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UBC Theses and Dissertations

Fractionation of particle suspensions in a viscoplastic fluid : towards a novel process Madani, Ario


The focus of this thesis is the separation or sorting of particle suspensions in a yield stress or viscoplastic fluid. Although the process is applicable to most industrial suspensions, the motivation of the work stems from pulp and paper industry,i.e. papermaking and microfibrillated cellulose (MFC) suspensions. The work is presented in four different yet complementary studies. In the first study, the concept of particle fractionation in a viscoplastic fluid is introduced. Here this novel principle is demonstrated, batch wise, by measuring the difference in centrifugal force required to initiate motion of an initially stable particle suspension in a gel. The criteria for motion is delineated as the ratio of the centrifugal force to yield stress as a function of particle size and orientation. Demonstration experiments are given to illustrate that the separation process is very efficient. In the second and third studies we demonstrate the principle on two industrial suspensions, i.e. a SBK (semi-bleached kraft) papermaking fibre and MFC. With papermaking fibres, it is shown that efficient separation, based upon cell wall thickness can be achieved. With MFC, it is shown that the process is more efficient than traditional separation techniques, i.e hydrocyclone and pressure screen. In the final study, we speculate regarding the conditions required to make a continuous process based upon the batch testings. Here, it is identified that a spiral Poiseuille flow would be sufficient to achieve separation. The questions addressed in this study are what is the size of the unyielded region for this flow field and what is the bound for transition to turbulent flow. It was found that the magnitude of the swirling flow does not affect the size of the plug and the axial velocity is decoupled from the rotational rate. In addition, the yielded region is always formed in the middle of the annular gap. To address the flow state, a linear stability analysis was performed using the method of normal modes. The flow was found to be linearly stable for all conditions tested.

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