Spray drying often involves the transformation of a spray of liquid material or wet particles into a dry powder or particulate material. This is achieved by spraying the wet material into a drying chamber, where the liquid droplets or particles are passed through a hot gas stream. For liquid sprays, the objective on the upstream side is to produce a spray of high surface-to-mass ratio droplets (ideally of equal size). In the dryer itself, the objective is to uniformly and quickly evaporate the moisture from these droplets, leaving behind dried particulate matter. Another goal is to avoid high temperature damage to the particles in the dryer. Evaporation keeps the product temperature to a minimum, but prompt removal of the product from the dryer is also helpful.

The geometry of the spray dryer

The surface mesh

Pathlines are used to illustrate the gas flow field

Numerical modeling of spray dryers poses challenges due to the simultaneous need for complex physics such as combustion, turbulence, and dispersed phase particle tracking. Strong inter-phase coupling exists between the particle and gas phases, owing to the high mass loading in the particle phase, the density difference between the phases, and the ongoing heat transfer with evaporation.

CFD can play an effective role in the optimization of parameters for this process, such as the initial droplet diameter, the location and orientation of sprays, and the mass flow rate of the sprayed material. In this example, the DPM capability of FLUENT is used to carry out a 3D simulation of a typical spray dryer.

Contours of evaporated water