REVISED STEADY-STATE MODEL FOR CHLORINE DIOXIDE BRIGHTENING THAT CONSIDERS EXTRACTION WASHER CARRYOVER EFFECTS

摘要

In an earlier investigation, a generalized model was developed to simulate the first chlorine dioxide brightening stage (D|). This stoichiometric model accurately predicts pulp brightness values for given bleach charges and vice versa for laboratory softwood pulps. The equation parameters are dependent upon the kappa number and brightness of the extracted pulp. In this study, the above model was refined to include the negative effects of extraction carryover to simulate more realistically a mill's D1 stage. It was found that extracted pulps that contain washer carryover 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. It was determined that the mill's washed pulps had 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 to 4.6 kg ClO2/t pulp per kappa unit of carryover when bleaching a pulp to 78 to 84% ISO. Application: Revised model forecasts pulp brightness for extracted softwood kraft pulps for a variety of chlorine dioxide, extracted kappa and washer carryover levels. Model can be used to analyze bleach plant data to determine extraction washer carryover and bleach consumption caused by this material. Other applications include bleach plant optimizations, process control strategies, and computer simulations.