Fast and Accurate Online Measurement of Differentiated Lignin and HexA Kappa Numbers Leads to Improved Fiberline Control, 2018 PEERS

It is well-known that Hexenuronic acid (HexA) groups cause increased consumption of bleaching chemicals, decreased brightness and greater brightness reversion particularly for hardwood pulps. Still, the control of a kraft pulp mill process is typically based only on measurements of kappa number and brightness, subsequently disregarding whether the kappa number or the brightness actually is due to lignin, HexA, or other constituents. The present study hows how a differentiated lignin and HexA measurement and unit operation process control concepts can be applied in key process stages. Combined with proper determination of lignin concentration in both the fiber and the liquor phase of the pulp slurry, a significant improvement can be demonstrated. 

The paper reports findings from laboratory A- and D-stage experiments on birch, to investigate the impact of dissolved lignin carryover, and to investigate the lignin and HexA reduction, respectively. The results showed that the dissolved lignin had a significant impact on the efficiency and that the lignin and HexA reductions were affected partly different. Case histories are also presented utilizing differentiated online lignin and HexA measurements which can be used to optimize the process conditions for a better pulp quality control. A concept is proposed for using the improved measurements combined with a model-predictive process control strategy for the fiberline, with the objective to reach target pulp quality at minimum operational cost, and with minimum environmental impact.  

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