Spouted bed seed coating: The effect of process variables on maximum coating rate and elutriation

The effect of bed hydrodynamics, particle and feed slurry properties on maximum slurry feed rate and elutriation during spouted bed coating of seed was studied. Experiments were performed in a 0.15 m diameter laboratory batch spouted bed using 13 different seed types and two fertilizer coatings - monocalcium phosphate (MCP) and tricalcium phosphate (TCP) - with a methylcellulose binder. Gas superficial velocity, bed height and temperature, orifice size, coating strength and flow rate were varied during experiments. The slurry feed rate is limited by spout collapse caused by the formation of embryo agglomerates in the spray zone at the bottom of the spout. The formation of agglomerates is determined by a balance between binding forces which are related to seed and coating properties, and shearing forces which are determined by bed hydrodynamics. The spout collapses before exit air from the bed is completely saturated with water. The relative humidity of the exit air at spout collapse increases with increasing excess air velocity and particle size, decreases with increasing feed slurry viscosity and is independent of bed height and temperature. Attrition of newly formed coating is the main mechanism to generate elutriated fines. The elutriation rate increases linearly with feed slurry rate and with gas velocity to the third power. It is a strong function of coating strength. Two attrition models - the inlet gas kinetic energy model and the excess energy model - were examined. The inlet gas kinetic energy model best explains the elutriation data from these experiments at constant orifice size (25 mm). However, when the inlet orifice size is decreased, the increase in elutriation is less than predicted by the increase in inlet gas kinetic energy. The mechanistic models used in this work may also be relevant to other spouted bed granulation and coating processes.