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Synthetic fibre fractionation in hydrocyclones Ho, Sheau Ling

Abstract

Literature on fibre fractionation was reviewed. Some equations of motion for spheres and cylinders moving through a fluid in a centrifugal field were solved. These solutions indicated that long, coarse fibres with low specific surface tended to move faster in a radial direction than did short, fine fibres with high specific surface. A model for a swollen particle was developed based on the inclusion of measurable values for specific surface and volume. For swollen particles it was noted that long, coarse fibres having low values of specific surface and high values of specific volume/specific surface ratio tended to move faster than short, fine fibres with high specific surface and low specific volume. In a given time particles in a hydrocyclone will move a certain distance towards the hydrocyclone wall. In the wall region there is a flow directed towards the rejects tip opening. Thus long, coarse fibres of low specific surface and high specific volume/specific surface ratio are more likely to be entrained in this reject bound flow and be rejected than their opposite counterparts. Calculation of anticipated fibre diameter based Reynolds numbers in a hydrocyclone indicated that they could be high enough that available drag coefficient formulations could be invalid. Equations for calculating separation efficiency for fibres in a hydrocyclone were derived. These allow estimates of how effective a hydrocyclone could be in concentrating coarse (short) fibres in the rejects stream and fine (long) fibres in the accepts. Experimental work, done in fractionating nylon fibre mixtures of known coarseness and fibre length, showed that the nylon fibres were similar in behaviour to wood pulp fibres in that there was a tendency for short, coarse fibres to be rejected. The hydrocyclone used tended to reject short fibres. Other types of hydrocyclones tend to reject long fibres. Fibre fractionation in terms of coarseness and fibre length was affected by the feed flowrate to the hydrocyclone and by consistency. As flowrate increased, maxima (minima) were observed in the differences between rejects and accepts coarseness (fibre length). The lower the consistency the greater the difference between rejects and accepts coarseness and fibre length. Separation efficiencies, in addition to being affected by flowrate and consistency, were also affected by fibre coarseness and by fibre length. Mass reject ratios were measured. These were dependent on feed flowrate, consistency, reject tip opening diameter, coarseness and fibre length. Computational Fluid Dynamics (CFD) models of the flow inside the hydrocyclones used in the experimental work, were utilized in predicting profiles of axial, radial and tangential velocities and pressure. While the predicted profiles seemed to be reasonable, they did not suggest any obvious reasons for the maxima observed when the difference between rejects and accepts coarseness was plotted against feed flowrate.

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