Thesis (Ph. D.)--University of Hong Kong, 2005.

Thesis (M.D.Sc.) - University of Queensland, 2005. / Includes bibliographical references.

Thesis (Ph. D.)--Institute of Paper Chemistry, 1934. / Includes bibliographical references (leaves 71-72). Also available online from Georgia Institute of Technology.

No description available.

No description available.

No description available.

M.Dip.Tech. / Please refer to full text to view abstract

The main purpose of this study was to determine the effect of contour height on the passage ratio of pulp through screen apertures, and determine which operating variable has the greatest affect on screen performance. In addition, a freeness model was to be developed, in hopes of helping to predict the freeness drop between feed and accepts. The study was conducted at The University of British Columbia (UBC) using a laboratory scale pressure screen. Slot velocity, feed consistency and contour height were the changing variables. Samples were collected from which passage ratio, freeness, fibre length and coarseness were determined. From the studies conducted it was found that slot velocity had the greatest influence on the screen operation. As the slot velocity increased a greater force was applied to the fibre to help push it through the screen aperture. However, this increase in slot velocity decreases the fractionation ability (separation of fibres into different lengths) of the screen. The second most important variable was the contour height. The main function of the contour height is to disrupt the flow of thick stock at the wall of the screen and allow for unhindered movement of fibre to the screen wall. The greater the contour height is, the greater the passage ratio (pulp fibre passing through screen). However, there is a decrease in fractionation. The third most important factor was the feed consistency. At low feed consistencies there is less crowding in the screen. Less crowding leads to more loosely-formed flocs, which are easier for the contour height and the rotor to dissipate and thus leads to unhindered movement. Thicker feed stock has a negative effect on passage A Freeness model was developed that showed that freeness had a power law relationship to passage ratio. The passage ratio was raised to a constant B, which is a function of the contour height and the feed consistency. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Passage of fibres suspended in water through apertures of dimensions greater than a fibre length and less than a fibre diameter has been examined at flow conditions approximating those in a pulp screen (large velocity parallel to the wall upstream of the aperture compared to the flow velocity within the aperture). Fibre behaviour was characterized in terms of three components: penetration of the leading tip into the aperture, rotation of the fibres on the downstream edge of the aperture, and fibre bending. Dimensionless numbers for each of these factors were derived from simple mass and force balances of fibres at an aperture entry. Experimental measurements of the magnitude of fibre passage were made on a single aperture located in a flow channel and in multiple apertures in a device simulating a commercial pulp screen in cross-section. For stiff fibres, it was found that fibre passage changed greatly with the ratio of fibre length to aperture width (L/W). When L/W was less than 2, the relationship between fibre passage and aperture velocity was approximately an exponential curve. The passage data correlated well with the penetration parameter. On the other hand, when L/W > 2, up to measured values of L/W = 6, the relationship between fibre passage and aperture velocity corresponded to a cumulative probability distribution curve. This latter behaviour, and the absence of a correlation with penetration and rotation parameters, was ascribed to contact between the tip of the rotating fibre and the upstream wall of the aperture. This was confirmed by experimental observations. Flexible long (L/W > 2) fibres showed behaviour between the two cases described above. The above observations were found to hold qualitatively for elevated concentrations up to a crowding factor of 4, multiple apertures, and the presence of pulses induced by a rotor of the type found in pulp screens. Accordingly, it appears that the findings are likely to hold in pulp pressure screens. An implication of this finding is that long stiff fibres may be best separated from short ones by choosing an aperture size such that L/W > 2 for the long fibre fraction and < 2 for the short fibre fraction. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Nitrogen, argon and oxygen adsorption isotherms at 78 °K have been determined on samples of solvent exchange dried, fully bleached, kraft pulp of western hemlock and Douglas fir. The solvent exchange drying sequence used was water-methanol-n-pentane with the n-pentane removed at room temperature. The pulp samples were in two groups: one group was beaten to various levels on a P.F.I. mill and solvent exchange dried from a water swollen state; the second group was air dried to different moisture contents and then solvent exchange dried. The presence of micropores (Dubinin definition) in solvent exchange dried cellulose has been shown. This finding was suggested by the results of Harris, who noted that adsorbents having an average pore radius of less than 18 Å indicated an average Kelvin pore radius of 18 Å. The finding of micropores in solvent exchange dried cellulose was substantiated by the pore volume distribution which indicated a very large volume of pores at 18 Å radius, and by the Dubinin plot technique for the measurement of micropore volume. Some solvent exchange dried wood pulp samples indicated that up to 70 percent of their total pore volume was present as micropores. The presence of micropores with the associated enhanced adsorption and restricted adsorption space complicates the analysis of isotherms so that the B. E. T. surface area and Kelvin type pore analysis cannot be considered reliable. An investigation into the effects of errors associated with assuming values for the physical properties of adsorbed molecules led to the conclusion that gas adsorption is useful for indicating trends or for use as a parameter for correlations of some pulp and paper properties but should not be used to postulate models of specified dimensions. The Kelvin type pore analysis and the Dubinin pore analysis have been shown to be very sensitive to the model assumed to describe the physical structure.of the wood pulp fibre. While gas adsorption techniques are of doubtful validity for solvent exchange dried pulp samples, the B.E.T. analysis, was found to be quite reliable on air dried paper sheets. The existence of the large volume of pores of approximately 18 Å radius (cylindrical pore model) or 25 Å wall separation (parallel sided fissure model) in solvent exchange dried cellulosic materials which have been found by other workers has been shown to be doubtful. This finding is substantiated by the accessibility data of Stone and Scallan which does not indicate a large volume of pores near the most common pore sizes found by gas adsorption. P. F. I. mill beating of the pulp was found to lower the surface area slightly, to shift the pore size distribution slightly to larger pore sizes and to substantially lower the volume of pores at the most common pore size. The Dubinin micropore volume analysis also indicated lowering of micropore volume with beating. These results led to the conclusion that beating affected the structure of the pulp fibre even in structural elements of the smallest size measured. Partial drying of the pulp handsheets prior to solvent exchange drying was found to lower the surface area by nearly two orders of magnitude and to shift the pore volume distribution strongly to the smaller pore sizes. The effects of .beating and drying were easily interpreted on the basis of the Stone and Scallan parallel sided fissure model of the structure of cellulose. Comparisons between isotherm shapes indicated solvent exchange dried wood pulp may have a structure similar to montmorillonite, which is known to have a flat plate structure. The Kaganer and "t"-plot analytical techniques were applied to the solvent exchange dried wood pulp as additional methods of obtaining estimates of the surface area. The surface areas determined by these methods were significantly larger than those found by the B. E. T. equation, however, the trends were found to be the same. Handsheets were made from the pulp samples and these sheets were subjected to physical tests. The surface areas of these sheets were determined and the bonded areas estimated. The bonded area was found to increase with the level of beating. Sheet density increased as bonded area increased, so did the breaking length and burst. Tear factor decreased as the bonded area increased. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate