Delignification and cellulose degradation kinetics models for high lignin content softwood Kraft pulp during flow-through oxygen delignification

The kinetics of oxygen delignification and cellulose degradation of a high kappa softwood Kraft pulp (kappa 65) are studied in a “flow-through” reactor where the pulp was held immobile within the reactor and a fresh oxygenated alkaline solution passes through the pulp mat at a set temperature. The feed solution maintains the alkali and dissolved oxygen concentration inside the pulp mat at nearly constant level equal to that in the feed stream. The rate of lignin removal is monitored by measuring the UV absorption in the outflow stream. The kinetics of delignification are obtained by fitting the experimental data to the power-law model with a lignin order of 3.5 in order to describe the rapid initial and subsequent slow delignification phases. The activation energy for lignin removal is 47 kJ/mol. The cellulose degradation kinetic was described by different models. The best cellulose model takes the cellulose degradation to be proportional to delignification due to attack of cellulose by oxygen based radicals generated via the phenolate anion lignin degradation pathway. The novelty of this model is that it also assumes that the fraction of the generated radicals which reach and attack cellulose is inversely proportional to the amount of residual lignin. This model leads to surprisingly simple kinetics and gives an excellent prediction of cellulose degradation kinetics.