Decentralized Control of a Pilot Flotation Column: A 3×3 System

Abstract

This paper details the application of a decentralized control strategy to a pilot flotation column working with a water-air system. The process is represented by a multivariable system composed of three inputs and three outputs. The selected control variables are the froth depth, air hold-up and bias-rate whereas the corresponding manipulated variables are the set-points of the local flow controllers of tailings, air and wash-water. All controlled variables are estimated using electrical-conductivity based techniques. Results of a new method for estimating bias-rate, based on the volumetric fraction of wash water under the interface, are presented. This method reduces the coupling problem associated to the bias estimation as a difference between feed and tailing water rates. Moreover, the nominal system transfer matrix can be represented by an upper triangular matrix, in such a way that the closed-loop system stability reduces to the stability of the independent control loops. A two degree of freedom control structure was implemented for the froth depth so that in the absence of disturbances the controller acts as a proportional controller whereas in the presence of a disturbance it becomes a PI controller. For gas hold-up and bias-rate, PI controllers were implemented considering the closed-loop time constants being equal to those in the open-loop case. The experimental results show expected closed-loop performances for froth depth and gas hold-up, however, the bias rate control loop was observed to be strongly dependent on the air flow rate, in some cases reaching the saturation point of the controller.