Operation Of Heat Exchanger Network Research Articles

Closed-Loop Energy Flow Redistribution for Optimal Operation of Heat Exchanger Networks

Closed-Loop Energy Flow Redistribution for Optimal Operation of Heat Exchanger Networks

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

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

Optimal operation of heat exchanger networks through energy flow redistribution

AbstractThis article presents a novel energy flow redistribution methodology to achieve optimal operation of heat exchanger networks. The proposed method aims to manipulate the propagation path of a disturbance through the network to reduce its impact on utility consumption. Specifically, an optimization problem is formulated to generate new duty targets for heat exchangers of the network when a disturbance is encountered. Subsequently, a feedback control system is designed to track these targets by manipulating bypasses around the process heat exchangers. The effectiveness of the proposed framework is illustrated with the help of three benchmark examples. The proposed approach can handle disturbances in inlet as well as target temperature, inlet flow and heat transfer coefficient of individual heat exchangers.

Optimal Operation of Heat Exchanger Networks

Optimal Operation of Heat Exchanger Networks

Improving the operation of heat exchanger networks through exergy analysis

Improving the operation of heat exchanger networks through exergy analysis

Improving the operation of heat exchanger networks through exergy analysis

Improving the operation of heat exchanger networks through exergy analysis

Optimal operation of heat exchanger network with stream splitting

Heat exchanger networks (HENs) are drivers of sustainability in chemical process plants. Due to their interaction with various process units, their operation is frequently subjected to disturbances in inputs and planned changes in targets. This motivates the need for optimal operation and control. This paper addresses optimal operation of a class of HENs involving stream splits. It is shown that these stream splits can be used to direct the energetic effect of a disturbance along a favourable path (e.g. minimum energy consumption) and thus allow for optimal operation. Specifically, a two-step strategy is proposed involving steady state optimisation for redirection of energetic effect of the disturbance based on the structure of the network. This is followed by implementation of a feedback control system to track the targets given by the optimiser. The effectiveness of the proposed framework is illustrated with the help of a benchmark example.

A New Diagram for Long-term Heat Exchanger Network Cleaning and Retrofit Planning

With the long-term operation of Heat Exchanger Networks (HENs), fouling is a frequent problem. This is related to decreasing long-term heat transfer performance. Fouling decrease the heat transfer coefficient and reduce the efficiency of heat exchange. Different fluid types have different characteristics in corrosiveness and contribute to different results for fouling. The current graphical tools cannot fully tackle the HEN cleaning and retrofit design in the long-term operation.This paper proposes a new graphical tool called Time vs Temperature Diagram (TTD) for HEN long-term management, taking fouling into consideration. It visualises the long-term temperatures change of each heat exchanger on hot and cold streams. The effect of heat exchanger cleaning can be shown visually in this diagram. This diagram can help to determine the cleaning and retrofit plan over a long period. An example is illustrated to show how to apply this diagram.

Energy saving potential of a simple control strategy for heat exchanger network operation under fouling conditions

A real-life benchmark system comprising a Heat Exchanger Network operated as part of Crude Distillation Unit is considered. In such systems, the crude oil stream is typically split into parallel branches and the oil mass flows through the branches are kept in constant proportion, i.e., at constant split ratio, by the process control system. In this paper, linear control systems (proportional-integral-derivative controllers) are considered and the proposed control strategy is to adjust the parallel flows so that identical temperature values are maintained at the outlets from two parallel branches and consequently, the heat recovery in the network is maximized. The aim is to enhance the energy efficiency of the system and minimise greenhouse gas emissions. A mathematical model of the heat exchanger network was built and validated on the basis of real-life data recorded during operation of the crude distillation unit. Using MATLAB/Simulink, closed-loop control was simulated to enable comparative evaluation of the studied strategy of proportional-integral-derivative control and its potential to achieve energy savings in the operation of the distillation unit under fouling conditions. Compared to the strategy of constant split ratio, the proposed strategy of equal outlet temperatures from the network branches was found to increase the total heat recovery by about 1.5%. In the studied operation period, the heat-recovery increase fluctuated in the range 150–1100 kW and the average daily energy saving was estimated at 18 MWh.

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.

Energy saving potential and the efficacy of using different control strategies for the heat exchanger network operation

Heat exchangers – typically arranged in heat recovery networks – belong to the devices most widely used in industrial production systems. The control performance of heat exchanger network (HEN) operation can be impaired by fouling which builds up on heat-exchange surface. Fouling leads to burning extra fuel to compensate for reduced heat recovery and induces increased costs of cleaning interventions, etc. In this work, a real-life benchmark system is considered. The controlled system represents a HEN coupled with a crude distillation unit (CDU). The mathematical model of HEN with stream splits and interactions between the branches, was built and validated based on real-life data recorded during CDU operation. The use of various control strategies (employing linear control systems with PID controllers) for HEN operation was studied with the aim to maximize heat recovery in the HEN involving splits. Using historical data on HEN operation, simulations of closed-loop control were performed in MATLAB/Simulink environment. The simulation results demonstrated the efficacy of the studied strategies and their potential to achieve energy savings in CDU operation.

Coordination between bypass control and economic optimization for heat exchanger network

The bypass is widely used for optimizing the operation of heat exchanger network (HEN) to maintain the control requirements. More attention on the bypass control strategy has been paid to the control performance without considering the economics. However, the economic efficiency is quite important to the industry with energy intensive production as energy prices to rise. Therefore, both the control performance and the economic efficiency should be considered simultaneously during the life cycle of HEN. In this work, firstly we proposed a methodology for two-stage coordination of bypass control and economic optimization (CBCEO). Then the one-step coordination between bypass control and economic optimization is developed based on two-stage CBCEO. Secondly, the margin of HEN is commonly optimized and can be calculated by the variation of bypass fractions, which is regarded as the objective function in this work. Then non-square relative gain array is utilized to obtain optimization variables and the optimal control structure is established. Thirdly, an optimization algorithm combining external penalty function with pattern search is used to solve this dynamic optimization problem (DOP). Finally, two case studies indicated that the proposed CBCEO strategy can achieve the purpose of effective control and optimal economic at the same time.

Robust Model Predictive Control of Heat Exchangers in Series

This study investigates using of robust model based predictive control (MPC) algorithms for optimal operating of heat exchangers in series from the stability and economic viewpoints. For the advanced controller design, the influence of uncertain parameters was taken into account. In order to design the robust MPC, the optimization problem with constraints was formulated in the form of linear matrix inequalities and then the convex optimization problem was solved using the semidefinite programming. The designed robust MPC strategies were based on the worst-case optimization and on the additional control input saturation. We investigated a case study with two various significant disturbances in the temperature of the input stream in the heat exchangers in series. Results revealed that the robust MPC improved control performance and ensured energy savings during the heat exchanger network operation.

Optimal operation of heat exchanger networks with stream split: Only temperature measurements are required

For heat exchanger networks with stream splits, we present a simple way of controlling the split ratio. We introduce the “Jäschke Temperature”, which for a branch with one exchanger is defined as TJ=(T−T0)2/(Th−T0), where T0 and T are the inlet and outlet temperatures of the split stream (usually cold), and Th is the inlet temperature of the other stream (usually hot). Assuming the heat transfer driving force is given by the arithmetic mean temperature difference, the Jäschke Temperatures of all branches must be equal to achieve maximum heat transfer. The optimal controlled variable is the difference between the Jäschke Temperatures of each branch, which should be controlled to zero. Heat capacity or heat transfer parameters are not needed, and no optimization is required to find the optimal setpoints for the controlled variables. Most importantly, our approach gives near-optimal operation for systems with logarithmic mean temperature difference as driving force.

Heat Exchanger Network Optimization Considering Pressure Drop Constraints based on Minimum Operating Cost

Considerable research effort has been reported in cost-optimal operation of heat exchanger network. However, most of them neglect the pressure drop influence and assume constant film heat transfer coefficients. Pressure drop of streams are important influencing factors for the performance of heat exchanger network operation. In this paper, a general cost-optimal operation model considering pressure drop constraints and removing the assumption of constant film heat transfer coefficients is proposed. It is necessary to determine the pumping power cost required as part of operating cost function. The extended model is applied to one example taken from previous research, and the results prove that the proposed method can obtain more real optimization results for HEN operational optimization problems.

Cost-Optimal Operation of Heat Exchanger Network Considering Pressure Drop Constraints

Considerable research effort has been reported in cost-optimal operation of heat exchanger network. However, most of them neglect the pressure drop influence and assume constant film heat transfer coefficients. Pressure drop of streams are important influencing factors for the performance of heat exchanger network operation. In this paper, a general cost-optimal operation model considering pressure drop constraints and removing the assumption of constant film heat transfer coefficients is proposed. It is necessary to determine the pumping power cost required as part of operating cost function. The extended model is applied to one example taken from previous research, and the results prove that the proposed method can obtain more real optimization results for HEN operational optimization problems.

Control structure design for optimal operation of heat exchanger networks

AbstractWhen only single bypasses and utility duties are used as manipulations, optimal operation of heat exchanger networks (HENs) can be categorized as an active constraint control problem. This work suggests a simple split‐range control scheme to implement the optimal operation. For a given HEN and information about the disturbances, the corresponding control structure can be found by solving an integer‐linear programming (ILP) problem with two objective functions providing optimal split‐range pairs (for tracking active constraints during the operation) and appropriate control pairings (for fast control action). A HEN case study is used to demonstrate the application of the proposed design technique. Dynamic simulation shows the ability to provide the optimal operation of the obtained control structure. © 2007 American Institute of Chemical Engineers AIChE J, 2008

On a New MILP Model for the Planning of Heat-Exchanger Network Cleaning. Part III: Multiperiod Cleaning under Uncertainty with Financial Risk Management

This paper is a follow-up to a two-part paper on the scheduling of heat exchanger network cleaning. In the first part (Lavaja, J. H.; Bagajewicz, M. Ind. Eng. Chem. Res. 2004, 43, 3924−3938), a new mixed-integer linear model for the planning of heat-exchanger cleaning in chemical plants was presented, where the net present cost is minimized by carefully choosing the appropriate cleaning schedules. In the second part (Lavaja, J. H.; Bagajewicz, M. Ind. Eng. Chem. Res. 2005, 44, 8046−8056), throughput loss was added to the model, thus allowing the throughput to be reduced when convenient. Because of the high importance of financial risk in almost any industry and the presence of uncertainty in some of the parameters involved in the operation of heat exchanger networks, a stochastic version of the model was developed and applied to crude fractionation units. The model considers uncertainty not only in the future price of the natural gas expended in the furnace but also in the actual value of the fouling rates of the crudes and in the schedule of change of feedstock during the operation.

Design And Operation Of Heat ExchangerNetworks

Unlike a single two-stream heat exchanger, design and operation of heat exchanger networks are usually an inverse problem. Methods for rating, sizing, simulation, regulation and synthesis of two-stream and multistream heat exchanger networks based on a general matrix solution are developed. The corresponding optimization problems are formulated which yield better results than those in the literature.

A methodology for improving heat exchanger network operation

This work proposes a methodology and workflow to assist in the analysis of an existing heat exchanger network design when the operating conditions vary. Its purpose is to facilitate the involvement of the engineer in the daily operation and maintenance of the network. In order to incorporate this new methodology into the operations workflow, the concept of a heat exchanger network unit operation is introduced. The main characteristic of this new unit operation is its complete knowledge of the different complexities of the heat exchanger network model. Moreover, the heat exchanger unit operation is capable of managing the network degrees of freedom to consider different problem formulations. Additional features of this unit operation include the automatic analysis of output variables when the operating conditions change and the analysis of the trends follow by these variables during a given time period. An example showing the application of these features to the preheat train of a crude distillation column is presented.