Debottlenecking level control for tanks in series

Abstract

Smooth operation of an industrial liquid–liquid extraction process with three tanks in series posed an interesting control problem: for control of each tank's liquid level through PI manipulation of each corresponding outlet flow rate, the conflicting objectives of keeping both level and flow rate fluctuations within bounds became impossible to satisfy when the main disturbance (feed flow rate) exceeded a certain magnitude. A solution had to be found, preferably using the existing PI controllers and no additional equipment such as flow sensors and advanced control hardware, to keep cost down. A solution was found based on implementation of a simple modified control structure that employs the same PI controllers. This structure essentially aggregates the three tanks into one larger tank, whose virtual level is controlled by a PI controller, supported by the remaining two PI controllers that keep the differences between tank levels to a minimum, so that the three tanks can behave as virtually one. In addition to theoretical analysis of the proposed structure, plant validation is presented. The questions of how this structure could be reached through numerical optimization are addressed, whereas the broader question of automating the intuitive process of solution generation is raised.