Powder Bed Structure and Imbibition - A Study of Spray Dried Dairy Powders

Sammanfattning: The formation of lumps during powder dissolution is undesirable in most cases. In a world where the use of dry products is increasing, incomplete powder dissolution and lump formation can for instance affect the color of a paint or the nutritional content of baby food. In this thesis, the phenomena governing lump formation of agglomerated spray dried dairy powders were investigated. The focus of the study has been to achieve an increased understanding of the spontaneous imbibition of powder beds and how bed structure can be linked to morphological parameters of powders. The desired outcome was to provide information to enable a more efficient and less energy consuming powder dissolution for the industry. The results show that it is possible to estimate bed properties like porosity, permeability and structure from morphological data of the powder particles forming the bed. The porosity of a bed is shown to be heterogeneously distributed at the microscopic scale and is, as a result, divided into larger voids (called void space) and more narrow ducts (called pore space). The void space is considered to largely affect the porosity of the bed, without causing any significant pressure drop of any fluid flowing through the bed. The pore space is considered to significantly influence such a pressure drop, and thereby the bed’s permeability, but has an insignificant impact on the porosity of the bed. As a result, the porosity of a powder bed of agglomerated spray dried powder shows no influence on bed permeability. Studies of spontaneous imbibition of such beds show that large amounts of air inherent in the porosity of the bed is trapped during imbibition. The large amounts indicate that the origin is the void space of the bed. In this thesis, a model is presented explaining the trapping of air by an occurrence of film flow. However, in order to get a complete understanding of lump formation, the presented model needs to be combined with the effects of dissolution and swelling of individual powder particles in contact with dissolution media. A first step of such work is presented as a part of this study, showing that the particle surfaces can be considered gelled during imbibition and that individual particles show a significant swelling after initial contact with the solvent