On the Course of Kraft Cooking - The impact of ionic strength

Doctoral thesis, 2017

Kraft pulping is the dominant process currently used to produce chemical pulps, and has been applied for more than 130 years. A large number of studies have been conducted to gain a better understanding of the delignification kinetics as well as the reactions of the wood components being dissolved during the cooking process. Nevertheless, these remain far from being well understood in detail due to the very complicated chemistry and morphology of wood. The main goal of this work is to further increase knowledge of kraft cooking. The focus here is to investigate the influence of ionic strength on the kinetics of kraft cooking. Kraft cooks of wood meal from Scots pine (Pinus silvestris) were performed in a small-scale flow-through reactor. This equipment was employed to minimize resorption phenomena and the additional degradation of the wood components dissolved in the hot cooking liquors during the course of delignification. In addition, it was possible to adjust the concentration of the ions (Na+, CO32- and Cl-) in the cooking liquor at any given time during the cook. The results obtained show that the rates of delignification and xylan removal in the flow-through kraft cooks decreased significantly when the ionic strength/sodium ion concentration in the cooking liquor was increased, while the removal rate of glucomannan was virtually unaffected by the level of the ionic strength. The reaction rates of hexenuronic acid were found to increase with increasing ionic strength. When [Na+] was altered at a given time during the kraft cook, the effect on both the rate of xylan removal and delignification was quite similar, even though some differences could be noted. Analyses of lignin precipitated from black liquors showed that the molecular weight of the lignin dissolved decreased with increasing ionic strength in the cooking liquor. Furthermore, a very wide molecular weight distribution was found for dissolved lignin fragments. The content of carbohydrates present in precipitated lignin was found to increase with cooking time and decrease with increasing ionic strength. The results indicate the possibility of the existence of several types of lignin-carbohydrate complexes. The findings suggest that the solubility and/or mass transport of wood polymers mainly govern the overall rate of kraft delignification at the fibre wall level.