Heat Exchanger Network Configuration Research Articles

A Systematic Heat Recovery Approach for Designing Integrated Heating, Cooling, and Ventilation Systems for Greenhouses

Ventilation heat loss is one of the most important factors contributing to energy performance of greenhouses. This paper suggests a systematic method based on dynamic pinch analysis (PA) to design an integrated heating, cooling, and ventilation system that uses ventilation waste heat in a cost-effective and energy efficient way. A heat recovery system including an air handling unit, borehole thermal storage, and a heat pump is proposed to investigate all heat integration scenarios for an entire year. In the first step, the heat integration scenarios are reduced to a few typical days using a clustering technique. Then, a generic methodology for designing a heat exchanger network (HEN) for a dynamic system, ensuring both direct and indirect heat recovery, is presented and a set of HENs are designed according to the conditions of typical days. Afterwards, the best HEN design is selected among all design alternatives using a techno-economic analysis. The whole procedure is applied to a commercial greenhouse and the best HEN configuration and required equipment sizes are calculated. It is shown that the best-performing design for the greenhouse under study produces primary energy savings of 57%, resulting in the shortest payback period of 9.5 years among all design alternatives.

Cost allocation evaluation of a multi-plant flexible heat exchanger network design based on fuzzy game

A multi-plant heat exchanger network (HEN) can recover waste heat across plant boundaries and reduce utility consumption in industrial parks. Conventional multi-plant heat integration using nominal conditions cannot handle uncertainties from the environment or plant operations that affect stream properties; therefore, a flexible HEN design is more practical. This study is focused on the evaluation of an already designed flexible HEN with a fixed network structure and the provided heat exchanger areas to maintain operability under disturbances. Although the capital cost is determined, utility consumption is uncertain to meet possible target temperatures. Consequently, the total annual cost (TAC) varies and forms a distribution that can give plant operators more information about possible costs as well as extreme parameter values. To provide the TAC distribution, a mathematical model is established to calculate utility costs after inputting different stream parameters into the designed HEN configuration. In addition, an interval Shapley value from the fuzzy game, which considers the contribution of each plant when the decision environment is uncertain, is used to generate a cost allocation interval. For example, the potential costs paid for Plant 1 and Plant 2 in Case study 1 ranged from $18,195.95 to $25,897.80 and $14,778.05 to $19,921.20, respectively. These results demonstrate that the proposed approach can be used to evaluate a provided flexible HEN design before construction.

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.

Multi-plant heat integration with shell-and-tube heat exchangers for multi-period operations

Multi-period heat exchanger network (HEN) can provide various system configurations for seasonal changes in demands, supplies, and different operation conditions of process streams. Current multi-period heat integration has mainly focused on the single-plant recovery, while a chemical industrial park usually consists of several plants that can be linked together through streams. In this study, multi-period HEN has been extended to a multi-plant scale to recover more waste heat. However, as the drawback of a multi-plant HEN, if an unplanned shutdown happens in one plant, other plants will be influenced due to the connected invalid heat exchangers and resort to extra utilities to reach target temperatures. To overcome this, a heat exchanger network with safety redundancy (HEN-SR) model is established for guaranteeing the continuous operation of remaining plants during an unplanned shutdown. Besides, to meet the industrial application, parameters of the real exchanger design are included, including temperature difference correction factor and the number of shell passes. The optimization results of two cases indicate that the mode switching of multi-period HEN-SR model can avoid huge economic losses caused by plant shutdowns, and the one-step synthesis method using a bi-level optimization strategy can find the HEN configuration with a lower total cost.

Two-tier control structure design methodology applied to heat exchanger networks

Because of its paramount importance in the successful industrial control strategy of a given heat exchanger network (HEN), the control structure designs for providing appropriate manipulated variable (MV) and controlled variable pairings have received considerable attention. However, quite frequently HENs with such control structures face the problem of hard constraints, typically holding the HENs at less controlled operating space. So both the MV pairings and the above control pairings should be considered to design a control structure. This paper investigates the systematic incorporation of the two pairings, and presents a methodology for designing such two-tier control structure. This is developed based on the sequential strategy, coupling an indirect-tier with direct-tier control structure design, wherein the intention is realized in the former stage and the latter is implemented for further optimization. The MV identification and pairing are achieved through variations in heat load of heat exchangers to design the indirect-tier control structure. Then the direct-tier control structure is followed the relative gain array pairing rules. With the proposed methodology, on the one hand, it generates an explicit connection between the MV pairings and the HEN configuration, and the quantitative interaction measure is improved to avoid the multiple solutions to break the relationship among all the control pairings into individuals; on the other hand, a two-tier control structure reveals control potentials and control system design requirements, this may avoid complex and economically unfavourable control and HEN structures. The application of proposed framework is illustrated with two cases involving the dynamic simulation analysis, the quantitative assessment and the random test.

Multi-plant direct heat integration based on the risk-based Shapley value and Alopex-based evolutionary algorithm

Current heat exchanger network (HEN) synthesis is mainly focused on minimizing the total annual cost (TAC), and then allocating the minimized TAC among plants. However, the obtained HENs usually have complex inter-plant heat matches, and if unplanned shutdowns occur, connected plants will suffer huge economic losses. Consequently, some plants may be unsatisfied with their allocated costs and prefer not to join this multi-plant coalition. In this study, a natural risk-based Shapley value curve is used to represent the variation of cost allocation under shutdown risks between 0% and 100%. Alopex-based optimization strategies are proposed to optimize the modified HEN model with core constraints that can restrict the Shapley value curve to the core range as feasible cost allocation solutions. Therefore, HEN configurations and acceptable cost allocation plans under any shutdown risks are determined simultaneously. Two cases demonstrate that the proposed methodology can reduce shutdown-induced economic losses and facilitate multi-plant cooperation.

STUDY OF HEAT EXCHANGER NETWORK CLEANING SCHEDULE DESIGN WITH HEURISTIC SCENARIO ANALYSIS

The crude oil refinery heat exchanger network (HEN) cleaning schedule is critical to maximize the energy recovery and simultaneously maintain the HEN performance. In reality, the HEN configuration consists of multiple heat exchanger units with different sizes, fouling rates, and initial efficiencies. The plant availability and the HEN efficiency may decrease to a certain level after a period of operation due to fouling and heat exchangers taken offline for cleaning. Therefore, cleaning and bypass procedure is deemed necessary. In this study, the heat exchanger performance represented by the overall heat transfer coefficient is evaluated based on a proposed heuristics algorithm for seeking the optimum cleaning schedule while incorporating rigorous cleaning rules. Four heuristic values for HEN cleaning schedule scenarios are proposed to evaluate the overall performance of the HEN. The additional heat duty on the process heater due to cleaning operation and the total annual cost are considered in a crude oil refinery HEN that consists of 11 heat exchanger units. The cleaning frequency of the heat exchanger is found to have a significant effect on HEN performance. The results from the scenario analysis suggest that there is a proper cleaning schedule for optimum operation of a crude oil refinery HEN. It is also indicated that the design of the cleaning schedule depends on fouling resistance and the capacity of related heat exchangers.

Modeling and optimization of inter-plant indirect heat exchanger networks by a difference evolutionary algorithm

Inter-plant indirect heat integration via intermediate fluid circuit is an efficient energy-saving and heat recovery method, but traditional optimization methods, such as sequential synthesis, may not provide the best solutions. This paper addresses a multi-plant indirect heat exchanger network problem using a two-layer simultaneous synthesis method. To reduce the nonlinear constraints in the simultaneous heat exchanger network model, the outer layer uses a differential evolution algorithm to determine the temperatures of the intermediate fluid, while the inner layer uses a deterministic method to obtain the heat capacity flow rate of the intermediate fluid and the heat exchanger network configuration. Optimization was performed to minimize the total annualized cost, as the sum of utility cost, heat exchanger cost, pump cost, and pipe cost. The differential evolution algorithm in the outer layer improved the simultaneous synthesis efficiency in the inner MINLP model and also provided better results in the case studies.

Non-structural model for heat exchanger network synthesis allowing for stream splitting

For more than three decades, heat exchanger network (HEN) synthesis has been primarily addressed by defining initial structures that embed different design alternatives, and near optimal HEN configurations are extracted from these structures during the optimization process. However, such initial structures are prone to missing necessary design alternatives and may require simplifying assumptions to ease the computational burden of optimization algorithms. This paper presents a non-structural model (NSM) for synthesis of HEN considering stream splitting and non-isothermal merging of branch streams. The model exhibits randomness in stream matching, generation and elimination by which potential matches are realized. Random walk algorithm with compulsive evolution is used for optimization of both integer variables (number of heat units) and continuous variables (heat duties and split fractions). The effectiveness of the approach is tested for small- and medium-size literature cases. The method demonstrates results comparable to or better than those reported in literature.

Simultaneous Synthesis of Heat Exchanger Network and Utility System Considering Inter- and Inner-stage Heaters

Heat exchanger network (HEN) and utility system are both important compositions in chemical industry. Heat exchanger network contributes to recover heat from process streams and sometimes consumes external heat resources when heat demand is not satisfied by itself. Rankine cycle-based utility system supplies steams to motivate processes if external heat is needed, meanwhile produces power in turbine. Thus, there is great necessity to synthesize HEN and utility system simultaneously. In this work, a stage-wise HEN superstructure model considering the optimal placement of heaters and matches of streams is presented to integrate with utility system, wherein heaters that consume utility steams are allowed to be placed inside each stage and also between adjacent stages. The selection and allocation of the steams in different pressure levels are optimized considering both power generation in utility system and heat recovery in HEN. A mixed-integer nonlinear programming (MINLP) formulation is formulated for the optimal design of HEN-utility system with minimum total annualized cost (TAC), aiming at trade-off amongst capital cost, fuel cost and income of selling power. At last, a case is studied to demonstrate the application of the proposed method, obtaining a desired HEN configuration with meaningful economic benefits.

Non-structural model of heat exchanger network: Modeling and optimization

Simultaneous synthesis of heat exchanger network (HEN) is an indispensable strategy to improve energy efficiency and realize high techno-economic performances. The structural models that involve a stage-wise superstructure have some limitations concerning the global optimization for the HEN, including the flexibility of configurations and computational efficiency of synthesis methods. Therefore, a novel non-structural model (NSM) is proposed in this work for the simultaneous synthesis of the HEN. Firstly, the concept of a non-structural HEN configuration is described based on an analysis of the drawbacks of structural models. Then, the mathematical formulation of the NSM is provided. Finally, a general implementation method is proposed to realize the simultaneous synthesis of an NSM-based HEN and make it a feasible synthesis method. Six medium and large scale case studies are used to demonstrate the flexibility of configurations in the proposed model and its performance concerning the global optimization of HEN synthesis using a random walk algorithm with compulsive evolution. The combination of the NSM and the heuristic method has led to better optimal solutions than those that exclude the stream splits as published in previous works, achieving high quality and efficiency in the synthesis of HEN problems.

Geothermal distribution network modeled as Heat Exchanger Network to be optimized with Mixed Integer Nonlinear Programming

Geothermal energy is commonly harvested at either shallower depth (below 150ft/45.72m) for residential purposes (with ground source heat pumps), or deeper depths (beyond 8000ft/2.43 km) for Enhanced Geothermal Systems. The in-between depths are rarely visited due to high drilling costs, and the water harvested being unable to power turbines. Recent studies powered by the data released by the National Geothermal Data System (NGDS) opened a new opportunity of harvesting the geothermal potential in post-production oil/gas boreholes in Pennsylvania. We are interested therefore in whether it is feasible to connect the different heat sources with different temperature availabilities to distribute to spatially scattered end-users. Presented in this paper is a project that generates heat exchanger network configurations through mixed nonlinear programming (MINLP) problem formulation and optimization in Python. With the case intentionally simplified, the computational costs of the optimization was found to be marginal.

The implementation of inter-plant heat integration among multiple plants. Part II: The mathematical model

It is a challenging task to solve a large-scale Inter-Plant Heat Integration (IPHI) problem, especially for simultaneous optimization for intra- and inter-plant heat integration. In the companion paper (Part I), a novel screening algorithm named Nearest and Largest Qrec-based Screening Algorithm (NLQSA) was proposed. It can be used to divide a large-scale IPHI problem into several small ones, each of which includes two or three plants, while keeping the theoretical maximum inter-plant heat recovery potential Qrecmax almost unchanged. NLQSA provides a prior solution before determination of inter-plant Heat Exchanger Network (HEN) configuration for each achieved small IPHI scheme. In this paper, a modified MINLP model with an objective of minimum Total Annual Cost (TAC) is proposed to determine the final inter-plant HEN configurations of achieved segregated IPHI schemes. With the addition of stream data extraction method and NLQSA which were proposed in Part I of this paper series, a complete three-step strategy is established in order to solve the large-scale IPHI problem. Theoretically, a large-scale IPHI problem can be solved no matter how many plants involved. A case study with seven plants is introduced to illustrate the feasibility and effectiveness of the proposed method.

Linking pinch analysis and bridge analysis to save energy by heat-exchanger network retrofit

Reduction of energy requirements in the process industries results in increased profitability and better environmental performance. Methods for heat exchanger network (HEN) retrofit are based on thermodynamic analysis and insights, numerical optimization, or combined approaches. Numerical optimization-based methods are highly complex and may not guarantee identification of the global optimum. Pinch analysis, which is an approach based on thermodynamic analysis and composite curves, is the most widely used in the industry. Its simplicity, the use of graphical tools, and the possibility for the user to interact at each step of the design process help identify solutions with consideration of practical feasibility. In the last few years, bridge analysis has been developed for HEN retrofit. It includes the following tools: (a) the definition of the necessary conditions to reduce energy consumption which are expressed in the bridge formulation, (b) a method for enumerating the bridges, (c) the representation of the flow rate of cascaded heat through each existing exchanger on the energy transfer diagram (ETD), and (4) the use of the Heat Exchanger Load Diagram (HELD) to identify a suitable HEN configuration corresponding to modifications. It has been shown that reducing energy consumption implies decreasing the flow rate of cascaded heat through the existing exchangers across the entire temperature range between the hot and cold utilities. The ETD shows all possibilities to reduce the flow rate of cascaded heat through a HEN. The objective of this paper is to link these tools with the composite curves and concepts from pinch analysis into a consistent method for HEN retrofit. First, the relations between the composite curves, the ETD and the HELD are described, and a method for HEN retrofit which combines the insights from pinch analysis with the recently developed tools is proposed. Then the method is used to reduce energy consumption in three case studies. Comparison between results from pinch analysis and bridge analysis shows that the latter identifies supplementary solutions to save energy. The proposed synthesis fills a gap in pinch analysis, and its concepts can be helpful in the development of software for HEN retrofit.

Process synthesis for cascade refrigeration system based on exergy analysis

Refrigeration system holds an important role in chemical/petrochemical processes. The traditional cascade refrigeration system (CRS) used in ethylene plants contains multiple refrigerants working at multiple temperature/pressure levels. In this study, a general methodology is developed for the optimal process synthesis of a CRS based on exergy analysis. This procedure involves four stages: (1) refrigeration system exergetic analysis; (2) optimization model development for simultaneous synthesis of refrigeration system and heat exchanger network (HEN); (3) HEN configuration; and (4) final solution validation. The exergy–temperature chart is used to comprehensively analyze a CRS. A mathematical model is presented to minimize total compressor shaft work of the HEN‐considered CRS, where multiple recycling loops satisfying all cooling/heating demands are simultaneously addressed. The optimal solution is examined by rigorous simulations to verify its feasibility and consistency. The efficacy of the developed methodology is demonstrated by a case study of a propylene CRS in an ethylene plant. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2471–2488, 2015

Efficiency improvement strategies for the feedwater heaters network designing in supercritical coal-fired power plants

Coal will continue playing a major role in worldwide electricity generation during next years. This trend will augment CO2 emission to the atmosphere. Improving power plant efficiency could alleviate the negative effect of coal consumption on CO2 emissions. Main efforts have been focused on supercritical boiler technology (once-through units) and materials development (austenitic steels) with the aim of accomplishing high steam parameters that allow this efficiency enhancement. However, improvement in supercritical steam parameters should be followed by an exhaustive review of the steam cycle design. This paper shows a strategy for the optimization of the feedwater heaters network and the flue gas heat recovery system design. Starting with the lay-out of a supercritical steam cycle using the best available technology, this paper analyses not only the steam cycle itself, but also its integration with the boiler cold-end. By means of thermodynamic optimization it is possible to propose new feedwater heat exchanger network configurations and reducing steam consumption from turbine bleeds, achieving optimum power plant efficiency. Simulations have been carried out using Aspen Plus software and optimization procedure is based on a sequential quadratic programming method that maximized overall plant efficiency taking turbine bleeds pressure as independent variables. Results show a feasible improvement of the overall plant efficiency of 0.7 points in comparison with state-of-the-art reference plant. This increase implies a direct reduction of CO2 emissions of about 1.3% compared with the best plant currently available. Moreover, an economic analysis confirms the feasibility of the proposals analysed and shows important additional yearly incomes.

A principle of stream arrangement based on uniformity factor for heat exchanger networks synthesis

Heat exchanger network synthesis (HENS) has been one of the most extensively studied synthesis problem in chemical engineering. The methodologies commonly used to solve a heat exchanger network (HEN) problem are based on heuristic methods instead of deterministic methods which, theoretically, only with feasible initial HEN configuration enumeration, can find global optima. But enumeration always leads to unacceptable computational time. This paper presents a principle of process stream arrangement and the related deterministic algorithm for synthesizing HEN. The implementation of the principle based on the uniformity factor which proposed to describe the degree of the energy distribution of HEN configuration under different process stream arrangement is aimed to decrease the space of configuration enumeration. Thus, through the part of smaller uniformity factor network structure are optimized, deterministic methods are effectively suitable for medium-size problem with global optima in a reasonable brief computational time. Notably, using examples of 8SP1, 10SP, 10SP1, 14SP1, we demonstrate that our approach finds superior networks compared to those known in the literature.

Design of optimum flexible heat exchanger networks for multiperiod process

Due to the rising of energy prices, energy saving became very important. Optimum design of Heat Exchanger Networks (HEN) is a successful way to minimize energy consumption. The present work discusses the design of optimal flexible heat exchanger networks that adapt with changes in streams’ start and target temperatures and heat capacity flowrates. For a process consisting of n periods, multiperiod LP and MILP models were used to determine the target utility requirements and the heat exchanger network configuration that achieves the minimum number of units and remain flexible to ensure minimum utility requirements at each period of operation. Applying these models on a multiperiod literature problem resulted in different solutions corresponding to different iteration runs. The optimum solution that realizes the least exchangers’ cost was compared with literature results for the same problem.

Multi-period design of heat exchanger networks

Heat exchanger networks are an integral part of chemical processes as they recover available heat and reduce utility consumption, thereby improving the overall economics of an industrial plant. This paper focuses on heat exchanger network design for multi-period operation wherein the operating conditions of a process may vary with time. A typical example is the hydrotreating process in petroleum refineries where the operators increase reactor temperature to compensate for catalyst deactivation. Superstructure based multi-period models for heat exchanger network design have been proposed previously employing deterministic optimisation algorithms, e.g. (Aaltola, 2002; Verheyen and Zhang, 2006). Stochastic optimisation algorithms have also been applied for the design of flexible heat exchanger networks recently (Ma et al., 2007, 2008). The present work develops an optimisation approach using simulated annealing for design of heat exchanger networks for multi-period operation. A comparison of the new optimisation approach with previous deterministic optimisation based design approaches is presented to illustrate the utilisation of simulated annealing in design of optimal heat exchanger network configurations for multi-period operation.

Optimal heat exchanger network synthesis using particle swarm optimization

Heat exchanger network (HEN) synthesis has been a well-studied subject over the past decades. Many studies and methodologies were proposed to make possible the energy recovery, minimizing the utilities consumption and the number of heat transfer equipment. Most of papers published in this subject are based on Pinch Analysis and mathematical programming. Some recent papers use meta-heuristic techniques like Genetic Algorithms or Simulated Annealing to solve the HEN synthesis problem and good results are found but with large computational effort. In this paper an optimization model for the synthesis of HEN is proposed. The approach is based on the use of Particle Swarm Optimization to determine the HEN that minimizes the total annualized cost, accounting for capital costs of heat exchangers and the energy costs for utilities and pumping duties. The algorithm is based on a superstructure simultaneous optimization model for the HEN synthesis considering stream splitting. Some examples from the literature were used to show the application of the proposed algorithm, and the results confirm the achievement of the optimum HEN configuration with little computational effort.