Flexible Heat Exchanger Networks Research Articles

Synthesis of flexible inter-plant heat exchanger networks: A decomposition method considering intermedium fluid circles

The traditional methods for synthesizing flexible heat exchanger networks (HENs) are not directly applicable to inter-plant HEN challenges, primarily due to the spread of system uncertainty across plants via intermedium fluid circles. This complicates the synthesis process significantly. To tackle this issue, this study proposes a decomposed stepwise methodology to facilitate the flexible synthesis of the inter-plant HENs performing indirect heat integration. A decomposition strategy is proposed to divide the overall network into manageable sub-networks by dissecting the intermedium fluid circles. To address the variability in intermedium fluid temperatures, a temperature fluctuation analysis approach is developed and a heuristic rule is introduced to maintain the temperature feasibility of the intermedium fluids. To ensure adequate flexibility and cost-effectiveness of the designed networks, flexibility analysis and network retrofit steps are conducted through model-based optimization techniques. The efficacy of the method is demonstrated through two case studies, showing its potential in achieving the desired operational flexibility for inter-plant HENs.

A Flexible Heat Exchanger Network Synthesis Method Adapted to Multi-Operation Conditions

This paper presents a flexible HEN (heat exchanger network) synthesis methodology for designing a multiperiod HEN with streams involving phase changes. The methodology is based on an MINLP (mixed integer nonlinear programming) model, identification of critical points, and flexibility index analysis considering phase changes. A nominal multiperiod HEN topology is constructed in the first step. Then each process operating condition’s critical points and flexibility index are calculated to verify the feasibility of the designed HEN under multiple operating conditions. To verify the validity of the method, the proposed methodology will be applied to a case study on an ammonia synthesis process heat transfer network design based on renewable energy. The results show that the method can obtain a flexible heat transfer network that is cost-effective and adaptable to multi-condition production for green ammonia synthesis.

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.

New Approach for Designing an Optimum and Flexible Heat Exchanger Network

New Approach for Designing an Optimum and Flexible Heat Exchanger Network

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.

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.

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%.

Flexibility analysis and design of heat exchanger network for syngas-to-methanol process

The heat exchanger network (HEN) in a syngas-to-methanol process was designed and optimized based on pinch technology under stable operating conditions to balance the energy consumption and economic gain. In actual industrial processes, fluctuations in production inevitably affect the stable operation of HENs. A flexibility analysis of the HEN was carried out to minimize such disturbances using the downstream paths method. The results show that two-third of the downstream paths cannot meet flexibility requirements, indicating that the HEN does not have enough flexibility to accommodate the disturbances in actual production. A flexible HEN was then designed with the method of dividing and subsequent merging of streams, which led to 13.89% and 20.82% reductions in energy consumption and total cost, respectively. Owing to the sufficient area margin and additional alternative heat exchangers, the flexible HEN was able to resist interference and maintain production stability and safety, with the total cost increasing by just 4.08%.

A simultaneous optimization of a flexible heat exchanger network under uncertain conditions

Heat exchanger networks are important for energy recovery and cost reduction. Flexible networks can handle environmental disturbances, and is more practical for industrial processes than the conventional HEN design with fixed parameters. Previous research on flexible HEN synthesis has mainly applied sequential synthesis, in which the structure determination and area optimization are conducted separately, or other multi-period methods in which only a few extreme operating conditions are selected; however, these methods are not sufficiently accurate for solving nonconvex problems. In this study, a bi-level simultaneous optimization strategy is proposed for the synthesis of flexible HENs under the nominal and all the extreme operating conditions. A binary particle swarm optimization is used in the outer layer to optimize the structural variables, and an Alopex-based evolutionary algorithm is used in the inner layer to determine the heat exchanger areas. Via this method, the result of a convex problem can be directly obtained. Furthermore, to deal with nonconvex problems, a flexibility analysis is conducted to adjust the heat exchanger areas on each critical point. The results of two cases with lower total annual costs than those reported previously are included to demonstrate the efficiency of the proposed method.

Hybrid Synthesis Method for Multi-period Heat Exchanger Networks

Developing a flexible heat exchanger network, that will remain operable in the face of potential variations in stream parameters, or variables, around some nominal values, especially for large problems, are difficult to solve simultaneously. A hybrid synthesis approach that systematically combines sequential and mathematical programming techniques may be more suitable to adopt. The proposed method presented in this paper entails firstly generating two representative single period networks using the Pinch Technology Stream Temperature versus Enthalpy Plot (STEP) approach and the multi-period stage-wise superstructure model for heat exchanger network synthesis. Streams for the representative network are obtained using the largest stream heat demand as criteria. The second stage of the proposed method entails generating a reduced multi-period stage-wise superstructure, using a combination of the matches obtained in the single period representative networks of the first step as initialising matches. The solution of the reduced superstructure of the second step, which is solved as a Mixed Integer Non-Linear Programming (MINLP) model, is then selected as the best multi-period network. The newly developed method of this paper is tested using an example from the literature. One of the solutions obtained is only 0.9 % higher, in terms of total annual cost, than the best solution presented in the literature but has the benefit of a fewer number of units.

Synthesis of Optimal Heat Exchanger Networks with Quantified Uncertainties and Non-isothermal Mixing

The primary objective of this study is to develop a simultaneous approach for the synthesis of flexible heat exchanger networks (HENs) with non-isothermal mixing assumptions. The HENs synthesis procedure presented in this study took into consideration quantified uncertainties in inlet temperatures and flow rates with an unpredictable time of shift. The proposed multi-period MINLP model was used to generate a HEN with optimized heat exchanger areas and total annualized costs attributed to utility duties. A framework for generating the flexible HEN over a specified range of variations in flow rates and stream temperature was proposed in this study. The framework was based on a two-stage strategy; a HEN design stage was first performed before the testing stage where the energy-saving potential of the synthesized HEN was established. The effectiveness of the proposed approach was tested for energy minimization using a case study in literature with variation in inlet temperature and flow rate. It was observed that the inclusion of non-isothermal parameters in the non-linear model resulted in a HEN that optimally works under fluctuating conditions without losing stream temperature targets while maintaining economically-optimal energy integration.

Synthesis of Flexible Heat Exchanger Networks Considering Gradually Accumulated Deposit and Cleaning Management

This paper presents a flexible heat exchanger network (HEN) synthesis methodology to cope with (i) the fluctuations of uncertain process parameters and (ii) the gradually accumulated deposit, which...

Optimization-Based Framework for Designing Dynamic Flexible Heat Exchanger Networks

With complex dynamic nature, Heat Exchanger Networks (HENs) should be operated successfully throughout the whole time horizon even facing the stochastic and the time-varying disturbances. In curren...

Synthesis of flexible heat exchanger networks: A review

Abstract Dealing with uncertainty is one of practical issues in design and operation of heat exchanger networks (HENs), arising the problem of flexible HEN synthesis. This paper addresses the state-of-the-art methods for flexible HEN synthesis based on sensitivity analysis, resilience analysis, flexibility analysis and multiperiod synthesis techniques as well. Each of these methods is summarized by presenting their general procedures and recent developments on modeling, solving strategies and applications. Some current topics related to flexible process synthesis have been briefly presented to provide several future research possibilities.

A Sequential Integration between Optimal Flexible Heat Exchanger Network Synthesis and Control Structure Design

In this work, the optimal synthesis and control structure design (CSD) problems for flexible heat exchanger networks (HENs) are integrated into a new sequential methodology. The proposed approach r...

Design of flexible multiperiod heat exchanger networks with debottlenecking in subperiods

In order to accommodate potential fluctuations of uncertain parameters, a systematic three-stage method for the design of flexible multiperiod heat exchanger networks (HENs) is proposed, which satisfies not only the requirement of the multiperiod operation but also the flexibility of the subperiod operation. In the first stage, an initial multiperiod HEN is constructed based on the synthesis of the single period HEN. In the second stage, the structure and heat transfer area arrangement of the initial multiperiod HEN are modified and optimized by considering both the multiperiod operational characteristics and the subperiod operational flexibilities. In the third stage, the flexibilities of the modified multiperiod HEN in each subperiod are examined and improved by solving a subperiod debottlenecking model. A case study of a HEN in a vacuum gas oil hydrotreating unit is carried out to illustrate the application of the proposed method. Results indicate that the heat transfer areas and total annual cost of the multiperiod HEN will be underestimated if the parametric fluctuations in subperiods are neglected. A cost-effective multiperiod HEN with sufficient flexibility in subperiods can be obtained by using the proposed method when the parametric fluctuations are taken into consideration.

A Three-Step Method to Improve the Flexibility of Multiperiod Heat Exchanger Networks

This paper addresses the design of flexible multiperiod heat exchanger networks (HENs) when the disturbances of operating parameters in subperiods occur. In this work, a three-step method is developed, where a multiperiod HEN synthesis step, a flexibility analysis step, and a subperiod debottlenecking step are involved. In the proposed method, a nominal multiperiod HEN is first determined through the representative subperiod method. The flexibility index of the nominal multiperiod HEN in each subperiod is then analyzed to identify the bottlenecks that restrict the flexibility of the HEN. Finally, a subperiod debottlenecking model is solved to finalize the design of flexible multiperiod HEN. The application of the proposed method is verified through a case study of a multiperiod HEN in a vacuum gas oil (VGO) hydrotreating unit. Results show that the proposed method is effective to obtain a cost-effective multiperiod HEN with sufficient operating flexibility.

Synthesis of flexible heat exchanger networks considering fouling resistance

The conventional synthesis of heat exchanger network (HEN) is performed under nominal conditions with fixed operating parameters. However, it is inevitable for the network configuration to be disturbed by uncertain operating conditions (for example, feed temperatures, heat capacity flow rates, etc.). At the same time, fouling is a non-negligible factor, which will cause sustained decrease to the overall heat transfer coefficient due to the growth of deposit over the heat transfer surface of the heat exchange units. In practice, the fouling resistance is often fixed to its maximum value instead of considering its dynamic growth. In order to make the synthesis more controllable and operable, this paper presents a flexible synthesis method, in which uncertain operating conditions and the growth of the fouling resistances are considered simultaneously. The methodology is sequentially implemented in two steps: the flexibility analysis and the flexible synthesis. By searching the critical directions, the improved flexibility analysis method for evaluating the flexibility index is proposed, which can reflect the simultaneous influence of uncertain operating conditions and fouling resistances. Then, the flexible HEN synthesis method based on the critical operating points identified by the aforementioned flexibility analysis method is presented, with the target of decreasing the complexity of the HEN to avoid over-synthesis and minimizing the total annual cost (TAC) simultaneously. The effectiveness of the methodology is demonstrated by a case study.

Synthesis of Flexible Multi-Period Heat Exchanger Networks Using a Reduced MINLP Superstructure Approach

This paper presents an extension to the reduced superstructure approach for synthesising multi-period heat exchanger networks previously presented in the literature. The extension entails identifying promising candidate matches by solving not only the single period stage-wise superstructure of each period of operation (i.e. sub-periods), but also solving intermediate sub-multi-period and the complete multi-period stage-wise superstructure for all periods combined. The resulting matches from the solution networks are used to populate the set of binary variables for the reduced multi-period superstructure model which is then solved as a mixed integer non-linear programming model. Results obtained from the extended model shows an improvement over existing results.

An Industrial Case Study Application in Synthesising a Feasible Heat Exchanger Network

Heat exchanger network synthesis (HENS) is an important part in the overall chemical process. HENS links the process flowsheet with the utility system and generally involves a large fraction of both the overall plant capital cost, operating costs in terms of energy requirements, which is a key factor for a profitable process. The aims of the synthesis consist of finding a network design that minimises the total annualised cost, i.e. the investment cost in units and the operating cost in terms of utility consumption. In HENS, the feasibility of the HEN design does not take into consideration. As a consequence, the HEN design may not be able to be implemented into industrial applications. It is essential to check the feasibility of a design before it is being implemented into the industry. The objective of this paper is to present the application of a new flexible and operable heat exchanger network (FNO HEN) methodology in synthesising a feasible heat exchanger network (F-HEN) using an industrial case study. The aim of this work is to verify the existing industrial HEN design in terms of the process design point of view as well as the process feasibility. The existing industrial HEN design is verified in terms of Tmin value that gives minimum external energy requirement (EER) and heat exchanger area (HEA) as well as simultaneously analyse the feasibility of the HEN design. Using the new developed FNO HEN methodology framework, HEN design target, which is the value of Tmin is determined in order to obtain the F-HEN design. From process design point of view, Tmin value determines the size of heat exchanger in the network as well as energy saving. A process simulator is used to check the process feasibility of the HEN designs. With the use of the F-HEN trade-off plot, which is a plot of EER and HEA at different value of Tmin. with additional of feasibility area, the optimal feasible HEN design which satisfies external energy requirement (operability), heat exchanger area (capital) and process feasibility has been successfully determined in an easy, systematic and efficient manner.