A Theoretical Model of the Kraft Pulping Process

Abstract

A theoretical model of a batch kraft pulp digester is developed that predicts, ~ priori, the lignin content distribution, the carbohydrate content distribution, and the effective alkali concentration distribution within pulp chips as a function of cooking time. From this information the kappa number, yield, screen rejects, and kappa number distribution of the pulp are calculated. The model also predicts the bulk phase effective alkali concentration as a function of cooking time. Input to the model are the initial hydroxyl concentration and sulfide concentration, the chip-thickness distribution, the temperature history of the cook, and the chemical composition and density of the species of wood to be pulped. Model predictions are compared with data from pulping experiments in which the reaction rates are kinetically controlled and with experiments in which the diffusion rate restricts the overall reaction rates. In both cases the comparison is good. Model results show that the normalized second moment of the lignin content is a better predictor of rejects than the fiber liberation point. Results also show that in indirect heated batch digesters the inter-chip mass transfer resistence is negligible. The pulping model is coupled with an optimization scheme to find the optimum pulping conditions. One result of the optimization is that an increase in productivity of 19 %can be obtained by optimization of the effective alkali concentration in a kraft cook. The effect of digester temperature gradients on the lignin content distribution of a pulp is shown to be significant, but the effect on screen rejects is negligible. Stratifying the chip by thickness to take advantage of temperature gradients is found to narrow the lignin content distribution, but has no effect on the screen rejects of a cook.