Particle Professor: What is density in particle technology?
What is density? For gases and liquids, this is fairly straightforward. However, in particle technology, the meaning of density isn't so clear. The British Standards Institute (BSI) has 14 definitions for density and ASTM International has more than 40.
Particles are often a combination of solids, cracks, pores, and surface irregularities that make using one density definition difficult. To make matters worse, the different density definitions are based on different values, and using the wrong density can lead to a very wrong understanding of your particles. For example, if the weight of a container of solids is known, how do you determine the solids height in this container? The height can be calculated by dividing the mass of solids by the solids density divided by the container's cross-sectional area. But the answer depends on what definition of density you use. If you use the absolute powder (or absolute material) density, the calculated height could be 3 to 5 times lower than what would be measured with a ruler. Using a particle density would result in a 40 percent error. Even using the bulk density could lead to an error in this calculation, depending on whether you used the correct bulk density. Thus, you need to understand which density is being provided before using correlations and models in designing a solids processing unit operation. In particle technology, just calling it density is too ambiguous.