(9a) Discretization of Temperature and Moisture for Determining Surface Stickiness of Single Droplets during Drying

Spray coating of temperature sensitive particles requires lower temperatures than typical in spray driers. To avoid subsequent agglomeration of wetted particles, it is important that the coating solution forms a skin that rapidly becomes non-sticky. This process of film formation and stickiness development, and the low processing temperatures, mean that existing kinetic models of single droplet drying do not adequately predict the required surface temperature and moisture needed to define the stickiness state of the coating film. Previous models generally only discretize moisture, because the mass transfer diffusion coefficient is often highly dependent on the moisture content which means, as drying proceeds, the Lewis number (?Ü/D) becomes extremely small. This justifies the generally accepted assumption that the surface temperature is equal to the drying air temperature. However, in this work both the droplet internal moisture content and droplet internal temperature are discretized. In these conditions a small temperature gradient exists. Model predictions show good agreement with kinetic data collected from a single droplet drying experiment. Discretization of temperature was found not to be important for modelling the drying kinetics of the single droplet experiment, but calculations on a typically spray droplet found in a coater, indicate that the surface temperature can vary significantly from the drying air temperature. This is expected to affect its stickiness state. In conclusion, a double-discretized model such as this can be used to optimise drying conditions and spray zone design.