Interactive effects of operational and particle parameters on the coating quality of Wurster-coated urea granules

Abstract

Bottom-spray Wurster fluidized-bed coating is widely used to produce controlled-release urea; however, coating quality depends not only on thermal and spray conditions but also on the interaction between particle properties and bed hydrodynamics. In this study, the combined effects of spray intermittency (duty cycle), particle surface porosity (PSP), batch load, and partition gap on urea coating performance were investigated using a Central Composite Rotatable Design (CCRD).

A lignocellulosic biopolymer derived from biomass was applied through cyclic wetting–drying in a Wurster fluidized-bed system. Degree of surface coverage (DoSC), agglomeration rate, and nutrient release time were selected as response variables. Response surface analysis revealed strong second-order and interaction effects, indicating that coating performance is governed by coupled wetting dynamics and particle circulation rather than by individual parameters. Surface coverage was maximized when spray intermittency matched particle circulation frequency and when PSP promoted droplet anchoring and film continuity. Agglomeration increased nonlinearly with spray intermittency and batch load due to enhanced surface wetness and collision frequency, whereas suitable adjustment of the partition gap shortened residence time in the wet zone and limited excessive coalescence.

Nutrient release time exhibited dome-shaped response surfaces, showing that prolonged release is achieved under conditions favoring uniform coating formation and reduced structural defects. This work establishes mechanistic links between hydrodynamics, wetting behavior, and coating performance. The results provide a rational basis for optimizing Wurster coating conditions to produce urea granules with high surface coverage, controlled agglomeration, and extended nutrient release.

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Babar Azeem, Interactive effects of operational and particle parameters on the coating quality of Wurster-coated urea granules, Powder Technology, Volume 482, 2026, 122733, ISSN 0032-5910, https://doi.org/10.1016/j.powtec.2026.122733.

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