Nutrient release characteristics and coating homogeneity of biopolymer coated urea as a function of fluidized bed process variables

The present study investigates the effect of fluid‐bed process parameters on the diffusion coefficient of nitrogen release and coating homogeneity of controlled‐release urea (CRU) produced in a rotary fluidized bed using polyvinyl‐alcohol‐modified starch as a coating material. An existing mathematical model was used to estimate the diffusion coefficient. The coefficient of variance of size distribution and coating mass variation are reported as a measure of coating homogeneity. Statistical analysis suggested that the most influential process variables that govern urea release characteristics and coating homogeneity were (a) fluidizing gas temperature and (b) coating time. A moderate spray rate combined with longer coating time yielded the lowest diffusion coefficient for nutrient release. Elutriation as the result of elevated fluidizing gas temperature allowed a higher diffusion coefficient due to lower coating thickness. Burst release patterns were observed for granules with coating imperfections. The augmented temperature of fluidizing gas had a negative effect on coating mass and size distribution of CRU granules but the influence of longer coating time was positive.