Format And Type Fonts CCHHEEMMIICCAALL EENNGGIINNEEEERRIINNGG TTRRAANNSSAACCTTIIOONNSS VOL. 45, 2015 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Sharifah Rafidah Wan Alwi, Jun Yow Yong, Xia Liu Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-36-5; ISSN 2283-9216 DOI: 10.3303/CET1545294 Please cite this article as: Chang C., Wang Y., Feng X., Zhang P., 2015, A two step methodology for Inter-Plant Heat Integration design, Chemical Engineering Transactions, 45, 1759-1764 DOI:10.3303/CET1545294 1759 A Two Step Methodology for Inter-Plant Heat Integration Design Chenglin Chang, Yufei Wang*, Xiao Feng, Ping Zhang State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China wangyufei@cup.edu.cn HRL (heat recovery loop) is an indirect method for transferring heat from one plant to another plant using intermediate-fluid circles. Inter-plant heat integration using HRL is a very special approach for energy conservation, as there are some additional factors should be considered, such as the capital cost of additional heat exchangers, pumps and pipelines for long distance, operation cost of pumping power and heat loss during the transportation. Moreover, when the number of plants involved in Heat Integration is large, the connection between plants have to be considered. These factors simultaneously determine the possibility and performance of Heat Integration. In this work, graphical targeting and mathematical programming is combined, a generalized MINLP model with economic objection is proposed to minimize the total annual costs (TAC) for Inter-Plant Heat Integration using HRL. As this work concentrates on heat recovery in low temperature range, hot water is selected as the heat transfer medium. The solved results can give the mass flow rate of intermediate-fluids, diameter of pipeline, temperatures of the heat transfer medium and the configuration of heat exchanger networks (HENs). An industry case study with three plants is used to demonstrate the model. 1. Introduction Heat recovery loop (HRL) is an Indirect Heat Integration method using intermediate-fluids and it has been considered as a viable energy saving method for processing plants. Bagajewicz and Rodera (1999) firstly studied Indirect Heat Integration using intermediate-fluid circles, i.e. dowtherms which needed not to be isothermal. They developed a systematic procedure to identify energy-saving target for inter-plant heat integration. Bagajewicz and Rodera (2000) developed another procedure for Inter-Plant Heat Integration and calculated targets for several industrial cases. An MILP problem was proposed to determine the optimal location of the fluid circuits. Bagajewicz and Rodera (2002) studied heat pumps system in multi- plant heat integration. Stijepović and Linke (2011) proposed an approach to enable the targeting of waste heat recovery potential in industry zone. Their study is concentrated on high temperature waste heat and the intermediate-fluid is steam. Atkins et al (2012) analysed Inter-Plant Heat Integration at a semi- continuous factory by the application of HRL. They developed a method to minimize the amount of heat exchanger area required for the HRL by optimizing the allocation of heat exchangers and the storage temperatures of the intermediate-fluid. Inter-Plant Heat Integration is an important research area, while most researches above only focused on energy reused perspective. Wang et al (2013) pointed out that distance had a significant influence on the Inter-Plant Heat Integration, while it was not fully considered in the conventional design. Some additional factors such as installation cost of pumps and pipelines for long distance, operation cost of pumping power and heat loss during the transportation decupling with the capital cost of additional heat exchangers determine the performance of inter-plant heat integration synchronously. Nevertheless, their studies only simulated the HRL design based on graphical targeting tools, but the mass flow rate and temperatures of intermediate-fluid circuits are not optimized, which had a great impact on the performance of integration. Combining graphical targeting and mathematical programming method, for an overview see (Klemeš and Kravanja, 2013), this work presents an MINLP model based on economic criteria to minimizing the total annual cost (TAC). In addition, this work focus on low grate heat reused and hot water is selected as the 1760 heat transfer medium. The solved result can give the mass flow rate of intermediate-fluids, diameter of pipelines, temperatures of the intermediate-fluid circuits and the structure of heat exchanger networks (HENS) automatically. 2. Proposed method 2.1 Graphical targeting tool The proposed method in this work included two steps for inter-plant heat integration using HRL. The first step is to determine the connection between plants through heuristic based graphical tools. It is known that when the number of plants is large, the possibility of connection between plants can be various. Using three plants as an example, based on the heat demand and heat required, there are three connection possibilities for the Inter-Plant Heat Integration (as shown in Figure 1). In this work, it is assumed that HENs within both plants are well established. Therefore, only the streams with cooler and streams with heater in the plants are considered to be integrated. All such streams are used for each plant to established Grand Composite Curves (Klemeš, 2013). Then based on the Grand Composite Curves for each plant, the connection between plants can be determined though the cascade utilization of energy. Figure 1: Connection possibilities for a three plants example 2.2 Mathematical programming model Based on mathematical programming, an MINLP model is established to minimizing the TAC. The superstructure is modified from the generalized MINLP model for cooling water system. The configuration showed in Figure 2 encompasses both series and parallel for heat exchangers. Additional Heat exchanger Existing heater Existing cooler Pump Source plant H1 H2 C1 C2 Sink plant Figure 2: Superstructure of Inter-Plant Heat Integration using HRL In the mathematical model, energy balance around each heat exchanger is expressed as: Plant A Plant B Plant C Heat Heat Plant A Plant B Plant C Heat Heat Plant A Plant B Plant C Heat Heat 1761 in w i i HP F f    i HPS (1) out i ii i HP f f     i HPS (2) in w j j CP F f    j CPS (3) out j jj j CP f f     j CPS (4) Mass and heat balances for mixers: out w ii i HP i HP F f       i i (5) out out w ii i i supp i H y HP P l F T f t       i i (6) out w jj j CP j CP F f       j j (7) out out w return jj j j CP j CP F T f t       j j (8) , out in i i i i i HP i HP i i i i f f f           i HPS (9) , in out out in i i i i i i return i HP i HP i i i i f t f t f T               i HPS (10) , out in j j j j j CP j CP j j j j f f f           j CPS (11) , in out out in j j j j j j j CP j CP j j j upp y j s l f t f t f T               j CPS (12) Energy balance around each heat exchanger:  in Hi i i iq F T T   i HPS (13)  out ini i i iq f t t   i HPS (14)  C inj j j jq F T T   j CPS (15)  in outj j j jq f t t   j CPS (16) The temperature difference:  1 in H in i i i i i dt T t z      i HPS (17)  1 out in out i i i i i dt T t z      i HPS (18)  1in out inj j j j jdt t T z      j CPS (19)  1out in Cj j j j jdt t T z      j CPS (20) Finally, the objective function for the total annual cost (TAC) and the complete model are as follows: 1762            1 1 0.3333 1 1 1 0.5 n i j n i HPS j CPS i i i in out in out i HPS i HPS i i i i j j CP I I TAC Min CCU qcu CHU qhu Pumping Costpipe Costpump I h h q z dt dt dt dt z                                                       1 1 0.3333 0.5 j j in out in out S j CPS j j j j h h q dt dt dt dt                           (21) 3. Case Study and result This case is a heat integration project for three existing plants: a Styrene plant, a Solvent plant and a Methanol plant. The cost data are shown in Table 1. In the table, Dout and Din are the outer and inner diameter of pipe, Wtpipe is the weight of pipe and Pcul is the cost of pipe. Table 1: Cost data for case study Items Value Electric cost 0.12 ($•kW -1 •h -1 ) Capital cost of heat exchanger Capital cost of pump 40sch pipeline 4,000+200•Area 0.83 ($•y -1 ) 450(q•H 0.5 ) 0.2 ($•y -1 ) Dout(m) = 1.052Din+0.005251 Wtpipe(kg/(m) = 644.3Din 2 +72.5Din+0.4611 Pcul($/m) = 0.82Wtpipe+185Dout 0.48 +6.8+265Dout I = 10 % n = 4 y Heat loss: 60 W/m Stream data is analysed by using the Grand Composite Curve shown in Figure 3. From the figure, it can be seen that the surplus heat of Styrene plant can be used as the heat source for Solvent plant, and the surplus heat of Styrene plant is not enough for the heat demand of both Solvent plant and Methanol plant, so the surplus heat of Solvent plant is used as heat source for the Methanol plant. Therefore, in this case, Solvent plant is not only a heat source plant, but also a heat sink plant. Figure 3: The Grand Composite Curve each plant By using Grand Composite curve, the connection between plants is obtained, as shown in Figure 4. The distance between plants is also shown in the figure. T, °C H, kW Styrene Solvent Methanol 1763 Industry zone 1000m 800m 1500m Solvent Methanol Styrene Industry zone 1000m 800m Solvent Methanol Styrene Figure4: The location and connection of the plants The detailed connections between plants are showed in Figure 5. The detailed economic performance is shown in Table 2. The minimized total annual cost is 2,004,939 $ and the heat recovered is 17,822 kW. The flow rate of intermediate-fluid is 221 t/h and 257 t/h for the two HRLs. From Table 2, it can be known that the investment for the pipeline and heat exchangers are the major part of the investment, because the distance between plants is relatively long, and the number of new heat exchangers is large, about 16 new heat exchangers. 136 ℃ 125 ℃ 165 ℃ C11 146 ℃ Styrene 130 ℃ 115 ℃ H11 H12 126 ℃ 90 ℃ 120 ℃ 70 ℃ H13 H14 65 ℃ 78 ℃ C24 70 ℃ Solvent C23 75 ℃ 183 ℃ C22 80 ℃ C21 95 ℃ 150 ℃ 70 ℃ 69 ℃ 64 ℃H22 67 ℃ 60 ℃ 65 ℃ 58 ℃ H23 H24 40 ℃ C34 30 ℃ C33 32 ℃ 41 ℃ C32 33 ℃ C31 35 ℃ 50 ℃ Methanol H21 45 ℃ 130 ℃ 30 ℃ H31 26 ℃ 2520 kW 3232 kW 2497 kW 2145 kW 915 kW 2119 kW 4597 kW 2763 kW 1772 kW 2518 kW 1634 kW 1504 kW 1667 kW 2925 kW 1634 kW 1202 kW 221 t/h 82 ℃ 221 t/h 122 ℃ 257 t/h 64 ℃ 257 t/h 39 ℃ Figure 5: The HENs of each plant 1764 Table 2: Annualized cost and profit of the project Items Solved results Annualized pipe cost 288,238 $•y -1 Annualized pumps cost 24,342 $•y -1 Heat loss 520 kW Pump power cost 8,286 $•y -1 Annualized heat exchanger cost 883,293 $•y -1 Energy saving benefit 17,822 kW 4. Conclusions Inter-plant heat integration using HRL can improve energy and economic efficiencies in an overall prospective. A two-step Inter-Plant Heat Integration methodology including a graphic tool and generalized MINLP model is established for HRL designs. By using the new methodology, the connection between plants and the detailed design for HRL can be obtained. From the results of case study, the investment for the pipeline and heat exchangers are the major part of the investment, and the total annual cost is 2,004,939 $. The heat recovery obtained in the case is also very promising, which is 17,822 kW. Acknowledgements Financial support from the National Basic Research Program of China (973 Program: 2012CB720500) and the National Natural Science Foundation of China under Grant No. 21476256 is gratefully acknowledged. References Atkins M. J., Walmsley M. R. W., Neale J. 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