A rewriting grammar for heat exchanger network structure evolution with stream splitting

The design of cost optimal heat exchanger networks is a difficult optimisation problem due both to the nonlinear models required and also the combinatorial size of the search space. When stream splitting is considered, the combinatorial aspects make the problem even harder. This paper describes the implementation of a two level evolutionary algorithm based on a string rewriting grammar for the evolution of the heat exchanger network structure. A biological analogue of genotypes and phenotypes is used to describe structures and specific solutions respectively. The top level algorithm evolves structures while the lower level optimises specific structures. The result is a hybrid optimisation procedure which can identify the best structures including stream splitting. Case studies from the literature are presented to demonstrate the capabilities of the novel procedure

A Simple Method for Finding Optimal Paths of Hot and Cold Streams inside Shell and Tube Heat Exchangers to Reduce Pumping Cost in Heat Exchanger Network Problems

In this paper, a simple method is presented for the synthesis and retrofit of heat exchanger networks (HENs) by considering pressure drop as well as finding proper path of streams inside heat exchangers (HEs) to reduce the pumping cost of network. Generally, HEN problems lead to MINLP models which have convergence difficulties due to the existence of both continuous and integer variables. In this study, instead of solving these variables simultaneously, a combination of Genetic Algorithm (GA) with Quasi Linear Programming (QLP) and Integer Linear Programming (ILP) was used for solving the problem. GA was used to find optimal HENs structure and streams paths, whereas continuous variables were solved by QLP. For the retrofit of HENs, a modified ILP model was used. Results show that the proposed method has the ability to reduce the cost of annual pumping due to considering optimal paths for streams in the HEs compared to the literature. This work is licensed under a Creative Commons Attribution 4.0 International License

A Conceptual Efficient Design Of Energy Recovery Systems Using A New Energy-area Key Parameter

Energy integration in petrochemical and refining industries is an effective concept to minimize dependence on heating and cooling utilities through networks of exchanger equipment. Pinch Analysis is very popular and successful technique to optimize heat recovery between heat sources and sinks. Yet, design of networks of exchangers is challenging and requires careful attention to energy consumption and exchanger areas. This work presents a graphical methodology to design exchanger networks taking into account both heat loads and transfer areas of exchanger units in one single information. A new parameter is introduced for design that is the ratio between the heat load and the exchanger area and is determined in kW/m2. It is defined as an energy-area parameter expressing how much heat the exchanger would transfer per every meter square of area. Such parameter will be valuable key in design to screen matches of exchangers providing that both the heat and area are considered. The higher the value of the parameter, the better the performance of the exchanger, i.e. maximum heat transfer rate for minimum exchanger area. The design methodology embedding the energy-area parameter guarantees HEN designs with energy targets and minimum areas. A case is studied for the production of 100,000 t/y of dimethyl ether. An optimum network is generated by applying the new parameter with less exchanger areas and hot utility of 25% and 30%, respectively compared with an automated design by Aspen Energy Analyzer®. Also, substantial savings of about 47% in the total cost of the network are earned

Assessing the impact of changing electricity and CO2 prices on synthetic fuel production: Multi-criteria heat exchanger network synthesis

Facing the urgent need to reduce anthropogenic climate change, especially the emissions from the transport sector, climate-neutral fuels are gaining attention. Synthetic fuels serve as a long-term solution for aviation, maritime and heavy machinery. However, large-scale use requires cost-effectiveness, efficient production and resilience to price changes. We use mixed integer linear programming to simultaneously optimize the cell voltage of the solid oxide electrolysis cell, the heat exchanger network and the internal heat supply of a 1 MW PtL-plant. PtL-efficiency and production costs are used as objectives. Multiple Pareto fronts are calculated within reasonable price domains considering falling and rising electricity and CO2 prices. Deriving sensitivities on electricity and CO2 prices enables us to quantify the impact of price changes on operational and design parameters. The results show that sensitivity to price changes has different impacts on design and operating parameters, which can lead to unattractive solution domains in the Pareto front. Current electricity and CO2 prices result in production costs ranging from 1.83-2.36 EUR/kg. In the best case, fuels can be produced at 1.42-1.97 EUR/kg and at 3.88-4.28 EUR/kg in the worst case. This paper supports decision-makers in planning PtL-plants to ensure synthetic fuel availability on a global scale

A framework for flexible and cost-efficient retrofit measures of heat exchanger networks

Retrofitting of industrial heat recovery systems can contribute significantly to meeting energy efficiency targets for industrial plants. One issue to consider when screening retrofit design proposals is that industrial heat recovery systems must be able to handle variations, e.g., in inlet temperatures or heat capacity flow rates, in such a way that operational targets are reached. Consequently, there is a need for systematic retrofitting methodologies that are applicable to multiperiod heat exchanger networks (HENs). In this study, a framework was developed to achieve flexible and cost-efficient retrofit measures of (industrial) HENs. The main idea is to split the retrofitting processes into several sub-steps. This splitting allows well-proven (single period) retrofit methodologies to be used to generate different design proposals, which are collected in a superstructure. By means of structural feasibility assessment, structurally infeasible design proposals can be discarded from further analysis, yielding a reduced superstructure. Additionally, critical point analysis is applied to identify those operating points within the uncertainty span that determine necessary overdesign of heat exchangers. In the final step, the most cost-efficient design proposal within the reduced superstructure is identified. The proposed framework was applied to a HEN retrofit case study to illustrate the proposed framework

Novel Approaches for Retrofitting Heat Exchanger Networks Subject to Varying Operating Conditions

The process industry is responsible for a significant share of the final industrial energy use in Sweden. In particular, the pulp and paper industry accounts for more than 50 %. In this context, a number of studies have shown that there is a substantial potential for energy savings in the pulp and paper industry. In particular, increased heat recovery is an important measure for increasing energy efficiency. Industrial process plants usually include heat recovery systems that are built to transfer heat between different process streams. Such heat recovery systems consist of Heat Exchanger Networks (HENs) which can be characterized by a high degree of complexity, e.g. through stream splits, recycle and closed circulation loops.Increasing heat recovery, e.g. by redesigning (retrofitting) the existing heat recovery system, can contribute significantly to meeting energy efficiency improvement targets for industrial process plants. One issue to consider when screening retrofit design options is that industrial heat recovery systems must be able to handle external variations, e.g. in inlet temperatures or heat capacity flow rates, in such a way that operational targets are reached. Consequently, there is a need for systematic retrofitting methodologies applicable to HENs subject to variation in operating conditions. The aim of this thesis is to propose new approaches for retrofitting HENs operating in multiple periods.Three different approaches have been developed and published in three papers appended to this thesis which allow for retrofitting HENs operating in multiple periods. These approaches can be applied independently and are applicable to HENs commonly present in process industry, e.g. pulp and paper industry. All approaches have in common that they require structural retrofit design proposals as input. For the structural proposal generation, graphical approaches (e.g. Pinch-based) may be utilized in order to take advantage of the designer interaction during the design process. The three approaches propose different strategies to evaluate and ensure feasibility of the design proposals when operating conditions vary, e.g. by means of design modifications. In this context, feasibility is achieved if predefined target values, e.g. stream target temperatures, can be reached for the entire span of variations. Furthermore, different strategies help the designer to identify the most promising proposal (or proposals) among the provided ones with respect to a defined objective, e.g. most energy-efficient or most cost-efficient

An Improved Method for Predicting Heat Exchanger Network Area

Successful application of pinch analysis to any process, be it for grassroots design or retrofit, depends upon the extent to which set targets are achieved in practice. This entails predicating the three stages of process integration namely targeting, synthesis and detailed design on the same basis. There exist gap between these three stages largely due to inaccuracies in film heat transfer coefficient and inability to replicate same at the various stages. This paper presents an improved methodology for area targeting that is consistent with detailed design of an exchanger not just because it is premised on the same basis of pressure drop constraints but, more importantly, because it allows, for necessary variation of stream properties with temperature. The validity of the methodology has been tested using two case studies from the literature. The results obtained in all studies reveal a difference of less than 2% between targeting, synthesis and detailed design with the new methodology. This is contrary to the difference of as high as 59% between targeting and detailed design obtained with the state-of-the-art methodology. There is therefore an excellent agreement between the three stages of process integration arising from the new methodology. Keywords: heat exchanger network, area targeting, synthesis, detailed design, pressure drop, film heat transfer coefficient

New Elements of Heat Transfer Efficiency Improvement in Systems and Units

Zvýšení efektivity výměny tepla vede k poklesu spotřeby energie, což se následně projeví sníženými provozními náklady, poklesem produkce emisí a potažmo také snížením dopadu na životní prostředí. Běžné způsoby zefektivňování přenosu tepla jako např. přidání žeber či vestaveb do trubek ovšem nemusí být vždy vhodné nebo proveditelné -- zvláště při rekuperaci tepla z proudů s vysokou zanášivostí. Jelikož intenzita přestupu tepla závisí i na charakteru proudění, distribuci toku a zanášení, které lze všechny výrazně ovlivnit tvarem jednotlivých součástí distribučního systému, bylo sestaveno několik zjednodušených modelů pro rychlou a dostatečně přesnou predikci distribuce a také aplikace pro tvarovou optimalizaci distribučních systémů využívající právě tyto modely. Přesnost jednoho z modelů byla dále zvýšena pomocí dat získaných analýzou 282 distribučních systémů v softwaru ANSYS FLUENT. Vytvořené aplikace pak lze využít během návrhu zařízení na výměnu tepla ke zvýšení jejich výkonu a spolehlivosti.Improved heat transfer efficiency leads to decrease in energy consumption which then results in lower equipment operational cost, reduced emissions, and consequently also lower environmental impact. However, common enhancement approaches such as adding fins or tube inserts may not always be suitable or feasible -- especially in case of heat recovery from streams having a high fouling propensity. Since heat transfer rate depends also on flow field characteristics, fluid distribution, and fouling which can all be greatly influenced by the actual shapes of flow system components, several simplified models for fast and accurate enough prediction of fluid distribution as well as applications for shape optimization based on these models were developed. In addition, accuracy of one of the models was further increased by fine-tuning it using data obtained by evaluation of 282 flow systems in the fluid flow modelling software ANSYS FLUENT. The created applications can then be employed during the design of heat exchange units to improve their performance and reliability.

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

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