A structural approach to the design of control systems in heat exchanger networks

Abstract

A new approach is proposed to address the following problems: (a) design the configuration of control loops in a network of heat exchangers (the DESIGN problem); (b) sequence the control action of the loops, to accommodate set-point changes and reject load disturbances (the OPERATIONAL problem). The approach proposed exploits the structural characteristics of a heat exchanger network (HEN) by identifying routes through the HEN structure that can allocate loads (disturbances, or set-point changes) to available sinks (external coolers or heaters). The resulting configuration of control systems is composed of feedback and feedforward loops. Several design issues such as the placement of bypass lines, the restrictions imposed by the existence of a “process pinch”, etc. are also discussed. An online, real-time planning of control actions is the essence of implementational strategies generated for the solution of the OPERATIONAL problem. The plan of control actions is generated by an “expert” controller, which selects what path(s) through the HEN is (are) to be used for each entering disturbance or set-point change, and what loops should be activated (and in what sequence) to carry the associated load (disturbance or set-point change) to a utility unit.