Utilization of inline TDS measurement system in pulp mill’s brown stock washing line, TAPPICon25

Efficient washing improves the recovery of cooking chemicals (Na and S) and wood-based dissolved organic material. It also reduces the additional consumption of chemicals in the subsequent bleaching stages. The operation of the brown stock washing of the pulp mill’s fiber line has a significant impact on the mill’s energy economy, material efficiency and environmental emissions.

By using the process refractometers, it can be measured the total dissolved solids (TDS) both from the washing liquor and from pulp suspension filtrate part directly from pipelines or pipe loops made over the pump. Additionally, 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 on-line efficiency calculation based on TDS measurements were built for pressure filter after cooking and 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, torque of the washer’s and washing liquid distributions were varied and the washer's efficiency values were monitored using real-time measurements.

From the long-term trends of the efficiency calculation, it could be detected process disturbances, and then find causes of disturbances. 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 of the mill. The study also found the effect of cooking-related carryover on the operation of the oxygen stage, and the differences in the washability of softwood and hardwood.

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