Prediction of the Effect of Operating Variables on the Rates of Delignification and Cellulose Degradation During Ozone Bleaching, 1998 Pulping Conference Proceedings

It has been shown by transmission electron microscopy (TEM) that the progress of delignification during pulp ozonation is that of a sharp ozone-lignin reaction front moving from the external fibre surface towards the lumen. The mathematical description of the movement of the lignin reaction front is given by the so-called "shrinking core" model with reacted layer diffusion control. The cellulose degradation takes place mostly inside the reaction front by radical species generated by the fast lignin-ozone reactions. A much smaller contribution to the cellulose degradation is that by attack of ozone and radicals (from further reaction between ozone and oxidized lignin) in the reacted fibre wall region. Based on this model, the decrease in kappa number and degree of depolymerization of cellulose can be described as a function ozonation time, ozone partial pressure and initial kappa number for high consistency, well-fluffed pulp. With the use of experimental data to quantify the rate constants, these equations are transformed into predictive formulae for the calculation of the optimum ozonation time and the kappa number and viscosity of the pulps subjected to the optimum ozone charge.