A study of particle histories during spray drying using computational fluid dynamic simulations

CFD models for short-form and tall-form spray dryers have been developed assuming constant rate drying, and the predictions have been validated against published experimental data and other simulations. This study indicates that a 3D model is more suitable for analysing a spray drying system than a 2D axi-symmetric model. In addition, particle tracking has been included using the source-in-cell method; particle time histories for velocity, temperature and residence time and their impact positions on walls during spray drying are presented. The tall-form spray dryer model CFD predictions show that more than 60% of the particles impacted on the cylindrical wall and this may affect the product quality; particles may adhere to the wall for appreciable times, drying out and losing their wet-bulb protection. It also predicts that the particle residence time (RTD) is rather different from the gas RTD. The primary residence time CFD study also revealed that particle residence times in the tall-form dryer were longer than for the short-form dryer. This study indicates that the short form dryer with a bottom outlet is more suitable for drying of heat sensitive products, such as proteins, due to the low amounts of recirculated gas and hence shorter residence time of the particles.