CHEMICAL ENGINEERING TRANSACTIONS VOL. 56, 2017 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Guest Editors: Jiří Jaromír Klemeš, Peng Yen Liew, Wai Shin Ho, Jeng Shiun Lim Copyright © 2017, AIDIC Servizi S.r.l., ISBN 978-88-95608-47-1; ISSN 2283-9216 An Industrial Case Study Application in Synthesising a Feasible Heat Exchanger Network Suraya Hanim Abu Bakara,b, Mohd Kamaruddin Abd Hamid*,a,b , Sharifah Rafidah Wan Alwia,b, Zainuddin Abdul Manana,b aProcess Systems Engineering Centre (PROSPECT), Research Institute of Sustainable Environment, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia bFaculty of Chemical and Energy Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia kamaruddin@cheme.utm.my Heat exchanger network synthesis (HENS) is an important part in the overall chemical process. HENS links the process flowsheet with the utility system and generally involves a large fraction of both the overall plant capital cost, operating costs in terms of energy requirements, which is a key factor for a profitable process. The aims of the synthesis consist of finding a network design that minimises the total annualised cost, i.e. the investment cost in units and the operating cost in terms of utility consumption. In HENS, the feasibility of the HEN design does not take into consideration. As a consequence, the HEN design may not be able to be implemented into industrial applications. It is essential to check the feasibility of a design before it is being implemented into the industry. The objective of this paper is to present the application of a new flexible and operable heat exchanger network (FNO HEN) methodology in synthesising a feasible heat exchanger network (F-HEN) using an industrial case study. The aim of this work is to verify the existing industrial HEN design in terms of the process design point of view as well as the process feasibility. The existing industrial HEN design is verified in terms of ΔTmin value that gives minimum external energy requirement (EER) and heat exchanger area (HEA) as well as simultaneously analyse the feasibility of the HEN design. Using the new developed FNO HEN methodology framework, HEN design target, which is the value of ΔTmin is determined in order to obtain the F-HEN design. From process design point of view, ΔTmin value determines the size of heat exchanger in the network as well as energy saving. A process simulator is used to check the process feasibility of the HEN designs. With the use of the F-HEN trade-off plot, which is a plot of EER and HEA at different value of ΔTmin. with additional of feasibility area, the optimal feasible HEN design which satisfies external energy requirement (operability), heat exchanger area (capital) and process feasibility has been successfully determined in an easy, systematic and efficient manner. 1. Introductions There are several methods in Pinch Analysis (PA) concept, such as graphical method, numerical method and mathematical programming method. In graphical method, Wan Alwi and Manan (2010) have modified the conventional graphic method by proposing Steam Temperature versus Enthalpy Plot (STEP). In numerical method, Escobar et al. (2013) proposed framework using numerical concept to synthesis HENs with operability considerations using computational efficiency. In mathematical modelling, Supiluck and Kitipat (2015) work on model formulations in finding effectiveness of initialisation strategy for both HEN synthesis and retrofit design. The concept of these methods is the same which is to find the optimal (target design) heat exchanger networks that can minimise the usage of heating and cooling utilities, as well as maximising the energy exchange among the process streams. Although there are many methods to fulfil the objective of PA however, feasibility and implementation to real industry can be questionable. It is needed to check the feasibility of a designed HEN (Abu Bakar et al., 2016). DOI: 10.3303/CET1756130 Please cite this article as: Abu Bakar S.H., Hamid M.K.A., Wan Alwi S.R., Manan Z.A., 2017, An industrial case study application in synthesizing a feasible heat exchanger network, Chemical Engineering Transactions, 56, 775-780 DOI:10.3303/CET1756130 775 The objective of this case study is to verify the existing industrial HEN design in terms of the process design point of view as well as the process feasibility. The existing industrial HEN design is verified in terms of ΔTmin value that gives the minimum value of HEA and EER as well as simultaneously verified the feasibility of the HEN design (Abu Bakar et al., 2015b). 2. Methodology 2.1 Process Descriptions The case study is adapted from Abu Bakar et al. (2015a) Fatty acid fractionation plant (FAFP). Figure 1 shows process flow diagram of the FAFP that used in this case study and Table 1 summarises the operating data for the process. Figure 1: Heat exchanger network (HEN) for fatty acid fractionation plant (Abu Bakar et al., 2015a) Table 1: Extracted data from the FAFP adapted from initial analysis of Abu Bakar et al. (2015a) Streams Type Stream names Temperature (°C) Cp (kJ/kg.K) Flowrate (kg/h) Temp (°C) FCp (kJ/K) Inlet Outlet Supply Target H1 111 105 128 60 1.882 101 68 190.08 H2 153 153B 140 65 2.176 14,657 75 31,893.63 H3 134 135 209 65 2.553 7,000 144 17,871.00 C1 101 105 70 130 2.053 37,000 60 75,961.00 C2 109 110 128 193 2.205 22,817 65 50,311.49 2.2 Problem statement The feasible HENs-synthesis problem for this industrial HEN design can be stated as follows: Given three hot streams (to be cooled) and two cold streams (to be heated), it is desired to synthesise a feasible network of heat exchangers that can transfer heat from the hot streams to the cold streams. Given also the heat capacity flow rate (flow rate x specific heat) of each process hot stream, FCP,u; its supply (inlet) temperature, Tus; and its target (outlet) temperature, Tut , where u = 1, 2, 3. The heat capacity, fcP,v, and supply and target temperatures, tvs and tvt , are given for each process cold stream, where v = 1, 2. In this feasible HENs-synthesis problem statement, focus is given to verify either the real industrial HEN design is optimal and feasible or not where the control structure are assumed to be fixed. In order to solve the problem statement stated above, method from (Abu Bakar et al., 2015a) has been adapted. Because in this case study the control structure has been assumed to be fixed, only Design Target, HEN Design analysis and Feasible Test were considered. Multi-objective function in the method has been redefined as shown in Eq(1), max(J) = w1(P1,1) + w3 ( 1 P3,3 ) + w3(1/P3,2) (1)  To achieve the process design objective, P1,1 is maximised. P1,1 is the performance criteria in maximising the energy recovery of the network 776  To achieve the steady state sensitivity objective, P2,1 is maximised, P2,2 is minimised and P2,3 is maximised. P2,1 is the flexibility, percentage of manipulated variables toleration, while P2,2 is the sensitivity of controlled variable, y with respect to disturbance variable, d and P2,3 is the controller structure pairing gain, the sensitivity of controlled variable, y with respect to manipulated variable, u.  To achieve the economic objective, P3,j is minimised. P3,1 is the capital cost and P3,2 is the operating cost.  w1, w2 and w3 are weight factors for each objective function to determine optimum value J 2.3 Feasibility Test All results obtained from Design Target and HEN Design Analysis stages were verified in terms of process design feasibility using process simulator. The output from Stage 2 is the output temperatures from all heat exchangers. Information transfer from GD to process simulator: 1. Temperature out from GD is transferred to simulator. However, heat exchanger in the process simulator, only required one degree of freedom (one variables value). The other temperature out is automatically calculated by the simulator. 2. Information temperature out obtain from GD may not be the same from process simulator calculation because of the Cp value. Cp value that obtained from GD is assumed the same after temperature has changed but Cp value in the simulator changed as temperature changes. There are two steps to do in the Feasibility Test. Firstly, is by calculating and analysing the ft correction factor for each heat exchanger in the network. ft correction factors lower than 0.75 it is considered as infeasible. Secondly by observing warning given in the Aspen HYSYS. Results from the process simulator for HEN design at ΔTmin = 10 °C show that heat exchanger HE3 has ft correction factor low problem and have to be redesigned. The yellow warning on the heat exchanger icon (in simulator) indicates that the heat exchanger has ft correction factor low problem. It can be concluded that HEN design at ΔTmin = 10 °C is not feasible from the process simulator point of view. The analysis should go back to Design Target stage to choose new value of ΔTmin. with increment 10 °C. Then, HEN at ΔTmin at 20 °C, 30 °C, and 40 °C were synthesised, however the feasibility of those candidates is still a question and it needs to be analysed. 3. Results and Discussions 3.1 Difference between the Original HEN and New HEN After Feasibility Test has been done, the only HEN that are feasible is HEN at ΔTmin at 40 °C. From the HEN of ΔTmin = 40 °C, that there are two different networks can be generated. These two different networks were obtained since one of the HEN designs was not following the Pinch Analysis (PA) stream splitting rules. It can be seen that the streams in the original HEN (Figure 2) do not have stream splitting while one of the streams in the new HEN (Figure 3) had split into three other streams. Red boxes in both figures show the different arrangement between the original and new HEN arrangements. Further details of the differences are tabulated in Table 2. There are two different criteria between both candidates: number of heat exchangers and HEA. The original HEN has less heat exchangers (two heat exchangers) compared to the new HEN (four heat exchangers). For the HEA, the new HEN has smaller value than the original HEN. From this result, it will be difficult to decide which network design is the best. Therefore, multi-objective function was calculated to select the best design using Eq(1). Table 2: Criteria of original HEN and new HEN designs at ΔTmin = 40 °C Criteria Original HEN New HEN PA matching rules Not followed Followed Number of Heat Exchanger 2 4 No of heater 2 2 No of cooler 3 3 Splitting No Yes EER value (kW) 1,606.5 1,606.5 HEA value (m2) 46.3 42.4 PFD Figure 2 Figure 3 777 101 103 153A 134A 153B 135 103A 105 111112 113 108 104 109 110 116 117 153 106 140 136 137 118 119 127 129 121 To Vac TWR TWS TWS TWR C16 98% Lights to V5 130 133 TWS TWR C16-18 C16-18 from other source LPS LPC MPS TWR MPR Feed Vessel1 Vessel2 To other unit TWR TWS To Vac To C12-14 LPS Condenser Reboiler Boiler DMW Column1 Column2 Cooler3 120 Lights to other unit Cooler2 Cooler1 Heat Exchanger2 Heat Exchanger1 Heater1 Heater2 134 Figure 2: Process flow diagram of original HEN for a unit in FAFP 111112 113 109 110 116 117 140 136 137 119 127 129 121 To Vac TWR TWS TWS TWR C16 98% Lights to other unit 130 133 TWS TWR To C16- 18 MPS MPR C1 Vessel1 Vessel2 To other unit TWR TWS To Vac To C12-14 LPS Condenser Reboiler Boiler DMW Column1 Column2 Cooler3 120 Lights to other unit U5 Cooler1 HE4HE2 HE1 U2 H3 10 HE3 U14 23 101 9 10 U3 H3 Out 8 5 105 H2 6 7 H2 Out 118 12 11 108 107 LPS LPCU1 Figure 3: Process flow diagram of new HEN for a unit in FAFP 3.2 Multi-objectives function calculation All objective function values were collected from Design Target and HEN Design Analysis stages and tabulated and results of multi-objective functions is presented in Table 3. Since all the values of the objective functions have different units, therefore all objective function values need to be normalised by dividing it with the largest value of each objective functions (Abu Bakar et al., 2013). Then, using the normalised objective function values, the value multi-objective J is calculated using Eq(1) for both new HEN and original HEN. It can be seen that the new HEN design has the highest value of the multi-objective function is the new HEN design that follows the PA matching rules. It is important for HEN design to follow PA rules in order to get the optimal HEN design. 778 Table 3: Multi-objective function calculation of Case Study 2 Maximum Energy Recovery (MER) (kW) Heat Exchange Area (HEA) (m2) External Energy Requirement (EER) (kW) Design/Control value, Px,x P1,1 P3,1 P3,2 New HEN 734.650 42.383 1,606.484 Original HEN 734.650 46.324 1,606.485 Normalise value, Px,xs P1,1s P3,1s P3,2s New HEN 1.000 0.915 1.000 Original HEN 1.000 1.000 1.000 Multi-objective function value, P1,1 1/P3,1s 1/P3,2s New HEN 1.000 1.093 1.000 Original HEN 1.000 1.000 1.000 3.3 F-HEN Trade-Off Plot The F-HEN trade-off plot is a plot of EER and HEA at different value of ΔTmin with additional of feasibility area (Figure 4). From the figure, the best HEN candidate (in terms of EER and HEA) that satisfies the design criteria can be identified at the intersection point between EER and HEA lines, which is approximately at ΔT min = 32 °C. The similar concept also has been used by Dimian et al. (2014) to identify the optimal HEN design using common trade-off plot which energy and capital cost versus ΔTmin. The optimal HEN design is not located inside the feasible area (Figure 4). It can be clearly seen that the feasible area for this case study is ΔTmin = 40 °C and above. Since the optimal HEN design is located outside the feasible area, it can be concluded that the optimal HEN design at ΔTmin = 32 °C is not feasible. It must be noted here that, the identification of the feasible area was based on the ΔTmin = 10 °C increment, which can be considered as too large for this analysis. This large increment value has a large possibility to overlook the optimal feasible design solution. The question either the design value at ΔTmin = 32 °C is feasible or not, still needs to be answered. For this reason, the feasibility analysis needs to be extended at ΔTmin = 32 °C. The results of the extended feasibility analysis have shown that the HEN ΔTmin = 32 °C is not feasible. Therefore, the cross point at 32 °C is still not in the feasible region. Figure 4: HEN trade-off plot with feasibility area for Case Study 4. Conclusions The existing industrial HEN design has been verified in terms of ΔTmin value that gives the minimum value of HEA and EER as well as simultaneously verified the feasibility of the HEN design. Throughout this case study, the HEN design that follows PA matching rules gives the best design results compared with the one that is not following the rules. The optimal solution for the feasible HEN design of the case study which satisfies external energy requirement (operability), heat exchanger area (capital) and process feasibility has been successfully analysed in this section using the developed FNO HEN methodology framework. The use of feasible trade-off plot helps in obtaining the optimal feasible HEN design in efficient and systematic manner. F ea si bl e A re a 779 Acknowledgments The financial support from Research University Grant (RUG) Tier 1 (Q.J130000.2546.12H67), Universiti Teknologi Malaysia (UTM) is acknowledged. Reference Abu Bakar S.H., Hamid M.K.A., Wan Alwi S.R., Manan Z.A., 2013, Flexible and Operable Heat Exchanger Networks, Chemical Engineering Transactions 32, 1297-1303. Abu Bakar S.H., Hamid M.K.A., Wan Alwi S.R., Manan Z.A., 2015a, Initial Analysis on Heat Exchanger Networks of Fatty Acid Fractionation Plant to Optimize Energy Recovery and Controllability, Chemical Engineering Transactions 45, 97-102. Abu Bakar S. H., Hamid M. K. A., Wan Alwi S. R., Manan Z. A., 2015b, Effect of Delta Temperature Minimum Contribution in Obtaining an Operable and Flexible Heat Exchanger Network, Energy Procedia 75, 3142- 3147. Abu Bakar S.H., Hamid M.K.A., Wan Alwi S.R., Manan Z.A., 2016, Selection of minimum temperature difference (ΔTmin) for heat exchanger network synthesis based on trade-off plot, Applied Energy 162, 1259-1271. Escobar M., Trierweiler J.O., Grossmann I. E., 2013, Simultaneous synthesis of heat exchanger networks with operability considerations: Flexibility and controllability, Computers and Chemical Engineering 55, 158- 180. Supiluck K., Kitipat S., 2015, Heat Exchanger Network Synthesis/Retrofit using MINLP Stage-wise Superstructure with Non-isothermal Mixing, Chemical Engineering Transactions 43, 1273-1278. Wan Alwi S.R., Manan Z.A., 2010, STEP - A new graphical tool for simultaneous targeting and design of a heat exchanger network, Chemical Engineering Journal 162 (1), 106-21. 780