The impact of self-sustained oscillations on particle residence time in a commercial scale spray dryer

Abstract : Spray drying is an established technique, which widely employs cylinder-on-cone type chambers. The air flow patterns inside such chambers are usually governed by transient fluctuations, which impact particle movement and drying history. To understand this impact, CFD using Lagrangian particle tracking through Eulerian flow-field was employed. The simulations revealed transient flow structures due to central jet deflections followed by rotating upwards sweep and formation of vortices. The self-sustained-fluctuations were intensified by high velocity of the main hot air inlet, while lower temperature thereof led to smoother fluctuations. Detailed numerical analyses on particles sampled at the outlet revealed that the distinct transient flow-field actually reduced the particle residence time compared to the residence time estimated via a simplified method ignoring flow-field fluctuations, while generating a wider distribution. No simple correlation was ascertained between the particle size and the residence time distributions, which indicates all sizes considered in the work are affected similarly by the air flow. These findings will benefit the designing of spray dryers.