An investigation into control techniques for cascaded plants with buffering, to minimise the influence of process disturbances and to maximise the process yield

[摘要] ENGLISH ABSTRACT: The Coal to Liquid facility, Sasol, Secunda operates as a train of processes. Disturbancesand capacity restrictions can occur throughout the plant and the throughputfluctuates whenever disturbances occur. When capacity restrictions occur in a subplantand more substances enter the sub-plant than can be processed, the extra substancesare flared or dumped and therefore lost. To reduce losses and extra costs andto maximise the throughput of the whole plant, supervisory control is implemented overthe whole plant system.Each process in the process train is controlled with regulatory controllers and the overallprocess is then controlled with a supervisory controller. These two sets of controllersoperate in two different layers of control, with the regulatory controllers the faster innerlayer. The supervisory control is the outer layer of the two control layers. Thesupervisory controller takes over the work of the human operator by deciding on thechanges in total throughput as well as the set points for each individual process. Theseset points for each process are then followed with the regulatory controllers. For theregulatory control of the system, different control methods are investigated and compared.The different control methods that are looked at are PI control, Linearised StateFeedback control, Fuzzy Logic control and Model Reference Adaptive Control.After an investigation into the various control methods Fuzzy Logic control was chosenfor the regulatory as well as the supervisory control levels. Fuzzy Logic control is a rulebased control method. Fuzzy variables are everyday terms such as very slow or nearlyfull. These terms are easy to understand by the operator and multi-variable control ispossible with Fuzzy Logic control without an accurate mathematical representation ofthe system. These facts made Fuzzy Logic control ideal for this implementation.To improve the profit of the Coal to Liquid facility the throughput was maximised. Thecombination of regulatory and supervisory controllers minimised losses and rejecteddisturbances. This resulted in a smoother output with maximum profit.

[发布机构] Stellenbosch University