Using multistage models to evaluate  how pulp washing after the first extraction stage impacts elemental chlorine-free bleach demand, TAPPI Journal November 2018

ABSTRACT: Steady-state models were constructed to predict the response of a southern U.S. softwood brown-stock to three- and five-stage elemental chlorine-free (ECF) bleach sequences. The models provided insight into how typical (EO) washing efficiencies from a vacuum drum unit affected pulp brightening and total chlorine dioxide consumption. When (EO) carryover was between 15% and 30%, the chlorine dioxide needed to reach target brightness increased by 8% to 15% for the D₀(EO)D1(EP)D₂ sequence (89% ISO) and by 15% to 23% for the D₀(EO)D₁ sequence (86% ISO) versus perfect washing. Use of (EO) filtrate as D0 shower water, such as in split-flow countercurrent washing, caused the bleach uptake to increase by 1.5 to 3.0 kg chlorine dioxide (ClO2)/ton pulp when compared to using cleaner D0 shower water sources. The ClO₂ consumed by 15% to 30% (EO) washer carryover is comparable to that consumed by typical carryover levels from brownstock washing (~10 kg Na₂SO₄/ton pulp). High (EO) carryover made ECF bleaching to higher brightness targets more difficult.

 Application: Multistage models are employed to estimate how typical levels of extraction washer carryover, as well as how extraction washer filtrate reuse in countercurrent washing, affects chlorine dioxide consumption and brightness development of a softwood kraft pulp when bleached with ECF sequences. These models could be adapted to simulate mill data, used to optimize bleaching operations, and used in process control strategies.

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