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APPLICATION OF SOFT SENSORS, DYNAMIC SIMULATION AND MODEL BASED CONTROL TO DIGESTION CHARGE CONTROL

Gorst, J. and Snow, T.

On-line measurement of Liquor Conductivity has been successfully used in numerous Alumina Refineries to provide improved control of the molar ratio (MR) of liquors leaving digestion (DIS). Further improvements in Charge Control have been made at the Gove Refinery using techniques such as soft sensors and model based control. This paper describes the successful development and implementation of a model based control strategy and an Operator Decision Support scheme for Digestion Charge Control. The applications make extensive use of various forms of soft sensors and explicitly incorporate process dynamics in internal models.

The soft sensors, using Recursive Least Square (RLS) and Kalman Filter estimators, perform three distinct functions; continuous calibration of the conductivity instrument, reconciliation of a dynamic mass balance, and the continuous inferred measurement of Digestion Feed Soda from on-line density measurements. The Operator Decision Support system uses the soft sensors to provide continuous predictions of the DIS MR, complementing the 2 hourly Laboratory analysis. The Control Application uses the soft sensors to perform feed forward control and Internal Model Control on a continuously calibrated conductivity instrument.

Considerable quality assurance was provided on the control application with the use of dynamic simulation and advanced control rapid prototyping tools. A high fidelity phenomenological dynamic simulation of the process and control system was developed and used to tune and evaluate candidate control strategies. The advanced control prototype was trialled on the simulation before being placed on-line. The Digestion process was perturbed with a multi-input pseudo random binary sequence to calibrate the process model. The extracted dynamics of the process were inherently incorporated into the soft sensors and model based control strategy.

The operator decision support has been on-line since December 1997 and the control scheme has been online since July 1998. These systems resulted in reduced variations in DIS MR.