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Mathematical modeling and simulation of suspension droplet drying

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2019
Özcan, Ahmet Furkan
Drying procedures like spray drying, fluidized bed drying etc. are widespread industrial applications. Such procedures mainly aim to produce high value pharmaceuticals and bulk commodities such as dried milk and detergent powders, which require controlled physical properties that are dependent on the drying process that they are subject to. The aim of this study is to model the kinetics of water droplet drying containing nanosized SiO2 particles and simulate the coupled heat and mass transfer processes via a computer program. The main physical mechanisms involve initial transfer of heat by convection from flowing air with constant properties and then thermal conduction and evaporation of water from the liquid interface, simultaneous diffusion of liquid and nanoparticles leading to agglomeration of solid particles near liquid interface and eventually formation of a shell structure, while a shrinkage in droplet size occurs. As the wet shell region widens with adhering nanoparticles from the inner region, at a certain point shell strength becomes able to sustain the capillary forces exerted by receding water and consequently solid particles in the shell region ceases to recede. Beyond this point, liquid interface recedes towards the center as water evaporation still continues through the pores of the dry crust region, which was a part of former wet shell itself. On the other hand, transfer diffusion of water molecules at the center of the particle towards the evaporation front causes a vacant region to form in the middle, provided that initial amount of solid particles is not sufficient to fill the entire volume of the dried structure. The essential focus in this study is on the kinetics of suspension droplet drying after the liquid interface recedes behind solid particles, namely the dry crust. Eventually all water molecules evaporate from the inner region of the particle leading to either a solid or hollow particle morphology depending on the initial solid fraction.