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 pos itions of the opening were measured under varied Rayleigh numbers from 4.2 x 10(7) to 5.2 x 10(10). A correlation for urea release with the angular or ientation of the opening was obtained. Urea release from a population of co ated particles was simulated using experimental data for individual particl es. 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 number s were also estimated using the average Sherwood numbers obtained from loca l values for various opening orientations. This model could be applicable t o the description of urea release from sulfur-coated urea into a quiescent water.