Permeability of powder beds formed from spray dried dairy powders in relation to morphology data

• An investigation of the permeability of beds formed from spray dried dairy powders
• No correlation between permeability and porosity of the investigated beds
• Division of porosity into the two definitions “pore space” and “void space”

The formation of lumps during industrial recombination of powder is a common problem in many industries. An important step in the recombination process is the wetting of the dry material. Only with an efficient wetting the individual powder particles will disperse and a complete dissolution will occur. In difference, a slow or incomplete wetting will not supply liquid to all portions of the powder, resulting in the formation of lumps.The resistance to wetting can be described by a material's permeability. This parameter is often estimated using the Kozeny-Carman equation, which models the permeability of a porous medium (such as a bed of powder) based on the porosity and specific surface area of the medium. This model has proven valid for a bed of packed spheres, but due to the model assumptions the accuracy of the model when utilized on beds formed from more complex, spray dried powders can be questioned.In this study, the permeability of powder beds consisting of spray-dried diary powders when exposed to an air flux has been characterized, using a method based on air permeametry. The results show that the Kozeny-Carman model is insufficient to estimate the permeability of such beds. Furthermore, the measured porosity of the bed had no influence on the permeability constant.Based on the results of this study a correlation between morphological powder parameters and permeability was developed. This correlation indicates that a small surface weighted average diameter of the particles as well as a narrow width of the particle size distribution have a positive effect on the permeability of a bed. In addition, incorporation of the structural parameter “BedRatio”, defining the basis of the bed to consist of agglomerates or primary particles (fines), indicates that a bed consisting of mainly agglomerates has a slightly larger permeability.

The complex structure of powder beds with a basis of mainly agglomerates (left image) and with a basis of mainly fines (middle image) compared to the rather homogenous structure of a bed formed from perfect spheres (right).The free volume in the beds formed from spray dried powders could be divided into pore space and void space. The pore space affects the pressure drop of flow through the bed significantly, whilst the latter has a significantly larger effect on the bed porosity but reduced influence on the pressure drop.Figure optionsDownload as PowerPoint slide