Spouted bed process for suplhur-coating fertilizers


The work undertaken was for the purpose of developing relationships between key variables in sulphur coating of urea to permit design of a semi-commercial scale plant. In order to generate study data, it was necessary to completely redesign and rebuild the existing spouted bed coating facilities owing to deficiencies encountered by previous researchers. The spouted bed (usually granulated urea) was contained in a cylindrical column, 0.154 m ID x 0.91 m high, with a conical bottom. Molten sulphur was sprayed through a specially designed nozzle into the bottom of the bed concurrently with the spouting air. The air could be preheated and special precautions were taken to remove particulate matter from the off-gas leaving the bed. The pilot plant was only capable of operating in the batch-wise mode. The product quality was expressed in terms of the 7-day dissolution of a sample immersed in water under conditions specified by the Tennessee Valley Authority. The dependence of the dissolution rate on the following operating parameters was examined: bed temperature (58-85°C), sulphur temperature (157-159°C), sulphur flow rate (34-260 g/min), atomizing air flow rate (0.402-0.785 m³/hr), bed depth (0.28-0.47 m). The bed temperature had the greatest effect on product quality. The quality increases (i.e. the 7-day dissolution rate decreases) up to a bed temperature of approximately 80°C and then decreases again. Similarly, the product quality improves with sulphur injection rate provided all other operating variables are kept constant. These measurements could be explained in terms of the transition of sulphur from the rhombic to the monoclinic form and also the droplet size which strongly affects the cooling rates of the sulphur film and hence the transition rates. Electron micrographs are presented to support these explanations. A technical and preliminary economic analysis was performed for two semi-commercial coating facilities with production capacities of 0.78 and 1.26 t/day. Both processes were assumed to operate in the batch-wise mode. The production costs per ton of sulphur-coated urea were estimated to be 523.00and523.00 and 448.00 for the two plants. However, relatively minor process modifications would enable continuous operation and lead to greatly increased plant capacities as well as reduced operating costs.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat

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