Utilization of inline total dissolved solids measurement system in a pulp mill's brownstock washing line, TAPPI Journal January 2026

 


Application: Without effective washing, the economic viability of chemical pulp production is affected. In this study, continuous TDS measurements were used to evaluate brownstock washing performance. Readers can learn the importance of control and monitoring of washing and the importance of accurate inline measurements. They can also learn new information about how different adjustments of the pressure filter affect washing results. This information could be beneficial for research and development engineers and production personnel.

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Author: Riku Kopra and Keijo Pyörälä
Utilization of inline total dissolved solids measurement sys
ABSTRACT: Efficient washing improves the recovery of sodium (Na) and sulfur (S) cooking chemicals, as well as wood-based dissolved organic material. It also reduces the additional consumption of chemicals in the subsequent bleaching stages. The operation of the brownstock washing of the pulp mill’s fiber line has a significant impact on the mill’s energy economy, material efficiency, and environmental emissions. Process refractometers can be used to measure the total dissolved solids (TDS) from both the washing liquor and the pulp suspension filtrate part directly from pipelines or pipe bypass loops. In addition, by measuring the three or four incoming/outgoing dissolved solids streams to the washers, together with consistencies and flow rate measurements, it is possible to build the real-time effectiveness calculation of the washer(s). In this work, an online efficiency calculation based on TDS measurements was built for a pressure filter after cooking and oxygen (O2) delignification. The washing performance was monitored over a longer period, and stepwise tests were conducted to find the optimal operating mode. The feed and washing consistencies, the washer’s torque, and the washing liquid distributions varied, and the washer’s efficiency values were monitored using realtime measurements. From the long-term trends of the efficiency calculation, process disturbances could be detected, and their causes could be found. Based on the results of the stepwise tests, an optimal operating model for the washer was found. By optimizing the washing consistency, the Y10 washing yield could be increased. More efficient washing with the same or even a smaller amount of washing liquid can relieve the operation of the evaporator, which is often a bottleneck in the mill. The study also found the effect of cooking-related carryover on the operation of the oxygen stage, as well as differences in the washability of softwood and hardwood.
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