A simplistic mechanistic model and effect of consistency on alkaline peroxide brightening of mechanical pulps

The multiple active site mechanistic model proposed earlier (Liu, Chem. Eng. Sci. 58 (2003) 2229) has been revisited and a simplistic single active site model has been developed. The effect of pulp consistency during brightening reaction has been examined based on the derived kinetic model. There are three competing factors affecting the brightening reaction arising from pulp consistency: effective chemical concentration in the brightening liquor, accessibility of chromophores to the active chemical agents, and frequency of the chromophore-chromophore contact. With the same brightening agent charges (on a dry pulp basis), increasing pulp consistency resulted in an increase in the effective brightening agent concentration in the brightening liquor and the brightening reaction rate increases as well as the final attainable brightness. Increase in pulp consistency under constant brightening agent concentration in liquor reduces brightening reaction rate and final attainable brightness due to the reduction of the accessibility of the brightening agents to the chromophore sites. Finally, increasing pulp consistency increases the frequency of chromophore contacts among different fibers and thus increases brightening reaction rate. Theoretical models are proposed for these effects and good agreement is found with existing experimental data. (C) 2004 Elsevier Ltd. All rights reserved.