Sulfur coating of urea in shallow spouted beds

Two mathematical models were developed to describe the operation of shallow spouted beds in which droplets of the coating materials (sulfur) are injected coaxially with the spouting gas (air) under steady-state, transient, batchwise and/or continuous conditions. Model I is based on a single zone and perfect mixing whereas model II is based on three zones, detailed hydrodynamics and inertial impaction. Both models predict the distribution of the coating material on the bed particles (urea) and the product quality (expressed in terms of seven day urea dissolution as measured by the TVA test). The results were compared with experimental data obtained from two laboratory coaters (column: 0.24 and 0.45 m dia.; included cone angle: 60 degrees; orifice: 3.2-38 nm dia.; sampling ports above cone apex: 0.28-0.56 m). Model II gave better agreement with experimental measurements, but model I is still useful for providing quick approximate estimates provided its inability to give the fraction of uncoated particles can be disregarded. The product quality was found to be primarily dependent on the average sulfur content and it is shown that the quality can also be increased by operating several coaters in series. (C) 1997 Elsevier Science Ltd.