Drying of droplets containing insoluble micro- and nanoparticles is a common technique in process technology and it is utilized different industries. Droplets dried in a spray dryer experience complex morphological evolutions, governed by the drying medium parameters (e.g., temperature, humidity), and determining the characteristics of dried material. The drying of suspension droplets is traditionally considered as a two-stage process. During the first drying stage a droplet is subjected to high-temperature gas and undergoes initial heating until an energy balance is achieved between the thermal energy obtained from the gas and the latent heat of the liquid evaporated from the droplet surface. The solid concentration in the droplet external layers increases until a solid wet shell is formed on the droplet surface. Once the liquid has been evaporated from the droplet surface, the second stage of drying commences. During the second stage, the vapor diffuses from the interior of wet particle, from intra-particle liquid interface, through dry crust towards the drying medium. The drying process stops once either equilibrium or the desired particle moisture content is achieved. This work proposes a novel mathematical model describing the entire two-stage drying process of single nanosuspension droplet. A new methodology of predicting the drying rate in the second drying stage was formulated. Several correlations for Nusselt number were examined to identify the most suitable for the considered conditions. The developed model was validated by experimental data on evolution of droplet mass and temperature during the drying of single silica nanosuspension droplet.
