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Sulfur coating of urea in shallow spouted beds Choi, Michael M.

Abstract

Sulfur coated urea (SCU) is an effective and economical slow-release nitrogen fertilizer, and its production in a spouted bed was investigated. SCU was produced by batch and continuous operations. Higher quality products were typically produced by the batch process, but at significantly lower production rates than the continuous process. In order to understand such operations, mathematical models describing the coating process were developed and verified through experiments. The production of SCU was studied in shallow spouted beds fitted with a pneumatic mol-ten sulfur spray nozzle located at the cone inlet. Bed hydrodynamics, coating mechanism, particle coating distribution and product quality were examined under the following conditions: Bed diameter of cylindrical section - 0.24 and 0.45 m; bed height - 0.11 to 0.63m; included cone angle - 60'; particle diameter - 2.1 to 2.8 mm; particle density 930 to1490 kg/m3; main spouting air 37 L(actual)/s; atomizing air S 0.87 L(actual)/s; urea feed rate - 7.6 to 20 g/s; sulfur injection rate - 2.1 to 6.1 g/s; orifice diameter - 21 to 35mm; bed temperature - 18 to 70°C; sulfur content < 60 %. The temperatures of atomizing air and molten sulfur were fixed for all runs at approximately 160 and 150 °C, respectively. The coating process was successfully modeled using mass and momentum balance equations, inertial sulfur droplet deposition as the dominant coating mechanism, and Monte Carlo simulations. The hydrodynamic model was based on the one-dimensional mass and momentum balances suggested by Lefroy and Davidson (1969) for gas and particle motion in the spout, the axial pressure correlation given by Morgan and Littman (1980), and the vector form of the Ergun (1952) equation for gas motion in the annulus. The effect of atomizing air entering through the spray nozzle was successfully incorporated into the model by considering the total momentum flux into the bed. Conical beds were found to behave similar to conical-cylindrical beds having a column diameter of 80 % of the maximum conical bed diameter. The dominant coating mechanism was deduced from the bed hydrodynamics and spray drop sizes produced by the pneumatic atomizing nozzle (type: internal mixing; Fluid Cap #40100; Air Cap # 1401110; manufactured by Spraying System Co.). The drop sizes were found to range from approximately 6 to 50 Am dia. The atomizing air flow rate did not affect the drop size distribution significantly under the conditions used in the present study. For the drop sizes produced and the hydrodynamic conditions prevailing in the spouted bed, inertial deposition was found to be the dominant mechanism for coating the bed particles. On the basis of the bed hydrodynamics and the coating mechanism, the particle coating distributions were calculated utilizing the Monte Carlo method, and the quality of SCU particles was estimated from the coating distributions. The simulation results, which were in good agreement with the experimental data, imply that the product quality improved with increasing bed diameter, spouting and atomizing air flow rates, and that it decreased with increasing urea feed rates. Some improvement in product quality was also observed after changing the urea feed location and reducing the spray angle. The model results also indicated that products with widely varying quality can be produced in a series of spouted beds at high production rates. This implies that the spouted bed is an effective and practical coating unit for producing SCU.

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