Comparison of single loop and QDMC-based control of crude oil distillation towers

Abstract

Significant economic benefits can result from improved control of crude towers because of their large throughput. In this paper, both conventional PI control, with and without decoupling, and model based QDMC control are applied to crude tower product quality variables. The conventional methods treated involve BLT tuning, detuned Ziegler-Nichols control, and detuned Ziegler-Nichols control plus decoupling. The various control methods are tested on a detailed non-linear simulation of a crude tower. It is shown that significant dynamic two-way interaction exists, particularly in the upper crude tower product quality loops, even though interaction is essentially one-way throughout the tower near steady state. Because of the dynamic interaction, PI controllers must be substantially detuned over what would be required for a one-way interacting system. Decoupling is shown to help improve the performance of the PI quality loops. QDMC is shown to give good transient performance, and it is considered easier to apply than traditional PI methods.