Abstract
Agglomeration in spray dryers occurs by overlapping the spray patterns of multiple liquid nozzles and the airstreams containing recycled fine powder. In this work, the role of the recycled fines stream is explored by measuring the extent of agglomeration. Two scales are investigated, a small-scale dryer with nominal capacity of ~ 1 kg/h and a pilot-scale dryer of ~ 75 kg/h. Experiments were designed to investigate a number of parameters at three levels: low, medium and high about midpoints that represent typical operation at each scale. These operating points are representative of those available, or potentially available, to operators of agglomerating spray dryers: spray flow rate, spray solids concentration, spray droplet size, powder flow rate, and powder particle size. Drying conditions were kept as constant as possible and all experiments used skim milk powder. A measure of success, called the agglomeration efficiency, was developed based on examining the agglomerated product size distribution relative to the spray and fines. The efficiency reflects the proportion of particles sizes that disappear from the spray size distribution and then appear in the agglomerated product after the fines have been censored from it. Comparisons were made between the reference case of natural agglomeration when the dryer was run without fines, and later forced agglomeration trials with fines. These confirm that fines addition does promote agglomeration. Trials containing fines show that the extent of agglomeration depends principally on the mass flux ratio of fines to spray followed by the particle size of the fines; this can be linked to the probability of interaction between spray and fines in the turbulent interaction zone. The concentration of the spray liquid has a weaker effect on influencing forced agglomeration, but has a major influence on the final product particle size because high solids concentrations promote natural agglomeration when sprays interact. This relates to the development of surface stickiness which influences the success of collisions. The two scales of dryer have the same trends and approximately scale.
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