Bulk Density and Porosity of Beds

When a powder just fills a vessel of known volume V and the mass of the powder is m then the bulk density of the powder is m/V. However, if the vessel is tapped in most cases the powder will settle leaving top empty space, so adding more powder would be necessary to end with the vessel completely filled. If the mass now filling the vessel is nt then the bulk density is m /V m/V. Clearly, this change in density just described has been caused by the influence of the fraction of volume not occupied by a particle, known as porosity. The bulk density is, therefore, the mass of particles that occupies a unit volume of a bed, while porosity or voidage is defined as the volume of the voids within the bed divided by the total volume of the bed. These two properties are in fact related via the particle density in that, for a unit volume of the bulk powder, there must be the following mass balance  

Equation 7.2 gives the porosity or voidage of the powder and whether or not this includes the pores within the particles depends on the definition of particle density used in such evaluation. 

---