Studies on liquid penetration into softwood chips : experiments, models and applications

Abstract

Latest developments in chemical pulping have been mainly driven by advances in pulping chemistry, disregarding the phenomena that take place at the front-end of cooking. In spite of the existence of several concepts and theories, some aspects still remain unclear and more research is needed to achieve a complete understanding of the process of liquid penetration into wood chips. The objectives of the present thesis were to provide more knowledge on the process of liquid penetration into softwood chips and the factors affecting it, to develop a model of penetration process and to evaluate the efficiency of chip presteaming.

It was found that the nature of the liquid and its properties play an important role in determining the efficiency of penetration. By optimizing the process conditions and applying certain "penetration aid" techniques, such as presteaming, it is possible to significantly improve the efficiency of penetration and to achieve a higher final penetration degree. Based on experimental data, a mathematical model was developed for the process of water penetration into softwood chips under isothermal conditions. The model was found to predict accurately the course and final level of penetration of water and fresh cooking liquor (white liquor) into the chips under defined conditions. However, the suggested model cannot be used as a general model for predicting the penetration of spent cooking liquor (black liquor).

Chip presteaming was examined from the viewpoint of the chip heating and air escape processes. A three-dimensional model was proposed to estimate the efficiency of heating the single chip by saturated steam. It was shown that the time required to heat the single chip with steam was very short, only a few minutes. However, the process of air removal proceeds considerable slower than chip heating process. Achieving a high degree of air removal may require quite long steaming times, up to an hour, especially with heartwood chips. Complete removal of air, however, may be difficult to achieve, even by applying optimal steaming conditions and long steaming times.

The knowledge acquired about the efficiency of penetration and presteaming processes was applied to examine contemporary cooking technology. Presteaming of the chips and application of a higher-pressure profile at the front-end of kraft displacement batch cooking resulted in reductions in the amount of rejects and the kappa number of the bulk pulp as well as improved uniformity of delignification.