Porosity

Porosity describes the proportion of voids in a material. In the context of powder processing and process engineering, the term refers to individual particles as well as bulk materials, piles and powder mixtures. Porosity has a significant influence on mechanical, thermal and mass transfer properties.

At the particle level, porosity describes the proportion of internal pores within a single particle. Porous particles have a larger internal surface area and a lower bulk density than compact particles. Particle porosity influences fracture behaviour, liquid absorption, reactivity, and heat and mass transfer.

At the bulk or packed material level, porosity describes the volume fraction of the voids between the particles. It results from particle size, particle shape, size distribution and packing structure. This external porosity determines, amongst other things, bulk density, gas permeability, flow behaviour and heat transfer in powder bed processes.

Formally, porosity ε can be defined as a dimensionless quantity:

ε = V_void​ / V_total

Here, V_void​ is the volume of the pores and interstices and V_total​ is the total volume of the system under consideration. In powder processing, porosity is not a constant material property. It can be specifically altered by mixing, compaction, agglomeration, granulation or drying. Mechanical stress can close or create new pores. The addition of liquid can fill pores or cause them to collapse.

The porosity of a powder is crucial for many processes. It influences drying rate, temperature control, reaction kinetics, dust behaviour and product stability. In process engineering, a distinction is therefore made between particle porosity and bulk porosity. Both phenomena are considered separately.