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The energy expended on pulp fibres during low consistency refining

University of British Columbia

Refining is a process in which the physical structure of the papermaking fibre is modified by repeated strain. While there is extensive literature on the changes in fibre morphology, there is little on the nature and magnitude of the forces which impose the strain. The objective of this dissertation was to estimate the force and energy expended on papermaking fibres in pulp refining. Both hydrodynamic and mechanical forces were considered. In the first half of the dissertation, a model is formulated to consider the hydrodynamic force acting on a single fibre trapped and transported on a moving bar edge. Equations were developed to predict the force as a function of fibre properties such as stiffness, length, and diameter, and fluid variables such as velocity and fluid viscosity. From this, the energy expended was estimated to vary within the range 10⁻¹² J to 10⁻¹¹ J for the cases studied. In the second half of the dissertation, theoretical and experimental estimates of the mechanical force acting on fibre flocs were developed. It was found that substantial force on fibres could only be imposed when the floc was compressed to near its zero void volume. Upon further compression of the floc, force was found to increase linearly with degree of compression. The theoretical estimates were found to agree reasonably well with the experimental measurements. The energy expenditure per fibre associated with these forces was found to be about 10⁻⁵ J. This compares favorably with estimates reported in the literature The energy associated with the mechanical force was approximately 6-7 orders of magnitude greater than that associated with the hydrodynamic force. This suggests that the latter contributes little to the refining action in pulp.

Thesis/Dissertation

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2009-06-09

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http://hdl.handle.net/2429/8872

Chemical and Biological Engineering

University of British Columbia

UBCV

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