Industrial Implementation of Motor Load and Freeness Control of Chemimechanical Pulp Refiners

A simple refiner control strategy aimed at minimizing variations in freeness of an ultra-high-yield sulfite (UHYS) pulp has been implemented in a newsprint mill. The feeder screw speed is being manipulated to compensate for variations in the bulk density of the chip feed and to maintain a constant motor load at the primary refiners. This ensures a more constant mass flow rate through the primary refiners, which translates into a more constant primary specific energy. This has resulted in a reduction in motor load fluctuations of about 60% and a reduction in freeness variations of 40%. The final pulp freeness is regulated by manipulating the plate gap of the secondary refiners using on-line freeness measurements made at the outlet of the latency chest. Control of the secondary plate gap has led to an additional 30% reduction of the variation in freeness. This two-stage refining control strategy has reduced the 2-sigma confidence interval for variation in pulp freeness from 48 mL to less than 14 mL, a reduction of more than 70%. The complete strategy has reduced the average refiner energy consumption per ton of pulp by 3.5% and has increased the average paper machine production by 2.5% by reducing the number of paper breaks. In addition, the strategy has compensated for changes in cooking conditions and their effect on the refining properties of the chips.