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Brogdon B.N.,Futurebridge
2010 TAPPI PEERS Conference and 9th Research Forum on Recycling | Year: 2010

This investigation evaluates how higher reaction temperatures and/or oxidant reinforcement of caustic extraction affect chlorine dioxide consumption during the ECF bleaching of hardwood pulps. A review of the available scientific literature is presented and the data obtained was critically analyzed. The addition of peroxide to an (EO)-stage roughly displaces an equal amount of ClO2 (by weight). Increasing the (EO) temperature from 70 to 90°C lowers the ClO2 usage by ≤0.5% on pulp. Unlike what is observed for ECF softwood bleaching, the presented findings suggest that aggressive oxidant-reinforced extraction stages do not result in significantly lower bleaching costs when compared to milder alkaline extraction stages. Likewise, the optimum amount of ClO2 used in delignification versus brightening varies from 40 to 80% of the total ClO2 bleach demand, and is dependent upon the brightness target and the hardwood kappa number.

Brogdon B.N.,Futurebridge | Brogdon B.N.,Light Green Machine Institute
Tappi Journal | Year: 2013

A generalized, steady-state model for hardwoods is proposed for predicting bleaching delignification and/or chlorine dioxide (ClO2) consumption for sequences that use oxidant-reinforced extraction. Published data for various hardwood species and mixtures were analyzed to develop the model. The kappa number data from these studies were normalized to their respective pre-D 0 kappa number, and the normalized kappa numbers were plotted against the bleach demand. This mathematical transformation allowed for various brownstocks and oxygen-delignified pulps with different kappa numbers to be modeled as a single curve based on an empirical relationship with fitted equation parameters. One of the two equation parameters could be expressed as simple functions of oxidant-reinforced extraction conditions (i.e., peroxide dosage). The model forecasts ClO2 usage reasonably well (±0.20% ClO2 on pulp) for conventional ClO2 delignification with extraction. Attempts to incorporate modified bleaching delignification processes that eliminate hexenuronic acids into the model were unsuccessful; data were insufficient to develop a relationship. This straightforward stoichiometric model contains relatively few fitted parameters to be determined. The model could be used with other steady-state brighteningstage models to predict bleach usage. Application: The hardwood stoichiometric model predicts ClO2 consumption during bleaching delignification for mills operating elemental chlorine-free bleaching sequences that use oxidant-reinforced extraction stages. Other applications of the model include bleach plant optimizations, delignification kinetics, computer simulations, and/or process control strategies.

This investigation evaluates how higher reaction temperatures or oxidant reinforcement of caustic extraction affects chlorine dioxide consumption during elemental chlorine-free bleaching of North American hardwood pulps. Bleaching data from the published literature were used to develop statistical response surface models for chlorine dioxide delignification and brightening sequences for a variety of hardwood pulps. The effects of higher (EO) temperature and of peroxide reinforcement were estimated from observations reported in the literature. The addition of peroxide to an (EO) stage roughly displaces 0.6 to 1.2 kg chlorine dioxide per kilogram peroxide used in elemental chlorine-free (ECF) bleach sequences. Increasing the (EO) temperature by Δ20°C (e.g., 70°C to 90°C) lowers the overall chlorine dioxide demand by 0.4 to 1.5 kg. Unlike what is observed for ECF softwood bleaching, the presented findings suggest that hot oxidant-reinforced extraction stages result in somewhat higher bleaching costs when compared to milder alkaline extraction stages for hardwoods. The substitution of an (EOP) in place of (EO) resulted in small changes to the overall bleaching cost. The models employed in this study did not take into account pulp bleaching shrinkage (yield loss), to simplify the calculations.

In 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% ClO2) 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. Application: The modified steady-state models predict chlorine dioxide consumption needed for bleaching delignification for a variety of incoming kappa numbers and washer carryover levels. Other applications of the model include bleach plant optimizations, process control strategies, and computer simulations.

Brogdon B.N.,Futurebridge
2013 PEERS Conference, Co-located with the 2013 International Bioenergy and Bioproducts Conference | Year: 2013

A generalized, steady-state model is proposed for predicting brightness and/or bleach consumption during chlorine dioxide brightening (D1 and D2) for softwood pulps treated with chlorine dioxide delignification sequences. The model is adapted from the D1 hardwood model of McDonough and co-workers, which related chlorine dioxide consumption to brightness gains with three tunable parameters. Literature data from various Elemental Chlorine-Free (ECF) bleaching studies were analyzed to find generalized relationships for the model's parameters. Data analysis revealed that the ultimate brightness limits for both D1 and D2 models, at excessively high bleach charges, was ~92% ISO. Another parameter for the D1 model was found to be a linear function of extracted kappa number, whereas for the D2 model, the parameter was a linear function of pre-D2 brightness. The third equation parameter for the D1 and D2 models was found to be a function of the other two parameters and the entering brightness value. The generalized brightening models predict D1 brightness to ±1% ISO units and D2 brightness to ±0.3% ISO units for a given amount of chlorine dioxide consumed. These relatively simple brightening models, which have a minimum number of equation parameters, can used in conjunction with other bleaching delignification models to simulate chemical consumption for a given ECF sequence. Copyright © 2013 by the TAPPI Press. All rights reserved.

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