Heterogenous Diffusion and Reaction Model of Kraft Delignification at the Cell Wall Level

This work explores the interplay between reaction and mass transfer kinetics during kraft pulping of birch wood meal. A novel delignification model has been developed, incorporating molecular weight-based diffusivities alongside traditional reaction kinetics to describe delignification at the cell wall level. Laboratory-scale reactor experiments provided pulping data for model validation at a range of industrially relevant process conditions. The developed model successfully described the delignification behavior during both constant and transient process conditions. By fitting the model to experimental data, the calculated rate constants (for both reaction and diffusion) revealed that the rate of delignification at the cell wall level is controlled by the diffusion of dissolved lignin. The results provide a plausible explanation to the slow rate of delignification at the end of the cooking operation. Furthermore, the reaction rate may also be limited by the access to OH– within the cell wall even during wood meal pulping.