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

摘要

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 D_0(EO)D_1(EP)D_2 (89% ISO) and by 15% to 23% for the D0(EO)D_1 (86% ISO) versus perfect washing. Usage of (EO) filtrate as Do shower water, such as in split-flow countercurrent washing, caused the bleach uptake to increase by 1.5 to 3.0 kg ClO_2/t pulp when compared to using cleaner Do 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 Na_2SO_4/t pulp). High (EO) carryover made ECF bleaching to higher brightness targets more difficult.