Single stage and two-stage sodium sulfite cooking were carried out on either spruce, pine or pure pine heartwood chips to investigate the influence of several process parameters on the initial phase of such a cook down to about 60 % pulp yield. The cooking experiments were carried out in the laboratory with either a lab-prepared or a mill-prepared cooking acid and the temperature and time were varied. The influences of dissolved organic and inorganic components in the cooking liquor on the final pulp composition and on the extent of side reactions were investigated. Kinetic equations were developed and the activation energies for delignification and carbohydrate dissolution were calculated using the Arrhenius equation. A better understanding of the delignification mechanisms during bisulfite and acid sulfite cooking was obtained by analyzing the lignin carbohydrate complexes (LCC) present in the pulp when different cooking conditions were used. It was found that using a mill-prepared cooking acid beneficial effect with respect to side reactions, extractives removal and higher stability in pH during the cook were observed compared to a lab-prepared cooking acid. However, no significant difference in degrees of delignification or carbohydrate degradation was seen.  The cellulose yield was not affected in the initial phase of the cook however; temperature had an influence on the rates of both delignification and hemicellulose removal. It was also found that the  corresponding activation energies increased in the order:  xylan, glucomannan, lignin and cellulose. The cooking temperature could thus be used to control the cook to a given carbohydrate composition in the final pulp. Lignin condensation reactions were observed during acid sulfite cooking, especially at higher temperatures. The LCC studies indicated the existence of covalent bonds between lignin and hemicellulose components with respect to xylan and glucomannan. LCC in native wood showed the presence of phenyl glycosides, ϒ-esters and α-ethers; whereas the α-ethers  were affected during sulfite pulping. The existence of covalent bonds between lignin and wood polysaccharides might be the rate-limiting factor in sulfite pulping. / The sulfite pulping process is today practised in only a small number of pulp mills around the globe and the number of sulfite mills that use sodium as the base (cation) is less than five. However, due to the increasing interest in the wood based biorefinery concept, the benefits of sulfite pulping and especially the sodium based variety, has recently gained a lot of interest. It was therefore considered to be of high importance to further study the sodium based sulfite process to investigate if its benefits could be better utilized in the future in the production of dissolving pulps. Of specific interest was to investigate how the pulping conditions in the initial part of the cook (≥ 60 % pulp yield) should be performed in the best way. Thus, this thesis is focused on the initial phase of sodium based single stage bisulfite, acid sulfite and two-stage sulfite cooking of either 100 % spruce, 100 % pine or 100 % pine heartwood chips. The cooking experiments were carried out with either a lab prepared or a mill prepared cooking acid and the temperature and cooking time were varied. Activation energies for different wood components were investigated as well as side reactions concerning the formation of thiosulfate. LCC (Lignin carbohydrates complexes) studies were carried out to investigate the influence of different cooking conditions on lignin carbohydrate linkages.

La valorisation de la biomasse lignocellulosique est aujourd’hui un enjeu majeur du fait de la réduction des ressources fossiles. Séparer chaque constituant pour les valoriser de la meilleure façon possible est l’objectif des bioraffineries papetières. L’effluent papetier, la liqueur noire, est actuellement brûlé pour produire de l’énergie, mais sa gazéification permettrait d’augmenter ces rendements énergétiques. Mais pour cela une cuisson sans soufre du bois est nécessaire, le soufre inhibant la gazéification.Cette étude a donc porté en premier lieu sur la faisabilité d’un fractionnement sans soufre du bois, plus ardu qu’un procédé kraft traditionnel contenant du soufre. Le travail a été effectué sur les bois résineux, ceux-ci étant plus difficiles à délignifier que les bois feuillus. Une étape d’autohydrolyse du bois, préalable au fractionnement à la soude, a été effectuée afin d’extraire les hémicelluloses pour une valorisation ultérieure. Les travaux ont montré que ce prétraitement permettait de délignifier le bois plus facilement et ainsi d’envisager un fractionnement sans soufre. La cellulose obtenue par ce procédé présente une pureté et un degré de polymérisation suffisants pour envisager son utilisation pour de la viscose ou pour des applications chimiques.La lignine ne présentant pas de différences structurelles majeures entre du bois préhydrolysé ou non, les complexes entre la lignine et les hydrates de carbone (LCC) ont été analysés. Il a été montré que la préhydrolyse modifie significativement la quantité et la composition de ces complexes, permettant d’expliquer par là les résultats obtenus. / The valorization of lignocellulosic biomass is nowadays a major issue due to the reduction of fossil resources. Separating each component to valorize them the best way as possible is the goal of the pulp and paper biorefineries. The effluent of the mill, called the black liquor, is currently burnt to produce energy, but gasification would increase the energy efficiency. For this, a sulfur-free cooking of wood is necessary, as sulfur inhibits gasification.Therefore this study first focused on the feasibility of cooking without sulfur, which is more difficult than a conventional kraft cooking containing sulfur. The work was done on softwood which is more difficult to delignify than hardwood. The wood was first pretreated with an autohydrolysis to remove hemicelluloses for further valorization.The results were conclusive for the production of cellulose pulp for chemical applications. To explain the differences obtained, structural differences of wood components were looked for. Lignin did not show major differences whether the wood was prehydrolyzed or not, whereas the complexes between lignin and carbohydrates (LCCs) showed significant differences, which would explain the results obtained.