Wet-core temperature and concentration profiles in a single skim milk droplet drying process

Abstract

Spray drying is a commonly used method to rapidly produce dry powder from liquids or slurries. In the dairy industry, this method is preferred for producing milk powder to prolong shelf life as well as to make the product denser for transportation. In the current work, an experiment accompanied by a numerical model is employed to study the drying kinetics of an industrial-scale skim milk droplet. A comprehensive four-stage evaporation model was validated against the experimental results and was then used to study the temperature and solute distribution profiles within the wet-core of the drying droplet. The results showed that the heat diffuses within the wet-core sufficiently quickly (as indicated by the Lewis number of 25.53), such that the uniform wet-core temperature assumption remains valid for this case study despite Biht=0.281. The uniform solute concentration assumption, however, does not appear to be an appropriate assumption (Bimt=77), especially for the later drying stages where the droplet becomes further concentrated and mass diffuses at slower rates. Furthermore, when the milk droplet dries faster under a higher evaporation rate, the solute had insufficient time to redistribute from the surface towards the centre, resulting in an uneven solute concentration profile. For the high evaporation rate presented in this study, a solute mass fraction of approximately 0.5 was calculated in the droplet centre, compared with 0.82 for a lower evaporation rate. Therefore, for obtaining a uniformly distributed solute in the final dried powder, a lower rate of evaporation is recommended.