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Date of Award

8-1994

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical Engineering

Supervisor

Dr. R.H. Pelton

Co-Supervisor

Dr. M.H.I. Baird

Abstract

The effect of polystyrene beads and nylon fibres on the permeability of bleached kraft pulp fibre beds was studied experimentally. Three types of experiments: 1) constant head filtration, 2) elastic permeation, and 3) fixed bed permeation were carried out. Four types of fibre beds were used: 1) pulp fibres only, 2) pulp fibres and plastic beads, 3) pulp fibres and nylon fibres, and 4) pulp fibres, nylon fibres and plastic beads. The beads used were 80 μm in diameter while the diameter and the length of the nylon fibre were 45 μm and 1.89 mm respectively. The beads were used as a model for dispersed air bubbles in pulp suspensions, while the nylon fibres were chosen because their specific surface area and specific swollen volume were close to those of the beads.

In a filtration experiment, a fibre bed was formed from the corresponding suspension while pressure drop across the bed and cumulative filtrate weight were recorded: Specific filtration resistance values for the suspensions were calculated from the filtrate weight data. It was observed that specific filtration resistance increased with bead content in the slurry while filtration resistance decreased with an increase in the content of nylon fibres in the suspension. A mathematical model based on permeation equations was derived to simulate the filtration process.

In a permeation experiment, the fibre bed formed in a filtration experiment was subjected to a complete recovery-compression cycle starting with the maximum pressure drop. Flowrate of water through the bed and the thickness of the bed were measured as a function of the pressure drop. Fibre beds of the four different compositions all displayed hysteresis in both the flowrate and bed thickness data. Bed thickness data were correlated by regression analysis and under flow induced compression the thicknesses were found to be linear or nearly linear functions of the pressure drop acting across the bed. The presence of plastic beads did not affect the structure and thickness of the beds, suggesting that the plastic beads were able to fit into the pores of the pulp bed without disrupting the structure. By contrast, nylon fibres increased both the permeability and the thickness of the bed because of the formation of a more open network structure. The mathematical model proposed by Jonsson and Jonsson was used to predict compression behaviour; however, the concave form of plot of thickness versus pressure drop during recovery was not predicted by any of the current bed compressibility models.

Permeability of the compressed bed formed in the filtration experiment was measured in a fixed bed permeation experiment. Permeability of the bed was used to calculate the specific external surface area and specific swollen volume of the bed materials using the Kozeny-Carman equation. It was found that for both the pulp/bead system and the pulp/nylon fibre system, the specific surface area and specific swollen volume of the bed were linear functions of the content of polystyrene beads or nylon fibres. The specific surface area and specific swollen volume of beds that contained pulp fibres, nylon fibres and plastic beads were predicted, within 7%, by the linear supposition of the specific surface area and specific swollen volume of the bed constituents. This analysis is important because it permits estimation of the effect of air bubbles on the permeability of pulp pads in various processes in the pulp and paper industry.

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