Spray drying is widely used in different areas of the process industry to produce powders from solutions and suspensions, ranging from heat sensitive materials like food powders to heat-resistant materials like catalysts. The main benefits of spray drying lie in the low production costs and the capability to produce well-defined particles regarding their sizes and shapes. Due to the complex mechanisms during the formation process of the particle, however, the material and process parameters to generate a specific product is still determined based on the experience of plant operators. Therefore, a model-based method for the determination of the unknown system parameters would offer great potential during the planning and operational phase of spray drying processes.
Single droplet drying
The drying process can be divided into two different phases. During the first stage the droplet shrinks due to the evaporation of water at the surface, while the remaining components move towards the core of the droplet until the solids concentration at the surface is large enough that a crust forms due to precipitation processes and inter-particle forces. At this point the second stage of the drying process commences indicating the drying behavior of a solid particle with a mostly liquid core. There are two main models describing the following drying mechanisms of the wet particles: The dry and wet shell drying model. While the dry shell drying model states a receding water vapor front inside the shell, the wet shell drying model regards a particle with a wetted particle surface, while a centrally located bubble grows inside of the particle. These drying mechanisms depend on various parameters like the solids concentrations, the size of the primary particles and the particle-particle interactions, influencing the resulting particle morphologies. A thin shell with low stability, for example, may lead to hollow particles that will inflate during the drying process due to capillary forces. To investigate the influence of different system parameters on the resulting particle morphologies and sizes, single droplet experiments are performed. The single droplet experiments can be conducted using acoustic levitation for contactless evaporation of the suspension droplets.
CFD simulation & Validation
CFD simulations are used to investigate the drying conditions for the droplets inside a spray dryer. The flow profiles along different particle trajectories regarding information like gas temperatures, moisture content and velocities can be coupled with the knowledge from the single droplet experiments to predict the produced particles. This way, the CFD simulations may be used to identify the ideal process conditions regarding atomization pressure or suspension mass flows to generate a pre-defined powder. Additionally, an assessment on critical process conditions especially regarding the occurrence of stickiness at the glass transition temperatures of the granular materials can be performed. The CFD simulation are validated using pilot scale spray drying experiments.