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UBC Theses and Dissertations

Continuous spouted bed process for sulphur-coating urea Tsai, Bobby S. E.


Sulphur-coated urea is a controlled-release, nitrogen fertilizer which has proven to be successful commercially. Current production technology used by Canadian Industries Ltd. and other firms is based on the Tennessee Valley Authority rotary drum process. This process is mechanically complex and the sulphur coated urea requires an additional wax coat to achieve the desired slow-release properties. By contrast, the spouted bed coating process developed at UBC promises to be mechanically simpler. Afer some initial difficulties, successful continuous coating was achieved in the present thesis project by means of a modified laboratory spouted bed facility. The facility can produce 9.6 kg/h of sulphur-coated urea for up to 3 hours. Coating is performed in a spouted bed, which consists of a cylindrical column (0.154 m I.D. by 0.91 m high) and a 60° conical base. Urea is fed into the bed through a feed tube inserted into the bed annulus. Molten sulphur is sprayed through a nozzle into the base of the bed co-currently with the spouting air. Product is discharged from slots around the perimeter of the spouting column and collected in a product cooler. Particulates from the off-gas leaving the spouted bed and the cooler are removed by a cyclone and water scrubber. Product quality is determined by a rapid dissolution test developed at UBC and a 7-day dissolution test developed by the TVA and modified at UBC. The product quality is found to depend on the bed temperature and the bed depth. In the range of 55 to 90°C the dissolution value decreases up to a bed temperature of approximately 80°C and then increases again. Coating performed at two different bed depths, 0.15 m and 0.25 m, show that superior product quality is achieved with the shallower bed. A set of optimal coating conditions is found: coating temperature of 80°C, bed depth of 0.15 m, spouting air flow rate of 0.65 m³/min and atomizing air pressure of 208 kPa. Under these conditions, modified 7-day dissolution values of about 30% are found for the UBC product compared to 88% for the CIL product with similar sulphur content and no wax. When coated with 30 wt% or more sulphur, the UBC product is able to meet the industrial standard of 25% of urea dissolved in 7 days.

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