Novel Technologies for Online Kappa Number Measurement Bring Opportunities for Improved Digester Control, 2016 Pulping, Engineering, Environmental, Recycling, Sustainability (PEERS) Conference Proceedings

Typically, more than 90% of the overall delignification is achieved during the cooking stage in a bleached kraft pulp mill. It follows naturally that any improvement in the digester operation, such as cooking selectivity and pulp evenness, may result in significant economic impact.  

All process control requires some sort of measurement to work. For a continuous kraft digester the primary control variable is the lignin content of the pulp fibers, typically expressed as Kappa number. Online analyzers are available for determination of lignin content using measuring principles employing absorption of ultraviolet light. Such systems have been the industry standard since their introduction in the 1980’s.  The traditional setup had  a central analyzer connected to a number of pulp samplers at different locations in the fiberline, thus operating sequentially and sharing the limited capacity of the central analyzer. The normal measurement update rate achieved is around 30 minutes, but it naturally depends on the number of positions and sequence program. In particular for continuous digester blowline applications, this rate of measurement update satisfies the control requirements given the relatively long residence time and slow dynamics of the digester. However, according to the results from several mill installations using a new generation of kappa number analyzers with one measuring system at each position in the fiberline, this common understanding of update rate can be challenged. The high update rate of the single point based system, typically below five minutes, provides means for a better assessment of the actual state of the process. It is through this picture of “actual state” the use of signal filtering, such as damping or moving averages to account for measurement noise related to pulp heterogeneity, doesn’t compromise a dynamic response. It has been shown how an increased accuracy of the kappa number measurement can be used to improve selectivity and subsequently increase pulp yield. Furthermore, data is presented showing how the high update rate data achieved can be used to assess digester operation performance by indicating the kappa number heterogeneity. 

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