A structured approach to heat exchanger network retrofit design

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(3) A Structured Approach to Heat Exchanger Network Retrofit Design. Proefschrift. ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties, in het openbaar te verdedigen op dinsdag 9 september 2008 om 12.30 uur door. Jozef Lambertus Benjamin VAN REISEN scheikundig ingenieur geboren te Vlaardingen.

(4) Dit proefschrift is goedgekeurd door de promotor: Prof. ir. J. Grievink Copromotor: Dr. ir. P.J.T. Verheijen. Samenstelling promotiecommissie: Rector Magnificus, Prof. ir. J. Grievink, Dr. ir. P.J.T. Verheijen, Prof. dr. H.L.M. Bakker, Prof. dr. ir. H. van den Berg, Prof. ir. M.W.M. Boesten, Prof. dr. T. Gundersen, Prof. dr. G.T. Polley,. voorzitter, Technische Universiteit Delft, promotor, Technische Universiteit Delft, copromotor, Technische Universiteit Delft, Universiteit Twente, Rijksuniversiteit Groningen, Norges Teknisk-Naturvitenskapelige Universitet, Norway, Universidad de Gaunajuato, Mexico.. Published by J.L.B. van Reisen, Vlaardingen An electronic version of this thesis is available from http://www.library.tudelft.nl E-mail author : J.L.B.vanReisen@TUDelft.nl. ISBN 978-90-8891-0555 Keywords: heat exchanger, heat exchanger network, heat exchanger types, conceptual design, retrofit, energy saving, methodology, targeting.. © 2008 by J.L.B. van Reisen All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilised in any form or by any means, electronic or mechanical, including photocopying , recording or by any information storage and retrieval system, without written permission from the author. Printed by BOX Press, Oisterwijk, The Netherlands.

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(7) v. Summary Energy saving is an important issue for both society and industries due to the increasing energy costs and the environmental concerns of climate change that require a substantial reduction of the global carbon dioxide emissions. Reduction of the energy consumption on short term requires the adaptation of existing installations (retrofit) to maintain their present value, in terms of money invested and energy captured. This thesis addresses energy saving retrofit, a specific retrofit class that aims for a reduction of the energy costs while maintaining the existing plant functionality. The work focusses on the retrofit design of heat exchanger networks (HEN), the main heat processing systems of chemical plants. Retrofit design has to combine an efficient reuse of the existing equipment and optimal addition of new equipment to get an affordable performance improvement. Advances in retrofit design methods and in new heat exchanger types, published in the past decennia, may improve the network retrofit potential, but are actually only applied occasionally. This thesis aims for a HEN retrofit design approach suitable for practical cases and work processes, including a systematic exploration of the potential of different heat exchanger types. The focus is on steady state, continuous processes with one operation mode. It involves the following research objectives: • define a general reference for the systematic definition of specific HEN design problems from a structured review of the general design problem; • get an appraisal of the network analysis and design methods available in literature; • establish a structured practical conceptual analysis and design method with exchanger type selection included. Heat exchanger network retrofit design is complex. It involves many case-specific details and constraints related to the existing equipment and operating practice. Additionally, the objective of retrofit design is generally only defined as a direction for improvement and may result in design alternatives ranging from operational improvement to essential network replacement. Determination of the optimum level of integration is part of retrofit design. This complexity is hardly recognized in current design methods that are mainly based on greatly simplified design problems. An extended problem analysis is given that shows the scope, the variables and the criteria for design. Additionally, their dependencies are discussed in the most extended form, based on a combination of heat transfer fundamentals, design standards and experiences. This description is a reference for a systematic definition of case-specific design problems and the selection of essential design variables and criteria, to make the design problems manageable and to select adequate design methods, depending on the designer’s background and work environment. A new general HEN retrofit design framework is proposed that takes into account the specific complexity of retrofit design. The approach is based on the generic design cycle and the general stage-wise process design framework of Siirola (1996). It guides the designer to master the design problem and control the design process. The design framework contains eight design phases starting with need identification and ending with the production and maintenance plan for the facility. Each design phase is divided in four stages: target, preliminary design, refined design and final design with increasing scope and detail. This thesis focuses on the third phase, the conceptual network design,.

(8) vi. which includes the main conceptual design activities that are generally regarded as the core network design. The four stages of this phase cover: • Target stage: Find the ultimate energy saving scope using grassroots targeting methods; • Preliminary Design stage Estimate the real scope, alternative saving options and the related main changes of the network using retrofit targeting methods; • Refined Design stage: Initial conceptual network design, a complete design in outlines; • Final Design stage: Final conceptual network design: the refined design verified and finalized with all necessary detail. An extensive structured literature review has been done to find the analysis and design methods that can be used in the given four design stages. They have been divided in three application groups: • Network Performance Analysis to determine the effectivity of designs, used in all four stages, • Targeting to support the design task of both the target and preliminary design stages with grassroots and retrofit targeting methods respectively, and • Network Design used in the refined design stage to create conceptual network designs. The available methods are outlined, classified and compared with available alternatives. Tables are provided to select adequate methods based on the relevant variables and criteria selected for a specific case, using the above-mentioned new HEN design problem analysis. The available performance analysis and grassroots targeting methods are adequate for conceptual network design. The retrofit targeting methods include the main design variables, but lack accuracy and the ability to include network structure and multiply utility levels. The network design methods are satisfactory, except for the limited focus on exchanger location, type and configuration details. Two new methods are proposed to overcome some limitations in the existing methods: Structural Targeting for retrofit targeting and the Retrofit Thermal Shifting Procedure for network design. Structural Targeting is a retrofit method that determines targets for utility use, number of units, exchanger area and topology based on saving, investment and complexity trade offs. The approach uses integrity zones based on the existing network to trade off saving potential and the need to integrate originally independent network parts. New methods are proposed to do area and saving-oninvestment targeting for cases with multiple utility levels. The Retrofit Thermal Shifting Procedure is a network design method that creates opportunities for the application of more efficient, advanced (multi-stream) heat exchangers. The method guides the adaptation of the heat transfer task of the existing exchangers to concentrate the required new area, generally at the locations with the lowest temperature differences. It gives guidelines for exchanger shifting, the minimisation of the number of tie-ins and for stream splitting. The new methods were effectively applied to two example cases, a simplified crude preheat train and an aromatics plant network. The entire new design approach for the conceptual network design phase, including the new analysis and design methods, was demonstrated with an industrial C2 C3 C4 separation section case study. The new design methods were found effective to identify an optimum saving scope and various independent simple retrofit options. The results of the research meet most of the aforementioned three research objectives. The review of the general HEN design problem is found to be a useful reference to determine a case specific problem.

(9) vii. definition with relevant design variables and criteria. The combination of this design problem review and the newly developed design framework gives a solid basis for the appraisal of the available literature on analysis and design methods. The new structured practical conceptual analysis and design method has proven suitable for general use, given its effectivity for a number of typical case studies..

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(11) Contents. ix. Contents Summary. v. 1 Introduction 1.1 Energy Conservation Incentives 1.2 Systematic Design Methods 1.3 Advances in Heat Exchanger Design and Equipment 1.4 Scientific Challenges 1.5 Research Objectives 1.6 Thesis Content and Set-up. 1 2 4 11 13 14 16. 2 The Heat Exchanger Network Retrofit Design Problem 2.1 Introduction 2.2 The Heat Exchanger Network Design Problem 2.3 Heat Transfer Fundamentals 2.4 Design Variables and Dependencies 2.5 Retrofit Design 2.6 Design Criteria 2.7 Applied Problem Definition. 21 22 22 26 31 40 47 52. 3 New Framework for Basic Retrofit Design 3.1 Introduction 3.2 Existing Design Approaches 3.3 New Retrofit Design Framework 3.4 Detailed Scope Definition of the Conceptual Network Design Phase in Retrofit. 55 56 57 61 66. 4 Network Performance Analysis 4.1 Introduction 4.2 Structure Visualisations 4.3 Composite and Driving Force Plots 4.4 Economic and Ecological Performance 4.5 Efficiency Numbers 4.6 Method Evaluation. 73 75 76 78 86 91 97. 5 Targeting Method Review 5.1 Introduction 5.2 Overview of Existing Methods 5.3 Basic Grassroots Targeting Methods 5.4 Retrofit Targeting Methods 5.5 Target Optimisation 5.6 Evaluation. 101 102 104 110 118 120 122.

(12) x. Contents. 6 Retrofit Targeting with Integrity Zones and Multiple Utilities 6.1 Introduction 6.2 Retrofit Structural Targeting 6.3 Retrofit Targeting with Multiple Utilities 6.4 Case Study: Aromatics Case. 131 132 133 146 153. 7 Conceptual Retrofit Design Method Review 7.1 Introduction 7.2 Existing Methods Overview 7.3 Grassroots-based Retrofit Design Methods 7.4 Design Methods Based on Retrofit Targets 7.5 Evolutionary Network Screening Methods 7.6 Mathematical Optimisation Methods 7.7 Evaluation of the Existing Design Methods. 159 160 161 166 166 168 170 173. 8 Retrofit Design with Alternative Exchanger Types 8.1 Introduction 8.2 Example Problem 8.3 Network Design Guidelines 8.4 Retrofit Thermal Shifting Procedure for Refined Network Design 8.5 Example Case: Simplified Crude Preheating Train. 181 182 183 185 197 201. 9 Case Study 9.1 Introduction 9.2 Design Basis 9.3 Target Stage 9.4 Preliminary Design Stage 9.5 Refined Network Design Stage 9.6 Evaluation. 215 216 216 218 221 227 232. 10. 235 236 242 244 247 249. Evaluation: Results and Prospects 10.1 Evaluation Based on Research Questions 10.2 New Method Evaluation 10.3 Evaluation Research Work Approach 10.4 Conclusion 10.5 Further Research. 11. Literature. 253. 12. Nomenclature. 271.

(13) Contents. xi. Appendix A. Summary of Assumptions and Limitations. 277. Appendix B. Glossary. 281. Appendix C. Auxiliary Heat Flow Curves. 287. Appendix D. Cross-reference Design and Review Variables. 291. Appendix E. Saving on Investment Relation. 293. Appendix F. Effect of Thermal Shifting and Splitting on FT. 303. Appendix G. PHITS Targeting and Analysis Software. 309. Appendix H. Case Details. 317. Samenvatting. 333. Dankwoord (Acknowledgement). 337. Curriculum Vitae. 339.

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(15) Introduction. 1. Chapter 1 Introduction This introductory chapter gives the context of the research in this thesis. It first describes the main trends in society and industries to reduce the use of energy resources. Next, it gives a brief overview of the main developments in process synthesis in general and heat exchanger network synthesis in particular. Additionally, it outlines the advances in the heat exchanger equipment available. Finally, it gives the relevance, the research objectives and the set-up for this thesis..

(16) 2. Chapter 1. 1.1 Energy Conservation Incentives Energy conservation has been an important topic for the society as a whole and for industries in particular since the early seventies. The first incentive was the availability and cost of energy resources both on the short and on the long term. Currently, the high energy costs are again an important driver, but additionally energy conservation is pushed by the demand for a reduction of carbon dioxide emissions. These emissions are mainly due to the use of fossil energy sources. There is growing evidence that an increased carbon dioxide content of the atmosphere causes enhanced global warming (International Panel of Climate Change (IPCC), 2007). To mitigate this global warming, carbon dioxide emissions have to be reduced substantially, which requires a reduction of the use of fossil energy resources. Alternatively, special techniques can be applied, like carbon dioxide capturing, that will further boost the energy costs. The alarming scientific reports of the potential consequences (IPCC, 2007) and campaigns to change the public opinion by personalities like Gore (2006), supported by the award of the Nobel Peace Prize 2007 to both the IPCC and Gore, have recently pushed the worldwide political willingness to invest in this emission reduction. Many governments, including the Dutch and the European Union, have set ambitious commitments to reduce carbon dioxide emissions. They will force their citizens and home industries in near future to increase their efforts and investments to meet these commitments.. ANA LY. S. ECONOMY & ECOLOGY CHANGES. PROCESS ADVANCES. IS. ,D. ES IGN & C. PLANT. RO L NT O. Process industries, that are traditionally very energy demanding, will have to contribute substantially to such a demand for emission reduction. In the past the Dutch government have agreed covenants with industries to reduce their emissions and save on use of fossil energy resources. New covenants, encouraging measures and controlling laws will be necessary to meet the European Union commitment to reduce the carbon dioxide emission by 20% in 2020 compared to 1990 (Council of the European Union, 2007). It will require substantial investments to apply state-of-the art technology where possible. Additionally, further research is necessary to find and develop new ways to increase the energy efficiency of processes. These include enhanced analysis, design and control techniques, new process routes and enhanced equipment, Figure 1.. EQUIPMENT ADVANCES. Figure 1 Drivers for plant performance improvement. Obviously, industries will continue their business and aim at maximum profitability with the lowest total cost of ownership. Any investment has to pay off and should not undermine the competitiveness. Increased energy efficiency reduces the operating costs but often at the expense of additional capital costs. The willingness to invest in energy efficiency will increase with higher energy costs, increased energy taxes, less secure energy availability and client or public opinion demand. Investments that will reduce the cost per unit product can generally be justified. Investments that give no or little reduction of cost per unit product are only viable when the less efficient alternative is abandoned by governmental restrictions or when energy efficiency coincides with other product improvements..

(17) Introduction. 3. Anyway, it is necessary to find the most efficient investments to get the desired reduced energy consumption, to meet the requirements both for individual companies and for the long term future of society as a whole. Opportunities to reduce the energy demand of an industrial plant exist throughout the lifetime of such plant (Grant, 2005, Bakker, 2004). Most cost-effective is energy efficiency enhancement during the grassroots design of the plant. The sooner energy efficiency is addressed during the initial design the lower the associated costs. Unfortunately, resource, knowledge, schedule and cost constraints together with design conservatism and conflicting requirements, generally limit the actual energy efficiency of the new plant. Once built, plant experience, changed markets and regulations and new technologies drive plant improvements and open opportunities to reduce the energy demand or at least the demand per unit product. There are three alternatives to get a better energy efficiency: improve plant operation, adapt the plant (retrofit) or replace it by a new plant, Figure 2.. New Plant. Improved Operation. Saving. Retrofit. New Processing Unit. Site Integration (Utilities, Heat & Power). Heat Exchanger Network Retrofit Equipment Items Retrofit or Replacement Improved Control Energy Management Investment. Figure 2 Schematic overview of the relative saving scope and relative investment for various plant improvement options. The first option, improved operation, relates to good energy management and adequate plant control. Energy management is the systematic monitoring and control of energy flows in the plant. It is essentially a combination of attention to the issue, insight in the process and proper information for and training of the operators. Adequate plant control requires good control algorithms and tuning together with adequate measurements and controllers. For both good energy management and adequate control, the required investments are low but the revenues can be large in some cases. The second option, retrofit of the plant, allows a further reduction of the energy consumption by changing the hardware of the plant. This can range from local equipment changes to full replacement of a processing unit. The cost of such adaptations are, generally, considerably high. Apart from the purchase of new equipment and the construction, also the loss of production to incorporate the plant modification contributes substantially to the cost of plant retrofit. The revenues must justify these expenses and thus the projects must carefully be selected. The third option, a new plant, is the most radical one. It has the advantage of allowing all opportunities that exist in grassroots design, but makes the existing investments worthless. On the other hand, production loss is generally limited with this option. Generally, plant or processing unit replacement is not economically feasible with the energy.

(18) 4. Chapter 1. conservation incentive only, but it may be a feasible option in case of a capacity increase or a product quality improvement. Both in new and retrofit design, progress is made in finding less energy demanding alternatives due to new design methods and the availability of new equipment. New design methods allow a more systematic generation and evaluation of alternatives. Much work on design methods is reported as part of Process Systems Engineering or Process Synthesis (Grossmann and Westerberg, 2000, Westerberg, 2004), but especially literature about energy efficiency is also labelled as Process Integration or Heat Integration (Dunn and El-Halwagi, 2003). Impressive progress in design methods for more energy efficient processes has been made especially in the last two decades of the twentieth century, as can be seen in the overview in the next section. Most methods deal, however, with grassroots design. The progress in retrofit design is also significant, but the scientific interest for this topic is limited compared to its industrial relevance, as many projects in industries are retrofit projects (Westerberg, 2004). Despite all progress, the application of systematic design methods in everyday’s engineering practice is limited (Barnicki and Siirola, 2004). The complexity of many design problems and most design methods require expertise and development time, that is often not available (Butterworth, 2004). Instead of inventing a new design, engineers tend to select the best-fit-of-theshelf solution. For retrofit this practice is less applicable and engineers are forced to elaborate a tailormade solution. Simple systematic retrofit design methods still need to be developed to facilitate this labourious task. New equipment can make new operations technically or economically feasible. The next section also shows some trends in the development of heat exchanging equipment, like compact and compact multi-stream heat exchangers. Most enhanced equipment is designed for special purposes, but in the past decade wider application and new designs have been investigated especially in the research area of Process Intensification (Stankiewicz and Moulijn, 2004). Process design methods have two roles in the introduction of new equipment (after Bakker, 2004). Initially, fundamental process design methods may identify some desired features of new equipment that is still to be designed. It can show the (economical) scope of the availability of a specific operation. New equipment can be developed for that operation, if this scope is sufficiently attractive. Secondly, process design methods can identify new opportunities for available (new) equipment in specific process design problems.. 1.2 Systematic Design Methods 1.2.1 General Process Design Methods Research on systematic process design or process synthesis investigates new process design methods to integrate processing units and processing tasks and their applications. Since its foundation in the 1960s (eg. Rudd and Watson, 1968), it has become an important area in process engineering. Reviews of the total field of process synthesis have been published by, among others, Nishida et al. (1981), Gundersen (1991) and Westerberg (2004). The fundamentals are well described in text books by Douglas (1988), Biegler, Grossmann and Westerberg (1997), Turton et al. (2003), Seider et al. (2004) and Smith (2005). Process Synthesis literature describes five main topics as shown in Table 1 (after Nishida et al., 1981 and Westerberg, 2004) or combinations thereof. Each topic is briefly described below..

(19) Introduction. 5. Table 1 Main topics of Process Synthesis PROCESS SYNTHESIS Object Class. Model. Synthesis Method. Evaluation Method. Work Process. • reactor paths • separation systems • heat exchanger networks • heat & power systems (utility systems) • complete flowsheets • mass exchanger networks • batch processes • control systems • processing tasks. • object representation • design space / variables - efficiency - economy - safety - flexibility - controllability - operability - reliability - availability - maintenance - uncertainty - retrofit • superstructures • abstractions • design space representations. • evolutionary • heuristic • hierarchical / decomposition • mathematical optimisation • artificial intelligence • target based. • criteria • objective (see design space under Model) • alternative selection methods. • mental design process • information representation • project execution • inter-discipline work. The Object Classes are the main objects that can be subject to process synthesis. These include all main parts and functions of a processing plant. In recent literature, there is increasing focus on processing tasks to represent plant functions, defined in terms of thermodynamic and transport processes, rather than on traditional unit operations. This allows the invention of new processing units. Additionally, the focus shifts to smaller entities on micro (including micro-organisms) and nano scale, also entering the area of Process Intensification (Stankiewicz and Moulijn, 2004, Bakker, 2004). The used Model of the object is the used definition and demarcation of the design space, including the specification of the relevant design aspects and variables considered for the object. Much work is done to enhance process modelling and to make models more accessible (Barnicki and Siirola, 2004). Many models have become equation-based instead of unit-based and dynamic modelling has become more state-of-the-art with tools like Aspen HYSYS Dynamics (2007) and gPROMS (2007). Optimisation based synthesis methods require the definition in advance of the set of all possible options, captured in a superstructure. The definition of adequate superstructures is the key to finding good solutions (Westerberg, 2004) and efforts are ongoing to define better ones in terms of captured solutions, captured details and easiness to solve. On the other hand, there are efforts to capture the key aspects of the design problem in understandable abstractions and (graphical) representations, that facilitate the designer to understand and explore the design space. These include the construction of attainable regions in reaction path analysis (Feinberg, 2002) and residue curve maps in separation analysis (Siirola, 1996). The Synthesis Method is the third topic of Process Synthesis (Table 1). It is the actual method to generate design alternatives systematically. The synthesis methods can be divided in six classes (after Nishida et al. 1981 and Gundersen, 1990):.

(20) 6. Chapter 1. • Evolutionary methods: a step-wise modification of a previously synthesised design leading to an improved design. • Heuristic methods: generate new alternatives using rules from common sense and experience. • Decomposition or hierarchical methods: decomposition of the design into subproblems which are simpler to solve. • Mathematical optimisation or algorithmic methods: extract the optimal alternative from a superstructure that includes all relevant alternatives by mathematical optimisation techniques. This class also includes structural parameter optimisation methods, tree-search and bounding methods and simulated annealing methods. • Artificial intelligence methods: use knowledge that can be retrieved systematically from an experts database. • Target-based methods: generate designs from targets and ideal designs obtained from the analysis and optimisation of simplifications of the design problem.. Two classes of the above-mentioned synthesis methods have especially developed during the past decades. The target-based methods have developed from an algorithmic / evolutionary method for heat exchanger networks, known as pinch technology, to a complete design approach (Smith, 2005). After heuristics, this method class has become the most generally applied in industries. Mathematical optimisation methods have developed along with the development of computing power and of more efficient problem formulations and optimisation algorithms (Biegler et al. 1997, Biegler and Grossmann, 2004, Westerberg, 2004). Currently, many real-size design problems can be solved on common PC’s. Much work is done on the interface of the synthesis method classes. Decomposition and targeting are used to create manageable mathematical optimisation problems (Westerberg, 2004) and mathematical optimisation is used to calculate complex targets (Briones and Kokossis, 1999a,b,c, Asante and Zhu, 1997) and to optimise parts of the design. The Evaluation Methods, the next main topic in Process Synthesis literature, cover the specification of criteria and objectives, the analyses to evaluate these criteria and objectives and the methods to evaluate and select the best alternatives. For many of the design aspects and variables mentioned under the topic Model, performance indicators have been defined to allow evaluation, both numerically and graphically (Westerberg, 2004). Analysis and evaluation approaches have been developed to include uncertainty, multiple base cases and multiple objectives. Additionally, there is a trend to identify (pareto) sets of solutions rather than a single optimum. The literature on Work Processes, the last topic mentioned in Table 1, investigates the improvement of the design process itself taking into account the needs of the designer to perform his task (data, tools), limited resources, project requirements, changing environments and information flow (Siirola, 1996, Biegler et al., 1997, Westerberg, 2004, Marquardt and Nagl, 2004). Developments in this area are generally the most visible for the average designer - the author has experienced that himself during his work within an engineering and contracting company in the past ten years -, as these are the basis for the developments in software available for the design of process units. Companies like Aspen Tech (Tipton et al., 2007) and Invensys SimSci-Esscor nowadays have application suites with an integrated environment to do process synthesis, simulation, detailed equipment design, Piping and Instrumentation Diagram development and information and document control. Information standards.

(21) Introduction. 7. like Cape Open allow applications to share information and models. Graphical interfaces try to guide the designer through the overwhelming number of possibilities. IT developments enable to easily share information with colleagues and other disciplines in-house and at the other side of the world. Meanwhile, the designer can access from his desk more information than ever before. 1.2.2 Retrofit Design Methods The previous section mentions retrofit design as a special class of process design. Actually, retrofit design, the redesign of existing installations to incorporate new technology or equipment to meet revised performance requirements as opposed to new or grassroots design, is a very important class. The major part of the design work in Western Europe and Northern America is related to plant retrofit (Barnicki and Siirola, 2004). Retrofit is a special design class with specific options (Figure 2) and numerous constraints. Grossmann et al. (1987) and Gundersen (1990) published general reviews on retrofit process design, including clear description of the retrofit design problem. The mentioned reviews list a number of reasons to retrofit a plant: • Increase throughput (debottlenecking); • Process a new feedstock or make a new product; • Improve product quality; • Reduce the energy consumption (energy saving retrofit); • Implement a new technology; • Improve process safety or reduce environmental impact; • Improve process operability, controllability, flexibility or maintenance. Both reviews stress the differences between retrofit and grassroots design: • Grassroots design can much easier be divided in conceptual phase and detailed phase. In the conceptual phase equipment details generally can be ignored. In retrofit design it is often very hard to make this decomposition as the existing equipment must be taken into account in any stage of design. • In grassroots design, many design alternatives generally exist near an optimal design that have similar economics. In retrofit, this is often not the case, as the existing equipment must be reused as much as possible and structural modification tend to raise the costs very rapidly. • Detailed rating models are required in retrofit to test the performance of the existing equipment, especially when the new operating conditions of that equipment will be rather different. • Retrofit design generally has to take into account specific constraints such as space limitations within the plant that does not allow new equipment to be installed. It is remarkable that after the mentioned two reviews no new review on general retrofit design is published. Recent general reviews on process synthesis (Westerberg, 2004, Barnicki and Siirola, 2004) just mention the importance of retrofit, as many projects are retrofit, but fail to give a review of developments. General text books on process systems design treat retrofit in a similar way, if mentioned at all. There is a significant imbalance between the importance of the topic and the research reported. A quick exploration of the recent work in the field, shows that the core of publications on retrofit design report case studies and no generally applicable methods. The literature on heat exchanger networks does include some retrofit design methods. These are discussed in the next section..

(22) 8. Chapter 1. 1.2.3 Methods for Heat Exchanger Network Design Heat exchanger network design is a major class within process synthesis. It is one of the shells of the onion model of process design used by Smith (2005) and Linnhoff et al. (1994), see Figure 3. The heat exchanger network balances the heat deficits (sinks) and surpluses (sources) within the process with the efficient use of utilities as external heating and cooling resources. Prior to heat exchanger network design the heat and mass balances of the process need to be fixed. Reactor This requires determination of the reaction units, the separations and the recycles. It is Separation and mainly executed before the design of the Recycle System utilities systems, but there is a strong Heat Recovery System dependency with these systems. Quite often the design of the heat exchanger network and the Heating and Cooling related utility systems is combined. The basics Utility System of heat exchanger network design is well Water and Effluent described in a number of text books, including Treatment System Smith (2005), Kemp (2007) and Shenoy (1995). A very complete description is also available Figure 3 Onion diagram for process design from Gundersen (2000). decomposition from Smith (2005) The literature on heat exchanger networks is massive. An annotated bibliography by Furman and Sahinidis (2002), covering the literature of the 20th century, contains 461 references. From 2000 to 2006 the Web of Science reports about 240 more references. The mentioned review gives some classification of the publications, but lacks a thorough review of the state-of-the-art. For such a review we have to go back to the reviews by Gundersen and Naess (1988) and by Jezowski (1994a,b). Heat exchanger network methods are mainly target-based or use mathematical optimisation. The target-based design methods use the concepts of (grand) composite curves and heat recovery pinch points as introduced by Hohmann (1971), Umeda et al. (1979) and Linnhoff and coworkers (Linnhoff and Flower, 1978a). Their simplification of the second law of thermodynamics analysis and the related graphical presentations pushed systematic heat exchanger network design into industrial practice (Dunn and El-Halwagi, 2003). The initially used minimum utility and minimum number of units targets have been extended with targets for minimum transfer area and total network cost (Ahmad and Linnhoff, 1984). These targets enabled trading off network cost and operating (utility) cost prior to heat exchanger network design, referred to as supertargeting. The first mathematical formulation of the heat exchanger network design problem for mathematical optimisation was given by Masso and Rudd (1969). Since this publication numerous more extended nd more efficient models have been published. Key contributions are from Cerda et al. (1983), Cerda and Westerberg (1983), Papoulias and Grossmann (1983) for the optimisation of utilities and number of heat transfer units and from Floudas et al. (1986), Floudas and Ciric (1989), Ciric and Floudas (1991), Yee and Grossmann (1990) and Yee et al. (1990) for full heat exchanger network synthesis..

(23) Introduction. 9. Table 2 Selected references for special issues in grassroots heat exchanger network design with focus on the literature after 2000. See Furman and Sahinidis (2002) for a complete bibliography of the literature on various topics until the year 2000. Special Issue. References. Reactors. Kravanja and Glavic, 1989. Distillation Columns. Linnhoff et al. 1983, Smith and Linnhoff, 1988. Furnaces. Hall and Linnhoff, 1994, Jegla et al. 2000. Heat Engines, Heat Pumps and Refrigeration Systems. Kravanja and Glavic, 1989, Linnhoff and Dhole, 1992a,b, Fonyo and Benko, 1996, Gomes and Wolf Maciel, 1996, Kimura and Zhu, 2000. Evaporator Systems. Urbaniec et al., 2000. Process Changes. Linnhoff et al., 1988. Optimal Operation. Glemmestad et al., 1999. Operational Flexibility. Kotjabasakis and Linnhoff 1986, 1988, Cerda et al. 1990, Cerda and Galli, 1990, and Galli and Cerda, 1991, Papelexandri and Pistikopoulos, 1993a,b, 1994a,b,c, Tantimuratha and Kokossis, 2004, Verheyen and Zhang, 2006, Picón-Núñez and Polley, 1995, Bochenek and Jezowski, 1999. Controllability or Resilience. Saboo et al. 1986a,b, Colberg and Morari, 1988, Yan et al, 2001. Detailed Exchanger Design. Polley and Linnhoff, 1988, Polley and Panjeh Shahi 1991 and 1996, Mizutani et al., 2003a,b, Ravagnani et al., 2003. Different Exchanger Types. Ahmad et al, 1990, Hall et al. 1990, Polley and Haslego, 2002a,b, Pua and Zhu, 2002, Sorsak and Kravanja, 2002a,b, 2004, Stehlik and Wadekar, 2002, Wang and Sunden, 2000. Multi-Stream Heat Exchangers. Yee et al. 1990, Wang and Sunden, 2001, Picón-Núñez et al. 2002 and 2006. Pressure Drop Constraints. Panjeh Shahi, 1992, Panjeh Shahi and Khoshgard, 2006, Nie and Zhu, 1999, Frausto-Hernandez et al. 2002, Picón-Núñez et al. 2006. Heat Transfer Enhancement. Polley et al. 1992, 1994, Nie and Zhu, 1999, Zhu et al. 2000, Zimparov, 2002. Heat Exchanger Fouling. Fryer et al. 1987, Kotjabasakis and Linnhoff, 1987, Wilson et al. 2002, Polley et al., 2005, Yeap et al, 2004, 2005. Generic Constraints (Safety, Operation, Etc.). Ahmad and Hui, 1991.

(24) 10. Chapter 1. Since the foundation of the basic approaches in both method groups, much work is published on more extended design objects, other design objectives and special design considerations similar to the ones mentioned for general process synthesis given in Table 1. The main developments with selected references are given in Table 2. The retrofit of heat exchanger networks has been subject of publications since the work of Tjoe and Linnhoff (1984 and 1986), who provided retrofit design targets, network analysis tools and a modification strategy for energy saving retrofits. Ahmad and Polley (1990) extended this work to the debottlenecking (capacity increase) case and Polley and Panjeh Shahi (1990) and Polley et al. (1990) to take into account pressure drop constraints. Asante and Zhu (1996 and 1997) introduced the concept of network pinch and a linear programming approach to find options to modify the network structure to increase the heat recovery potential. Since then, a number of extensions and applications of this method have been reported. The first mathematical formulation of the heat exchanger network retrofit problem is published by Yee and Grossmann (1987 and 1991) and Ciric and Floudas (1990a,b). These formulations have been extended and revised, in a similar way the grassroots formulations have developed, to include special issues like special exchanger types, heat transfer enhancements and the retrofit of multi-stream exchangers (Bulatov, 2005). The number of publications is however much more limited than for grassroots design. A complete overview of the retrofit design methods for heat exchanger networks is given in Chapter 7, whereas targeting methods that support retrofit design are reviewed in Chapter 5. Exergy is an important concept in thermodynamics to qualify the usefulness of heat and is also used to improve process design, often referred to as second-law analysis, thermo-economics (El Said, 2003 and Valery et al. 2006) or exergo-economics (Sciubba, 2005). The pinch based analyses mentioned above are essentially simplifications of the exergy concept, that covers the principal quality component (temperature) for heat exchanger networks. Exergy has been used for the design of heat exchanger networks (Sama, 1995a,b and 1996, Sciubba et al. 1999), but these applications seem rather academic, as there is hardly any added value compared to the much easier pinch-based methods. Most literature on thermo- or exergo-economics covers the design of entire processes including the heat exchanger network. Especially reported is the design of the heat exchanger network in relation to separation system design (eg. Taprap and Ishida, 1996) and heat and power system design (eg. Feng and Zhu, 1997, and Kimura and Zhu, 2000). For the latter a pinch-like approach is also available that uses the Carnot factor instead of temperature in (grand) composite curve plots (eg. Dhole and Linnhoff, 1993b and Staine and Favrat, 1996)..

(25) Introduction. 11. 1.3 Advances in Heat Exchanger Design and Equipment 1.3.1 Heat Exchanger Design The advent of still growing personal computer power has dramatically changed the heat exchanger design process (Butterworth, 2004). The main software packages in the field from HTRI, HTFS and BJac give easy access to generally accepted models for shell-and-tube heat exchangers that can be used for design and optimisation. More special features for common exchangers and more exchanger types have come available in these tools. Additionally, some vendors of special heat exchange equipment, like Alfa Laval, have made available their models for heat exchanger design through special software, to allow independent designers to explore the possibilities of these exchangers. The interface between the heat exchanger design packages and process simulation software is also significantly improved and even allow to include the exchanger design and rating models seamlessly in any process simulation. If the above-mentioned generic design packages are insufficient, designs can be checked and optimised using Computational Fluid Dynamics (CFD) analysis. Besides, better models to estimate fouling have come available, that allow - though still to a limited extent - a reduction of the generally applied design margins on transfer area. As a consequence, heat exchanger design can be better fit for purpose nowadays and the performance of existing equipment and designs can be estimated more accurately. General applicable design margins can (partly) be replaced by specific performance checks for all relevant design cases. Generally, this will make the equipment cheaper and the design more reliable. 1.3.2 Heat Exchange Equipment The shell-and-tube exchanger is by far the most applied exchanger type in the process industry. Despite the many alternatives, this type is still the most selected one for new equipment (Reay, 1994, Butterworth, 2004). The design of shell-and-tube exchangers is well described in general text books like Kakaç and Liu (2002). New features have come available for shell-and-tube exchangers in the past decades, mainly to enhance the heat transfer capacity and to reduce the fouling tendency. These new features include the helical baffles, marketed as Helixchangers (Master et al. 2006), tube enhancements like Twisted Tubes (Zimparov, 2002) and the application of tube inserts like HiTran (Polley, 1993 and Zimparov and Penchev, 2006). There are many advanced types of exchangers as alternatives to shell-and-tube exchangers. Most of these types are initially developed for special applications, but their applicability is steadily extended. Table 3 shows the main, more generally applicable exchanger types with available specialties, the maximum allowable operating conditions and some size characteristics, based on Hewitt (1992), Reay (1994), Shah and Mueller (2000), Wadekar (2000), Hesselgreaves (2001) and Thonon and Tochon (2004). Note that exchangers for special applications like air coolers and fired heaters are excluded. In literature, there is special interest for compact heat exchangers, a special exchanger class that includes all exchangers with an area density above 700 m2/m3 (Reay, 1994). Reay (1994) summarized the main advantages and limitations of these compact heat exchangers, see Table 4. The high area.

(26) Description. commonly applied. tubes with in- and/or external fins. pack of corrugated metal plates hold together by a frame and sealed by gaskets. The plates have holes for the in- and output of the exchanging streams. like plate-and-frame are assembled by welding many different types available also partially welded and plate-in-shell. crimped or corrugated finned plates sandwiched between flat metal separator plates. stacked perforated plates, diffusion bonded. assembled plates in which flowchannels are chemically etched. plates wrapped to form concentric spiral passages; fluids flow from and towards the centre to give a countercurrent flow; different types are used for condensers and evaporators.. Type. shell-and-tube. tube-fin. plate-and-frame. welded plate. plate-fin. matrix. micro-channel/ printed-circuit. spiral-plate. low fouling but cleaning may be difficult. high heat-transfer surface densities extreme conditions possible hard to clean. like plate-fin but easier to make in stainless steel. low ǻT possible multi-stream ability high heat-transfer surface density. wider range of operating condition than plateand-frame not possible to disassemble for cleaning configuration if fixed (not extendable). easy to clean easy to extend only mild conditions limited multi-stream ability. common for clean two-phase application. may be enhanced with finned tubes and special baffles or tube can be equipped with tube inserts; varieties for all kind of applications available. Specialties. Table 3 Overview of the main available exchanger types for general application in process industries. < 2500 m2 100 - 800 m2/m3. > 1000 m2 100 - 800 m2/m3. < 25 bar -25 to 175C. < 300 bar -200C to 900C. < 30 bar < 400C. < 1000 bar -200C to 900C (stainless steel). < 1000 bar < 800°C. < 500 m2. < 1000 m2 1000-5000 m2/m3. like plate-fin. < 100 bar; -273 to 150C (Al) < 9 m3 < 200 bar;-273 to 650C (stainless < 5900 m2/m3 steel) < 4 bar, <1300 °C (ceramic). <3300 m2/m3. <100 m2/m3. Size Characteristic. not reported. <1000 bar <800°C. Operating Condition.

(27) Introduction. 13. density compared to normal shell-and-tube exchangers (< 100 m2/m3), the large amount of area that can be in a single unit (up to 10,000 m2, compared to generally much less than 1000 m2 for shell-andtube exchangers) and the ability to integrate more than two streams allow a different way of heat exchanger and process design, that fits within the process intensification research area (Thonon and Tochon, 2004). The resulting advantages make compact heat exchangers an interesting alternative, especially in case of retrofit, where space and weight limitations can be an issue. Table 4. Advantages and limitations of compact heat exchangers after Reay (1994). Advantages smaller volume. lower weight and cost and less space requirements. multi-stream and multi-pass possible. less space requirements, less piping and lower pressure drops. improved effectiveness. lower approach temperature allowed. tighter temperature control. better quality control for temperature dependent flows. less hold-up. improved safety, less product loss, lower chemicals use in case of chemical cleaning. Limitations limited choice/application range. often restricted to special applications. not too well-proven. conservative industries use proven solutions. fouling concern. apply to clean fluids only or take special precautions/designs. 1.4 Scientific Challenges The developments in society and science outlined in the previous sections show the increasing importance of energy saving to reduce operating costs and fight the worldwide threat of climate change. The recently set target savings are high and require exploration of all available options and development of new techniques. In the developed countries much of these savings must be realised in existing installations. Saving options that were economically unattractive before, like most energy saving retrofits, will also become more relevant. Retrofit design, including heat exchanger network retrofit design, will thus become more important and design methods must be extended to exploit systematically the potential of technological advances like the ones in heat exchanger design. Essential for proper systematic design is a clear description of the design problem. Ignorance of relevant aspects is a clear handicap for design (Westerberg, 2004). No clear description of the heat exchanger network retrofit design problem is available in literature. Various important aspects have been mentioned, distributed over many publications, that had been ignored before. It is difficult to get a clear picture of the design problem from these publications. Both scientific and practical design work will gain transparency, if derived from a general all-inclusive problem definition. For research, it will be easier to demarcate the design space and to set the limitations. For practical design, there will be less risk of overlooking important design variables, constraints and opportunities..

(28) 14. Chapter 1. There is a large number of analysis and design methods in literature that can be applied to heat exchanger network retrofit. There is, however, no independent overview of these methods and the way they relate to each other. Consequently, it is difficult to determine what methods are applicable for specific heat exchanger network retrofit tasks and which one will be the most appropriate to use. A good overview can facilitate this method selection and additionally may identify potential need for new methods. Heat exchanger network retrofit with different exchanger types, including advanced heat exchangers, has been subject of a few publications in the past decades. An overview will be given later in this thesis. All publications present a mathematical model that can be optimised to get the highest saving or the best economy. The retrofit of multi-stream heat exchangers is described in one publication (Bulatov, 2005). This is also a mathematical model, which allows the determination of the most optimal extension of an existing multi-stream heat exchanger. These methods are useful as they allow exploration of the potential of different heat exchanger types and make the methods more generally applicable. They fail, however, to give insight in the way the advantages of various exchanger types are best exploited and how this would affect the heat exchanger network design. Bulatov (2005) does not address the use of new multi-stream heat exchangers in retrofit. No other publications are known that address this topic. The methods for heat exchanger network retrofit with equal exchanger type for all heat exchangers have significantly been improved and extended since the start of the research of this thesis. It is remarkable that all recent methods rely to some extent on the optimisation of a mathematical network model. Unfortunately, even the simplest approach, that uses a set of linear (LP) models developed by Zhu and coworkers (Asante and Zhu, 1996 and 1997) are sometimes cumbersome to use as reported by Phipps and Hoadley (2003) and as experienced during the research of this thesis. Design methods with more complex models are likely to be more cumbersome to use, especially when applied to industrial design problems. As mentioned in Section 1.2.2, industrial retrofit design problems generally have to meet specific constraints and the original designs generally contain specific particularities that must be taken into account as well. Retrofit design methods must be sufficiently flexible to accommodate the requirements of specific cases. Existing methods apparently provide insufficient flexibility or their adaptation requires too much expertise and access to customizable tools, that are not available.. 1.5 Research Objectives The previous section shows a need for additional research on the energy saving retrofit of heat exchanger networks to make it easier to execute such a retrofit for a process engineer in a common working environment with limited resources and expertise. Besides, additional research is necessary to understand and exploit the opportunities provided by currently available advanced, compact and multistream heat exchangers to increase the energy saving. Based on these needs three main objectives have been defined for this thesis:.

(29) Introduction. 15. Objective 1 Get a clear definition of the design space of the energy saving retrofit design problem, i.e. a definition of all relevant design issues and design variables and their relations, based on a structured analysis. The related research questions are: Question 1a: What design variables and criteria are useful to represent relevant design issues? Question 1b: What level of detail is necessary in conceptual network retrofit design to include all design issues and design variables that are relevant in this design stage? Question 1c: Can we use the definition of the design space as a useful basis to clearly define actual heat exchanger network retrofit design problems? Objective 2 Get a structured overview of the available analysis and design methods for the retrofit of heat exchanger networks. The related research questions are: Question 2a: Can we define a unified framework to fit in the analysis and design methods for heat exchanger network retrofit? Question 2b: What blank areas exist in current analysis and design methods? Question 2c: Can we define a recommended practice for the use of heat exchanger network retrofit analysis and design methods? Objective 3 Develop a practical conceptual analysis and design method to do energy saving retrofit of heat exchanger networks and to allow exploitation of the properties of different exchanger types. ‘A practical method’ we define as a method that allows a process engineer to handle in a controlled way design problems with practical size and practical complexity, as identified in the structured analysis of the design problem (Objective 1), and that takes into account the common working environment of a process engineer with limited access to data and limited resources. The related research questions are: Question 3a: Can we systematically define and effectively incorporate all relevant design issues (refer to Objective 1 and Question 1a)? Question 3b: Is it possible to set up a practical method that is sufficiently controllable, flexible and simple also for common industrial problems? Question 3c: Can we define general application rules for various exchanger types in retrofit to make best use of the advantages of each type? Question 3d: Can we effectively adapt the network retrofit design to exploit the advantages of advanced (compact) two- and multi-stream heat exchangers?.

(30) 16. Chapter 1. 1.6 Thesis Content and Set-up 1.6.1 Overview of Research Activities Part of the research questions, posed in the previous section have been subject of some early design studies (van Reisen, 1994, van Reisen and Verheijen, 1996). The main focus of this study was the application of compact multi-stream heat exchangers in the energy saving retrofit design of heat exchanger networks. This study was initiated and supported by the Netherlands Advanced Heat Exchanger knowledge network (NLAHX) and the Dutch Society for Energy and Environment (NOVEM, currently known as SenterNovem) and also executed at the Delft University of Technology. After this study the focus of the research has been changed to energy saving retrofit of heat exchanger networks in general and especially to the application of different types of heat exchangers including compact two- and multi-stream exchangers. During additional case studies, partially reported in van Reisen and Verheijen (1996), some extensive literature surveys and 10 years work as process engineer in an engineering and contracting company, the knowledge and experience was gained to get answers to the posed research questions. The work on the first two objectives described in the previous section is mainly based on an extensive literature survey. For this survey there have been three main sources: the Chemical Abstract (SciFinder) and the Compendex databases and the Web of Science. The literature until March 2007 that was available from the library of the Delft University of Technology has been included in this work. Part of it has been reviewed in this chapter. Other relevant parts will be reviewed in the subsequent chapters, as outlined in the thesis set-up description below. The literature survey has focussed on pinch-based and mathematical optimisation methods, as they are well-established for the analysis and design of heat exchanger networks. An overview of other limitations set on the research and this thesis is given in Appendix A. Apart from the literature survey, the research is mainly based on a number of simple literature cases and three industrial cases, two of them extracted from running plants. One of these industrial cases is included in this thesis. The other case from a running plant was only useful as a learning case for this thesis, as the significant savings were only possible by process modifications, which have been excluded from the final scope of this thesis. For the third industrial case only limited data was available and therefore this case was also used as a learning case only. A heat exchanger network analysis computer program has been developed to support the research and the case studies. This program, called PHITS, has been written in Object Pascal as available initially in Borland Pascal and later in Borland Delphi and Turbo Delphi, see Appendix G. Besides, the various versions of the heat exchanger analysis and design software from AspenTech have been available for the research. The latest version used is AspenPinch Version 10.2. 1.6.2 Set-up of Thesis Overall Set-up The research work for this thesis started with the elaboration of case studies and the exploration of the possible advantages of advanced heat exchangers. This resulted in a new retrofit design method . Later this evolved to a survey of the available analysis and design methods. Finally, the research focussed on.

(31) Introduction. 17. CHAPTER 1 Introduction Context, Objectives, Research Questions. CHAPTER 2 Design Problem Exploration and Definition New concise description Definition of Objects Variables Relations Criteria. CHAPTER 4 Network Performance Analysis Method Review Method Selection New Method: Section 4.3.6. CHAPTER 3 Design Approach New Retrofit Design Approach Conceptual Design Stages: - Target - Preliminary - Refined - Final. CHAPTER 5 Targeting Methods Grassroots and Retrofit Method Review Method Selection CHAPTER 6 New Targeting Method Background Description Illustrating Case. CHAPTER 7 Refined Design Methods Method Review Method Selection CHAPTER 8 New Design Method Background Description Illustrating Case. CHAPTER 9 Case Study. CHAPTER 10 Evaluation and Conclusion Evaluate Reviews, New Developments, Research Questions, Objectives Preferred Methods and Alternatives Conclusions and Recommendations New methodological development from thesis research Main flow. Further reading. Uses information from. Figure 4 Schematic setup of the thesis with new methodological developments marked shaded..

(32) 18. Chapter 1. the elaboration of the extended definition of the heat exchanger network retrofit design problem and the set-up of a general design framework. This thesis is structured in the reverse order, to get the setup of a design guide. It starts with the problem definition and subsequently elaborates the design methodology from a high-level approach to a number of detailed design methods, Figure 4. The research resulted in new methodological developments throughout the design process. These new developments are presented as part of the overall design approach and they are therefore distributed over the entire work. Figure 4 shows how different parts of the thesis are related and how the reviews of existing methods and the new methods are organised. The heavy arrows show the route map for reading recommended for less experienced designers and researchers, who need a guide for heat exchanger network retrofit design. Experts may prefer to concentrate on the new developments, shaded in Figure 4. These parts may be read largely independently. A proper understanding of the overall structure of the design approach will, however, be helpful to understand the set-up of the thesis and the individual parts. Therefore, it is recommended to study Chapter 3 prior to the chapters thereafter. Set-up per Chapter Chapter 2 gives an extensive description of the core conceptual network design problem. It is a systematic compilation of information from literature and own experiences combined with a new systematic analysis and definition of the network design problem. This essentially covers Objective 1. Chapter 2 gives the definitions of the relevant objects, design variables, their relations and the design criteria. These are essential input for the reviews of existing methods and the development of new methods in the subsequent chapters. Chapter 3 is the backbone of this thesis. It gives the new basic design approach for heat exchanger network retrofit design as a special case of a plant retrofit project. The conceptual network design phase, which is part of that basic design approach, is elaborated in detail to get four well defined design stages. The first two stages require targeting methods, which are subject of Chapter 5 and 6. The third stage requires network design methods, which are subject of Chapter 7 and 8, in which these network design methods are referred to as ‘refined’ design methods. Chapter 3 gives a significant contribution to all three objectives. It extends the structured analysis of the design problem with a clear split in stages (Objective 1), it gives the main structure to organise the method reviews (Objective 2) and the high level design approach that integrates the more detailed methods elaborated in the subsequent chapters. Chapter 4 gives an overview of the relevant network performance analysis tools that are available in literature (Objective 2). Additionally, it gives an introduction to the main concepts in heat exchanger network design. These tools are used in all stages of conceptual network design. Tables and method summaries are provided to guide method selection by a designer. Some performance analysis tools need the results from targeting as reference values. This way Chapter 4 links to Chapter 5. One new network performance analysis tool, developed during the research of thesis, is included in Chapter 4. Chapter 5 and 6 give the targeting methods mainly used in the first two stages of conceptual network design. The existing methods are reviewed in Chapter 5 (Objective 2), whereas a new retrofit targeting method is presented in Chapter 6 (Objective 3). Chapter 5 includes some method overview and method assessment tables to guide method selection..

(33) Introduction. 19. Chapter 6 presents a new retrofit targeting method. It gives a number of guidelines that are subsequently elaborated to a new targeting method. This method is illustrated by a case study from literature. Chapter 7 and 8 give the network design methods used in the third, refined design stage of conceptual network design. These include the methods that are commonly referred to as network design methods, which specify conceptually the matches between streams. Chapter 7 reviews the existing methods (Objective 2), whereas Chapter 8 presents a new design method (Objective 3). The set-up of Chapter 7 is the same as of Chapter 5, with similar overview tables. Chapter 8 presents a new retrofit design method for refined network design. It presents a number of design guidelines, which are subsequently captured in a design procedure. Also this method is illustrated by a case study from literature. In Chapter 9 the entire new design approach, based on the new developments, is applied to an industrial case study. Chapter 10, finally, gives a discussion of the results of the research, including an attempt to answer the research questions posed in the previous sections of this chapter, an evaluation of the new developments, an overview of the main conclusions and recommendations for further research..

(34) 20. Chapter 1.

(35) The Heat Exchanger Network Retrofit Design Problem. Chapter 2 The Heat Exchanger Network Retrofit Design Problem A well-defined problem is the key to successful design. This chapter elaborates the four essential elements to get a well-defined problem - goal, starting point, design space description and design test method for heat exchanger network design. It covers both the generic grassroots and generic energy saving retrofit heat exchanger network design problems and gives the differences between these two types of design. The elaboration and comparison are given in a qualitative, descriptive format. The design space is described in terms of objects and arguments and their relations at macro (network), meso (unit) and micro (surface) level. This results in a number of design aspects and design variables. For the test method the possible evaluation criteria are summarised. The generic problem defined this way is complex and requires trade offs and evaluations of many design aspects at different detail level. Specific cases require specific problem definitions with focus on the main issues for that case to keep the actual design problem manageable. This chapter gives some guidelines to get a case specific problem definition from the generic definition. Finally, this chapter gives the specific definition of the energy saving retrofit design problem of heat exchanger networks used in this thesis.. 21.

(36) 22. Chapter 2. 2.1 Introduction Design cannot succeed without a clear problem definition. This definition clarifies the task(s) and restrictions a design has to meet and allows evaluation of any proposed solution. Even a well-defined design problem generally has numerous solutions. Also most designs can serve numerous objectives, depending on the use of these designs. Consequently, we can only determine whether a design process has succeeded, if we test the design with predefined test methods to predefined objectives and criteria. Biegler et al. (1997) give four elements that are required to get a well-posed design problem: • the design goal; • the starting point for the design; • a description of the design space. • a test method to determine if the design meets the goal; In this chapter we will elaborate these four elements for the heat exchanger network design problem. First we will elaborate the general heat exchanger network design problem and some heat transfer fundamentals. Next, we will elaborate the retrofit design problem, showing the differences with the general grassroots design problem, followed by the design criteria that are required to test the design and some guidelines to use the general problem description to setup the design problem definition for a specific case. Finally, we will define the heat exchanger network retrofit design problem that we will use in this thesis.. 2.2 The Heat Exchanger Network Design Problem In Chapter 1 we showed the place of heat exchanger network design in the overall process design using the Onion Diagram, Figure 3, that gives a decomposition strategy for the entire conceptual process design. This decomposition gives a rough demarcation of the scope of the heat exchanger network design problem with interfaces with the core process design and the utility system. If we look at the main specialisms that generally contribute to heat exchanger network design, Figure 5, we get a rough demarcation of the detail of the design. Heat exchanger network design has its place between process design and the thermal design specialisms. The process design specialism takes care of the setup of the main elements on the process flow sheet and performs the process simulations. Often, there is client involvement at this level. Thermal or heat exchanger design determines all details of a heat exchanger that influence the thermal performance. The thermal design often requires involvement of heat transfer equipment vendors, especially in case advanced heat exchangers are applied. Figure 5 shows the relation and the main information flows between the above mentioned specialisms. We can define the goal of heat exchanger network design, based on its place in the design process using the definition of heat exchanger network given in Section 1.2.3: Heat exchanger network design is the conceptual design of a process system consisting of heat exchangers to balance the heat deficits (sinks) and surplus (sources) within a process with the efficient use of utilities as external heating and cooling resources..

(37) The Heat Exchanger Network Retrofit Design Problem. Need. CLIENT. 23. Solution. PROCESS ENGINEERING Goals Models Stream & Utility Data System Constraints. Utility Requirements Improvement Options Network HEAT EXCHANGER NETWORK DESIGN. Exchanger Types Type Constraints Exchanger Details Exchanger Performance. Design Basis for Heat Exchangers. THERMAL DESIGN VENDOR Figure 5 Heat exchanger network design and related specialisms Table 5. Generic grassroots heat exchanger network design problem definition. Goal:. heat balance specified heat sinks and sources at minimum yearly cost. Criteria. sum of yearly utility cost and annualised investment cost heat balance feasible heat transfer safe and operable design. Starting Point. set of hot and cold streams with fixed source and target temperature, massflow and composition; physical properties and heat transfer characteristics as function of temperature; maximum pressure drop per stream.. Design Space. variables and model: refer to Table 6 case specific constraints. The tables 5 and 6 give a more comprehensive elaboration of the generic heat exchanger network design problem with the essential elements discussed in Section 2.1 above. The test method is summarised by the criteria to be used for evaluation. Obviously, we need to specify an adequate test for the mentioned criteria prior to design to avoid ambiguity in the design. The design space is represented by the relevant objects and main attributes of these objects, given in Table 6. The relations between these objects and attributes is discussed in the subsequent sections. Note, Table 5 gives the generic design problem definition for a steady state case with a single operating case, in line with the scope of this thesis (Appendix A). Actual cases require information about the variability of the input and possible different operating modes including startup and shutdown as part of the starting point.

(38) 24. Table 6. Chapter 2. Main objects and attributes for conceptual grassroots heat exchanger network design. Object (parent). Contains objects. Attributes (type). Input. Output. Stream. feed product. feed, product Ts , Tt (cont), ǻp (cont) duty (cont) massflow (cont) properties (cont) heattr. charac. (disc). Ts , Tt , max ǻp massflow properties heattr. charac.. duty ǻp. Utility (Stream). feed product. feed, product Ts , Tt (cont), ǻp (cont) duty (cont) massflow (cont) properties (cont) heattr. charac. (disc) cost (cont). Ts , Tt , max ǻp massflow properties heattr. charac.. duty ǻp massflow annual oper cost. location. location. type (disc) EMAT (cont) streams included (disc) utilities included (disc) up&downstr units (disc) FT (cont) htc (cont) size (cont) material (disc) cost (cont) no. shells in series (disc) location (cont). allowable types EMAT. Feed/Prod Match. set of streams set of utilities. stream included - Ts, Tt, massflow, duty, ǻp utilities included - Ts, Tt, massflow, duty, ǻp up- & downstream units annual oper. cost type htc, FT , size material investment no. shells series. Stream split. stream (disc) up&downstr units (disc) split fractions (cont). stream up- & downstream units split fractions. Stream mixers. stream (disc) up&downstr units (disc). stream up- & downstream units. set of streams see match see match see match set of utilities topology stream splits/mixers set of streamincluded splits / mixers matches included set of matches heattr. charac. = heat transfer characteristics; EMAT = Exchanger Minimum Approach Temperature; Ts ,Tt = source , target temperature; ǻp = pressure drop; htc=heat transfer coefficient; FT=non counter-current correction factor; disc = discrete; cont = continuous Network (Match).

(39) The Heat Exchanger Network Retrofit Design Problem. 25. definition. Additionally, the goal, criteria and design space definition should be adapted to include the time dependency. Table 6 gives the relevant objects in the first column. If appropriate a parent object is given in brackets, eg. a utility is a stream. An object has all the attributes of its parent, thus a utility has all the attributes specified for the stream object. The second column of the table gives the object (sets) that are part of the object and that need to be specified to get a complete specification of the holding object, eg. the specification of the streams and utilities that are part of a match are also part of the specification of that match. The third column gives the attributes of the object that are relevant for design. For each attribute the type (disc = discrete, cont = continuous) is given in brackets. In the last two columns the input and output data for the object are given. If the attribute value is an internal value or the output is an estimate, the item is printed italic. A similar representation of the design space of the thermal engineer is given in Table 7. It shows how thermal design fills in the details of the heat exchangers and what is used as input, which must be provided by the heat exchanger network design. Note, that some estimates used in heat exchanger network design are not used for thermal design and consistency is thus not guaranteed. Table 7 Thermal and hydraulic design of heat exchangers Object (parent). Contains objects. Stream. Attributes (type). Input. Output. Ts , Tt (cont), ǻp (cont) duty (cont) massflow (cont) properties (cont) fouling coefficient. Ts , Tt , max ǻp massflow properties fouling coefficient. duty ǻp actual. Heat exchanger. set of streams surface. type (disc) streams included (disc) size (cont) material (disc) number of shells (disc) number of passes (disc) surfaces (various) flow paths (various) in/outlet (various). streams included - Ts , T t , massflow, duty, max ǻp constraints. type material number of shells number of passes surface details flow path details fluid side / flow path per stream htc clean / fouled pressure drops area FT , effective ǻT sizes investment installation requirements construction details. Surface. materials of construction. surface extensions (various). streams included - properties - conditions. wall thickness surface extensions.