On low consistency refining of mechanical pulps

The aim of this thesis is to develop comprehensive knowledge to fill the gaps in the understanding of three key aspects of low consistency refining of mechanical pulps. Firstly, the fibre shortening mechanisms are formally studied by using a comminution model. Fibre length distribution data from before and after refining with a variety of pulp types, net-powers, feed flow rates, angular velocities and plate geometries was analyzed. Fibres' cutting rate and cutting location were found to be highly correlated with refiner gap. Plate geometry was also demonstrated to have a role in the fibre cutting location. Secondly, the relationship between net-power and gap was described using a correlation built entirely from pilot-scale refining data. Results showed that a properly defined dimensionless net-power number is crucial to compare different refiner sizes under the same grounds. The developed correlation was compared to industrial-scale data showing that the correlation is well suited for predictions. Key assumptions of the correlation were validated using bar-force sensor measurements data. Finally, the framework developed in the first two parts of this thesis were used together with pressure screening models available in literature to theoretically analyze refining systems typically found in TMP lines. Fibre length was used to assess each system performance in terms of refiner gap, reject ratio and refiner power. Moreover, the impact of some design aspects such as refiner size, recirculation and split-ratios was also described.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat