Powder and Bulk Engineering
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Publication Date: 02/2010
Pages: 3
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Particle Professor: The danger of particle size assumptions

Because particle size distributions often are represented as one average, mean, or median number, all too often we don't understand the role of particle size distribution. We forget how a powder's whole particle size distribution and any change in it can impact a unit operation or the entire process.

A classic example is filling a hopper with powder. A bimodal powder (containing two components with different particle sizes) or a powder with a wide particle size distribution can segregate during hopper filling. The smaller particles migrate toward the center and bottom while larger particles move toward the hopper wall and upper region. This segregation is similar to the Brazil nut effect often seen in a newly opened can of mixed nuts, where these large nuts appear at the top as a result of vibration during shipping. In fluidized-bed equipment, such as a dryer, cooler, or reactor, a different form of particle migration occurs based on the bed's superficial gas velocity (that is, the velocity of the gas flowing through the fluidized-bed equipment before the powder is added). If the superficial gas velocity in the fluidized bed is too low, the larger or denser particles flow toward the bed's bottom while smaller particles migrate to the top. By using a representative particle size, we miss the ability to predict these segregation effects.

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