Spray drying is a common processing method for rapidly producing dry powder from liquids or slurries by evaporating the solvent. This process involves the atomisation of the liquid into hot air and evaporating the resulting droplets in a drying chamber. This technique is employed in a wide range of industries including the dairy industry for production of milk powder in order to prolong the shelf life as well as to make the product denser for transportation. While complete evaporation is the desired outcome of the process, partially wet droplets may deposit on the chamber surface which have been linked to biofouling. This reduces the e ciency of the process and poses a serious health and safety hazard as well as a reduction in the quality of the nal powder.

In this research, the evaporation of a single milk droplet as well as the spray drying process of milk droplets were numerically simulated under conditions representative of the real world. The aim was to develop a mechanistic model of the physics underlying the removal of water from a milk droplet as well as the process of spray drying of milk. This would provide a clear understanding of the drying kinetics and shed light on the spray drying process to optimise it and to reduce any hazards associated with poor drying of the nal product.

It was found that heat di uses more rapidly than mass during the evaporation process of a 100 μm milk droplet, which implies that the temperature distribution tends to be more uniform than the concentration distribution within the droplet. Under conditions in which the droplet evaporates rapidly, the concentration gradient is large, however, a slower rate of evaporation results in a relatively uniform distribution of the solute in the nal particle structure. By applying a high evaporation rate condition presented in this study, a solute mass fraction of approximately 0.5 was calculated in the droplet centre, compared with 0.82 under a condition of lower evaporation rate. According to the spray drying analysis, the results indicated that larger droplets remained in the chamber for a shorter period of time and left with a higher moisture content compared to the smaller ones. Decreasing the air ow rate in the chamber increases droplets residence time, which results in a higher percentage of droplets leaving the chamber being fully dried, similar to what occurs when the air ow temperature is increased. Based on examination of the droplet deposition on chamber surfaces, nearly 52% of droplets hit the wall at an angle of less than 8° and 55% of droplets impacted the walls with a velocity magnitude of less than 1m=s under the conditions in this study.