Modeling and Measurement of The Particle Size Dependent Residence Time Distribution in Spray Drying
Spray drying is one of the most commonly used methods to produce a powder out of a liquid and is widely used in the food industry. During spray drying, the liquid is atomised into hot air and dried in the drying chamber within seconds to minutes. The residence time, which is the contact time of the product with the drying medium, is of great importance as it is a trade-off between the removal of moisture and thermal damages of the product. Therefore, the residence time distribution (RTD) is a key parameter for the resulting product quality. For the RTD, especially the size of the particles is of importance as their inertia is leading to different trajectories and thus residence times. In addition, the required minimal drying time is proportional to the particle size.
Despite the significance of the RTD, current measurements techniques are insufficient as they do not allow for a continuous and particle size resolved measurement. Hence, a full 3D model of a pilot plant spray dryer (1.2 m diameter, 2.85 m height) was developed using COMSOL Multiphysics®. The air flow pattern in the drying chamber was modelled using CFD and the RTD for different particle size fractions was obtained using the Particle Tracing Module with a particle counter at the dryer outlet.
Two different air distributors (centrifugal and parallel flow) as well as different inlet air velocities were modelled and the obtained RTD compared to a novel measurement technique for continuous and particle size resolved measurement of the RTD.
This model will enable predictions about product quality and therefore give decision criteria for optimal process parameters.
