Heat Exchanger Network Design Considering ... - Science Direct

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ScienceDirect Energy Procedia 61 (2014) 2469 – 2473

The 6th International Conference on Applied Energy – ICAE2014

Heat Exchanger Network Design Considering Inherent Safety Irene Chana, Sharifah Rafidah Wan Alwi a,*, Mimi H. Hassimb, Zainuddin Abd Manana, Jiří Jaromír Klemeš c a

Process Systems Engineering Centre (PROSPECT), Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, MALAYSIA b Institute of Hydrogen Economy (IHE), Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, MALAYSIA c Centre for Process Integration and Intensification – CPI 2 , Research Institute of Chemical and Process Engineering, Faculty of Information Technology, University of Pannonia, Egyetem u. 10, H-8200 Veszprém, HUNGARY

Abstract Pinch Analysis (PA) has been one of the most established methods since the 1970’s for the design of a maximum heat recovery network. It has been one of the vital tools for maximising heat recovery in a process plant and for minimising the impact of rising energy cost as well as the environmental emissions. By combining the thermodynamic insights into the process heat recovery bottleneck with the HEN capital and operating cost tradeoffs, the PA has become an energy-saving tool widely used in the industry . However, further studies are needed to incorporate the safety aspect into HEN design using PA. Not fully considering the potential process hazards when selecting matches of hot and cold streams in a HEN can introduce process operation risk and consequently necessitate high investment in materials of constructions for heat exchangers. This work presents a new method for HEN design that incorporates the inherent safety index during the selection of heat exchanger matches in order to reduce the potential hazards of the optimal HEN design. Application of the extended PA on a case study shows that inclusion of the inherent safety feature has managed to localise the area of hazards, reduce the requirement for special materials of construction, and ultimately reduce the HEN capital cost by 10%. © Published by Elsevier Ltd. This © 2014 2014The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the Organizing Committee of ICAE2014

Keywords : Heat exchanger network; inherent safety; Pinch Analysis; maximum energy recovery; Inherent Safety Index

1. Introducti on Pinch Analysis (PA) is a systematic and holistic methodology for maximising heat recovery in process systems. PA provides targets for the maximu m heat recovery, the min imu m heating and cooling requirements as well as the minimu m number o f heat-exchange units; e.g. coolers, heaters and heat exchangers [1].

* Corresponding author. Tel.: +6-07-5536025; fax:+6-07-5588166. E-mail address: [email protected]

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of ICAE2014 doi:10.1016/j.egypro.2014.12.025

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Irene Chan et al. / Energy Procedia 61 (2014) 2469 – 2473

There has been a lot of research done to improve the operability of HEN designed using the PA. it presently attracts high interest – see e.g. work of Escobar et al. [2] who introduced a framework that yields a HEN design that is guaranteed to operate with the designed control system under varying conditions. However, further studies and development are needed to integrate the safety aspect with PA during HEN design. This is one of the imp lementation issues of PA highlighted by Chew et al. [3]. Safety element is typically considered when HAZOP analysis is conducted on a completed HEN design. The consideration of safety at the latter stages of the process lifecycle may lead to the additional investment on add-on safety systems. Among the typical safety issues associated with HEN design are the possibility of contamination, leakage, runaway react ion and even explosion. Therefo re, it would be h ighly efficient to consider safety in the early stage of HEN design by analysing the chemical and physical properties of each heat exchange match. By doing this, the HEN can be made fundamentally safer and less dependent on external safety counter-measures. This is consistent with the inherently safer design concept [4]. The Inherent Safety Index (ISI) includes a wide range of safety assessment indices and factors, making the ISI relat ively mo re accurate [5]. One of the latest index used in inherent safety assessment is the Process Stream Index (PSI) that focuses on the exp losion-related inherent safety assessment. The PSI calculates, compares and priorit ises the level of explosion-related inherent safety of process streams during simulat ion work [6]. To date, research on the inherent safety assessment has yet to be applied to a heat exchanger network. This paper presents a new approach to obtain an inherently safer HEN design. The approach uses the extended graphical STEP (Stream Temperature vs Enthalpy Plot) method [7] that has been integrated with the Inherent Safety Index (ISI) by Heikkila et al. [8]. Note that the proposed new approach can also be extended to the algebraic STEP technique known as the Segregated problem table algorith m (SePTA) [9] to complement the STEP method. 2.0 Methodol ogy This paper extends the methodology by integrating the ISI and the STEP graphical approach in order to obtain an inherently safer HEN design. This procedure is described using Case Study 1. The first step is to collect the process stream data as well as the safety data. The heat capacity flowrate and temperature data were collected fro m the plant mass and energy balances which were obtained from the design data and verified as well as reconciled with the operations data from the plant Distributed Control System (DCS). Information on safety was obtained from the plant chemical material safety data sheet (MSDS). The second step assessing the safety level of the individual process streams using the Inherent Safety Index (ISI) scoring system [8]. The stream is given a score based on their physical and chemical properties, and the total index of each stream is calcu lated by using Equation 1. The considered properties are flammability, exp losiveness, temperature and pressure of the streams. The higher the ISI score, the higher is the hazard probability. IISI = IT + IP + IFL + IEX

(1)

where, IISI = Total inherent safety score, IT = Process stream temperature score, IP = Process stream pressure score, IFL = Process stream flammability score, IEX = Process stream explosiveness score. The next step is to construct hot and cold STEPs. The procedure to construct the STEP is presented in [7]. Instead of using the size of FCp as a criterion to construct the hot and cold STEPs, the streams are matched based on its ISI score. A hot stream with a high ISI score is matched with a cold stream with a high ISI score. Subsequent matches are made by pairing hot and cold streams with the next highest ISI scores. In this way, the hazard fro m h igh ISI score is not distributed throughout the HEN, thereby reducing the total area of hazard.

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The Pinch temperature and the minimu m ut ility targets are determined fro m the constructed STEP in the follow up step. The Pinch temperature is the point where hot and cold STEP pairs touch at the same temperature [1]. The enthalpy overshoot of hot STEPs give the minimu m co ld utility requirement wh ile the enthalpy overshoot of the cold STEPs give the minimum hot utility requirement [7]. The tube side and the shell side fluids are analysed in order to select the best material of construction in the final step. The material of construction selection is selected based on the stream properties (temperature, pressure, stream corrosiveness etc.). In order to calculate the HEN capital cost, the total area of the HEN is obtained first. Depending on the surface area, the type of heat exchanger and the materials of construction, the appropriate correlation is employed to compute the HEN capital cost as demonstrated in [10]. 3.0 Results and Discussion Table 1 shows the data for Case Study 1 and the ISI scoring fo r each stream. Note that streams H2 and C2 give the h ighest ISI score. Th is is due to the high temperature and pressure emp loyed by both streams. The streams also consist of chemicals that are highly flammable and explosive. Before considering the ISI, streams H2 and C2 are d istributed everywhere in the network as shown in Figure 1(a). This has caused the hazard to be distributed throughout the HEN. Th is makes it co mplicated for p lant designers to focus the efforts to mitigate hazards on a specific area of the process. In Figure 1(b), STEP 1 groups all streams with lower ISI scores in one area while STEP 2 groups all streams with higher ISI scores in another area. STEP 3 in Figure 1(b) shows the H1 -C2 stream match. This match is the result of stream splitting to achieve the global Pinch Point (where all hot and cold STEP pairs pinch at the same temperature). Figure 1(b) shows that the hazards from streams H2 and C2 are now concentrated on STEP 2, and are no longer distributed t hroughout the network. Table 2 shows that the capital cost for HEN without inherent safety consideration is USD149, 079. This is 10.42% higher than the HEN that considers inherent safety. The saving has been obtained because concentrating the hazards into a smaller area has managed to reduce the investment on the stainless steel material that was needed for the entire HEN when inherent safety was not considered. Case Study 1 proves that considering inherent safety in HEN can lead to savings. Further savings can be expected fro m the reduced add-on safety features of the HEN design with ISI, as co mpared to the design without the ISI consideration. Shariff et al. [6] highlighted that, in avoiding hazards, critical streams must be identified and considered for imp rovement. Therefore, inherent safety has to be considered in order to reduce the risk of loss of production and capital, on top of the need to conserve the resources and protect the environment . T able 1 Data for Case Study 1 Stream H1 H2 H3 C1 C2 C3

Heat Capacity Flow Rate, FCp (kW/°C) 30 15 10 40 20 10

Supply T emp (°C) 170 150 60 80 20 20

T arget Temp (°C)

Pressure (bar)

Flash T emp (°C)

60 30 30 150 135 80

5 30 5 1 30 5

-187 -

Explosive limit (vol %) 4 – 75 16 – 20 5 – 15 -

T otal Index Score 3 8 2 2 8 2

T able 2 Capital Cost for HEN with and without considering inherent safety.

Capital Cost

Considering Inherent Safety (USD) 133, 538

Without Considering Inherent Safety (USD) 149, 079

Percentage Difference (%) 10.42

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(a)

Without considering inherent safety

(b) Considering inherent safety Fig. 1. ST EP for Case Study 1

4. Conclusion A new method to design an inherently safer HEN has been developed. By incorporating the inherent safety index in the STEP-guided graphical select ion of heat exchanger matches, the potential hazards of the optimal HEN design have been reduced. Case Study 1 shows that inclusion of the inherent safety feature has managed to localise the area of hazards, and ultimately reduces the HEN capital cos t by 10%. Acknowledgements The authors would like to thank UTM for providing the research fund for this project under the Vote No. Q.J130000.2427.00G56 and fro m the Hungarian State and the European Union project TAMOP-4.2.2.A11/ 1/ KONV-2012-0072 – Design and optimization of modernization and efficient operation of energy supply and utilization systems using renewable energy sources and ICTs., and fro m the Slovenian Research Agency (Program No. P2-0032). References

[1] Klemeš, J. J. (Ed). Handbook of Process Integration: Minimisation of Energy and Water Use, Waste and Emissions. Woodhead Publishing Limited/Elsevier, Cambridge, UK 2013. [2] Escobar M., Trierweiler J.O., Grossmann I.E.. Simultaneous Synthesis of Heat Exchanger Networks with Operability Considerations: Flexibility and Controllability. Computer and Chemical Engineering 2013; 55:158-180. [3] Chew, K. H., Klemeš, J., Wan Alwi, S. R., Manan, Z. A.. Industrial Implementation Issues of Total Site Heat Integration. Applied Thermal Engineering 2013; 61: 17-25. [4] Shariff A. M., Wahab N. A., Inherent Fire Consequence Estimation Tool (IFCET) for Preliminary Design of Process Plant. Fire Safety Journal 2013; 59: 47-54. [5] Rahman, M., Heikkilä, A. M., Hurme, M., Comparison of Inherent Safety Indices in Process Concept Evaluation. Journal of Loss Prevention in the Process Industries 2005; 18: 327-334. [6] Shariff, A.M., Leong, C. T ., Zaini,D.. Using Process Stream Index (PSI) to Assess Inherent Safety Level during Preliminary Design Stage. Safety Science 2012; 50: 1098-1103. [7] Wan Alwi, S. R., Manan, Z. A., STEP—A New Graphical Tool for Simultaneous Targeting and Design of a Heat Exchanger Network. Chemical Engineering Journal 2010; 162(1): 106-121. [8] Heikkilä, A. M., Inherent Safety in Process Plant Design. PhD Thesis, Helsinki University of Technology, Finland; 1999. [9] Wan Alwi, S. R., Manan, Z. A., Misman, M., Chuah, W. S., SePTA—A New Numerical Tool for Simultaneous Targeting and Design of a Heat Exchanger Networks. Chemical Engineering Journal 2013; 57: 30-47. [10] Seider, W. D., Seader, J. D., Lewin, D. R., Wildago, S., Product and Process Design Principles: Synthesis, Analysis and Evaluation. 3 rd ed. Wiley & Sons, Hoboken: New Jersey, USA; 2007.


Biography of Presenter Associate Professor Ir Dr Sharifah Rafidah Wan Alwi is the Director of Process Systems Engineering Centre (PROSPECT) of Un iversiti Teknologi Malaysia (UTM ). She is also a Certified Energy Manager, a trainer for Energy Management Certificat ion and an En ergy Professional for Malaysia. Sharifah has been involved in 36 R & D and consultancy projects for various companies and government agencies and has trained engineers fro m more than 100 co mpanies in the field of e nergy and water minimisation. She specialises in process systems engineering with emphasis on resource conservation. Dr Sharifah has more than 120 publications consisting of refereed journals, conference papers, magazine art icles, book chapters, monographs and patents.

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