M. Määttänen: Studies on phenomena involved in impregnation of industrial wood chips

Abstract: 

In chemical pulping, cooking chemicals are used to dissolve lignin in wood to release the fibres from the wood tissue. Chemicals must be present in the parts of the wood tissue where delignification takes place before the cooking temperature is reached, as otherwise the quality of the pulp will suffer. The transport of the cooking chemicals takes place by two mechanisms. Penetration fills the fibre cavities with liquid under a pressure gradient enabling faster diffusion of the chemicals into the wood. Diffusion refers to the movement of ions through a liquid from a higher concentration to a lower concentration. Although both mechanisms occur in parallel, their separate and combined influences on impregnation have been seldom studied in the same research. The objectives of this thesis were to clarify and deepen the understanding of the impregnation of industrial chips and the phenomena occurring in the course of impregnation. Therefore, a new experimental method was developed. The experiments were divided into penetration and impregnation experiments. The data collected from the experiments were combined for calculating corrected penetration degree values and the mass and alkali balance involved in impregnation. Based on the data, quantitative results and deeper understanding for impregnation were achieved. The research work confirmed the three defined hypothesis dealing with the influence of the physical properties of industrial chips and the processing conditions on impregnation and the influence of the chemical diffusion and the dissolution of wood on the degree of penetration. Practical outcomes and guidelines for the impregnation of the industrial wood chips were the following. The moisture content and the basic density of wood chips determine the initial penetration degree (PDi) of wood chips: a low basic wood density and a low moisture content result in higher amounts of air inside the wood, which means a lower initial penetration degree of the wood. The lower PDi hinders the whole impregnation process. Therefore, it is recommended to use pre-steaming as a common practice and more emphasis must put on the use of fresh/moist wood chips.The prerequisites for fast diffusion are a high degree of penetration and a high concentration gradient between impregnation liquor and the alkali inside wood chips. On a laboratory scale, for high-concentration liquor, 15 minutes of impregnation time is sufficient for an adequate alkali charge for kraft cooking, but in the case of low-concentration liquor, even 60 minutes is insufficient. Part of the impregnated alkali is consumed during the impregnation, and more at higher temperature. Therefore, there is risk of running out of alkali if the temperature is raised quickly to the cooking temperature after low alkali impregnation. For successful impregnation, proper pre-steaming, a low impregnation temperature and moderate alkali concentration should be used for all wood species.

Follow the remote defence: TBC

Opponent: Professor Richard Gustafson, University of Washington, The United States of America

Supervisor: Professor Olli Dahl,  Aalto University, Department of Bioproducts and Biosystems

Link to electronic thesis: Studies on phenomena involved in impregnation of industrial wood chips