Coordination of Bypass Control and Economic Optimisation for Heat Exchanger Network with Stream Splits

Abstract

The bypass control is an online adjustment strategy widely used on heat exchanger network (HEN) to maintain the operating requirements. Once the operating conditions are varied, the controllers will adjust the fractions of bypass until satisfying control objective and keep the value until the next varying. The designed margin has to be increased thus economic efficiency becomes poor obviously. It is essential to take both control performance and economic efficiency into account during the process of operation. Several researchers have put forward coordinate control or coordinate optimization in chemical processes. However, these researches mainly studied on the coordination of different variables for controlling or optimization without considering the relationship between control performance and economic optimization. In this work, firstly we proposed a methodology for coordination of control and economic optimization for the HEN with stream splits. Usually the number of operating variables equals to controlled variables. The fractions of bypass are selected as operating variables and adjusted for achieving the control targets. In addition, the splits as available degrees of freedom have greater adverse effects on pressure drop, so only can be regulated within a tiny range and may suitable as optimization variables for economic optimization. Then a coordination scheme of bypass control and economic optimization for HEN with stream splits is designed on the basic of previous research. In this case, the control objective is to keep outlet stream temperature at the set point and the optimization objective is to release margin minimum which can make cost lower. The fractions of bypass are adjusted for controlling and the split ratios are regulated for economic optimization simultaneously, so adequate control and optimal economic can be achieved at the same time. The simulation results on a HEN with stream splits confirm the superiority of coordination method between bypass control and economic optimization, which ensure economic optimal meanwhile control performance promising though a certain control performance is sacrificed.