Model predictive control of a heat-exchanger network

GONZÁLEZ, ALEJANDRO HERNÁN; ODLOAK, DARCI; MARCHETTI, JACINTO LUIS

Río de Janeiro, Brasil

Congreso; 2nd Mercosur Congress on Chemical Engineering, 4th Mercosur Congress on Process System Engineering; 2005

Abstract: Control of Heat-exchanger networks has been matter of research for many years. Though some strategies going from hierarchical control structures to low-level flexible control systems have been proposed to solve the problem, the use of Model Predictive Control has not been extensively evaluated in these process systems. This work discusses the online optimisation and control of a Heat Exchanger Network (HEN) through a two-level structure: the high level performs a supervisor online optimisation that provides the optimal steady-state conditions, and the low level uses the flexibility and adaptability of constrained Model Predictive Control (MPC) to dynamically guide the process up to the set points. Since MPC is a multivariable control technique capable of handling control-input constraints, it is neither necessary to define a variable-pairing approach nor to include individual-loop protections to avoid close-loop saturations. The coordination between the supervisor program and MPC is achieved through the definition of an extended cost-function that provides the ability of driving the system to optimal conditions, and preserves the original predictive control advantages. The proposed MPC algorithm uses an input-output linear time-invariant representation to perform the output predictions, and to account for constraints. On the other hand, the supervisor program basically delivers key desired manipulated-variable positions to MPC, leading to minimum utility consumption. The proposed method was successfully tested using a rigorous simulation of a typical HEN system of the process industry.