Supply and target-based superstructure synthesis of heat and mass exchange networks

Includes abstract.Includes bibliographical references.This study presents three new methods for superstructure synthesis of heat exchanger networks (HENs) and mass exchanger networks (MENs) that involve the use of key parameters in HEN and MEN to define superstructure intervals. The key parameters are the stream supply and target temperatures/compositions. The Supply-Based Superstructure (SBS) uses the supply temperatures/compositions of both the hot/rich streams and the cold lean streams. The Supply and Target-Based Superstructure (S&TBS) uses the supply temperatures/compositions of hot/rich streams and the target temperatures/compositions of cold/lean streams for the definition of the superstructure intervals. The Target and Supply-Based Superstructure (T&SBS) uses the target temperatures/compositions of hot/rich streams and the supply temperatures/compositions of cold/lean streams for the definition of the superstructure interval boundaries

The synthesis of Combined Heat and Mass Exchange Networks (CHAMENs) with renewables considering environmental impact

Process synthesis is used to evaluate different potential designs to select the most suitable that fulfils some process goals. There is ever-increasing pressure to reduce operating cost and emission of pollutants as energy prices continue to increase and more regulations are set by government. To address these concerns, optimisation methods based on heuristics, pinch technology and mathematical programming can be adopted. Since the early 90s, mathematical programming has gained significant attention to solve large and complex problems. Extensive studies have been conducted for heat exchange network synthesis (HENS), which was first used to optimise utility usage and operating costs. Many existing mass exchange network synthesis (MENS) methods are derived from HENS techniques since analogies exist between the two networks. Integrating the synthesis of mass and heat exchange networks in what is known as combined heat and mass exchange network synthesis (CHAMENS) can be beneficial because mass transfer is affected by operating temperature. However, very little research has been done in this area of process synthesis due to their complex nature. It is even more challenging to find literature involving the regeneration of multiple recyclable MSAs in a network synthesis context. Furthermore, the few studies that have considered CHAMENS have done the optimisation considering economic performance alone, whereas the consideration of environmental impact as an additional objective can help attain a more sustainable process. This thesis builds on current knowledge of CHAMENs synthesis methods by considering CHAMENs with detailed regeneration networks (RENs) involving multiple recyclable MSAs, multiple regenerating streams, and solar thermal as an alternative energy source, using a multi-period synthesis approach. Simultaneously optimising a combination of these networks is not a trivial task due to the resulting large model size having many binary and non-linear terms and the interactions among them. Stage-wise superstructure (SWS) synthesis approaches for heat and mass exchanger networks are adopted in this thesis for the synthesis of CHAMENs. A new superstructure for RENs, which is equivalent to that of a MEN, is presented in this thesis. The combined superstructure, which involves multiple MSAs, multiple regenerants, and multiple hot and cold process streams, is integrated with solar thermal energy as a renewable energy option. The availability of solar thermal energy is simplified by discretizing into two time periods of daytime and nighttime operations. The proposed CHAMEN model is also extended to handle multi-objective optimisation (MOO) of environmental impact and economic objectives to identify the optimal network configuration. Two examples were solved, and the results obtained showed that the implementation of integrated solar panels and thermal storage tanks could reduce the environmental impact of the combined networks by 76% and 26% for case studies 1 and 2 respectively. However, such eco-friendly infrastructure resulted in increased total annual cost (TAC) values of 36% and 15% respectively for the two case studies. These results indicate that by using the methodologies developed in this thesis, trade-offs can be established between economics and environmental impact as objectives

Synthesis of optimal heat and mass exchange networks using a two-step hybrid approach including detailed unit designs

This PhD thesis develops a methodology for the synthesis of optimal heat and mass exchanger networks through a novel hybrid method. The two-step procedure makes use of simplified exchanger models in a network optimisation step, followed by a detailed design where the exchangers found in the network synthesis step are modelled in detail. Subsequent iterations of the network design step are then updated with information from the detailed network designs. The algorithm has certain advantages over previous methods in that the network optimisation is based on more realistic representations of the actual units therein and also that the method increases the likelihood of attaining a globally optimal solution through the generation and assessment of multiple candidate networks throughout the algorithm. The method can be used in a variety of applications and is demonstrated to be effective for large problems and multi-period scenarios. The thesis also shows that the method can be used in conjunction with multiple individual unit optimisation techniques including heuristics and fully explicit optimisation methods

Development of a techno-economic energy model for low carbon business parks

To mitigate climate destabilisation, global emissions of human-induced greenhouse gases urgently need to be reduced, to be nearly zeroed at the end of the century. Clear targets are set at European level for the reduction of greenhouse gas emissions and primary energy consumption and for the integration of renewable energy. Carbon dioxide emissions from fossil fuel combustion in the industry and energy sectors account for a major share of greenhouse gas emissions. Hence, a low carbon shift in industrial and business park energy systems is called for. Low carbon business parks minimise energy-related carbon dioxide emissions by enhanced energy efficiency, heat recovery in and between companies, maximal exploitation of local renewable energy production, and energy storage, combined in a collective energy system. Moreover, companies with complementary energy profiles are clustered to exploit energy synergies. The design of low carbon energy systems is facilitated using the holistic approach of techno-economic energy models. These models take into account the complex interactions between the components of an energy system and assist in determining an optimal trade-off between energetic, economic and environmental performances. In this work, existing energy model classifications are scanned for adequate model characteristics and accordingly, a confined number of energy models are selected and described. Subsequently, a practical categorisation is proposed, existing of energy system evolution, optimisation, simulation, accounting and integration models, while key model features are compared. Next, essential features for modelling energy systems at business park scale are identified: As a first key feature, a superstructure-based optimisation approach avoids the need for a priori decisions on the system’s configuration, since a mathematical algorithm automatically identifies the optimal configuration in a superstructure that embeds all feasible configurations. Secondly, the representation of time needs to incorporate sufficient temporal detail to capture important characteristics and peaks in time-varying energy demands, energy prices and operation conditions of energy conversion technologies. Thirdly, energy technologies need to be accurately represented at equipment unit level by incorporating part-load operation and investment cost subject to economy of scale in the model formulation. In addition, the benefits of installing multiple units per technology must be considered. A generic model formulation of technology models facilitates the introduction of new technology types. As a fourth important feature, the potential of thermodynamically feasible heat exchange between thermal processes needs to be exploited, while optimally integrating energy technologies to fulfil remaining thermal demands. For this purpose, thermal streams need to be represented by heat –temperature profiles. Moreover, restrictions to direct heat exchange between process streams need to be taken into account. Finally, the possibility for energy storage needs to be included to enhance the integration of non-dispatchable renewable energy technologies and to bridge any asynchrony between cooling and heating demands. Starting from these essential features, a techno-economic optimisation model (Syn-E-Sys), is developed customised for the design of low carbon energy systems on business park scale. The model comprises two sequential stages. In the first stage, heat recovery within the system is maximised, while energy supply and energy storage technologies are optimally integrated and designed to fulfil remaining energy requirements at minimum total annualised costs. Predefined variations in thermal and electrical energy demand and supply are taken into account, next to a carbon emission cap. At the same time, heat networks can be deployed to transfer heat between separate parts of the system. In the second stage, the model generates an optimal multi-period heat exchanger network enabling all required heat exchanges. Syn-E-Sys builds upon a multi-period energy integration model that can deal with restrictions in heat exchange. It is combined with a generic technology model, that features part-load operation as well as investment cost subject to economy of scale, and a generic energy storage model. The technology model can be manipulated to represent various thermal or electrical energy conversion technology units, and serves as a building block to model more complex technologies. The storage model covers electrical as well as thermal energy storage, taking into account the effect of hourly energy losses on the storage level, without increasing the number of time steps to be analysed. For this purpose, time sequence is introduced by dividing the year into a set of time slices and assigning them to a hierarchical time structure. In addition, a more complex model for storage of sensible heat is integrated, consisting of a stack of interconnected virtual tanks. To enable the optimisation of the number of units per technology in the energy system configuration, an automated superstructure expansion procedure is incorporated. Heat transfer unit envelope curves are calculated to facilitate the choice of appropriate temperature levels for heat networks. Heat networks that are embedded within this envelope, completely avoid the increase in energy requirements that would result from the heat exchange restrictions between separated parts of the energy system. Finally, the heat exchanger network is automatically generated using a multi-period stage-wise superstructure. Two problems inherent to the heat cascade formulation are encountered during model development. As a first issue, heat networks can form self-sustaining energy loops if their hot and cold streams are not completely embedded within the envelope. This phenomenon is referred to in this work as phantom heat. As a second issue, the heat cascade formulation does not prevent that a thermal storage releases its heat to a cooling technology. To demonstrate the specific features of Syn-E-Sys and its holistic approach towards the synthesis of low carbon energy systems, the model is applied to a generic case study and to a case study from literature. The generic case study is set up to demonstrate the design of an energy system including non-dispatchable renewable energy and energy storage, subject to a carbon emission cap. For this purpose, the year is subdivided into a set of empirically defined time slices that are connected to a hierarchical time structure composed of seasons, daytypes and intra-daily time segments. The results obtained by Syn-E-Sys show a complex interaction between energy supply, energy storage and energy import/export to fulfil energy demands, while keeping carbon emissions below the predefined cap. The model enables optimisation of the intra-annual charge pattern and the capacity of thermal and electrical storage. Moreover, an optimal heat exchanger network is automatically generated. In the second case study, heat recovery is optimised for a drying process in the paper industry. To avoid the energy penalty due to heat exchange restrictions between two separated process parts, heat transfer units need to be optimally integrated. Firstly, a simplified version of the original problem is set up in Syn-E-Sys and the obtained results correspond well to literature. Subsequently, the original problem is extended to demonstrate the optimal integration of heat transfer units in a multi-period situation. In conclusion, Syn-E-Sys facilitates optimal design of low carbon energy systems on business park scale, taking into account the complex time-varying interactions between thermal and electrical energy demand, supply and storage, while the potential for heat recovery is fully exploited

A pinch-based method for defining pressure manipulation routes in work and heat exchange networks

Aiming for more energetically efficient and sustainable solutions, academic attention to work and heat integration (WHI) has grown in the last decade. Simultaneous models for work and heat exchanger network (WHEN) synthesis often derive from heat integration (HI) frameworks. However, it can be noted that simultaneous optimization models for WHI are considerably more complex to solve than in the HI case. The design of efficient pressure manipulation routes (i.e., allocation and sizing of compression and expansion machinery) in process streams prior to heat exchange match allocation can make the optimization procedure more efficient. This work proposes a systematic procedure based on a model that employs Pinch Analysis concepts for defining these routes based on capital and operating cost targets. The solution approach is a hybrid meta-heuristic method based on Simulated Annealing (SA) and Particle Swarm Optimization (PSO). The obtained routes are then converted into a HI problem by fixing pressure manipulation unit sizes. The detailed HI solution is finally transferred into a WHI optimization model as initial design. In the two tackled examples, the total annual costs (TAC) predicted by the Pinch-based model differed by 0.5% and 1.2% from the final optimized WHEN obtained in the detailed WHI framework.The authors gratefully acknowledge the financial support from the Coordination for the Improvement of Higher Education Personnel – Processes 88887.360812/2019–00 and 88881.171419/2018–01 – CAPES (Brazil) and the National Council for Scientific and Technological Development – Processes 305055/2017–8, 428650/2018–0 and 311807/2018–6 – CNPq (Brazil)

EVALUATION OF TOTAL ANNUAL COSTS OF HEAT EXCHANGER NETWORKS USING MODIFIED PINCH ANALYSIS

This study presents pinch analysis of some heat exchanger networks (HENs) problems using Hint integration (HINT) software. Three examples reported to have been solved using different approaches by various researchers to obtain the least possible total annual cost (TAC) were solved using the Hint software.  In this work, the use of remaining problem analysis (RPA) contained in the heat integration software was used to carry out matching and the general optimization of the networks for minimum TAC for the three problems solved. The results obtained after solving the first problem using RPA based heat integration gave a minimum total annual cost (TAC) of $237, 510 /yr. which is the fourth when compared with the lowest solution that shows the minimum TAC of$235.400 /yr. in that example. However, the TAC obtained in solving problem  2 and 3 were $562, 333 /yr. and$2.881 M/yr. respectively and they are the least total annual cost obtained when compared with what have been obtained using mathematical programming and non-RPA based Hint software. The overall assessment of the various approaches used to solve these problems when compared with the results obtained in this study shows that HINT software is able to obtain TAC that are within the same range as those obtained using mathematically based technique. http://dx.doi.org/10.4314/njt.v35i3.1

Energy Induced Separation Network Synthesis of an Olefin Compression Section: A Case Study

When latent heat is transferred in a heat exchanger network, the formation of the second phase creates an opportunity for separation. This network is known as a Heat Induced Separation Network (HISEN). HISENs have been extended to include pressure adjusting devices for improving the thermodynamic feasibility of the network. This extended network is termed an Energy Induced Separation Network (EISEN). Most examples of EISENs in the literature are environmental pollution treatment case studies which do not require liquid phase mass integration or shaft power integration. They assume a predetermined extent of separation and mostly are based on conceptual methods of design. This paper explains how the optimization framework must be developed in order to systematically address the general characteristics of EISENs. The framework is illustrated using a case study of the synthesis problem of an olefin compression section

Mathematical modelling of simultaneous water and energy minimisation considering water management hierarchy options

Water and energy are closely interlinked together. The goal to reduce water and energy simultaneously has been a growing research. However, previous studies only consider maximising water reuse and, in some cases, also include water regeneration. This study aims to develop a mathematical model to design water and energy network that further reduces the water consumption, considering the whole water management hierarchy (WMH) schemes. This includes elimination, reduction, reuse, outsourcing and regeneration. Two steps solution is proposed, which involves solving two MINLP models. First, water and energy minimisation network considering WMH schemes and direct heat transfer is designed. The obtained network is then improved by inclusion of indirect heat integration to minimise the objective cost function. Two cases of thermal data extraction are studied for heat integration, Case A extracts individual streams based on supply and targeted temperature, whereas Case B extracts stream after mixer based on mixer temperature and targeted temperature. Streams which temperature load is satisfied in direct heat transfer were excluded for heat integration. The proposed method has been tested with literature case study. The implementation of all possible WMH scheme yields a lower freshwater consumption and wastewater generation. The model selected 35% and 15% of reduction for demand 3 and demand 1 respectively. Case A yields a lower total operating cost but slightly higher investment cost compared to Case B. Case B result in a simpler heat exchanger network, but degradation of the potential energy causes more heating and cooling. Case A is chosen as the optimal network and exhibits 13% reduction of the total cost compared to the literature case study

MINLP Optimization Algorithm for the Synthesis of Heat and Work Exchange Networks

This paper introduces a new optimization model for the simultaneous synthesis of heat and work exchange networks. The work integration is performed in the work exchange network (WEN), while the heat integration is carried out in the heat exchanger network (HEN). In the WEN synthesis, streams at high-pressure (HP) and low-pressure (LP) are subjected to pressure manipulation stages, via turbines and compressors running on common shafts and stand-alone equipment. The model allows the use of several units of single-shaft-turbine-compressor (SSTC), as well as helper motors and generators to respond to any shortage and/or excess of energy, respectively, in the SSTC axes. The heat integration of the streams occurs in the HEN between each WEN stage. Thus, as the inlet and outlet streams temperatures in the HEN are dependent of the WEN design, they must be considered as optimization variables. The proposed multi-stage superstructure is formulated in mixed-integer nonlinear programming (MINLP), in order to minimize the total annualized cost composed by capital and operational expenses. A case study is conducted to verify the accuracy of the proposed approach. The results indicate that the heat integration between the WEN stages is essential to enhance the work integration, and to reduce the total cost of process due the need of a smaller amount of hot and cold utilities.The financial supports by the Brazilian agency “Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES” (process nº 10758/12-7), and the Spanish Ministry of Science and Innovation and Ministry of Economy and Competitiveness (under project CTQ2012-37039-C02-02) are gratefully acknowledged

A Multi-Period Synthesis Approach to Designing Flexible Heat- Exchanger Networks

This chapter presents a new synthesis method for designing flexible heat-exchanger networks. The methodology used involves a two-step approach: In the first step, a multi-period network is designed for a large number of critical operating periods using a finite set of operating points which lie within the uncertain parameter range, while considering the impact of potential fluctuations in periodic durations of each of the chosen critical points on the network. In the second step, the flexibility of the resulting multi-period network of the first step is tested using very large, randomly generated set of finite potential operating points together with their periodic durations. The key criteria used in determining the finite set of operating points that would participate in the initial multi-period network synthesis of the first step are the nominal operating points, the extreme operating points in terms of heat-load requirements as well as their length of periods. This implies that the resulting flexible network can feasibly transfer heat irrespective of possible fluctuations in periodic durations for any of the potential process-operating points. The solutions obtained using the new approach compare favourably with those in the literature