Heat Exchanger Network Synthesis Research Articles

Matrix Non-Structural Model and Its Application in Heat Exchanger Network without Stream Split

Heat integration by a heat exchanger network (HEN) is an important topic in chemical process system synthesis. From the perspective of optimization, the simultaneous synthesis of HEN belongs to a mixed-integer and nonlinear programming problem. Both the stage-wise superstructure (SWS) model and the chessboard model are the most widely adopted and belong to structural models, in which a framework is assumed for stream matching, and the global optimal solution outside its feasible domain may be defined by the framework. A node-wise non-structural model (NW-NSM) is proposed to find more universal stream matching options, but it requires a mass of structural variables and extra multiple correction strategies. The aim of this paper is to develop a novel matrix non-structural model (M-NSM) for HEN without stream splits from the perspectives of global optimization methods and superstructure models. In the proposed M-NSM, the heat exchanger position order is quantized by matrix elements at each stream, and a HEN structure is initialized by the random generation of matrix elements. An approach for solving HEN problems based on a matrix real-coded genetic algorithm is employed in this model. The results show that M-NSM provides more flexibility to expand the search region for feasible solutions with higher efficiency than previous models.

Synthesis of multiperiod heat exchanger networks: n-best networks with variable approach temperature

In industrial processes, the stream parameters of the heat exchanger network (HEN) synthesis problem can vary during different periods. As a result of the change in inlet and outlet temperatures and the flow rates of the streams, the optimal HEN and the optimal heat transfer areas of the heat exchangers can be different in each period. In our previous work, the standard HEN synthesis for minimum utility consumption was extended to consider varying stream parameters, where bypasses at the heat exchangers are used to ensure the exit streams meet the expected outlet temperatures. This requires optimization of the total annualized cost based on the maximal heat transfer areas of the heat exchangers for each period. Minimum approach temperature is applied to ensure that the areas of the heat exchangers stay within reasonable limits in the solution network. Since minimum approach temperature affects both the structure of the HEN and the total annualized cost, its value needs to be determined during the optimization procedure. The current work proposes a procedure for HEN synthesis which determines the best, n-best, or all feasible HENs for all periods considering variable approach temperature. The proposed method extends the P-graph-based HEN synthesis method for determining all feasible networks in all periods. Three case studies are used to demonstrate the application of the proposed method. The case studies show that while the commonly applied minimum approach temperature of 10 °C does indeed gives near-optimal results, some problems need different values, such as 25 °C or 30 °C.

Integration and optimization of methanol-reforming proton exchange membrane fuel cell system for distributed generation with combined cooling, heating and power

The methanol-steam-reforming proton exchange membrane fuel cell system is an attractive option for distributed cogeneration due to its low emissions, quiet operation, and low-cost fuel storage. To further increase its energy efficiency, waste heat can be utilized for combined cooling, heating, and power generation. However, the additional equipment, processes, and streams required for cogeneration make the system design complex, with a large number of degrees of freedom. To address this challenge, we propose an equation-based optimization framework for the simultaneous heat integration and flowsheet optimization of the combined cooling, heating, and power system based on the methanol-steam-reforming proton exchange membrane fuel cell. The framework comprises a detailed modelling of methanol steam reforming reaction, fuel cell performance, cooling/heating cogeneration systems, heat integration, heat exchanger network synthesis and energetic-economic performance evaluation. Additionally, the framework incorporates the sizing of the corresponding equipment, including the total length of the reformer, scale of proton exchange membrane fuel cell stack, and absorption cooling apparatus. Furthermore, it takes into account the operating conditions, such as the temperature and pressure of methanol steam reforming reaction, the operating temperatures and pressures of the fuel cell stack and absorption cooling system. We apply the framework to a 1000 kWe combined cooling, heating, and power generation system, and the integrated design achieved an energy efficiency of 88.50% and a levelized cost of electricity of 0.2374 $/kWh. The results show that the simultaneous heat integration and flowsheet optimization can increase the system's energy efficiency by 5.45 percentage points, exergy efficiency by 2.22 percentage points, and decrease the levelized cost of electricity by 4.50% compared to a conventional design.

Techno-Economic Analysis of a Hybrid System with Carbon Capture for Simultaneous Power Generation and Coal-to-Hydrogen Conversion

Under the growing pressure of decarbonization across industrial sectors, power plants face great challenges in reducing carbon emissions. To improve energy efficiency and reduce the carbon emissions of thermal power plants, a hybrid system with carbon capture for simultaneous power generation and coal-to-hydrogen is proposed, which features comprehensive consideration of the energy demands of power plants and coal-to-hydrogen units. In the proposed system, the reduction of energy demand is realized by heat exchanger network synthesis (HENs) of the internal streams, and a carbon capture unit is then integrated for further reduction of carbon emissions. The techno-economic feasibility of the integrated system is analyzed and discussed by using energy consumption, economics, and emission reduction as major performance indexes. The results show that energy integration saves 108.2 MW of heat, while nearly 90% of carbon emissions are reduced for the system. When comparing the total energy consumption of the subsystems and a stand-alone carbon capture unit, the consumption for cooling and heating utilities in the integrated system is reduced by 8.81 and 9.11%, respectively. In addition, the total overnight cost and the annual operation cost using the proposed integrated system are reduced by 8.51 and 22.69%, respectively, and the cost of hydrogen production decreases by 5.99%. The economics of the integrated system is significantly affected by the market prices of coal and hydrogen. The integrated system has significant advantages in reducing energy consumption and carbon emissions with higher economic performance, which can provide guidance for the low-carbon transition of coal-fired power plants.

Intelligent search strategy applied in heat exchanger network synthesis

Heat exchanger network (HEN) synthesis has been acknowledged as an effective design technique to achieve significant energy savings in a wide range of industrial and engineering applications. In this regard, heuristic methods have been demonstrated as a powerful means to solve the HEN synthesis problem. However, because the new structure is generated at random, the structures that can effectively make the objective cost descend are limited in comparison to the entire evolutionary process. In addition, such heuristic methods are usually time-consuming processes. In this work, an intelligent search strategy is proposed to increase the efficiency of heuristic methods. Based on the defined objective cost, this strategy could rapidly and effectively find a route that could satisfy cost minimization. It can also achieve further evolution aimed at highlighting potential structures. The proposed search strategy can increase the ability of searching and reduce time consumption. Considering the random walk algorithm with compulsive evolution as an example, the principle and impact of intelligent search were clearly demonstrated. Furthermore, applying the intelligent search strategy to the random walk algorithm with compulsive evolution to some cases in the existing literature was found to yield better solutions.

Enabling in-depth analysis in heat exchanger network synthesis via graph-theoretic tool: Experiences in Swinburne University of Technology Sarawak Campus

The ability and capability to analyze and benchmark alternative designs on top of the optimal network are deemed valuable competencies for current and future chemical engineers. In this context, a process graph (P-graph)-inspired tool – P-HENS is introduced to an integrated plant design unit in an undergraduate chemical engineering degree program at Swinburne University of Technology Sarawak Campus in Malaysia. The energy recovery aspect is one of the key design elements in the integrated plant design unit. The introduction of P-HENS, which is capable of mathematically determining multiple optimal and sub-optimal solutions is considered useful for the students to (i) identify plausible heat exchanger networks (HENs) structures that may be overlooked using conventional approaches and (ii) enable a more in-depth analysis to justify the selected design. Overall, the implementation of P-HENS shows positive outcomes, where this free-of-charge software complements the learning of conventional manual approaches used in HENs synthesis. Furthermore, recommendations suggested by the users (students) are collected and compiled for potential future software development. This work serves as an essential reference for other chemical engineering educators who are teaching pinch analysis or heat integration-related courses.

Critique – Comparison between P-HENS and Aspen Energy Analyzer for heat exchanger network synthesis

This critique evaluates the use of the P-HENS software for developing recommended heat exchanger network designs compared to the commercial simulator Aspen Energy Analyzer. The similarities between the two software when carrying out an energy integration are discussed, as well as their advantages and possible improvements to be implemented in the tool.

Flexible heat integration system in first-/second-generation ethanol production via screening pinch-based method and multiperiod model

In sugarcane biorefineries, energy integration plays a fundamental role regarding by-product destination and final sales profitability. In biorefineries, the by-product of the sugar/ethanol production process – sugarcane bagasse – can be burned in boilers to produce steam to supply the thermal demands of the plant. Normally, the quantity of bagasse from the plant exceeds the quantity necessary for supplying the energy requirements of the plant. This surplus can be used for either (i) additional electricity production and selling to the grid or (ii) second-generation (2G) ethanol production. To maximize profit, the plant should be flexible to produce either electricity or 2G ethanol according to the current market prices of these products. In this work, an approach is presented for designing a flexible heat exchanger network (HEN) based on a multiperiod synthesis. It involves assessing probabilities of occurrence for different bagasse use scenarios. These values are obtained from a pinch-based approach for screening optimal heat integration in a range of ethanol/electricity prices. A multiperiod HEN synthesis problem is then solved with variable flow rates. Results show that maximum bagasse diverted to 2G ethanol production is increased from 66% to 77% with the new HEN compared to the original design considered.

Integrated synthesis and control of heat exchanger networks with dynamic flexibility consideration

• Dynamic flexible aspect is taken into account in early integration stage. • A sequential iterative algorithm for the problem solution is devised. • Stream matches, heat transfer areas and bypass fractions are optimized. • Results indicate considerable cost (−14.95%) and control performances (+61.83%). • Major advantages found for case studies with multiple disturbances. Flexible synthesis and control which can be both used to reduce the influences of disturbances on heat exchanger networks are often considered as two separate issues assisting network development at different stages. A proper integration of these separate yet connected tasks carries the promise of achieving superior designs. Currently, the study of integration of heat exchanger networks is still towards a sequential procedure consisting of initial flexible synthesis based on steady-state economic calculations followed by a controller. To overcome this challenge, based on the stage-wise superstructure, an optimization framework in this work is presented to address dynamic flexible synthesis and advanced control simultaneously for maximizing performance in face of disturbances. The framework is based on a sequential iterative procedure that decomposes the overall problem into two stages. The first stage is performed by the dynamic flexibility analysis where the design variables are chosen. In the second stage, the optimization variables are adjusted during the discrete time intervals on the realizations of the control actions and the variation ranges of outlet stream temperatures. The sequential iterative is to map the temperature regulations to the network configuration retrofits. The application to two case studies indicates that the proposed framework returns solutions which are considerably better all in terms of dynamic flexibility, economics and control performance than those published in literature. Compared to previous literature studies, the optimized solutions feature a total annual cost reduction up to 14.95%, a decrease in control action up to 48.58% and an increase in control performance up to 61.83%. Moreover, the application to two case studies indicates that allows solving a real-world problem with up to 26 hot streams and 29 cold streams (leading to models with a size of 523 binary variables and 1655 equations).

Heat Integration for Phenols and Ammonia Recovery Process of Coal Gasification Wastewater Considering Optimization of Process Parameters

A heat integration optimization method that considers the changes in process parameters is proposed to find the global optimal process scheme for a coal chemical company’s phenols and ammonia recovery process. The phenols and ammonia recovery process is simulated by Aspen Plus, and a programming method for heat exchanger networks synthesis that can simultaneously optimize process parameters and heat integration is constructed by Matlab. Taking the total annual cost as the objective function, the following process parameters are optimized: the hot feed temperature and cold/hot feed ratio of sour water stripper, the temperature of three-step partial condensation system, the feed temperature and column pressure of both solvent distillation column and solvent stripper. The result shows that, compared with the heat integration process under original process parameters, the new heat integration process saves 14.3% energy consumption and reduces the total annual cost by about 15.1%. The new heat integration process provides guidance for the optimization of the phenols and ammonia recovery process. The proposed heat integration optimization method based on changing process parameters is an effective and practical tool that offers good application prospects.

Multi-Objective Optimal Design and Operation of Heat Exchanger Networks with Controllability Consideration

Controllability reflects the ease that a process can be controlled in practical operating environment. However, an unclear influence between the HEN synthesis and the control structure selection has been not investigated for the work of controllability of heat exchanger network (HEN). To address this challenge, this paper proposes a multi-objective optimization method by considering both the quantitative measures of economic and controllability, i.e., minimizing the total annual cost (TAC) and the relative gain array number (RGAn). This method is developed using a HEN synthesis procedure where a model-based superstructure is employed to involve the set of the network configuration alternatives and all the potential control structures. The effects of minimum approach temperature (ΔTmin) on the multi-objective optimization problem are investigated to distinguish the consistent and opposite variations of TAC and RGAn. The consistent change enables us to solve the single objective optimization problem for economical HEN design as well as for taking controllability into account. The opposite change prompts the Pareto front of the two objectives in order to develop a trade-off strategy. Results indicate that this method helps in the determination of the relationship-based nature between network configuration and control structure to yield a HEN design with economic and controllability considerations.

Generalized simultaneous optimization model for synthesis of heat and work exchange networks

In this work, a new superstructure for simultaneous synthesis of work and heat exchange networks (WHEN) is presented. In the proposed model, an explicit modeling of the identity changes of the process streams is developed. The main idea behind this approach is to allow the pressure-change streams act as low-pressure hot streams as well as high-pressure cold streams in different stages of the WHEN without using any predefined manipulation routes. In this sense, using this novel approach, all possible thermodynamic paths of the process streams are considered into the synthesis problem. The novel formulation is done without incorporating new binary variables and nonlinear constraints in the problem. Therefore, the proposed superstructure allows to improve the obtained solution and to reduce the computational burden. These improvements are shown by using three case studies with different size and complexity. In most cases, savings of approximately 1%–7% in total annualized cost were observed.

An enhanced heuristic algorithm with coordination evolution strategy for waste-mass minimization and heat integration network design

The issue of clean production and energy efficiency has become a major concern, especially in the chemical and processing industries. In this paper, an enhanced optimization algorithm satisfying heat-waste minimization and waste-mass minimization is proposed. First, a nodes-based non-structural model and random walk algorithm with compulsive evolution for mass exchanger network synthesis are constructed. The optimization mode of continuous variables is analyzed after analyzing the characteristics of the mass exchange process. To enhance the optimization ability of the heuristic algorithm, a novel coordinated evolution strategy combining the dynamic operations of the walk length optimization and participating evolution probability adjustment is proposed. The proposed strategy can adjust the walk length via an instant objective function change and simultaneously controls the participating evolution of the number of units for effective coupled evolution. The quantified data show that the proposed strategy assists the algorithm in enhancing the local and global optimization abilities, with the final TACs of a benchmark aromatic plant case and phenol absorption process achieved being 125,457 $/yr and 334,696.6 $/yr, respectively. To evaluate the effectiveness of the proposed strategy, it was successfully applied in two industrial-scale cases of heat exchanger network synthesis, where the TAC of the aromatics plant case achieved was 2,892,800 $/yr while the TAC of a gasoline distillation case was 5,089,402.7 $/yr.

Heat Integration and Heat Exchanger Network Design with Aspen Energy Analyze

The considerable amount of energy used in industrial processes is typically used for heating and cooling. The synthesis of heat ex-changer networks (HEN) is a critical topic in this industry in order to reduce energy and capital costs in chemical plants. Heating and cooling systems in industry must be designed efficiently, which can be done using technologies like pinch analysis to create an optimal heat recovery network. Heat pinch analysis is a systematic method for designing thermally efficient systems. A designer can determine the minimum heating and cooling requirements as well as the maximum heat recovery (MHR) potential by locating a thermodynamic bottleneck, or the pinch point for heat recovery. This Paper uses Aspen Energy Analyzer for HEN Design.Heat integration network design is a cost-effective strategy that helps industries to cut costs by reducing the number of external utilities needed and increase profitability by lowering energy consumption. The HENs’ composite curves, grand composite curves, grid representation, and target reports were created using the Aspen Energy Analyzer. The software suggested ten designs, which were then optimized based on total annualized cost. By comparing it to other optimized designs, the best design is determined.

Enhanced superstructure optimization for heat exchanger network synthesis using deterministic approach

Heat Exchanger Network (HEN) synthesis is primarily formulated as a mixed integer non-linear programming (MINLP) problem based on method of stage-wise superstructure (SWS). Approaches to obtain an optimal HEN configuration can adopt deterministic algorithms. But, as a large scale problem, it is difficult to solve due to the complexities arisen from nonlinearities of SWS. To overcome the model nonlinearities, stochastic algorithms and meta-heuristic approaches have been proposed in the literature to tackle the problem. However, it reaches a near-optimal HEN configuration from a series of stochastic solutions, which are obtained by the execution of many computational procedures with extensive time resource, and a global optimum will not be guaranteed from only randomly generated results. In this paper, an enhanced SWS approach is proposed, in which new temperature and heat duty constraints are updated to reduce the redundant combinations and avoid conflicted calculation of non-isothermal mixing energy balance. The present model is also extended to allow flexible stream splitting for practical applications. Then, a deterministic-based global solver (GAMS/BARON) is applied in solving three case studies. The results show that the proposed approach can provide cost-efficient HEN solutions with lower TAC than using existing stochastic and deterministic algorithms.

Heat exchanger network optimisation considering different shell-side flow arrangements

Heat exchanger network synthesis (HENS) is an effective tool for heat recovery in chemical and petrochemical industries. This study aims to show a method for HENS with the consideration of different shell-side flow arrangements in shell-and-tube heat exchangers. The proposed MINLP model is modified from the stage-wise superstructure model, incorporating newly developed correlations for shell-side pressure drop calculation for the helical baffle. The objective is to minimise the total annual cost (TAC) with a trade-off between the cost of different shell-side heat exchangers, utility cost, and pumping cost. The selection of baffle design is decided by the saved pumping cost and the increased area cost. The proposed model is tested from different points of view: with/without utility constraints and different statuses of the streams. Three case studies are presented and compared with the results from the literature. The proposed method with different shell-side baffles can reduce the heat transfer area and pressure drop. For fixed utility consumptions, the TAC in a case study is decreased by 8.9% with mixed baffle types. Although the unit per area cost of the helical baffle is higher than the segmental baffle, the increased investment cost could be compensated by the reduced operation cost, especially for plants with high viscosity streams and long-term cost-saving.

Synthesis of multiperiod heat exchanger networks: Minimum utility consumption in each period

Heat exchanger networks in real industrial processes often have to be designed for varying parameters of the process streams that renders multiperiod heat exchanger network synthesis essential in industrial realization. However, three standing issues must be considered in the synthesis procedure. (1) The generated network should be efficient in all periods, preferably consuming minimum utility. (2) This network should also be structurally as simple as possible for minimizing the investment cost and making the process controllable. Furthermore, (3) all or the n-best networks that fulfilled the first two principles must be considered for the selection of the most preferred industrial realization. The method proposed in the current work satisfies all three requirements simultaneously; it has been the only known method with these properties. The method primarily relies on the formerly developed P-graph-based heat exchanger network synthesis procedure. Three case studies demonstrate the application of the proposed method.

A novel two surfaces hybrid approach for Multi-period heat exchanger networks synthesis by combination of imperialist competitive algorithm and linear programming method

• A two-surface hybrid approach was introduced for multi-period HENs synthesis. • It was combined linear programming (LP) + imperialist competitive algorithm (ICA). • The ICA is used to find the structural variables on the 1st surface. • The continuous variables are handled with a combined LP and ICA on the 2nd surface. • The relatively linear behavior improved its ability to deal with HENs problems. In this paper, an efficient hybrid approach to deal with multi-period heat exchanger networks (HENs) synthesis, which is inherently formulated as a mixed-integer non-linear programming (MINLP) model, is proposed. In this novel two surfaces approach, the creation and determination of optimal HENs structures are accomplished by an imperialist competitive algorithm (ICA) on the first surface. In each HEN structure, there is a sequence of stages containing the addresses of the exchanger(s). The HENs generated by the ICA are sent to the second surface, where their minimum total annual cost (TAC) is calculated as an overall objective function. This surface works on two levels. For all periods of each HEN, the local optimal solutions are determined based on the maximum energy recovery at the outer level, including an external search loop and a linear programming (LP) model. Based on the outer level results, the ICA is re-used, at the inner level, to find the final minimum TAC of the network. As a result, the MINLP model is transformed into a relatively linear LP + ICA hybrid model, which is easily solvable. The results demonstrate that this approach can sometimes reduce the network TAC even by over 7.2% compared to the literature.

Parameters for cost estimation in shell and tube heat exchangers network synthesis: A systematic literature review on 30 years of research

Heat exchangers are essential elements used in a wide range of engineering applications. Heat Exchanger Networks (HEN) synthesis and optimization is an important field in process integration and different objective functionshave been used to calculate the network total annual cost. This paper presents a comprehensive review in terms of the published literature on HEN cost calculations, highlighting papers related to the design and optimization of HEN. The main research in HEN’s parameters for total cost calculation and its contribution to how these costs are formed, through a systematic literature review is presented in the present paper. It can be noteda gap in this field indicating that apart from a few studies published after 2005 and despite the emergence of new studies on changes in HEN cost functions, the use of more traditional cost calculation models still prevails in the current literature. This review also identifies the current challenges regarding this research field through analysis and comparison. Finally, issues related to cost parameters of objective functions concerning HENoptimization are highlighted and scope for future research is discussed.

Multiperiod Heat Exchanger Network Synthesis With Pinch-Based Strategies and Metaheuristics

Heat exchanger networks (HENs) are a widely studied subject. These systems may undergo important variations in their operating conditions. Such uncertainties lead networks to require some degrees of flexibility. An efficient approach to address such uncertainties is the development of multiperiod solutions. However, these are difficult to develop since one must synthesize a network that is simultaneously feasible under a given number of conditions. This work presents a method based on pinch insights and a hybrid meta-heuristic. It employs the “spaghetti” network concept as initial estimations for single-period networks, which are refined and merged in a single solution that is able to operate under multiple period conditions. The merged solution is refined as well with a multiperiod HEN model, and a final multiperiod network is achieved. The method was able to outperform previous literature regarding total annual costs (TAC) for the multiperiod HEN developed. The case study approached is one of the largest multiperiod cases for HEN synthesis in the literature (15 streams, 4 periods). The obtained solution has TAC 3.5% lower than the literature solutions.