Control strategies for a supercritical fluid extraction process

Abstract

This paper addresses alternative control strategies for a supercritical fluid extraction (SFE) process. Alternative control strategies are generated by applying different plant decompositions for the SFE process to produce a decentralised controller for the whole process. Two techniques of plant decomposition are introduced, based on physical unit operations and based on process dynamics. Screening alternative plant decompositions are then performed by observing two indicators: the gap β and the maximum stability margin b max. These indicators are also useful for deciding a “realistic” performance specification for each subsystem. This study shows that the performance of a control strategy depends on how the SFE process is decomposed and how the performance specifications are posed in each subsystem. Hence, the “best” control strategy for the SFE process can be determined in a systematic manner by combining the “best” plant decomposition with a “realistic” performance specification for each subsystem. As a result, two control schemes are selected as alternative control strategies for the SFE process rather than the use of a centralised controller.