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Chapter: Pharmaceutical Engineering: Powders

Powders are employed in many pharmaceutical processes.



Powders are employed in many pharmaceutical processes. They are more dif-ficult to handle and process than liquids and gases primarily because their flow properties are fundamentally different. Unlike fluids, a particulate mass will resist stresses less than a limiting value without continuous deformation, and many common powders will not flow because the stresses imposed, for exam-ple, by gravity are insufficiently high. Often additional processes that improve flow, such as granulation and fluidization, are adopted to facilitate powder transport and powder feeding.

Another important property of powders is the manner in which the par-ticles of a powder pack together to form a bed and its influence on bulk density. The latter is the ratio of the mass of the powder to its total volume, including voids. Unlike fluids, it varies greatly with the size, size distribution, and shape of the particles because these affect the closeness of packing and the fraction of the bed that is void. Vibration and tapping, which cause rearrangement of the particles and a decrease in the void fraction, increase the bulk density. In several processes, these factors are important because the powder is subdivided and measured by volume. Variation of bulk density then causes variation in weight and dose. The variation in the weight of compressed tablets is an excellent example of this effect. The manner of packing also influences the behavior of a bed when it is compressed.

Finally, in a static condition, there is no leveling at the free surface of a bed of powder. Nor is pressure transmitted downward through the bed. Instead, the walls of the containing vessel carry the weight of the bed.

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