Multiperiod Heat Exchanger Networks Research Articles

Process integration of a multiperiod sugarcane biorefinery

Process integration in sugarcane biorefineries allows reducing steam consumption. As a consequence, the bagasse surplus can be diverted to second generation ethanol production. Furthermore, sugarcane plants can vary the production of ethanol and electricity, depending on the demand. For those reasons, equipment present in the plant might be required to operate under different conditions. This study presents the energy integration of a sugarcane biorefinery. A Mixed Integer Nonlinear Programming (MINLP) optimization model is proposed to solve the problem of synthesizing a Heat Exchanger Network (HEN) able to periodically operate under the distinct conditions required in the biorefinery, i.e., a multiperiod HEN. For solving the MINLP problem, a hybrid metaheuristic approach was used, which combines Simulated Annealing and Rocket Fireworks Optimization. The proposed strategy achieved lower HEN total annualized cost (TAC) when compared with the project energy integration that is commonly found in Brazilian plants. This reduction in TAC, in particular in utilities demand, allows the surplus bagasse to be available for the most suitable application: produce 2G ethanol or more electricity.

Efficient multiperiod heat exchanger network synthesis using a meta-heuristic approach

Multiperiod heat exchanger networks (HEN) are required in plants with seasonal alterations to operating conditions. Like for single-period HEN, the synthesis of multiperiod HEN can be formulated as a mathematical programming optimization problem. However, since the network needs to feasibly perform heat integration under different process conditions, additional constraints are required and problem complexity is increased. Studies on the subject based on mathematical programming often use deterministic approaches and rely on commercial solvers. In this work, a meta-heuristic two-level method based on Simulated Annealing and Rocket Fireworks Optimization (SA-RFO), originally developed for single-period HEN synthesis, is adapted to handle multiperiod HEN optimization. A new post-optimization (PO) strategy is coupled with the main method in order to improve the results. Four case studies are investigated and results are compared to the literature. The solutions achieved presented lower total annual costs (TAC) than those obtained by other methods and the new PO scheme was able to significantly improve the results.

Multi-Period Heat Exchanger Network Design Based on Periodic Heat Storage Approach

Multi-Period Heat Exchanger Network Design Based on Periodic Heat Storage Approach

Synthesis of multiperiod heat exchanger networks with timesharing mechanisms using meta-heuristics

Plants with cyclical variations in operating conditions require multiperiod heat exchanger networks (HEN) able to perform heat integration efficiently in all those periods. In such cases, however, overdesign issues arise, i.e., a heat exchanger might be too large for the service it must perform in a given period, requiring fractions of the process streams to by-pass such units. In this work, timesharing mechanisms (TSM) are considered for circumventing such issues, which means heat exchanger services are not fixed throughout the different periods of operation and a single piece of equipment can perform heat integration between different pairs of streams in each cycle. Two schemes are considered with the present meta-heuristic approach to obtain a preliminary solution before applying the service switching concepts. Then, systematic service rearrangement and equipment re-sizing in a new optimization stage are performed in order to find areas more suitable for the different timesharing arrangements. The latter is a new proposed model, not present in previous works that consider timesharing schemes. Four cases from the literature were investigated and results with lower total annual costs (TAC) than those reported previously by other works were achieved.

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.

Towards low carbon business park energy systems: A holistic techno-economic optimisation model

In this paper, an energy model is developed customised for the design of low carbon energy systems on business park scale. The model comprises two sequential stages: In the first stage, heat recovery within the system is maximised, while utility system and energy storage are optimally integrated and designed to fulfil remaining energy requirements at minimum total annualised costs. Simultaneously, heat networks can be deployed to transfer heat between separate parts of the system. Intra-annual variations in thermal and electrical energy demand and supply are taken into account, next to a carbon emission cap. In the second stage, the model generates an optimal multi-period heat exchanger network (HEN) enabling all required heat exchanges. The proposed model builds upon a multi-period energy integration model that can deal with restrictions in heat exchange. This model is combined with a generic technology model that can be manipulated to represent the various thermal or electrical energy conversion technology units in the utility system. Moreover, simple models for thermal and electrical energy storage are included that allow for calculation of storage levels subject to energy loss over time, without increasing the number of time steps to be analysed. A method for automated superstructure expansion is incorporated in the solution procedure to enable the optimisation of the number of units per technology in the configuration of the utility system. The HEN is automatically generated using a multi-period stage-wise heat exchanger network model. The energy model is applied to a literature example and to a generic case study demonstrating its holistic and integrated approach towards the synthesis of low carbon energy systems.

A systematic strategy for multi-period heat exchanger network retrofit under multiple practical restrictions

Abstract A systematic strategy for retrofit of the multi-period heat exchanger network (HEN) on the basis of the multi-objective optimization is developed. In this three-stage procedure, a simplified multi-objective optimization model of the multi-period HEN is first established and then solved to target the retrofit, aiming to minimizing the total annual cost and total annual CO 2 emissions. The obtained Pareto front represents series of retrofit targets under different emission limitations, from which the most desirable one can be selected. The matching of the existing and the required heat exchangers is further implemented to finalize the retrofit, which will meet the practical retrofit requirements and matching restrictions. The application of the proposed procedure is illustrated through a case study of a HEN in a vacuum gas oil hydro-treating unit.

Integrating renewables into multi-period heat exchanger network synthesis considering economics and environmental impact

Abstract This paper presents a further development of synthesis methods that considers economics and environmental impact in the integration of renewable energy into the optimisation of heat exchanger networks involving multiple periods of operations and multiple options of utilities. The multi-period process stream parameters, and those of the utility sources are integrated in a systematic approach using an expanded version of the simplified stage-wise superstructure multi-period model. Two examples were used to demonstrate the benefits of the expanded synthesis method and the quality of solutions obtained were judged by representation on a Pareto curve and by the use of a modified goal solution method. It was found that various combinations of utility sources were selected for use at various periods/seasons of operations, while utilities from solar photovoltaic were not selected for use at any of the periods/season of operation due to its relatively high cost and limited periods of availability.

Optimal synthesis of multiperiod heat exchanger networks: A sequential approach

Heat exchanger network (HEN) synthesis is an important research field in industrial processes. It is possible to minimize utilities usage as well as pollutant emissions by an optimal HEN synthesis. In multiperiod HENs, the same heat transfer devices must be able to operate under different operating conditions. The synthesis of multiperiod HEN can be formulated as an optimization problem. In the present paper it is used a sequential approach to solve the problem of synthesizing multiperiod HEN, considering heat capacities and stream temperatures variations into different operation periods. In this approach, multiperiod HEN synthesis is decomposed into three sequential steps, considering three optimization models. The novelties of the proposed approach are a modification in a well-known superstructure from the literature, with the inclusion of new by-pass streams, and an improvement in the NLP model of the third step. Two benchmark literature examples are studied and the obtained results prove the approach applicability, showing better values and network topologies.

Heat Exchanger Network Optimization for Multiple Period Operations

In this paper an optimization model is presented for the synthesis of a heat exchanger network (HEN) for multiperiod operations. A literature very well-known stagewise superstructure is used, but isothermal mixing assumption is not made and a timesharing procedure is adopted. A MINLP problem is solved separately for each period of operation. The final multiperiod HEN is synthesized automatically considering the greatest areas and not fixing matches in each device in different periods, which avoids excessive heat exchange areas. Heat exchangers are designed to be feasible in practice, with a minimum acceptable area. Three literature problems were used to test the applicability of the proposed model. The objective function aims to minimize the total annualized cost (TAC). During implementation of the model, inconsistencies found in the literature were corrected. Results indicate that lower TACs were obtained in the present paper and each heat transfer device is feasible in practice.

Synthesis of multi-period heat exchanger networks based on features of sub-period durations

In this work, a representative sub-period method is proposed for synthesis of multi-period heat exchanger networks (HENs). For the multi-period HENs with a large number and significantly different durations of sub-periods, the sub-period with the longest duration is taken as the representative sub-period. The cost-effective HEN structure in this representative sub-period can be obtained by solving the single-period HEN synthesis problems, and the operational parameters are then optimized to meet the requirements of other non-representative sub-periods on the fixed HEN structure achieved by the former step. In particular, for synthesis of multi-period of HENs with similar durations of sub-periods, a simplified model method is also proposed. In this method, the multi-period HENs are finalized by introducing the commonly shared heat exchangers in the multi-period HENs model. Both of the proposed methods avoid directly solving the multi-period HEN model, and hence the computational loads are reduced. An industrial case study for the synthesis of multi-period HENs is carried out to illustrate the procedure of the proposed methods. The effectiveness and advantages of the proposed methods are demonstrated by the comparison with the results obtained by the methods in literature. The effects of period duration on the cost-effectiveness of the multi-period HENs are discussed and highlighted.

Two-step hybrid approach for the synthesis of multi-period heat exchanger networks with detailed exchanger design

In this study a novel methodology for multi-period heat exchanger network synthesis is presented. The new synthesis method aims to systematically generate many candidate networks and, through the use of more detailed individual heat exchanger designs and their evaluation over all periods, guide the network optimisation to more realistic designs. This is done by using the multi-period mixed integer non-linear programming (MINLP) stage-wise superstructure (SWS) model and modifying it to include correction factors. These correction factors enable the MINLP optimisation of the overall cost of the designed network, which uses only shortcut models of the individual exchangers, to be guided by more detailed models of the individual heat exchangers that comprise the network. The designs obtained at the topology optimisation stage thus more accurately represent an actual network. The correction factors take into account aspects of the real design, such as TEMA standards, FT correction factors, number of shells, and changes in overall heat transfer coefficients. Each exchanger is designed to function over all periods of operation, and if this is not possible, extra exchangers are designed for the periods that cannot be satisfied. The methodology is applied to a case study that demonstrates the benefits of the proposed approach.

Minimizing investment cost for multi-period heat exchanger network retrofit by matching heat transfer areas with different strategies

Multi-period heat exchanger network (HEN) retrofit is usually performed by targeting and matching heat transfer areas. In this paper, based on the reverse order matching method we proposed previously, three strategies of matching heat transfer areas are proposed to minimize the investment cost for the retrofit of HEN in multi-period, in which replacement of heat exchangers, addition of heat exchangers and addition of heat transfer areas are performed. We demonstrate the procedures through three scenarios, including maximum number of substituted heat exchangers after retrofit, minimum additional heat transfer areas in the retrofitted HEN, and minimum investment cost for retrofit. The strategies are extended to a single period HEN retrofit problem. The results of multi-period and single period HEN retrofit problems indicate the effectiveness of the strategies. Moreover, these results are better than those reported in literature. The strategies are simple and easy to implement, which are of great benefit to large-scale HEN retrofit in practice.

Target-Oriented Methodology on Matching Heat Transfer Areas for a Multiperiod Heat Exchanger Network Retrofit

Heat exchanger network retrofit problems in a multiperiod operation are commonly solved in a stepwise manner. The matching of heat transfer areas is generally used to finalize the retrofit after the determination of the retrofit target of the heat exchanger network. Conventional heat transfer area matching methods are either experience based or model based methods, which may lead to indeterminate solutions or scale limitations in the optimal matching strategy. In this work, a systematic approach and algorithms for matching heat transfer areas in a multiperiod heat exchanger network retrofit are proposed. The methodology is based on the reverse order matching method and the matching algorithms of a bipartite graph. The proposed approach achieves four main objectives: to maximize the number of substituted heat exchangers, to minimize the increases in heat transfer area, to minimize the investment cost after a retrofit, and to minimize the investment cost given the preference rankings restrictions of heat ex...

Retrofit of Heat Exchanger Networks for Multiperiod Operations by Matching Heat Transfer Areas in Reverse Order

This work presents a retrofitting approach for heat exchanger networks (HENs) for single-period and multiple-period operations, aiming at the improvement of operation flexibility of HENs. A two-step method is proposed to solve the problems of redundant heat transfer areas in multiperiod HENs and the incomplete utilization of existing heat-transfer areas in HEN retrofit. In this approach, a multiperiod HEN design model is first solved to target the retrofit. Then, the final HEN is obtained by matching the required heat exchangers with the existing ones in reverse order. The proposed matching procedure is provided in detail and mathematically proven. Three case studies are employed to illustrate the advantages of the proposed method for solving the problems in the multiperiod HEN retrofit. In addition, the proposed method is extended to solving single-period HEN retrofit problems. Results indicate that the proposed method makes full use of the existing heat exchangers to reduce retrofit costs and increases the energy utilization efficiency of the HEN in multiperiod operations. The proposed method is simple and easy to implement. It can reduce the computational load and the difficulties in solving practical industrial problems.

A heuristic Lagrangean approach for the synthesis of multiperiod heat exchanger networks

In this work, we apply a decomposition technique to address the multiperiod heat exchanger network synthesis. Each period corresponds to changes in operating conditions, i.e. inlet temperatures and heat capacity flowrates. The problem size grows quickly with the number of streams and the number of periods resulting in a large scale problem to solve. In order to reduce the problem to a manageable size, we propose a specialized heuristic algorithm that relies on the concept of Lagrangean decomposition. We present an iterative scheme where feasible solutions are postulated from the Lagrangean decomposition subproblems, and the Lagrangean multipliers are updated through a subgradient method. Numerical examples are used to illustrate the proposed approach and the solutions are compared with the full space solutions with no decomposition. In general, slightly better solutions are found but in the terms of computational effort, its use is only justified as the problem size is increased.

A New Approach to Generate Flexible Multiperiod Heat Exchanger Network Designs with Timesharing Mechanisms

Seasonal changes in demands, supplies, and also in the operating conditions of a chemical process may call for different system structures to optimize the performance of its heat exchanger network (HEN). To produce a multiperiod HEN design, the traditional approach is to solve a single mathematical program that minimizes the total annual cost (TAC). This objective function, that is, TAC, is usually the sum of the annualized capital costs and annual utility costs determined according to given durations of all periods in a year. As a result, the conventional designs are often suboptimal, since the period lengths may have to be adjusted in response to the unexpected disturbances during actual operations. A new design approach is taken in the present study to circumvent the aforementioned drawback. In particular, a single-period model is first constructed and solved to produce the optimal design for each period individually. A timesharing strategy is then applied to integrate all such single-period designs so as to reduce the overall capital investment as much as possible while still keeping the utility consumption rates in every period at the minimum levels. In addition to their economic benefits, the new designs should be considered to be more flexible, since they are optimal despite unforeseen changes in the operation schedule. Finally, the numerical results of extensive case studies are also reported in this paper to demonstrate the effectiveness of the proposed approach.

Heuristic Approach to Incorporate Timesharing Schemes in Multiperiod Heat Exchanger Network Designs

Heat exchanger network (HEN) synthesis has been widely recognized as an effective design method for significant energy saving in industrial processes. Although traditionally its sole objective is to minimize the total annual cost, there are additional implementation issues which must be addressed in realistic multiperiod applications. In particular, if a conventional programming-based procedure is adopted for generating the optimal HEN structure, each embedded match inevitably calls for different heat-transfer areas in different periods. Since the usual practice is to select the largest unit, its operation in one (or more) period may be grossly inefficient because the corresponding overdesign level is simply too high. To circumvent this drawback, a modified mathematical model and a set of timesharing heuristics have been developed in this work to generate practical network configurations that can handle all heat duties. On the basis of the extensive case studies performed so far, it can be observed that this proposed approach is especially effective for multiperiod HEN design problems in which the process conditions vary significantly.

Interval Based MINLP Superstructure Synthesis of Multi-Period Mass Exchanger Networks

A new method for the synthesis of mass exchanger networks for multi-period operations is presented in this paper. The new technique which is called multi-period interval based MINLP superstructure (MIBMS) for mass exchange network synthesis (MENS) is adapted from the interval based MINLP superstructure for MENs presented by Isafiade and Fraser (2008). Process parameters such as supply and target compositions and flowrates of streams can vary over a specified range due to environmental or economic reasons. In this paper, the IBMS model for MENS is modified to handle variations in the aforementioned set of parameters by including the index ‘p' in the IBMS model and using the maximum area approach in the objective function as presented by Verheyen and Zhang (2006) for multi-period heat exchanger networks synthesis (HENS). The index ‘p' represents each period of operation which in this paper can be unequal. The maximum area approach ensures that each mass exchanger connecting the same pair of streams in more than one period is able to transfer mass in such streams for all the periods. It should be known that this technique applies to multi-period problems where the process parameters for each period of operation as well as the duration are specified. The method does not handle scenarios where the process parameters are uncertain. The new technique is applied to three examples.

Design of Crude Distillation Plants with Vacuum Units. II. Heat Exchanger Network Design

This is the second part of a two-part paper. In this part, a multiperiod heat exchanger network design model is proposed to handle two different crudes, a light crude and a heavy crude, and the results are presented. This model considers the existence of a preflash drum, which is used only in the light crude period. The model contains a topological constraint so that all periods share the same heat exchange network but not necessarily at the same temperature levels.