Control of holdup and solute concentration in an Oldshue-Rushton extraction column

An experimental application of multiloop control of an extraction column of the Oldshue-Rushton type is reported in this article. The employment of an ultrasonic technique for the control of the dispersed-phased volume fraction (holdup) of an equilibrated system in the same extraction column has been described in an earlier work. This work is extended here to include control of holdup during mass transfer and simultaneous control of the solute concentration at the exit of the extractor. The control objective is to control the outlet concentration and to maintain a high value of the holdup under safe operation in reference to flooding. Transfer function models are identified from step tests about a nominal operating point. The outlet concentration is controlled by manipulating the continuous-phase flow-rate, and the holdup is controlled by manipulating the agitation speed. The Dahlin controller is used in both loops and is proved to work satisfactorily for both servo and regulatory control even when the process is not represented well by the transfer functions. The process monitoring and control demonstrated in this article can be applied to any new or existing extraction column equipment for safe and optimum operation.