UBC Theses and Dissertations
Understanding the dynamics of pulp fibre suspension dewatering Paterson, Daniel Thomas
Dewatering of pulp fibre suspensions is a fundamental process in many unit operations in the production of pulp and paper. A theoretical understanding of the dynamics of these networking fibre suspensions can prove valuable in the design of industrial equipment and supplement the general field of compressive rheology. This project aims to provide a general understanding of the consolidation, in particular, how the network of fibres responds to the stresses experienced during dewatering events. This begins with assessing the robustness of our previous modelling effort, which extended the traditional multi-phase, deformable porous media frameworks, through the accommodation of a rate-dependent constitutive model for the solid effective stress (bulk viscosity). Robustness studies include: testing a large variety of pulp types, investigating low concentration dynamics, and using industrial and lab equipment. Results from these studies motivated extending studies of the solid network’s response during high speed dewatering and shear stress application. For each study, a combined theoretical and experimental approach was undertaken to formulate appropriate model equations, independently calibrate the required material parameters, and collect experimental dynamic dewatering results. Model robustness for varying pulp suspensions at intermediate concentrations utilized a Darcian flow cell to calibrate permeability, a uni-axial dewatering experiment to determine their compressive yield stress, and dynamic dewatering experiments at modest rates to characterize the suspension’s bulk viscosity. The low concentration investigation introduced experiments for calibrating permeability and compressive yield stress around the suspension’s gel point and utilized gravitational drainage experiments to gauge bulk viscosity’s importance. In both investigations, the inclusion of a sizable bulk viscosity was necessary to effectively represent the dewatering behaviour. Dewatering dynamics in the Twin Roll press, collected at a pilot-scale facility, primarily highlighted the limitations of our previous modelling efforts. Rebuilding of the uni-axial experiment and constitutive model for the solid effective stress was undertaken to capture the solid network’s elastic response evident at elevated dewatering rates. Additionally, a unique apparatus was developed to experimentally calibrate a pulp suspension’s significant shear yield stress at concentrations above traditional rheometer approaches.
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