Fluid mechanics inside spray nozzles: The key to targeted product design and improved energy efficiency in spray drying processes

Spray drying is widely used for powder production due to its scalability and cost efficiency, but it also involves a large energy consumption and high initial costs. The spray drying process can be divided into three stages: atomization of the liquid, drying of the droplets, and separation of the powder. The atomization step is critical to the entire process, as it determines the size of the drying droplets and, consequently, the dried particles. In the scope of this work, two atomization nozzle concepts were investigated: the Air-Core-Liquid-Ring (ACLR) and the pressure swirl nozzles. The former is a type of internal-mixing twin-fluid nozzle, while the latter belongs to the pressure-nozzle category. The ACLR nozzle has the potential to reduce total energy consumption in a spray drying process by up to 30%, due to its ability to handle and atomize highly viscous feeds. However, the free-surface interaction between air and liquid inside the ACLR nozzle causes unstable flow conditions, which can lead to a wide droplet size distribution. In this dissertation, these unstable flow conditions inside the ACLR nozzle were minimized by adjusting the process conditions and improving the nozzle geometry. In the case of pressure swirl nozzles, an interesting field of research is the spray drying of emulsions. In this process, a key performance parameter is the final oil droplet size in the dried powder, as it affects both powder flowability and the shelf life of the product. Based on this, a correlation was established in this work between the oil droplet sizes obtained after atomization and the deformation stresses generated inside the nozzle. The investigations conducted in this dissertation involved both experiments and CFD modelling. Overall, this work demonstrates how CFD and fluid mechanics can be used, within the broader topic of spray drying, to address existing gaps in our understanding of the process function and performance of different spray nozzles.