Investigation of urea release from a coated sphere into a quiescent liquid

Urea (a fertilizer) release from a single opening on a coated sphere into a quiescent liquid was determined by measuring the changes of the weight of the particle with time. Mass-transfer coefficients at different angular positions of the opening were measured under varied Rayleigh numbers from 4.2x10 7 to 5.2x10 10 . A correlation for urea release with the angular orientation of the opening was obtained. Urea release from a population of coated particles was simulated using experimental data for individual particles. A model for the time-varying fraction of urea released was subsequently obtained. It correlates well with experimental data. Overall mass-transfer coefficients for a population of urea particles at various Rayleigh numbers were also estimated using the average Sherwood numbers obtained from local values for various opening orientations. This model could be applicable to the description of urea release from sulfur-coated urea into a quiescent water.