Revised steady-state model for chlorine dioxide brightening that considers extraction washer carryover effects, TAPPI JOURNAL March 2016

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ABSTRACT: In an earlier investigation, a generalized model was developed to simulate the first chlorine dioxide (ClO₂) brightening stage (D₁). That stoichiometric model accurately predicts pulp brightness values for given bleach charges and vice versa for laboratory softwood pulps. The equation parameters are dependent on the kappa number and brightness of the extracted pulp. In this study, the earlier model was refined to include the negative effects of extraction carryover to simulate more realistically a mill’s D₁ stage. Extracted pulps that contain washer carryover were found to brighten as if the pulp had a kappa number equal to the sum of the extracted kappa and the kappa value of extraction dissolved solids. If this higher kappa (i.e., apparent or wet kappa) is used in place of the extracted kappa with the D₁ model, the equations suitably predicted the bleached brightness for a given bleach charge. The modified expressions were used to quantify the amount of carryover and to calculate carryover bleach consumption for a softwood D0(EOP)D₁ fiber line. The mill’s washed pulps were found to have carryover levels of 1.4 kappa units, which was consuming 48% of the total D₁ bleach charge. Additional analyses revealed that extraction carryover consumes 2.5-4.6 kg ClO₂/ton pulp per kappa unit of carryover when bleaching a pulp to 78% to 84% ISO.

Application: This revised model forecasts pulp brightness for extracted softwood kraft pulps for a variety of chlorine dioxide, extracted kappa, and washer carryover levels. It can be used to analyze bleach plant data to deter-mine extraction washer carryover and bleach consumption caused by this material. Other applications include bleach plant optimizations, process control strategies, and computer simulations.