Format And Type Fonts CHEMICAL ENGINEERING TRANSACTIONS VOL. 39, 2014 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Peng Yen Liew, Jun Yow Yong Copyright © 2014, AIDIC Servizi S.r.l., ISBN 978-88-95608-30-3; ISSN 2283-9216 DOI: 10.3303/CET1439246 Please cite this article as: Angsutorn N., Saikhaw P., Chuvaree R., Siemanond K., 2014, Heat exchanger network synthesis on gas separation plant no.2 (GSP2) in Thailand, Chemical Engineering Transactions, 39, 1471-1476 DOI:10.3303/CET1439246 1471 Heat Exchanger Network Synthesis on Gas Separation Plant No.2 (GSP2) in Thailand Natchanon Angsutorn a , Penprapa Saikhaw a , Rungroj Chuvaree b , Kitipat Siemanond* a a Chulalongkorn University, The Petroleum and Petrochemical College, Wangmai, Patumwan, Bangkok 10330, Thailand b PTT Public Company Limited, Chatujak, Bangkok 10900, Thailand kitipat.s@chula.ac.th Energy conservation is one of the most common concerns in gas separation plants. It plays a key role in energy and operating cost saving of their high-energy consuming processes. Although many research works on heat exchanger network (HEN) synthesis has been extensively studied for more than 40 years, most of them have limitations on computational time and feasibility due to their mathematical difficulties. Thus, they are impractical for large problems as industrial cases. In this work, a strategy for HEN synthesis was presented and applied to the industrial case of gas separation plant no.2 (GSP2) in Thailand consisting of twelve hot and eleven cold process streams. The strategy for HEN synthesis is a combination of Pinch Technology, the well-known thermodynamics-based approach, and mathematical programming based on the stage-wise superstructure model. HEN was synthesized in two parts; above and below pinch, corresponding to the optimal pinch temperature as well as the heat recovery approach temperature (HRAT) predicted by Pinch Analysis in order to assure near-optimal design. The HEN was then improved by a mathematical model applying relaxation technique based on a concept of loop and path toward better design with less complexity. According to the industrial case of GSP2, the proposed strategy for HEN synthesis is effective where a good solution can be obtained with reasonable computational time. 1. Introduction The world has been facing an energy crisis for a few decades. The rising cost of energy has a significant impact on industry, especially gas separation plant (GSP). According to its high-energy consuming, energy conservation is one of the most common concerns to save an operating cost. Heat exchanger network (HEN) synthesis, heat integration between hot and cold process streams, has been widely applied for that purpose. Research works on HEN synthesis have been done for more than 40 years by many researchers (Klemeš and Kravanja, 2013); Linnhoff and Hindmarsh (1983) introduced the thermodynamics based technique to predict an optimal heat recovery approach temperature (HRAT) for inventing good HEN design. Yee and Grossmann (1990) developed the mixed integer nonlinear programming (MINLP) formulation of stage-wise superstructure considering all trade-offs simultaneously. Barbaro and Bagajewicz (2005) presented a rigorous mixed integer linear programming (MILP) formulation for HEN synthesis relying on transportation/transshipment concepts. Since mathematical formulations for HEN synthesis contain nonlinear equations, most of them are impractical for large problems as industrial cases due to large computational time and lack of feasibility. To overcome this limitation, heuristics, thermodynamic methods and optimization should be used in a combination to narrow solution space and numerical complexity (Anantharaman et al., 2013). This research work developed the three-step approach for HEN synthesis and applied to the industrial case of gas separation plant no.2 (GSP2) in Thailand. The strategy is a combination of thermodynamic principles based on Pinch Technology concept (Linnhoff and Vredeveld, 1984) and mathematical optimization based on stage-wise superstructure model (Yee and Grossmann, 1990). 1472 2. Methodology There are three steps to synthesize HEN as shown in Figure 1. In the first step, an optimal HRAT was predicted and then used to synthesize HEN to assure near-optimal design in the second step. After that the HEN was improved toward better design with less complexity in the last step. 2.1 Targeting by Pinch Analysis Pinch Analysis predicted the optimal HRAT as well as Pinch location providing the best value of objective function based on vertical heat transfer area and minimum number of process heat exchangers. In this study, Grassroots Potential Program (Siemanond and Kosol, 2012) was used as an automated tool for this step. The program was implemented on Microsoft Excel incorporating with Visual Basic for Applications (VBA). 2.2 HEN synthesis by stage-wise model with pinch temperature The stage-wise model with pinch temperature is the MINLP stage-wise model (Yee and Grossmann, 1990) where pinch temperatures of hot and cold process streams are located in the model as shown in Figure 2. The model synthesized a HEN by using the pinch temperatures from previous step to restrict utility load. The HEN was designed in 2 parts; above and below Pinch. This simplifies the model to be optimized only number of heat exchangers and their area. 2.3 HEN improvement by stage-wise model with topology control According to the heuristic rule of the Euler’s general network theorem observed by Hohmann (1971) expressed in a simple relationship, as shown in the following equation where is minimum number of units and is number of process streams and utilities. The number of heat exchangers in the previous step is often large because of the existence of pinch in the HEN; as a result, the designed HEN is complex and unfavorable for construction. In this step, the relaxation technique was introduced to improve the HEN from previous step and reduce its complexity, achieving a better value of objective function. The Pinch Point was removed to allow cross-pinch heat transfer, and the problem was then re-optimized with the control of matching from the previous step. The number of heat exchangers should be reduced with some shift of HRAT. Targeting Pinch Analysis HEN Synthesis Stage-wise Model with Pinch Temperature Start Stop Stage-wise Model with Topology Control HEN Improvement Figure 1: The proposed three-step strategy for HEN synthesis (1) 1473 tC2,K2 tC1,K2 tH2,K2 tH1,K2 tC2,K1 tC1,K1 tH2,K1 tH1,K1 tC2,K3 tC1,K3 tH2,K3 tH1,K3 tC2,Kn tC1,Kn tH2,Kn tH1,Kn tC2,Kn+1 tC1,Kn+1 tH2,Kn+1 tH1,Kn+1 tC2,KP tC1,KP tH2,KP tH1,KP tC2,Kn-1 tC1,Kn-1 tH2,Kn-1 tH1,Kn-1 H1-C1 H1-C1 qH1,C1,K1 H1-C2 H1-C2 H2-C1 H2-C1 H2-C2 H2-C2 qH1,C2,K1 qH2,C1,K1 qH2,C2,K1 H1 H2 HU-C1 HU-C2 H1-CU H2-CU tH2,IN tH1,IN tC2,OUT tC1,OUT tH2,OUT tH1,OUT H1-C1 H1-C1 qH1,C1,Kn H1-C2 H1-C2 H2-C1 H2-C1 H2-C2 H2-C2 qH1,C2,Kn qH2,C1,Kn qH2,C2,Kn Stage K1 Stage K2 Stage Kn tC2,IN tC1,IN C1 C2 qH1,CU qH2,CU qHU,C1 qHU,C2 Location K1 Location K2 Location K3 Location Kn Location Kn+1 Location Pinch Location Kn-1 Stage Kn-1 Pinch point Figure 2: Stage-wise model with Pinch temperature 3. Case Study The case study is the industrial case of GSP2 in Thailand consisting of twelve hot and eleven cold process streams, one cooling and one heating utility. The data for HEN synthesis are shown in Table 1, 2, and 3. The objective function of all steps is to minimize net present cost (NPC). The mathematical models were implemented in the General Algebraic Modeling System (GAMS) 24.2.1 and solved with the MINLP solver DICOPT using CONOPT 3 and CPLEX 12.6 as nonlinear programming (NLP) solver and MILP solver, respectively. The default options were used for all solvers. The CPU times are reported corresponding to runs performed in Notebook PC Model SVS15135CHB with Intel(R) Core(TM) i5-3230M CPU @ 2.60 GHz processor and 4.00 GB of ram memory. The results shown in this paper are the screened results where the heat exchangers with no heat duty have been removed. Table 1: Stream data Table 2: Utility data Stream FCp Tin Tout h (kW/°C) (°C) (°C) (kW/m 2 -°C) H1 98.19 -14.74 -37.78 1.35 H2 794.90 9.24 6.17 8.48 H3 4,421.41 53.20 51.27 0.80 H4 145.99 83.28 52.00 0.80 H5 321.20 58.48 52.00 0.80 H6 8.18 171.10 91.42 9.50 H7 90.03 -39.21 -48.21 0.80 H8 276.10 33.00 -40.00 11.05 H9 89.18 50.00 26.82 2.36 H10 6.54 95.65 26.11 1.96 H11 24.13 72.47 26.11 2.35 H12 15.55 49.29 26.11 0.62 C1 13.78 15.10 85.50 11.05 C2 271.91 67.64 80.90 0.10 C3 1,171.93 164.30 170.10 1.96 C4 723.88 99.67 106.70 0.33 C5 196.32 82.85 86.17 0.34 C6 3,255.13 -39.59 -38.59 0.62 C7 13.77 -20.22 22.78 0.33 C8 7.05 -50.00 5.00 2.36 C9 147.80 -53.33 21.67 0.80 C10 81.82 -42.33 26.67 0.80 C11 172.61 21.67 44.85 0.80 Utility Tin Tout h (°C) (°C) (kW/m 2 °C) HU1 250.00 180.00 0.337 CU1 -55.00 -54.00 0.620 Table 3: Cost data Cost Data CU1 $/kW d 0.066 HU1 $/kW d 0.033 Heat exchanger cost $ 4,838.50 + 68.5A Splitting cost $/split 20,000 Project life time y 20 Interest rate % 10 Yearly operating days d/y 350 EMAT (exchanger minimum approach temperature) °C 3 1474 4. Result and discussion The result of targeting step is shown in Table 4 and Figure 3. The optimal HRAT was 4.9 °C corresponding to the best NPC of $ 5,228,430 and the hot and cold pinch temperatures of 72.54 °C and 67.64 °C. Note that NPC in the first step was calculated based on vertical heat transfer and minimum number of heat exchangers. The Hot and Cold Pinch temperatures were then used for HEN synthesis in the second step getting the result as shown in Table 5 and Figure 4. The computational time is reasonable (13,294 s or 3 h 41 m 34 s). The utility load is identical because the HRAT was fixed at the same value. The number of heat exchangers is larger than the minimum number of units about one unit predicted by targeting step. The area of heat exchangers is slightly more than the predicted one because of non-vertical heat transfer in the actual network. This ensures the reliability of the predicted HRAT. The designed HEN was then improved in the last step, as the result shown in Table 6 and Figure 5. It required very short computational time because the topology, involving binary variables, was controlled in the optimization reducing the problem size. With a penalty of heat exchanger area, utility load and number of heat exchangers were decreased giving the better NPC. According to the previous step, two of heat exchangers were disappeared (E12 and E16); as a result, the total number of heat exchangers is more than the minimum number of heat exchangers of HEN without pinch based on Euler’s general network theorem (24 units) about one unit. It can be noticed that there is no splitting in the HEN because of the expensive splitting cost. Table 4: Summary report of targeting step Pinch Analysis results Optimal HRAT °C 4.9 Hot pinch temperature °C 72.54 Cold pinch temperature °C 67.64 Optimal cooling utility load kW 17,441 Optimal heating utility load kW 14,019 Optimal number of heat exchangers Units 26 Optimal total vertical area m 2 4,294.6 Payback period y 0.43 NPC (Objective function) $ 5,228,430 Figure 3: Composite Curves at optimal HRAT Table 5: Summary report of HEN synthesis step Table 6: Summary report of HEN improvement step Stage-wise model with pinch temperature results Number of cycle cycles 2 Total CPU time s 13,294 Average CPU time per cycle s/cycle 6,647 HRAT °C 4.9 Total cooling utility load kW 17,441 Total heating utility load kW 14,019 Total number of heat exchangers Units 27 Total area of heat exchangers m 2 6,275.6 Total splitting splits 0 Total number of splits splits 0 Payback period y 0.58 NPC (Objective function) $ 5,368,967 Stage-wise model with topology control results Number of cycle Cycles 2 Total CPU time s 2 Average CPU time per cycle s/cycle 1 HRAT °C 4.4 Total cooling utility load kW 17,350 Total heating utility load kW 13,928 Total number of heat exchangers Units 25 Total area of heat exchangers m 2 6,551.8 Total splitting splits 0 Total number of splits splits 0 Payback period y 0.58 NPC (Objective function) $ 5,351,506 -100 -50 0 50 100 150 200 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 T e m p e ra tu re ( °C ) Cumulative Enthalpy (kW) Hot Composite Cold Composite Pinch Position 1475 5. Conclusion The strategy for HEN synthesis was presented in this paper. The method consists of three steps; targeting, HEN synthesis and HEN improvement. From this study, it can be concluded that the heuristics from Pinch Analysis can help mathematical optimization to generate the good HEN solution for large-sized problem with reasonable computational time requirement. The study also proves that the relaxation technique can help reduce the complexity of HEN. Therefore, the proposed strategy is the effective method for HEN synthesis, even for industrial problems. For future work, non-isothermal mixing should be taken into account in the HEN improvement step to get more possibility of better solutions. Acknowledgements Authors would like to express our gratitude to The Petroleum and Petrochemical College, Chulalongkorn University, National Center of Excellence for Petroleum, Petrochemicals and Advanced Materials, The ASAHI Glass Foundation, PTT Public Company Limited, and Research and Researcher for Industry for funding support. S tr e a m FCp kW/°C T °C T °C H1 H2 H3 H4 98.19 -14.74 794.90 9.24 4,421.41 53.20 145.99 83.28 H5 H8 H7 H6 321.20 58.48 276.1 33.00 90.03 -39.21 8.18 171.10 H9 H10 H11 89.18 50.00 6.54 95.65 24.13 72.47 H12 15.55 49.29 C1 C2 C3 C4 13.78 85.50 271.91 80.90 1,171.93 170.10 723.88 106.70 C5 C6 C7 196.32 86.17 3,255.13 -38.59 13.77 22.78 C8 C9 C10 C11 7.05 5.00 147.80 21.67 81.82 26.67 172.61 44.85 -37.78 6.17 51.27 52.00 52.00 -40.00 -48.21 91.42 26.82 26.11 26.11 26.11 15.10 67.64 164.30 99.67 82.85 -39.59 -20.22 -50.00 -53.33 -42.33 21.67 HU3 CU1 HU2 HU4 Heating Utility Process-Process (Above Pinch) Cooling Utility 17,441 kW 2,038 kW 6,797 kW 5,089 kW Heat Exchanger Area m 2 E1 E2 E3 E22 HU3 CU1 HU1 HU22,484.8 59.6 9.3 301.2 782.7 2.2 195.1 599.1 Heat Exchanger Area m 2 E4 E5 E6 38.6 90.5 133.7 E7 E8 E9 257.2 310.4 54.1 E10 E11 E12 79.9 8.7 37.2 E13 E14 E15 65.3 38.2 94.4 E16 E17 E18 4.4 38.6 64.0 E19 232.8 E20 E21 1.0 12.0 E1 E1 1,568 kW E2 E2 652 kW E3 E3 151 kW E4 E4 420 kW E9 E9 592 kW E5 E5 E6 E6 1,988 kW E7 E7 2,418 kW E8 E8 5,953 kW 2,440 kW E10 E10 2,081 kW E15 E15 815 kW E11 E11 E12 E12 638 kW E13 E13 2,714 kW E14 E14 491 kW 304 kW E16 E16 160 kW Hot Pinch Temperature 72.54°C Process-Process (Below Pinch) 67.64°C Cold Pinch Temperature Heat Exchanger Area m 2 Heat Exchanger Area m 2 HU1 95 kW E21 E21 360 kW E17 E17 E18 E18 453 kW E19 E19 1,448 kW E20 E20 27 kW 1,576 kW E22 E22 810 kW HU4 280.5 Heat Exchanger Area m 2 Heat Exchanger Area m 2 Heat Exchanger Area m 2 Figure 4: Grid diagram of GSP2 case study from HEN synthesis step 1476 S tr e a m FCp kW/°C T °C T °C H1 H2 H3 H4 98.19 -14.74 794.90 9.24 4,421.41 53.20 145.99 83.28 H5 H8 H7 H6 321.20 58.48 276.1 33.00 90.03 -39.21 8.18 171.10 H9 H10 H11 89.18 50.00 6.54 95.65 24.13 72.47 H12 15.55 49.29 C1 C2 C3 C4 13.78 85.50 271.91 80.90 1,171.93 170.10 723.88 106.70 C5 C6 C7 196.32 86.17 3,255.13 -38.59 13.77 22.78 C8 C9 C10 C11 7.05 5.00 147.80 21.67 81.82 26.67 172.61 44.85 -37.78 6.17 51.27 52.00 52.00 -40.00 -48.21 91.42 26.82 26.11 26.11 26.11 15.10 67.64 164.30 99.67 82.85 -39.59 -20.22 -50.00 -53.33 -42.33 21.67 HU3 CU1 HU2 HU4 Heating Utility Process-Process Cooling Utility 17,350 kW 1,970 kW 6,797 kW 5,089 kW Heat Exchanger Area m 2 E1 E2 E3 E22 HU3 CU1 HU1 HU2 2,746.9 59.6 12.2 301.2 780.8 1.7 188.8 599.1 Heat Exchanger Area m 2 E4 E5 E6 48.4 90.5 130.2 E7 E8 E9 272.2 312.8 54.1 E10 E11 81.0 8.7 E13 E14 E15 66.7 38.4 94.4 E17 E18 40.3 97.6 E19 232.8 E20 E21 1.0 12.0 E1 E1 1,636 kW E2 E2 652 kW E3 E3 154 kW E4 E4 443 kW E9 E9 592 kW E5 E5 E6 E6 1,919 kW E7 E7 2,488 kW E8 E8 6,022 kW 2,440 kW E10 E10 2,081 kW E15 E15 815 kW E11 E11 E13 E13 2,805 kW E14 E14 422 kW 301 kW Heat Exchanger Area m 2 Heat Exchanger Area m 2 HU1 72 kW E21 E21 360 kW E17 E17 E18 E18 1,091 kW E19 E19 1,448 kW E20 E20 27 kW 1,645 kW E22 E22 810 kW HU4 280.5 Heat Exchanger Area m 2 Heat Exchanger Area m 2 Heat Exchanger Area m 2 Figure 5: Grid diagram of GSP2 case study from HEN improvement step References Anantharaman R., Jordal K., Berstad D., Gundersen T., 2013, The role of process synthesis in the systematic design of energy efficient fossil fuel power plants with co2 capture, Chemical Engineering Transactions, 35, 55-60. 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