USING MULTISTAGE MODELS TO EVALUATE HOW PULP WASHING AFTER THE FIRST EXTRACTION STAGE IMPACTS ELEMENTAL CHLORINE-FREE BLEACH DEMAND, 2018PEERS

Steady-state models were constructed to predict the response of a southern US softwood brownstock to three- and five-stage elemental chlorine-free (ECF) bleach sequences.  The models examined 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 bleach needed increased by 8% to 15% for the D₀(EO)D₁(EP)D₂ (89% ISO) and by 15% to 23% for the D₀(EO)D1 (86% ISO) versus perfect washing.  Usage 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 ClO₂/t pulp when compared to using cleaner D0 shower water sources.  The bleach consumed by 15% to 30% (EO) washer carryover is comparable to that consumed by typical carryover levels from brownstock washing (ca. 10 kg Na₂SO₄/t pulp).  High (EO) carryover made ECF bleaching to higher brightness targets more difficult. 

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