Spray dryer modelling

Abstract

Both spraying and drying are critical to spray dryer performance. Models are developed which explain the very different performance of a spray dryer when large droplets of film forming materials are created using a Rayleigh resonance atomiser. The droplet diameter distribution from this "Acoustic Atomiser" is inadequately described by previously reported spray size distribution functions, but well described by the Stable distribution. The alpha parameter of this distribution was found to tend towards the Gaussian limit for low viscosity fluids and the Lorentz limit with increasing viscosity, consistent with behaviour as a simple and damped forced harmonic oscillator respectively, and hence with the physics of the atomisation process. Droplet drying kinetics dominate model predictions. A device using an ultramicrobalance to measure droplet drying kinetics with unprecedented accuracy and range has been designed. A scaling and residence time analysis model was able to account for experimental spray dryer observations. Sprayability even of complex fluids is predicted adequately by the Ohnesorge diagram, provided that extensional rather than shear viscosity is plotted. A new determination of the transient apparent extensional viscosity from arbitrary CaBER time-diameter curves has successfully been used for fluids too complex to analyse using previously published rheological models.