CONTROL STRATEGIES FOR A SUPERCRITICAL-FLUID EXTRACTION PROCESS

This paper addresses alternative control strategies for a supercritica l fluid extraction (SFE) process. Alternative control strategies are g enerated by applying different plant decompositions for the SFE proces s to produce a decentralised controller for the whole process. Two tec hniques 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 and the maximum stability margin b(max). These indicators are also us eful for deciding a ''realistic'' performance specification for each s ubsystem. This study shows that the performance of a control strategy depends on how the SFE process is decomposed and how the performance s pecifications 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'' pe rformance specification for each subsystem. As a result, two control s chemes are selected as alternative control strategies for the SFE proc ess rather than the use of a centralised controller.