Microstructure of spray dried particles at different scales of drying
Abstract: Popular Abstract in English In our society powder is found to be a common state of material. Indeed, the powder state is found to be convenient to store and transport large quantities of material. One of the major techniques to produce powder is spray drying, also for thermally sensitive materials such as foods and pharmaceuticals. In spray drying a liquid is vaporized into a hot air stream, which dries the droplets to powder particles in less than a second. Even though spray drying is a well-established method to produce dry powder numerous challenges are still encountered to have a full understanding of the process. Spray drying is a complex method with several important steps: To create a droplet the feed passes through the nozzle and undergoes shear stress which can influence its properties. Further, in a very short time, the droplets are expelled into the drying chamber and come into contact with the hot air. The droplets undergo fast water evaporation, dimensional changes and temperature changes. Adsorption of surface active materials at the droplet surface and phase segregation can occur while drying, influencing the dried particle properties. Furthermore, the droplets are prone to hit each other or the wall of the dried chamber. If this occurs before the droplet surface is dry agglomeration or deposition, respectively, will result. Finally the solution composition and the spray dryer settings can influence the type of powder obtained. Powders can have various morphology and composition. The studies of spray-drying at production scale are expensive, consume vast amount of test material and may thus not be possible. Single drop drying studies are a way to simplify testing, using minimal amount of material. Single particle dryers are used to dry one droplet at the time, and typically operate with larger droplets than in a spray dryer. One type of dryer is based on the principle of ultrasonic levitation where a drop is held in levitation in a steady acoustic wave. In this way, the droplet can be dried without physical contact with any other object, and still the particle can be collected after drying is completed. The overall objective of this thesis was to establish to what extent and how the particle morphology, surface and internal microstructure in single particle drying can predict the corresponding properties in spray drying at different scales. The first objective of this work was to provide a deeper understanding on how the surface morphology of powder particles depends on the surface properties of the components. The second objective was how the internal microstructural depends on key properties of the components. The results show that interfacial properties of the components in the formulation can explain the dried particle morphology, although other factors such as the drying conditions also play a role. Particle surface composition is found to be influenced by the adsorption rate of macromolecules in the liquid feed. Even though spray drying is a fast drying process we found that ingredients can phase separate while drying. Further, although the particle size and drying time differ, analogies between individually dried particles, laboratory dried particles, pilot plant dried particles and full scale dried particles were found in terms of powder morphology, surface composition and internal composition.
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