Control of particle size distributions in spray dryers with two-fluid nozzles
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A tall-form spray dryer, equipped with a Spraying Systems ¼ J, full-cone, two-fluid nozzle, was instrumented and commissioned for the purpose of investigating control of the spray droplet size distribution and the effect of such control on the product powder properties. A Malvern Instruments Particle Sizer provided online measurement of the droplet size distribution. The volume-surface mean diameter of pure water and 2wgt% sodium nitrate sprays was successfully controlled by a software-based, proportional-integral controller. Electron microscope photographs indicated that control of the powder volume-surface mean diameter might be achieved but was not attainable with the available equipment. The distribution log span measurement proved too noisy to control adequately. A theoretical model of the variation in droplet volume concentration and droplet diameter as functions of radial distance from the spray axis was developed using laser-obscuration data from the Particle Sizer. Errors in the measured volume-surface mean diameter and log span of up to 50% were predicted as a result of radial non-homogeneities in the spray. Use of the model in a control algorithm would allow control of the true volume-surface mean diameter and log span. The evolution of the volume-surface mean diameter and the log span, as functions of axial distance from the atomizing nozzle, were measured. The volume-surface mean diameter was found to decrease as droplet breakup occurred, pass through a sharp minimum and increase with droplet agglomeration. The distribution log span increased with droplet breakup, decreased as droplets agglomerated and increased again with drying. Electron micrographs of dried sodium nitrate sprays showed evidence of two distinct drying mechanisms resulting in predominantly porous spheres or dense, crystalline particles.The crystalline particles were thought to be produced by a mechanism similar to the graupel formation of snowflakes.