Optimal operation of heat exchanger networks through energy flow redistribution

Abstract

This 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.

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

“…Thermodynamic feasibility requires maintaining a positive thermal driving force across any heat exchanger. Accordingly, the following constraints can be obtained where δT is a user-defined parameter.…”

“…For a traditional HEN control problem, the output vector y includes the target temperature for relevant process streams, and the input move vector corresponds to the changes in heater/cooler duty. In a sequential (two-step) implementation of EFR, 11 the output vector is augmented by either duty or outlet temperature of each heat exchanger with bypass or split and the input vector additionally includes heat exchanger bypass fraction and process stream split fraction. The set points for these additional variables come from the solution of the EFR optimization problem given in eq 7.…”

“…The EFR methodology can be used to reduce the impact of this disturbance on the overall utility cost. The methodology uses stream splits to manipulate the energetic effect of the disturbance in such a way that there is a reduction in the consumption of costly hot utility.…”

“…These efforts can be broadly classified into two directions based on the frequency of optimization. The approach of self-optimizing control uses offline optimization to identify key variables (or combinations thereof) which can be maintained at a constant value to achieve (near) optimal operation. − The other approach of real time optimization (RTO) uses online optimization to obtain a new operating point, which is tracked using a feedback control system. − The present work pursues this direction.…”

“…In previous work, , we presented a novel concept of energy flow redistribution (EFR) to optimize HEN operation in the presence of the above-mentioned disturbances. Specifically, the propagation path of the disturbance through the network is manipulated (by changing the duties of the individual heat exchangers) to achieve minimum utility consumption.…”