Optimal operation of heat exchanger networks (original) (raw)
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On-line optimization and choice of optimization variables for control of heat exchanger networks
Computers & Chemical Engineering, 1997
The paper discusses optimal operation of a general heat exchanger network with given structure, heat exchanger areas and stream data including predefined disturbances. A method that combines the use of steady state optimization and decentralized leedback control is proposed. A general steady state model is developed, which is easily adapted to any heat exchanger network. Using this model periodically for optimization, the operating conditions that minimize utility cost are found. Setpoints are constant from one optimization to the next, and special attention is paid to the selection of measurements such that the utility cost is minimized in the presence of disturbances and model errors. In addition to heat exchanger networks, the proposed method may also be applied to other processes where the optimum lies at the intersection of constraints.
A simple strategy for optimal operation of heat exchanger networks
2006
The objective of this work is to propose a systematic procedure to find a control structure for optimal operation of heat exchanger networks. Optimal operation in this context requires that 1) all controlled temperatures are kept at their targets and 2) utility cost is minimized. The degrees of freedom of heat exchanger networks are analyzed and used to identify if the operation is structurally feasible and if the utility cost can be optimized. The LP problem formulation for optimal operation of HENs implies that optimal operation is always at active constraints in terms of target temperatures and zero or maximum heat transfer (such as fully open or fully closed bypass valves). The information from an offline optimization is used to identify the set of nominally active constraints, and then split-range and selective controls are used to track the active constraints during operation for optimality.
Optimal setpoint control of complex heat exchanger systems
Computers in Industry, 1985
This paper presents an approach for calculating the best way of distributing the streams following through a certain class of complex heat exchanger systems in order to achieve maximum heat recovery within the system. A computer code has been developed by which the described method is demonstrated, off-line, for two real cases. This program can be readily integrated into an overall , on-line computer control system for any complex process consisting of an exchanger system of this class. Using an accurate and detailed heat exchanger model, the exit temperatures of each exchanger are calculated by a simple mathematical procedure based on Gilmour's design method. This procedure has been included in a general model for the complete scheme of the system. The scheme is made up of a series of heat exchanger groups with parallel paths in each group. The optimal distribution of the streams within a group is found by the direct search method of Hooke and Jeeves, modified to include constraints; while the overall optimization of the system is achieved via dynamic programming.
Multivariable Optimal Control of a Heat Exchanger Network (HEN) with Bypasses
IASTED Technology Conferences / 696:MS / 697:CA / 698: WC / 699: EME / 700: SOE, 2010
Heat exchanger networks present an interesting control problem due to coupling among process streams. In this work, the linear quadratic regulator (LQR), a feedback optimal control technique, is used to control stream temperatures on a laboratory scale heat exchanger network, through bypass manipulation, in a multivariable system. The LQR design was based on a mathematical model of the plant and its performance was compared to traditional PID control and to dynamical decoupling. Experimental tests were performed to evaluate the controllers, involving regulatory and servo problems. The performance of the different controllers was quantitatively compared by using the integral absolute error. Although LQR is not a new control methodology, the results obtained in this work suggest that LQR is an interesting alternative to control HEN when compared to the PID and to the dynamic decoupler. Moreover, one of the main advantages of the LQR is its tuning simplicity, since only one parameter is sufficient for this application.
Multivariable Optimal Control of a Heat Exchanger Network with Bypasses
Brazilian Journal of Chemical Engineering, 2016
Heat exchanger networks present an interesting control problem due to coupling among process streams. In this work, the linear quadratic regulator (LQR), a feedback optimal control technique, is used to control stream temperatures on a laboratory scale heat exchanger network, through bypass manipulation, in a multivariable system. The LQR design was based on a mathematical model of the plant and its performance was compared to traditional PID control and to dynamical decoupling. Experimental tests were performed to evaluate the controllers, involving regulatory and servo problems. The performance of the different controllers was quantitatively compared by using the integral absolute error. Although LQR is not a new control methodology, the results obtained in this work suggest that LQR is an interesting alternative to control HEN when compared to the PID and to the dynamic decoupler. Moreover, one of the main advantages of the LQR is its tuning simplicity, since only one parameter is sufficient for this application.
Heat Exchanger Network Optimization for Multiple Period Operations
Industrial & Engineering Chemistry Research, 2016
In this paper an optimization model is presented for the synthesis of a heat exchanger network (HEN) for multi-period 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 multi-period 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 the implementation of the model,
Control and Operation of Heat Exchanger Networks using Model Predictive Control
Abstract—While, during the past decades, many strategies for heat exchanger network synthesis and design have been developed, much less effort have been dedicated to developing online optimal control strategies to tackle their complex and distributed dynamics. Since past optimal control design efforts predominately revolves around centralized control ideas they typically suffer from high computational demands.
OPTIMAL CONTROL OF A HEAT EXCHANGER'S TEMPERATURE BY USING OF GENETIC ALGORITHM AND ARTIFICIAL NEURAL NETWORK, 2014
Heat exchanger (HE) problems have been one of the interesting fields for researchers in the recent 30 years. Theirs important effects on industrial usage in many serious places such as power plants, refineries and… are deniable. Theirs outputs have direct effect on many devices which are placed after it. Variation of these outputs should always be in a steady range. Disturbances can change HE's outputs so control of the outputs and fixing them in a steady range is a significant task. We present a new method which can be used for all HEs. This method can give the designers the optimal bypass quantities which can control the HEs output in the presence of disturbances. The obtained results by using this method on a HE show that it can restore the output to its primary quantity and also shows the desirable function of our method.
Performance comparison of different control strategies for heat exchanger networks
Polish Journal of Chemical Technology, 2018
In this article, the dynamic responses of heat exchanger networks to disturbance and setpoint change were studied. Various control strategies, including: proportional integral, model predictive control, passivity approach, and passivity-based model predictive control were used to monitor all outlet temperatures. The performance of controllers was analyzed through two procedures: 1) inducing a ±5% step disturbance in the supply temperature, or 2) tracking a ±5 o C target temperature. The performance criteria used to evaluate these various control modes was settling time and percentage overshoot. According to the results, the passivity-based model predictive controllers produced the best performance to reject the disturbance and the model predictive control proved to be the best controller to track the setpoint. Whereas, the ensuing performance results of both the PI and passivity controllers were discovered to be only acceptable.