Thesis for the Degree of Master of Science


Year 2008
Leena Länne

Fault Tolerant Control of a Dearomatization Unit

A faulty actuator can - in the worst case scenario- produce a severe safety risk. Faults in actuators are more difficult to control than faults in sensors because actuators provide the actual process variables that have a direct influence on the process itself. This thesis studies the fault-tolerant aspects of model predictive control and aims to develop a fault-tolerant model predictive control system that is able to perform also in the presence of actuator faults.

This study begins with a survey of the methods for ensuring plant safety In addition, it examines the structure of a fault-tolerant control system on a more general level and considers the advantages of using a model predictive controller. This study also discusses examples of fault-tolerant control methods with a model predictive controller in the presence of actuator faults.

A model predictive fault-tolerant control system was developed. This study chose a solvent dearomatization unit from the Neste Oil Oyj Naantali Refinery as an object for the simulations. The model predictive controller NAPCON developed by Neste Jacobs was implemented to control the quality of the distillation product. Faults in actuators were simulated and their impact for control investigated. The two situations involving actuator faults were the obstruction of the valve that controls the reflux flow back to the column and the valve that controls the feed to the columns side stripper. Different methods to improve the temperature and quality control of the distillation column were tested under the fault scenarios with the dynamic process simulator PROSimulator.

On the basis of these simulations, the developed fault-tolerant control system was written and compiled as a separate program module and connected to the same test platform with the controller and the simulator. The system was then tested with simulations. The results indicated that the developed fault-tolerant control system was able to improve the quality control of the studied distillation column in the presence of the previously prescribed actuator faults.

Keywords: model predictive control, fault-tolerant, actuator fault

This info last modified 26 Sep 2017 by Jerri Kämpe-Hellenius