Model for Predicting Yield in Oxygen Delignification of Softwood Kraft Pulps, 1999 Pulping Conference Proceedings

Thomas J. McDonough, J. Thomas Burton, Andrew E. Yethon

A predictive model has been obtained for yield in the high consistency oxygen delignification of softwood kraft pulps. The model is based on a large collection of experimental data and is generally applicable to southern and western North American softwood pulps, as shown by its ability to predict effects of time, temperature, degree of delignification and unbleached kappa number for the entire set of 15 diverse softwood kraft pulps evaluated. It was also shown to accurately predict the absolute yield level for 13 of the 15 pulps. The model predicts that, at constant degree of delignification, increasing the temperature will decrease yield, but only at temperatures above 120 o C. It also predicts that increasing time will have no effect at high degrees of delignification, but will decrease yield at low degrees of delignification. When kappa number reduction is maintained constant at 40%, very little additional yield loss is predicted when the unbleached kappa number is increased from 30 to 50. High kappa pulps become increasingly vulnerable as the extent of delignification is increased. At low and moderate degrees of delignification, specific shrinkage is sharply lower for high kappa pulps, but this advantage disappears as the extent of delignification is increased. Neither oxygen pressure nor substitution of oxidized white liquor for caustic was observed to affect yield. Comparison with literature data suggests that the model will also be applicable to medium consistency oxygen delignification.