Robust control of a crude unit

The product quality control is a main issue in a crude unit which performs the initial distillation of the crude oil into several boiling range fractions. The specifications vary according to the processing capabilities and the market demand. The crudes have greatly different physical characteristics. A quadratic dynamic matrix control (QDMC) has been used to design a controller which is able to guarantee different setpoints with the same crude, the same setpoint with different crudes and both of them. Four detailed models (with 5 input variables and 4 control objectives) of the same crude unit working with different crudes have been derived from literature. Some of the 20 input-output transfer functions of each model are nonminimum phase. Validation of the controller has been done by (i) assuming a “design” model for the design of the controller, (ii) using three different models for the “true” crude unit, (iii) requesting a significant variation of the setpoint (naphtha/kero, kero/LGO, etc., cutpoints). Some guidelines about the choice of the design parameters (prediction horizon, control horizon, input/output weights, input/output constraints, etc.) are given in order to achieve both stability robustness and performance robustness