IMPROVED STEADY-STATE MODELS FOR CHLORINE DIOXIDE DELIGNIFICATION SEQUENCES THAT INCLUDES WASHER CARRYOVER EFFECTS

摘要

[n previous studies, generalized steady-state models were proposed to approximate the chlorine dioxide demand needed for the delignification of softwood and hardwood pulps, where the kappa number entering the bleach plant can fluctuate. However, these expressions neglect the impact of dissolved solids with the stock that originate from incomplete pulp washing. In this study, the original elemental chlorine-free (ECF) models are modified to include the effects of carryover from brownstock or post-oxygen washing. The stoichiometric bleach consumption caused from carryover was calculated from various literature sources based upon its composition. The majority of the bleach demand (ca. 70%) is caused from the dissolved lignin contained in the brownstock carryover, with the remainder being caused from the inorganic sulfur constituents (e.g., sulfide and thiosulfate). When the impact of brownstock carryover was taken into account, the modified models accurately predicted the amount of chlorine dioxide consumed for a given delignification level (ca. ±0.1% C1O2) versus actual bleach plant data. The improved models can also be used to gauge the level of washer carryover entering the bleach plant if this parameter is not regularly monitored by the mill. Additionally, these modified expressions could be integrated into advanced process control strategies for ECF bleaching where the washer carryover or dissolved lignin entrainment is measured with on-line sensors.