Dry chip feedrate control using online chip moisture, TAPPI JOURNAL May 2018

ABSTRACT: Normal practice in continuous digester operation is to set the production rate through the chip meter speed. This speed is seldom, if ever, adjusted except to change production, and most of the other digester inputs are ratioed to it. The inherent assumption is that constant chip meter speed equates to constant dry mass flow of chips. This is seldom, if ever, true. As a result, the actual production rate, effective alkali (EA)-to-wood and liquor-to-wood ratios may vary substantially from assumed values. This increases process variability and decreases profits. In this report, a new continuous digester production rate control strategy is developed that addresses this shortcoming. A new noncontacting near infrared–based chip moisture sensor is combined with the existing weight-ometer signal to estimate the actual dry chip mass feedrate entering the digester. The estimated feedrate is then used to implement a novel feedback control strategy that adjusts the chip meter speed to maintain the dry chip  feedrate at the target value. The report details the results of applying the new measurements and control strategy  to a dual vessel continuous digester.

Application: A new method of dry chip feedrate control could help mills stabilize and potentially increase  production rates and achieve better production quality when combined with existing liquor dosing controls.

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