Modeling chlorine dioxide bleaching of chemical pulp (Aalto University)

ABSTRACT  This doctoral thesis deals with the phenomenon-based modeling of pulp bleaching. Previous bleaching models typically utilize one or two empirical correlations to predict the kinetics in kappa number development. Empirical correlations are simple to develop, but their parameters are often tied to the validation system. A major benefit of physico-chemical phenomenon models is that they are valid regardless of the reaction environment. Furthermore, modeling the bleaching processes at molecular level provides a new way to examine the relative importance of various phenomena, the validity of theories and the bottle necks of bleaching applications. The first part of the thesis introduces a model for the pulp suspension environment and describes the physico-chemical models in use (reaction kinetics, mass transfer, thermodynamics). The second part deals with inorganic oxychlorine reactions related to chlorine dioxide bleaching. The reaction kinetics and mechanism are documented for iron- mediated chlorite decomposition, chlorous acid self-decomposition, and for the reaction between hypochlorous acid and chlorous acid. The rate coefficient temperature dependency is reported for the two latter reactions. The reaction kinetic models were utilized in assessing the potential chlorate formation routes encountered in pulp bleaching. It was concluded that chlorous acid self-decomposition is unlikely to contribute to chlorate formation in bleaching applications. The iron-mediated chlorite decomposition and the reaction between hypochlorous acid and chlorous acid are expected to produce chlorate. The last part introduces a model for chlorine dioxide delignification. Here the pulp suspension model was combined with a broad set of chemical reactions describing the ClO2bleaching chemistry. The incorporated reactions cover lignin oxidation, lignin chlorination, hexenuronic acid degradation, lignin dissolution, oxidation of extractives and the essential...

Impregnation, vapor phase and methanol as means of intensifying the softwood kraft pulping process (Helsinki University of Technology)

ABSTRACT The objective of the research was to find ways to shorten the cooking time, i.e. intensify the kraft pulping process. The reason for undertaking such a study lies in the long standing trend of ever increasing reactor size in the kraft pulping industry. The huge digester size in use presently has lead to severe problems in understanding the behavior of the chip column inside the digester. An intensified process with a drastically shorter pulping time would give a more manageable process and greater freedom in reactor design. The study was performed using a new experimental digester giving a much greater control over temperatures than what can be achieved with other types of digesters. This enabled experiments that clarify the impact of impregnation, heat-up time, cooking temperature and cooking time to a greater degree than what has been possible earlier. The research on actual intensification centered on understanding the impact of impregnation and the impact of alcohols(methanol) onthe overall rate of pulping. This research supports earlier research that shows how the cooking time can be shortened using alcohols as additives in pulping. It also supports results showing that a fast process can be achieved by using impregnation with high concentrations of cooking chemicals followed by a cooking stage performed with direct steam heating. The fact that the effects work in synergy so that the fastest pulping process identified was one that employed high concentration impregnation followed by heating using methanol steam is a new finding. The decrease in cooking time compared to a conventional liquid phase batch process without proper impregnation is close to 70%. The present research was...

Studies on industrial digesters and cooking performance – Methods and applications (Helsinki University of Technology)

ABSTRACT Industrial pulp cooking reactors, digesters, were introduced in the early stages of chemical pulping technology for chemical liberation of fibres from wood chips and other lignocellulosic materials. With cooking temperatures of 20 – 80 °C above the boiling point of water and strong chemicals such as sodium hydroxide, suiphide or suiphite, digesters had to be sturdy and big. When the first rotating digesters were replaced by stationary batch digesters or continuous chip plug digesters, the digester content was no longer mixed during the cooking process. Because of the harsh process conditions and stationary chip material it has been very difficult to make observations of the physical and chemical conditions prevailing inside industrial digesters. As a result, very little attention has been paid to gaining a more thorough understanding of the performance of industrial digesters. For example, compared to small laboratory digesters, industrial digesters are generally known to produce pulp with significantly poorer pulp strength, uniformity, reject pulp percentage etc. The objective of the present work was to develop new techniques and to improve old ones for examining the industrial cooking process in order to obtain more knowledge about digester performance, and to make digesters more attractive as objects of scientific research. In the first part of the work, the main emphasis was on examining the cooking liquors which flow out from digesters for recirculation or to other processes. The liquors were assumed to represent the situation in the reacting chip bed with a minimum delay. A new carrier-flow method for cooking liquor analysers was invented, developed and taken into use. In this method, a small liquor sample is...

Chemical characterization of lignosulfonates (North Carolina State University)

ABSTRACT Softwood (pinus silvestris) lignosulfonates from the acidic stage of a three-stage sulfite cook were fractionated and purified by ultrafiltration and ion exclusion chromatography. The fractions were characterized by 13C NMR spectroscopy together with conventional chemical methods. In order to ascertain correct signal assignment of the 13C NMR spectra of lignosulfonate fractions, the chemical shifts of model compounds were studied. It was demonstrated that the principles of Karplus-Pople theory are valid for guaiacyl type lignin models, when guaiacol and various guaiacyl ethers are used as reference or parent compounds. The spectral data of various model compounds, including the substituent chemical shifts in the aromatic region of the spectra, is tabulated and discussed. 13C NMR spectroscopic analyses of lignosulfonate fractions suggested a considerably higher degree of condensation of C-6and/or C-5, then what has been reported for milled wood lignins (MWL) isolated from spruce and for kraft pine lignins. The degree of condensation in the lowest molecular weight fractions (MW < 4000) seemed to be affected by the post-cook treatment of spent liquor. The degree of sulfonation decreased substantially with increasing molecular weight. The frequency of phenolic hydroxyl groups varied only slightly in the medium weight range (MW 5 000 – 20 000), but was considerably lower in the highest molecular weight fractions (MW > 20 000). The number of methoxyl groups per assumed C9-unit was almost constant (0.98-1.07) throughout the studied molecular weight region. Gas chromatographic – mass spectrometric analyses of the KMnO4 – NaIO4 oxidation products obtained from methylated lignosulfonate fractions indicated substantial condensation at C-6 during acidic pulping. The degree of condensation at C-6 was remarkably higher than in spruce MWL...