Advances in recycle mill supervisory control at Avenor Thunder Bay

Abstract

This study examined supervisory control systems at Avenor Inc. Recycle Pulp Mill Thunder Bay making improvements in process control and automation. Supervisory controls were implemented for improving the control of a talc conveyor metering system to monitor and regulate the addition of talc to maintain a target talc concentration on mill pulp; in the implementation of a feed forward pulper discharge consistency control system to improve stock consistency control during pulper dilution and discharge to the retention chest; and to improve the operation of the Pulper Head tank Level Control System to reduce the impact of pulper head tank inventory changes on grey water chest inventory control and mill water makeup. Implementation of the talc supervisory control system achieved the desired steady state control of the talc concentration and reduced maximum excursions in the mill talc concentration to within 25% of setpoint. The talc control system eliminated the need to replace a malfunctioning density transmitter, resulting in capital cost savings. Feed forward pulper discharge consistency controls reduced the variance in phase 5 stock consistency entering the retention chest by 93%, eliminating undesirable valve oscillation on downstream consistency control loops associated with the high variability in the pulper discharge consistency. A proposed feed forward pulper head tank supervisory level controller demonstrated the possibility of reducing the variance in the pulper head tank feed by over 95%, thereby significantly reducing the impact of pulper head tank feed flow changes on mill water makeup and inventory control in the grey water chest. Potential Sources of Flotation Inlet Brightness variation were also examined to determine the cause of excessive brightness variation and suggest possible solutions through the available on-line distributed control system mill technology.