The Effect of Particle Shape and Scale up on Just Suspended Impeller Speed

The goal of this study is to determine the minimum stirrer speed required to generate suspensions of spheres, rods, and plates.  There has been essentially no research done to determine the effect of particle shape and minimum stirrer speed, so there is a need for a correlation relating the shape of particles and a required impeller speed for off bottom suspension. As a result of this lack of knowledge, industries use more energy than is required to reach the just suspended state. In this study experiments were performed holding the density of the material and fluid constant. It was found that at equivalent solid-liquid mass fractions and particle volumes, spheres were the most difficult to suspend requiring about 1.1 times higher stirrer speeds than both rods and plates.  In addition, the required speed to just suspend rods and plates of equal particle volume was very similar.  Rods with diameters from 0.8 to 2 mm and lengths from 10 to 30 mms were tested, and it was concluded that the minimum dimension of the rod has the most significant impact on predicting just suspended impeller speed. Plates with various dimensions were tested, and it was concluded that both the side length and thickness have an impact on the just suspended impeller speed. Correlations of just suspended impeller speed and a volume/surface area shape factor show a maximum for plates and sphere at the 0.33 mm value, but further tests are required to confirm this behavior for rods.  Scaled up testing was done using a geometrically similar tank that was 2.5 times larger than the small scale tank.  The best method of predicting scale up just suspended speed for our system was to hold the Froude number equal.  This gave an overall error of less than 3%.