Small-scale top-spray fluidised bed coating: Granule impact strength, agglomeration tendency and coating layer morphology

The degree of agglomeration and impact strength of Na2SO4 cores (all in the size range of 200–300 μm) coated with aqueous solutions of sodium sulphate and dextrin as a function of several process and formulation variables have been investigated. The coating process was performed in a modified small-scale GEA Aeromatic-Fielder Strea-1 top-spray fluidised bed, and the impact strength was tested in bulks using a Pneumatic Impact Gun, in which granule samples of 0.5 g were exposed to ten repeated impacts at 10 m/s. Using an unreplicated double 24 − 1 fractional factor design, two regression models were derived from experimental data describing quantitatively the degree of agglomeration and impact strength respectively. The agglomeration model suggests in accordance with previous studies that increasing the nozzle pressure as well as coating-solution-dry-matter-concentration decreases the tendency of agglomeration. The consistency between the agglomeration model and new experimental data is concluded to be satisfactory. The impact strength model indicates increasing impact strength with increasing nozzle pressure, coating solution–dry-matter-concentration and bed temperature. These tendencies may be influenced by the large extent of droplet penetration, as observed in additional coating droplet penetration and coating layer morphology studies in the present paper. The validation of the impact strength model shows that the model clearly follows overall experimental tendencies. The derivation of these quantitative models may be seen as a first step towards the development of processes for production of unagglomerated enzyme granules with high mechanical strength and contribute to an improved understanding of fluid bed processes and products.