CHEMICAL ENGINEERING TRANSACTIONS VOL. 81, 2020 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Petar S. Varbanov, Qiuwang Wang, Min Zeng, Panos Seferlis, Ting Ma, Jiří J. Klemeš Copyright © 2020, AIDIC Servizi S.r.l. ISBN 978-88-95608-79-2; ISSN 2283-9216 Study on Energy Saving of Epichlorohydrin Unit Based on Entransy Theory Li Xiaa, Jianyang Linga, Zhen Xua, Zhixiang Zhanga, Xiaoyan Suna, Xiaorong Caob, Shuguang Xianga,* aState Key Laboratory Base for Eco-Chemical Engineering, College of Chemical Engineering, Qingdao University of Science and Technology, Zhengzhou Road No.53, Qingdao 266042, China bChemistry and Chemical Engineering Faculty, Qilu Normal University, Jinan, 250013, Shandong, China xsg@ qust. edu. cn The mathematical models for calculating the energy utilization efficiency of heat exchanger networks (HENs) based on entransy analysis method was built. The energy utilization efficiency of HENs for epichlorohydrin unit were calculated by exergy analysis and entransy analysis. Aiming at the maximum energy recovery, the entransy analysis of epichlorohydrin unit’s HENs was done. By taking three different △Tmin, 10 K, 15 K and 20 K, the results show that the exergy efficiencies are 61.01 %, 59.28 % and 57.27 %. The entransy transfer efficiency is 42.81 %, 42.13 % and 41.00 %. Using the entransy analysis method, utilities were saved by 19.41 %, 18.01 % and 15.70 %. Using the exergy analysis method, utilities were saved by 16.59 %, 14.86 % and 12.02 %. The results of different methods were analysed and compared. Exergy is more complicated than entransy in the calculation process. The exergy analysis method can not complete HENs Synthesis. But the entransy analysis method not only can complete HENs synthesis, but also can calculate the energy utilization efficiency of HENs. The entransy analysis method is more suitable for analyzing the energy utilization of epichlorohydrin unit. 1. Introduction With the increase of global energy consumption and carbon emissions, the problem of global resource shortage and environmental pollution is becoming more and more serious. Energy consumed by industry accounts for about 53 % of the total global energy consumption. About 72 % of the energy sources are consumed in the manufacturing process (Xia et al., 2019). Saving energy and reducing carbon emissions is of great significance to promote the sustainable development of global economy. The first law of thermodynamics can reflect the conservation of energy, but does not reflect energy utilization efficiency. Rant (1956) introduce a new thermodynamic parameter which was called “exergy” to solve this problem. Ahern (1980) proposed exergy analysis method which can effectively reflect energy utilization efficiency by analyzing the value, cause and location of the exergy loss. Linnhoff (1990) proposed the concept of exergy loss into HENs. After years of development, the exergy analysis methods were used for HENs synthesis of petrochemical industry, such as graphic method (Stijepovic et al., 2014), formula method (İpeka et al., 2017), and objective function method (Miladiet et al., 2016). However, because the exergy calculation process is too complicated, the application of exergy analysis method to HENs of petrochemical industry is limited. A new physical quantity named “entransy” was proposed by Guo et al. (2007). Entransy dissipation is described the heat transfer process irreversibility by analyzing the characteristics of the heat transfer process. Hu and Guo (2011) defined entransy transfer efficiency to analyze heat transfer of exchangers. Chen et al. (2012) proposed the temperature-heat flow diagram to intuitively describe the change rule of entransy in the process of heat transfer. Cheng et al. (2014) made use of the temperature-heat flow rate diagram (T- Qdiagram) to research the entransy dissipation rates of chemical processes. Xia et al. (2017b) proposed a method based on entransy for setting energy targets of heat exchanger network. The entransy transfer DOI: 10.3303/CET2081185 Paper Received: 30/04/2020; Revised: 11/06/2020; Accepted: 17/06/2020 Please cite this article as: Xia L., Ling J., Xu Z., Zhang Z., Sun X., Cao X., Xiang S., 2020, Study on Energy Saving of Epichlorohydrin Unit Based on Entransy Theory, Chemical Engineering Transactions, 81, 1105-1110 DOI:10.3303/CET2081185 1105 http://www.sciencedirect.com/science/article/pii/S0360544217302359#aff1 efficiency can indicate the reasonable heat utilization of heat exchanger network. Xia et al. (2018) proposed the design method of HENs based on entransy theory. In this work, the mathematical models for calculating the energy utilization efficiency of heat exchanger networks (HENs) based on entransy analysis method was built. Aiming at the maximum energy recovery, the entransy analysis of epichlorohydrin unit’s HENs was done. The optimal energy utilization efficiency for heat exchanger networks of epichlorohydrin unit was determined. 2. The entransy transfer efficiency for heat exchanger networks The entransy is defined as, 𝐸𝑣ℎ = 1 2 𝑄𝑣ℎ𝑈ℎ = 1 2 𝑄𝑣ℎ𝑇 = 1 2 𝑚𝑐𝑣𝑇 2 (1) where Qvh is thermal capacity of an object with constant volume, cv is specific heat capacity at constant volume, Uh represents the thermal potential. Assume heat exchanger containing two streams is operated in steady state. The temperature drop of the stream is dT, and the heat flow generating is dQ (Xia et al., 2017a ). The quality of entransy for the heat exchanger output is, d𝐸𝑣ℎ = 𝑇𝑑𝑄𝑣ℎ (2) T is a state quantity. δQ is a process quantity. So entransy T·δQ is obviously a process quantity. The quality of entransy for HENS can be obtained by the integration of the cold and hot Composite Curves with the Q - axis in the T - Q diagram (Xia et al., 2017d). For the heat exchanger networks, the entransy of all the hot streams is: 𝐸𝐻 = ∑𝐸ℎ,𝑖 𝑛 𝑖=1 (3) 𝐸ℎ,𝑖 = 1 2 ∙ 𝑄ℎ,𝑖 ∙ 𝑇ℎ,𝑖 = 1 2 ∙ 𝐶𝑃ℎ,𝑖 ∙ (𝑇ℎ,𝑖,𝑖𝑛𝑙𝑒𝑡 2 − 𝑇ℎ,𝑖,𝑜𝑢𝑡𝑙𝑒𝑡 2 ) (4) where EH denotes the entransy of all the hot streams in heat transfer network, Eh,i denotes the entransy of the i hot stream, Qh,i denotes heat of the i hot stream, CPh,i denotes heat capacity flowrate of the i hot stream. The entransy of cold streams for the heat exchanger networks is： 𝐸𝐶 = ∑𝐸𝑐,𝑖 𝑛 𝑖=1 (5) 𝐸𝑐,𝑖 = 1 2 ∙ 𝑄𝑐,𝑖 ∙ 𝑇𝑐,𝑖 = 1 2 ∙ 𝐶𝑃𝑐,𝑖 ∙ (𝑇𝑐,𝑖,𝑜𝑢𝑡𝑙𝑒𝑡 2 − 𝑇𝑐,𝑖,𝑖𝑛𝑙𝑒𝑡 2 ) (6) Where EC denotes the entransy of all the cold streams in a heat transfer network，Ec,i denotes the entransy in the i cold stream, Qc,i denotes heat of the i cold stream, CPc,i denotes heat capacity flowrate of the i cold stream. The entransy dissipation ∆E is: ∆𝐸 = ∑𝐸ℎ,𝑖 𝑛 𝑖=1 − ∑𝐸𝑐,𝑖 𝑛 𝑖=1 = 1 2 ∙ 𝑄ℎ,𝑖 ∙ 𝑇ℎ,𝑖 = 1 2 ∙ ∑𝐶𝑃ℎ,𝑖 ∙ (𝑇ℎ,𝑖,𝑖𝑛𝑙𝑒𝑡 2 − 𝑇ℎ,𝑖,𝑜𝑢𝑡𝑙𝑒𝑡 2 ) 𝑛 𝑖=1 − 1 2 ∙ ∑𝐶𝑃𝑐,𝑖 ∙ (𝑇𝑐,𝑖,𝑜𝑢𝑡𝑙𝑒𝑡 2 − 𝑇𝑐,𝑖,𝑖𝑛𝑙𝑒𝑡 2 ) 𝑛 𝑖=1 (7) The entransy transfer efficiency is： 𝜂 = 𝐸𝐶 𝐸𝐻 = ∑ 𝐶𝑃𝑐,𝑖 ∙ (𝑇𝑐,𝑖,𝑜𝑢𝑡𝑙𝑒𝑡 2 − 𝑇𝑐,𝑖,𝑖𝑛𝑙𝑒𝑡 2 )𝑛𝑖=1 ∑ 𝐶𝑃ℎ,𝑖 ∙ (𝑇ℎ,𝑖,𝑖𝑛𝑙𝑒𝑡 2 − 𝑇ℎ,𝑖,𝑜𝑢𝑡𝑙𝑒𝑡 2 )𝑛𝑖=1 (8) According to the different temperature differences, make use of the Eq (8) to the calculate the entransy transfer efficiency of HENs. The maximum heat transfer capability of hot streams of HENs is determined. It can be seen from Eq(8) that the greater the heat transfer temperature difference between cold and hot 1106 streams, the greater the entransy dissipation, the greater the entransy transfer efficiency, and the worse the energy utilization effect. 3. Comparison of entransy analysis and exergy analysis in HENs of epichlorohydrin unit The epichlorohydrin was produced with propylene and chlorine as raw materials. The process diagram of epichlorohydrin produced by the high temperature chlorination method is shown in Figure 2 - 1, Figure 2 - 2, Figure 2 - 3, Figure 2 - 4 of the master thesis Liu X. (2018). The hot and cold stream data of HENs is shown in Table 1 and Table 2. 3.1 Entransy analysis of existing HENs According to the data of Table 1, the cold and hot Composite Curves of epichlorohydrin unit are plotted in the T-Q diagram, as shown in Figure 1. The entransy of cold streams and hot streams of epichlorohydrin unit are 8.27×106 kW·K and 1.17×107 kW·K. The entransy of cold utilities and hot utilities is 6.44×106 kW·K and 4.66×106 kW·K. The entransy recovery is 3.62×106 kW·K. The entransy dissipation is 1.62×106 kW·K. The efficiency of entransy transfer is 30.98 %. 3.2 Exergy analysis of existing HENs Set the pressure as 0.1013 MPa, the ambient temperature T0 = 298.15 K. The exergy of cold streams and hot streams for epichlorohydrin unit are 4,028.30 kW and 5,163.72 kW. The exergy loss is 1,970.8 kW, the exergy efficiency of epichlorohydrin unit is 61.83 %. The total rate of exergy loss is 38.17 %. Table 1: The data for hot streams Stream Stream description Supply Temperature / K Target Temperature / K CP / kW·K-1 1 propylene 380 331 6.45 2 propylene 404 355 7.17 2A propylene 355 313 35.10 3 inert gases 313 303 0.00 4 propylene 313 283 9.74 6 the gas product of the reaction 743 303 8.07 8 heat conduction oil 477 391 13.61 9 the gas product of the reaction 303 263 19.41 11 propylene 283 263 2.58 12 crude chloropropene 342 313 2.25 13 chloropropene 318 318 3,396.32 13A chloropropene 318 313 5.69 14 crude D-D mixture 392 313 0.34 15 low boiling point mixture 318 316 746.38 16 chloropropene 344 308 1.86 21 dilute hydrochloric acid 341 313 6.72 22 dilute hydrochloric acid 323 309 12.03 23 washing water 361 309 55.03 24 washing water 348 303 58.44 27 epoxy wastewater 358 338 193.51 28 epoxy Steam 371 357 591.13 28A epoxy Steam 357 313 15.53 29 the top of prefractionator’s low boiling point mixture 360 328 17.03 29A the top of prefractionator’s low boiling point mixture 328 313 1.86 32 epichlorohydrin 327 327 13,150.50 32A epichlorohydrin 327 313 6.14 34 low boiling point mixture 353 313 3.60 36 epichlorohydrin 400 313 0.09 37 epichlorohydrin 355 313 1.38 38 high boiling point mixture 366 313 0.18 1107 3.3 The maxium energy recover HENs According to the data of Table 1 and Table 2, minimum approach temperature △Tmin is given as 10 K, 15 K and 20 K, the results of exergy analysis and entransy analysis are shown in Table 3 and 4. Synthesis of epichlorohydrin unit HENs based on entransy theory is shown in Figure 2. From the results, it was seen that the quantity of entransy varies with the variation of minimum approach temperature. When ΔTmin is given as 10 K, 15 K and 20 K, the exergy efficiency of epichlorohydrin unit are 61.01 %, 59.28 %, 57.27 %, saving utilities are 16.59 %, 14.86 %, 12.02 %. It is indicated that the greater the temperature difference, the lower quality of exergy. When ΔTmin is 10 K, 15 K and 20 K, the entransy transfer efficiency of epichlorohydrin unit are 42.81 %, 42.13 %, 41.00 %, saving utilities is 19.41 %, 18.01 %, 15.70 %. It is obvious that the greater the minimum approach temperature difference, the larger entransy dissipation, and the lower entransy transfer efficiency. Table 2: The data for cold streams Stream Stream description Supply Temperature / K Target Temperature / K CP / kW·K-1 5 propylene 283 286 352.30 5A propylene 286 553 6.41 7 heat conduction oil 391 477 13.61 10 the top gas of prefractionator 230 291 5.50 17 the tower bottoms of prefractionator 270 303 10.55 18 the tower bottoms of propylene stripping tower 340 342 158.46 19 the tower bottoms of D-D separation tower 389 392 500.57 20 the tower bottoms of M-C separation tower 343 347 461.64 25 the tower bottoms of propylene absorber 360 378 311.49 26 dichloropropanol aqueous solution 319 353 171.72 30 the tower bottoms of hydrogen chloride absorber 383 384 1,563.17 33 the tower bottoms of fractionating tower 394 395 1,178.61 35 the tower bottoms of propylene absorber 400 401 1,504.78 39 the tower bottoms of propylene scrubber 365 366 50.01 40 water 363 418 16.13 Table 3: The results of exergy analysis method for epichlorohydrin unit Items △Tmin = 10 K △Tmin = 15 K △Tmin = 20 K Hot streams exergy / kW 4,811.63 4,631.73 4,449.20 Cold streams exergy / kW 4,234.98 4,331.42 4,426.63 Hot utilities exergy / kW 2,106.93 2,150.19 2,220.92 Cold utilities exergy / kW 1,591.43 1,624.66 1,680.17 Exergy loss / kW 1,875.90 1,885.90 1901.10 Exergy efficiency / % 61.01 59.28 57.27 Saving exergy / % 16.59 14.86 12.02 Table 4: The results of entransy analysis method for epichlorohydrin unit Items △Tmin = 10 K △Tmin = 15 K △Tmin = 20 K Hot streams entransy / kW·K 11,670,394 11,670,394 11,670,394 Cold streams entransy / kW·K 8,274,245 8,274,245 8,274,245 Hot utilities entransy / kW·K 3,277,686 3,357,630.23 3,489,182 Cold utilities entransy / kW·K 5,663,815 5,739,316.464 5,864,135 Entransy recovery / kW·K 4,996,559 4,916,614.77 4,785,063 Entransy dissipation / kW·K 1,010,020 1,014,462.766 1,021,196 Entransy transfer efficiency / % 42.81 42.13 41.00 Saving entransy / % 19.41 18.01 15.70 1108 In the entransy analysis, the calculations of entransy and entransy dissipation are not affected by ambient temperature. But in the exergy analysis, calculations of exergy and exergy loss must take into account ambient temperature and pressure. The calculation process of exergy is also complex, considering physical and chemical exergy, as in other cases. Figure 1: Composite Curves diagram of epichlorohydrin unit 1 2 6 36 19 25 30 33 35 7 8 40 39 14 106.95 130.56 280 470 119.28 80 126.79 116.33 87.33 110.34 120.28 126.50 117.69 117.69 90 92.28 110.79 105.15 119 121.79 126.60 203.84 203.84 144.85 93.21 103.064 68.647 3 309.05 88.51889.439 H1 H2 H3 H4 H5 H6 1 24 5 7 6 5 26 20 27 18 17 9 11 10 3 4 12 13 16 21 15 24 22 29 32 34 37 38 97.64 57.57 40.17 10 40 45.48 40 40 70.0173.44 84.95 64.96 67.0168.36 -3.4129.39 -43.2618 30 -10 10 -10 40.17 30 40.17 10 69.03 40 45.04 40 45.09 43 70.48 35 67.85 40 50.05 36 74.65 30 86.39 40 54.22 40 79.82 40 82 40 92.98 40 46.44 68 94.840 92.162 91.800 89.857 12.66 28 C1 C2 C3 C4 C5 C6 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 8 9 C8 C9 C10 C11 23 88.03 36C781.744 30 10 11 12 14 15 163.62 117.73 182.68 77.64 97.64 96.62 77.64 97.64 97.64 13.26 13 Figure 2: Synthesis of epichlorohydrin unit HENs based on entransy theory The variation trend of entransy transfer efficiency is same as exergy efficiency. But saving utilities of entransy analysis is more than exergy analysis. To sum up, entransy analysis and exergy analysis can calculate the energy utilization efficiency of the HENs of epichlorohydrin unit. Compared with exergy analysis method, entransy analysis is simpler and more efficient. 0 10000 20000 30000 40000 200 300 400 500 600 700 800 T / K Q / kW Hot Stream Cold Stream 1109 4. Conclusion The mathematical models for calculating the energy utilization efficiency of heat exchanger networks (HENs) based on entransy analysis method was built.The energy utilization efficiency of epichlorohydrin unit’s HENs was calculated by entransy analysis method. Aiming at the maximum energy recovery, the entransy analysis of epichlorohydrin unit’s HENs was done. Synthesis of epichlorohydrin unit HENs based on entransy theory was done. By taking three different △Tmin, 10 K, 15 K and 20 K, the entransy transfer efficiency is 42.81 %, 42.13 % and 41.00 % , the utilities are saved by 19.41 %, 18.01 % and 15.70 %. The greater the minimum approach temperature difference, the larger entransy dissipation, and the lower entransy transfer efficiency. The results of entransy analysis method was compared with the exergy analysis method. By taking three different △Tmin, 10 K, 15 K and 20 K, the exergy efficiency are 61.01 %, 59.28 % and 57.27 %. Exergy analysis method is calculated under certain ambient temperature and pressure, but entransy analysis method cannot need reference state. Exergy is more complicated than entransy in the calculation process. Besides, the entransy analysis method not only can complete HENs synthesis of epichlorohydrin unit, but also can calculate the energy utilization efficiency of HENs. 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