UBC Theses and Dissertations
Modeling the maximum capacity of a pulp pressure screen Salem, Hayder Jabber
Pressure screens are used as a means of separating pulp fibres from contaminants. They are also used to improve pulp quality by fractionating fibres by length. Both functions are limited by the capacity of the device. Three studies were conducted in this thesis to understand the factors that affect maximum capacity. Capacity is determined by the complex hydrodynamics in the region between the screen rotor and the screen wall. To better understand this flow, the stream-wise velocity and aperture velocities were measured using particle image velocimetry. The vortex generated above the aperture and its size is shown to be strongly dependent on the aperture velocity, wall roughness and, to some extent, on the rotor speed. The vortex diminishes in size at higher aperture velocities that increase the exit layer height. The experiments also show that the reversal flow through the slot decreased with lower rotor speeds and increased slot velocities. This observation challenges the existing models of apertures being cleared simply by flow reversal driven by a suction pulse caused by flow acceleration between the foil and cylinder. In its place, this study identifies elements of a more sophisticated flow model that considers such factors as the depletion of the zone below the rotor and flow disturbances in the wake of the foil. The effects of screen cylinder geometry, pulp type, rotor and flow velocities on capacity were also investigated. Five types of screen cylinders were tested using different ratios of softwood/hardwood kraft pulp and different reject rates. It was found that the average fibre length has a significant impact on capacity. A comprehensive understanding of pulp screen capacity remains elusive. The present research has, however, provided insights which move away from the simplistic ''backflush'' models used in the past and supports a more sophisticated model that also considers: 1) the dynamics of fibre accumulation and removal rates of the slot entry, 2) the importance of a small-scale perturbations created by turbulent flow for fibre removal, and 3) the mechanics of fibre trapping on the downstream edge of the slot. These advancements also provide some direction for future equipment developments.
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