Effect of Agitation Geometry on Measurement of Drainage Resistance Coefficients, 2007 TAPPI Papermakers and PIMA International Leadership Conference

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The measurement of pulp drainage rates and retention at industrial conditions is an important issue in the papermaking process. Drainage resistance coefficients were measured for a mechanical pulp furnish in a rapid drainage device. A cone and plate geometry to impose a shear field over the wire was compared to a simple impeller agitator. The flow pattern in the drainage cell was varied by adding baffles to the apparatus and by changing the rate of stirring. Agitation in the un-baffled tank configurations using both stirring configurations gave rise to tangential flow, large vortices in the drainage apparatus and non-uniform annular mats. Adding baffles to the apparatus reduced the tangential flow and gave rise to more axial flow during drainage. A baffle-tank apparatus equipped with an impeller stirrer provided the most uniform mat formation and had the lowest coefficient of variation for both the measured drainage resistance and retention coefficients. The effect of shear rate on the drainage resistance coefficients and fine particle retention are reported.