Simulering och optimering av sågspånskok i laboratorieskala

The aim of this thesis was to simulate and optimise sawdust cooking which was carried out on a laboratory scale and to compare two cooking methods, i.e. a conventional cooking method (reference cook) and Messing & Durkee-cooking (M & D-cook). In addition, impregnation experiments were carried out for comparing the impregnation of sawdust and pin chips. Further, the different wood raw materials were studied regarding fibre dimensions.

The theoretical part consists of the morphological and chemical structure of wood. Chemical pulping is discussed and the weight is given to kraft pulping. The terminology used in kraft pulping is described and the chemical components, impregnation, delignification and the concept of the H-factor are explained. The properties of sawdust and the possible uses of sawdust pulp are also discussed.

The experimental part consists of the description of the used wood raw material, impregnation and cooking liquors, apparatus, cooking performances, pulp handling, impregnation experiments and the determining of fibre dimensions. In addition, pulp bleaching and beating are discussed.

The fibre length of wood raw material is affected by the particle size of the sawdust and therefore the wood raw material could be grouped into three classes regarding fibre length. Sawdust was impregnated faster than pin chips because of the sawdust’s substantially greater surface.

Repeatability of the M & D-cook was achieved by installation of a check valve between the digester and the heat exchanger unit. The conventional sawdust cooking could be shortened by heating the cooking liquor in a separate tank and transfer the liquor into the digester by nitrogen gas overpressure.

Sawdust appeared to be a heterogeneous raw material, identical cooking conditions resulted in different cooking results. This resulted from fluctuations in heartwood and sapwood proportion and the dry matter content which depend on wood species, storage method and season.

Air-drying the raw material and adjustments to the cooking procedure minimised factors which interfered with the cooking. Pulp with kappa number 26 and residual alkali concentration around 8 g/L was obtained under the following conditions: liquor-to-wood ratio 4:1, total EA-charge 25 %, H-factor 1200 at 170 °C and 20 % chip addition.

When comparing M & D-cooking with reference cooking, it was evident that M & D-cooking had the following benefits: lower kappa number at the same H-factor, higher screened yield, lower shive content and higher viscosity at the same kappa number (residual lignin content).

 Bleaching of the pulps marked no significant differences between pulps regarding kappa number and brightness, but the viscosity of the bleached M & D-pulp was still higher than the reference pulp which also consumed more chlorine dioxide than the M & D-pulp.

PFI-beating of the bleached pulps did not show any substantial differences between the pulps either. M & D-pulp showed higher tear index, Scott Bond, Zero-span and opacity whereas reference pulp showed higher tensile index and air resistance at a given sheet density.