The production of fine powders and particles, such as milk powder, food additives like vitamins, pharmaceutical ingredients, and industrial ceramics, heavily relies on spray drying technologies. In standard spray drying, a liquid or slurry feed is atomized into fine droplets using high-pressure nozzles within a chamber filled with hot gas. This process achieves rapid drying by subjecting droplets to intense energy transfer, facilitating liquid evaporation in a short time. However, modern industrial requirements demand advancements in energy efficiency as well as product quality, necessitating improvements in spray drying technology. Energy efficiency is a crucial factor in spray drying, as conventional systems operate with high inlet gas temperatures between, consuming significant energy for gas heating. Spray drying systems can optimize energy utilization by reducing drying time without compromising product quality. Alongside energy efficiency, product quality in spray drying is paramount. Specific attributes, such as particle porosity, size, and density distribution, must be controlled to meet industry standards. Achieving precise product characteristics necessitates a detailed understanding of the drying process and its parameters, requiring advanced control strategies to meet strict quality specifications.
One promising technique to improve spray drying efficiency is foam spray drying, which involves injecting inert gas (e.g., nitrogen) into the feedstock at high pressures (typically above 50 bar). This pressurized feed is then atomized, releasing pressure at the nozzle outlet and allowing the feed to expand [1-2]. The rapid pressure depletion causes gas-saturated droplets to form bubbles, which alter the drying dynamics and the characteristics of the final product. Compared to standard spray drying, foam spray drying increases dryer throughput, reduces residence time, and modifies product properties. Although foam spray drying has been empirically explored, existing studies often focus on specific products without providing a reliable physics-based model to generalize findings across different parameters and scales.
Foam spray drying involves complex heat, mass, and momentum transfers between three phases (liquid, gas, and solid) within each droplet/particle, as well as interactions with the surrounding hot gas environment [3]. Compared to conventional spray drying, foam spray drying introduces additional complexities due to the presence of gas bubbles within the liquid matrix. These bubbles affect heat transfer by creating localized regions of lower thermal conductivity, alter mass transfer by influencing the diffusion of water vapor and dissolved gases, and impact momentum transfer by disrupting the liquid flow. Furthermore, bubble dynamics including nucleation, growth, and collapse can significantly modify the drying process by redistributing liquid and altering the capillary flow. These unique factors make foam spray drying more challenging to fully understand compared to non-foamed spray drying [4]. Therefore, focusing on a single slurry droplet, typically ranging from 20 to 180 μm in diameter, allows to study physical effects behind drying dynamics, which can be scaled up to the entire dryer using computational models.
In foam spray drying, individual solution or slurry (the focus of present study) droplets, containing liquid saturated with gas and dispersed solid particles, undergo rapid phase changes as they encounter atmospheric pressure within the drying chamber. The sudden pressure depletion initiates bubble nucleation within the droplets. At the same time, the gas solubility in the liquid phase decreases sharply, prompting the release of dissolved gas into those nucleated bubbles. This process varies based on nozzle type, which controls the pressure release rate, the magnitude of the pressure depletion, and droplet size, influencing both bubble nucleation and growth mechanisms [1].
The drying process of foam spray drying is influenced by the unique presence of bubbles within the slurry droplets, raising critical questions about their role in flow dynamics, drying kinetics and the eventual formation of solid structures and porosity. This study investigates these questions by exploring the coupled effects of heat, mass, and momentum transfer during foam spray drying. Through a dynamic pore network model, we examine the interactions between bubbles and liquid flow, highlighting their impact on drying kinetics and bubble dynamics. The findings provide new insights into the complex physics of foam spray drying and establish a foundation for optimizing this process for industrial applications.