Heat Exchanger Network Synthesis Research Articles

Stochastic optimization-based approach for simultaneous process design and HEN synthesis of tightly-coupled RO-ORC-HI systems under seasonal uncertainty

The use of waste heat powered wastewater desalination by coupling an organic Rankine cycle (ORC) with reverse osmosis (RO) has been recognized as a promising solution to desalination. Herein, a tightly-coupled system that allows for optimally customizing the RO-ORC to a background process via heat integration (HI) is developed to minimize the expected desalination cost, while accounting for the seasonal wastewater variability. Moreover, the developed RO-ORC-HI system can improve the membrane permeability by preheating the feed wastewater. To achieve these goals, a stochastic optimization-based solution strategy is proposed by sequentially considering (1) a Pinch-based Duran-Grossman model embedded with uncertainty realization for performing optimal HI during process optimization; (2) a flexible multi-scenario heat exchanger network (HEN) synthesis model that minimizes the total annualized cost of HEN based on a customized stage-wise superstructure. Finally, the behaviors of the proposed system and solution strategy are illustrated through its comparison with a deterministic solution.

A trust region framework for heat exchanger network synthesis with detailed individual heat exchanger designs

We develop a trust region filter strategy for simultaneous optimal design of heat exchanger networks that includes detailed design of shell-and-tube heat exchangers. The strategy first solves a mixed-integer nonlinear programming (MINLP) formulation with shortcut models to generate candidate network topologies, which are then used in a non-isothermal mixing nonlinear programming (NLP) suboptimization with detailed optimal exchanger design models embedded using a modified trust region filter (TRF) algorithm. An integer cut based strategy is used to bound the solutions from MINLP and the NLP which aids in convergence to the solution of the overall simultaneous design problem. Under assumptions, the TRF based strategy can guarantee convergence to near optimal solutions of the overall design problem. The presented solution strategy is thus able to find optimal heat exchanger network designs based on the simultaneous optimization of the network topology and mass and energy balances, together with detailed shell-and-tube heat exchanger optimization, including the number of shell and tube passes, pressure drops, and tubes, tube lengths, etc. The proposed strategy is tested on three literature based case studies and their results are compared with previous studies to showcase its performance.

Optimization route arrangement: A new concept to achieve high efficiency and quality in heat exchanger network synthesis

Heuristic algorithms are widely used in the global optimization of heat exchanger network synthesis (HENS). However, since heuristic algorithms are often characterized by a divergent search and a combination of multi-parameters, it is difficult to weigh and balance the global and local search in the process. In existing algorithms, the global and local search weight is commonly fixed during optimization. As a result, the algorithms hardly distribute the structural and continuous variables optimization suitably, sometimes even becoming invalid. Even more so, such algorithms fail to realize functions enhancing the searching ability. Therefore, this paper proposes a new, phased, modular concept that supports functional and sub-functional combinations to timely escape the stagnant status and obtain satisfying solutions with high efficiency and high quality. The concept's main goal is to achieve flexible combining and sequencing of different functions during the optimization process. Hence, the optimization route arrangement emphasizes the functions’ distribution, which differs from the sequential methods decomposing the whole problem. Each route's components are not unique, and the structure's current needs can change the modules’ sequences and categories. Such coordination of modules improves the algorithm's exploration and exploitation abilities, enabling the proposed method to attain better results within less computational time and rendering. Therefore, the proposed method is more suitable in large-scale HENS owing to much local optima involved in cases. Three cases are used to demonstrate the proposed routes’ validity, which shows that the optimization route arrangement successfully decreases the total annual cost and improves computational efficiency.

Comparative Study of Thermal Waste Recovery Systems Deployed in Three Different Chemical Units

Waste thermal energy is enough amount of energy that is rejected to the atmosphere in the form of flue gases, streams of air, and liquid rejected from industries. It arises from the equipment, less efficient processes, and limitations due to the laws of thermodynamics on operations. It is obvious that it is not possible to regenerate all waste energy but most of the time some waste heat can be used to achieve useful purposes. Waste heat recovery is the most important key to carry out most of the research areas. The major areas of research and it is necessary to make the process more energy-efficient in chemical industries. To save energy Heat Exchanger Network’s synthesis (HEN) is essential. They are designed to reach energy targets. HEN design is the thermal integration between cold and hot utilities by pinch analysis at minimum temperature difference. HENs are important for utility saving because it helps in recovering heat from hot streams to others which reduces the utility consumption and requirement. The heat exchangers design with simplified models for different industries using pinch technology by which most thermal recovery is obtained and then some HEN network is required for a particular targeted area. In this research improvements in energy recovery systems and HENs, synthesis helps in capital savings and pollutant emission can also be reduced.

Simultaneous optimisation of large-scale problems of heat-integrated water networks

This work presents a simultaneous mixed-integer nonlinear programming (MINLP) optimisation model and an efficient iterative solution strategy that can be successfully applied to various heat-integrated water networks (HIWNs), including large-scale problems with a large number of water streams. These problems are highly nonlinear, non-convex and combinatorial. To circumvent such difficulties, including network complexity as well as identifying the roles of water streams in the heat exchanger network (HEN) whether they are hot or cold, a modified convex hull formulation proposed by Ahmetović and Kravanja [1] is applied. The overall model combines the water network (WN), wastewater treatment network (WTN), heat integration (HI), and heat exchanger network synthesis (HENS) models. This model is iteratively solved in three steps including targeting and design steps. The proposed model and solution strategy are tested on large-scale problems. To the best of our knowledge, the results obtained for all the problems in this paper are better than those reported in the current literature.

Synthesis of stream‐split heat exchanger networks using non‐structural model considering serial equipment in stream branches and submixing of substreams

AbstractThis paper proposes a non‐structural model with stream splits, for heat exchanger network (HEN) synthesis, that allows for submixing of substreams within splits and series connection of exchanger units in a substream. In a submix, one substream totally combines with another non‐isothermally, and by allowing this feature, the designer incurs only piping costs, which are significantly low compared to other HEN costs, and benefits from increased possibility of manipulating substream temperatures. An algorithm employing random walk principles and compulsive evolution is applied for HEN optimization. The method's randomness, which enhances its explorative searching power and efficiency, is an attractive property that matches the proposed model. Four case studies from the literature are solved and annual cost savings of $1800, $7769, and $4771/year are achieved for three of them in comparison with published best solutions, attesting to the effectiveness of the proposed model.

An improved algorithm for synthesis of heat exchanger network with a large number of uncertain parameters

This paper presents an improved method for the Mixed Integer Nonlinear Programming (MINLP) synthesis of flexible Heat Exchanger Network with a large number of uncertain parameters. Typically, such a problem is written as a multi-scenario two-stage stochastic model with recourse which is difficult to solve because the size of the model grows exponentially with the number of uncertain parameters. The exponential growth could be avoided by decomposing the model into simpler problems that are solved sequentially in a small number of scenarios.In this work, the determination of the first-stage variables (process topology and unit sizes) and the second-stage variables (operating and control variables) is decomposed into several steps where they are determined separately. This is achieved by iteratively solving smaller two-scenario MINLP and one-scenario Nonlinear Programming (NLP) subproblems. In this way, the size of the problem remains independent of the number of uncertain parameters.The innovation of this study is the introduction of correction factors for the second-stage variables into the objective function when determining the first-stage variables. In this way, better trade-offs are found that mitigate the unpleasant effect of decomposition. The synthesis of the heat exchanger network shows that the implementation of correction factors improves the optimal result by 7.6%.

An analytical model to study the dynamic response of heat exchanger network

An analytical model for the heat exchanger network (HEN) has been proposed in this work to study the dynamic performance of HEN in the design stage. The model relies on the Laplace transform to obtain the system outlet temperature function when facing operational changeover. Equipped with the decomposition strategy and pathway analysis, the model can reach the system outlet temperature function in the time domain in an easy way and free of the numerical difficulty concern caused by the inverse Laplace transform process that may occur toward complicated expressions. The potential bias of the model has been illustrated both in static and dynamic aspects by changing the operational conditions of a heat exchanger. The analytical model has been applied in a four streams HEN synthesis problem to investigate the trade-off result between the economic cost and the response time. Moreover, the comparison with the simulation result toward selected HENs has shown that the proposed model may act as a pre-selection tool to predict the dynamic performance of various HENs during the design stage.

Multiple-solution heat exchanger network synthesis using P-HENS solver

Analysis on the alternative designs on top of the optimal network has proven valuable and meaningful for the decision-makers in determining the most suitable options which fulfill a wide range of objective functions. On the basis of an extension of the P-graph framework, a procedure was developed previously for multiple-solution heat exchanger network (HEN) synthesis. This procedure is capable of generating the n-best HENs depending on predefined structural constraints, for example, the maximum number of heat exchangers used for the entire system, the maximum number of sequential heat exchangers on each stream, the maximum number of stream splittings per stream. Since the choice of a parameter influences the effect of other parameters on the result, it is difficult to find the proper set of parameters for the solver that result in all plausible solutions to the original problem. Naturally, this issue emerges in any HEN synthesis problem, its systematic treatment is essential. The purpose of the current work was to develop a heuristic framework for determining the most reasonable parameters of the HEN generation algorithm and guiding the designer through the optimization process. The outcome of this study not only benefits the researchers and industrial practitioners of this field but may also be extended for educational purposes.

A surrogate-based optimization framework for simultaneous synthesis of chemical process and heat exchanger network

Heat-integrated process synthesis is fundamental to achieve higher energy efficiency. The well-known sequential-conceptual methods have been widely adopted to solve the synthesis problem in a hierarchical manner. However, the natural hierarchy fails to consider complex interactions between the unit operation and the heat integration. To address this issue, a surrogate-based optimization framework is proposed for simultaneous synthesis of chemical process and heat exchanger network. An artificial neural network (ANN)-based surrogate model, derived from the simulation data generated via rigorous mechanism modelling approach, is established for process units to replace their complex realistic models. With surrogate model formulation incorporated into heat integration, an enhanced transshipment-based mixed integer nonlinear programming model is introduced to synthesize heat exchanger network with variable flowrates and temperatures, aiming at the maximized annual profit. Finally, two example studies are investigated to demonstrate the effectiveness of the proposed framework.

Node dynamic adaptive non-structural model for efficient synthesis of heat exchanger networks

Balancing the trade-off between structural diversity (solution space) and optimization efficiency is an urgent challenge in the modeling and global optimization of heat exchanger network synthesis problems. This paper presents a node dynamic adaptive non-structural model without stream splitting to enhance optimization efficiency under the premise of ensuring an adequate solution space. The node-based non-structural stream matching mechanism can be used to make the solution space scalable and achieve the efficacy of multi-stage stage-wise superstructure with a finite number of nodes. Two node dynamic adaptive strategies (uniformly distributed and random) are established to periodically adjust the number of nodes in process streams and the node distribution of existing heat exchanger units, thereby preventing a decrease in computational efficiency due to an excess number of preset nodes while still satisfying the solution space requirements. The node dynamic adaptive non-structural model is optimized using the random walk algorithm with compulsive evolution. The proposed method was applied to four well-known heat exchanger network case studies. High-quality solutions were obtained, and they are more economical than most of the optimal results previously reported in the literature. These results demonstrate the enhanced computational efficiency and applicability of the proposed synthesis method based on a node dynamic adaptive non-structural model for efficiently solving large-scale heat exchanger network synthesis problems.

Heat Exchanger Network synthesis considering prohibited and restricted matches

In the industry, there are frequently prohibited and restricted matches in the Heat Exchanger Networks (HENs). These matches could cause risk cost when failures happen. Some matches of streams can be entirely prohibited, but some may only have a certain level of restriction. The margins are set up, based on the risk cost assessment, whether to exchange the heat and by which type of equipment between specific streams. This study proposes a method based on the Advanced Grid Diagram to benefit from an optimised synthesis plan for HEN considering prohibited and restricted matches. A pre-designed HEN could be determined in the synthesis stage, not considering the prohibited or restricted matches. A follow-up topology evolution is performed to trade off the prohibitions and evaluate the restrictions. A Benefit Diagram has been developed to show how many economic benefits can be gained by implementing new heat exchangers. It is used for comparison between allowing all restricted streams and avoiding restricted streams to find the potential risk cost margin. The method is applied to a case study to analyse the restricted and prohibited matches. Optimal HEN design results offer the designers a guide to evaluate to which extent should implement the restricted matches.

Passivity based control of heat exchanger networks with application to nuclear heating

Due to the common existence of heat exchanger networks (HENs) in fossil, nuclear and chemical plants, the synthesis and control of HENs play a key role in guaranteeing the plant operation performance. With comparison to the HEN synthesis (HES) that optimizes the topology and steady operation point of a HEN, the control of HEN provides satisfactory transient performance under load variations as well as interior and exterior disturbances. Without a properly designed control law, the optimized operation point of HEN cannot be maintained stably, which shows the importance of HEN control. In this paper, a dynamic model of HEN is first given based on the thermodynamics of a single heat exchanger and the directed graph describing the interconnection amongst multiple heat exchangers as well as heat sources and sinks. By adopting the total entransy as the corresponding storage function, it is also shown that the dynamics of HEN is strictly passive. Then, a passivity-based control of HEN is newly proposed, which can provide globally asymptotic stability while taking a simple form. Finally, the designed HEN control law is applied to the regulation of a HEN for nuclear district heating, and numerical simulation results not only verifies the theoretical results but also shows the satisfactory control performance.

Application of energy-saving hybrid distillation-pervaporation process for recycling organics from wastewater based on thermoeconomic and environmental analysis

Acetonitrile and n-propanol play an indispensable role in the manufacture of perfume and photosensitive materials, and the demand for these chemicals in China is growing. In this work, a series of processes was explored for achieving sustainable and green separation of wastewater containing acetonitrile and n-propanol. The solvent with high separation performance was determined by the combination of relative volatility and biological toxicity. Based on the sequential iterative optimization sequence and taking the minimum total annual costs (TAC) as the objective function, the program written by Visual Basic software was used to optimize the parameters of the basic process. The steam recompression assisted distillation and pervaporation (PV) technology are combined to reduce the energy consumption. Through the Heat Exchanger Network Synthesis (HENs) option, the heat recovery in the process was further improved. Furthermore, exergy and environmental analyses were further used to assess the feasibility of several separation processes. The results showed that compared to the basic process, the TAC and exergy loss of vapor recompression assisted distillation process were reduced by 43.6% and 12.65%, respectively. The use of heat integration and PV technology further reduced energy consumption. The TAC and emissions of novel hybrid process were saved by 74.6% and 53.87%, respectively, which indicates the feasibility of the processes for continuously updated cleaner production.

The design of the total site exchanger network with intermediate heat carriers: Theoretical insights and practical application

The present work is focused on finding a cost-effective solution to the synthesis of Heat Exchanger Networks (HEN) for Total Site Integration (TSI). The possible solutions for minimum-investment retrofit of the Total Site HEN are also discussed. The feasibility of intermediate heat carriers number is also analysed for better plants interconnection considering the optimal amount of the recovered heat.The presented case studies illustrate the pathway selection of site recovery system synthesis with a certain number of heat exchangers, heat transfer area, intermediate utilities, and interconnection between production processes, heat consumers, heat generation facilities and other members of an integrated regional network. Case studies demonstrate that the use of two proposed approaches allows minimising unit numbers and heat transfer area. The first one provides 18% less heat transfer area and the second one suggest network with 41% less unit. The economic results are compared and discussed. The results of this work can be used for site recovery network development as well as a decision-making tool during interplant heat integration and improvement of side-wide heat recovery. The results of this paper are potentially interesting for both research and engineering staff. The current work provides the methodological improvement when designing the Total Site Heat Recovery and better estimation of the HEN capital cost.

Simultaneous integration of heat pumps and different thermal energy storages into a tightened multi-period MILP HENS superstructure formulation for industrial applications

Design optimization of industrial installations with help of mathematical programs offers high potential for energy savings and cost reduction, which steadily gains importance. Heat exchange network synthesis (HENS) is one of the commonly used and promising methods. This paper deals with simultaneous integration of heat pumps and two different storage types into mixed integer linear programming (MILP) HENS by extending a multi-period superstructure formulation proposed by Beck and Hofmann (2018c). Further measures for tightening the solution space for the coefficient of performance (COP) are introduced. This improves the accuracy of the heat pump (HP) approximation, reduces computational effort and prevents solutions with uneconomical low COP. The obtained approach allows to design cost efficient heat recovery systems with storages (ST) and HP for the improvement of the energy recovery capabilities. A constructed test case was used to analyze the performance of the method and to show its possible potential.

Efficient Strategy for the Synthesis of Work and Heat Exchange Networks

The work and heat exchange network synthesis problem refers to the problem of determining a network of heat exchangers and pressure equipment that minimize the total annual cost or exergy consumption, where the thermodynamic paths (i.e., pressure-change and temperature-change paths) are variable. In this work, an efficient optimization strategy is presented, consisting of two synthesis phases. In the targeting phase, a novel targeting model is introduced to identify the optimal thermodynamic paths of process streams. The genetic algorithm in the outer level is applied to adjust the streams’ temperatures, and the golden section search method in the inner level is used to adjust the hot utility consumption. With the obtained thermodynamic paths, the detailed heat exchanger network is determined using a global search algorithm in the design phase. The integrated methodology is applied to four cases. Except for the illustrating example, results show a decrease of 4.26% in the total annual cost (TAC) for the liquefied natural gas (LNG) example, 48.7% in the TAC for the postcombustion CO2 capture example, and 1.05% in the TAC for the two-stage membrane separation process example.

Investigation of loops and paths as optimization tools for total annual cost in heat exchanger networks

This research investigates the effectiveness of loops and paths as embedded in a modified pinch package (Aspen Energy Analyzer), that comprises a blend of traditional pinch technique with mathematical programming, in simultaneous optimization of total annual cost (TAC) in heat exchanger network synthesis (HENS). It uses composite curves, grand composites curves, supertargeting and looping system just as in pinch, as well as linear programming and mixed integer linear programming (MILP) in the design of HENs. The tool was adopted in solving some literature problems that had earlier been solved using pinch and mathematical programming techniques. The results obtained compared well with those of various authors that used different techniques as shown in the tables of cost comparison in this paper. The loop and path optimization technique adopted in this research obtained the least TAC in four out of five problems solved in this paper. This shows that loop and path optimization as available in a software-combining pinch and mathematical programming can be more effective than various other methods that have been adopted in the literature. It further demonstrated that no particular technique can return the lowest TAC for all HENS problems.

A New Nodes-Based Model for Optimization of Heat Exchanger Network Synthesis

Searching the global optimal value in heat exchanger network synthesis (HENS) becomes more difficult along with the increasing scale of cases. In structural optimization model, such as stage-wise superstructure, solution domain can be enlarged by adding the stages of network, which would involve multiple integer and continuous variables in optimization and result in a decreasing computational efficiency. Thus, a new nodes-based model with stream splits is proposed, which can promote the quality of result and decrease time consumption at the same time. The characteristic of this model is that it changes the matching mode of heat exchangers by setting the number of nodes on streams to quantify the positions of heat exchangers. The proposed nodes-based model has more flexibility in generating and eliminating the heat exchangers and strong ductility in exploring the solution domain. The random walk with compulsive evolution is modified for enhancing its searching ability and applying modified random walk algorithm with compulsive evolution (RWCE) into the new proposed model. The obtained results of Case 1, Case 2 and Case 3 are superior to those reported in the literature, which decreases 106 USD·a−1, 1253 USD·a−1, 23 444 USD·a−1, respectively, verifying the robust effectiveness of this new model in solving HENS problem.

Multiperiod work and heat integration

The synthesis of multiperiod heat exchanger networks (HEN) is a well-studied topic in heat integration. Several methods for identifying heat exchanger network designs that are able to feasibly operate under multiple conditions have been presented. Multiperiod models are certainly a notable form of achieving such resilient designs. In work and heat integration, however, solutions presented so far are for nominal conditions only. This work presents a step-wise optimization-based multiperiod work and heat exchange network synthesis framework. Hybrid meta-heuristic methods are used in the optimization steps. The methodology is able to obtain work and heat exchanger networks (WHENs) that are able to operate under multiple known scenarios. A set of critical conditions for stream properties in work integration is proposed. When these scenarios are modeled as finite operating periods (which are here referred to as non-nominal periods), a WHEN which can feasibly operate under nominal and critical conditions can be obtained. An example is tackled in two cases: the first, with one nominal and six critical, non-nominal periods; the second with two nominal and twelve non-nominal periods. Note that with that number of periods, the problem is considerably more complex than in multiperiod HEN synthesis (which usually comprises three or four periods). Solutions obtained with the present method are compared to those obtaining by simply merging single-period solutions obtained for each period individually. Capital investments are 30.2% and 58.2% lower in Cases 1 and 2 than in straightforwardly merged solutions. The capacity utilization parameters also demonstrate that the overdesign issue is notably reduced in these solutions.