-pentane, n-hexane are dry and methanol, ethanol, isopropanol are wet fluids, respectively.
4. Calculation results To compare the different working fluids the separation of a mixture of n-heptane (50 mol %)- toluene is considered. This is a mixture of low relative volatility where the heat pump system can be economical. The specified product purity (xD,av) is 0.98. The number of theoretical stages is 50 (excluding the total condenser and the reboiler). The pressure drop of the column is 0.1 bar. The reboiler is of commercial type (DIN AE1000). At the beginning of the process the binary feed (charge) is filled in the reboiler and the column is empty. During the start-up total reflux is applied. The start-up ends when the xD reaches 0.9975. During the next process step (production) the reflux ratio is 12. First the flow rate of the heating steam is determined for the BD without VC which determines the duration of the heating up and that of the production. The highest heat duty must be provided during the start-up otherwise the length of this step (without product withdrawal) would be too high. The flow rates of the WFs are determined so that the duration of heating up is equal to that of the BD. Hence the duration of the production step is practically the same if the dew point of the different heating media (water steam and WFs) equals. The size of the compressor is determined for the heating up (highest heat duty). Polytropic compression with efficiency of 90 % is assumed. 4.1. Case 1 The dew point is minimal (126 °C). The duration of the heating up, from which the molar flow rates of the WFs (VWF) are determined: 305 min. Before compression the WFs are superheated to minimal extent (if necessary at all). The batch operation time (BOT) is 1,402 min in all cases. The cost of compressor is proportional to the power of its motor (MP). The calculation results are shown in Table 2.
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Table 2.Comparison of the different WFs (condensation at 126 °C). Working fluid steam (2.39 bar) n-pentane n-hexane methanol ethanol i-propanol
Pin [bar]
DT [°C]
Pout [bar]
Tout [°C]
VWF [kmol/h]
MP [kW]
Qce [MJ]
SMP [MJ]
COP
PBP [Y]
4 1.53 2.01 1.21 1.03
15 32 0 0 0
10.24 4.46 7.54 5.09 4.45
127.4 126.1 203.6 153.4 131.6
5.55 10 7.6 6.3 6.0 5.7
8.53 7.69 8.58 8.53 8.03
-501 -456 -109 -163 -211
717 646 720 717 675
2.6 2.9 2.6 2.6 2.8
36.9 24.0 37.7 36.8 28.2
By the results we can conclude that -The lowest MP is required for the n-hexane, it is nearly the same for all other WFs. -The flow rate of WF is highest for the n-pentane. -Since the compressor is operating with the same power during the heating up and the production, therefore during this latter when the heat duty of the distillation is lower, the compressor provides too much heat and extra cooling must be applied (the heat to be withdrawn is Qce). -The average value of COP is low, because the compressor is still operated with high power in the production step whilst the heat duty is already low. Hence the payback time (PBT) is very high. 4.2. Case 2 The dew point of WFs is equal to that of heating steam of 4 bar (143.7 °C). The duration of the heating up is 211 min. The WFs are superheated to a minimal extent. BOT is 802 min in all cases. The calculation results are shown in Table 3. By the results we can conclude that: -Because of the higher compression ratio a compressor of higher power is needed. -The lowest MP is required for the isopropanol and ethanol. Table 3. Comparison of the different WFs (condensation at 143.7 °C, without control) Working fluid steam (4.0 bar) n-pentane n-hexane methanol ethanol i-propanol
Pin [bar]
DT [°C]
Pout [bar]
-
-
-
4 1.53 2.01 1.21 1.03
19 24.5 0 0 0
14.23 6.47 11.9 8.3 7.28
Tout [°C]
VWF [kmol/h]
MP [kW]
Qce [MJ]
-
-
143.7 143.8 250.1 178.3 149.6
11 8.2 6.1 5.7 5.8
13.04 11.57 11.79 11.26 11.22
-311 -278 -60 -87 -121
SMP [MJ]
COP
PBP [Y]
627 556 567 542 539
3.1 3.5 3.4 3.6 3.6
15.5 11.6 12.0 10.9 10.9
Table 4. Comparison of the different WFs (condensation at 143.7 °C, with control)
Working fluid n-pentane n-hexane methanol ethanol i-propanol
Pin [bar] 4 1.53 2.01 1.21 1.03
DT [°C] 19 24.5 0 0 0
Pout [bar] 14.23 6.47 11.9 8.3 7.28
Tout [°C] 143.7 143.8 250.09 178.29 149.63
VWF, max [kmol/h] 11 8.2 6.1 5.7 5.8
MP [kW] 13.04 11.7 11.82 11.25 11.4
Qce [MJ] -244 -216 -42 -63 -93
SMP [MJ] 496 437 405 402 420
COP 3.9 4.4 4.8 4.8 4.6
PBP [Y] 12.9 10.1 10.0 9.3 9.6
1200
G. Modla, P. Lang
0.7
VWF [kmol/h]
Vf 0.6 0.5
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Figure 4. The evolution of the vapour fraction in the WF leaving the reboiler and flow rate of WF
-The WF flow rates only slightly varied but in different ways (both increase and decrease occur). -The average COP increased, therefore PBT decreased but it is still too high. 4.3. Case 3 Case 2 is so modified that during the production the increase of the vapour fraction (above 0.05) in the WF leaving the reboiler (Figure 4a) is prevented with a controller modifying the flow rate of the WF (Figure 4b). The results are shown in Table 4. Due to the increase of COP the PBP decreased further for each WF.
5. Conclusion The effectiveness of different working fluids (n-pentane, n-hexane and methanol, ethanol, iso-propanol) for vapour compression HP system integrated to real batch distillation columns was compared. The separation of a mixture of low relative volatility (n-heptane - toluene) was simulated in a rigorous way with the dynamic modules of the ChemCad. Cost calculations were also performed. For the minimal condensation temperature of heating media (minimal compression ratio) the payback time was very high for all WFs. By increasing this temperature and by decreasing the flow rate of the WF during the production by the aid of a controller the payback times considerably decreased. The lowest payback time (somewhat less than 10 y) was obtained for ethanol and isopropanol.
Acknowledgement This work was supported by the Hungarian Research Funds (OTKA, No.: K-106268).
References D. Bruinsma, S. Spoelstra, 2010, Heat pumps in distillation, Distillation and Absorption Conference 2010, Eindhoven, 21–28. H. Chen, D. Yogi Goswami, E. K. Stefanakos, 2010, A review of thermodynamic cycles and working fluids, Renewable and Sustainable Energy Reviews, 14, 3059–3067. A. K. Jana, D. Maiti, 2013, Assessment of the implementation of vapour recompression technique in batch distillation, Separation and Purification Technology, 107, 1-10. G. Modla, P. Lang, 2013, HP systems with mechanical compression for batch distillation, Energy, 62, 403-417.