Effect of process parameters on coating mass variability and nitrogen-release kinetics of controlled-release urea granules produced in a Wurster fluidized-bed

Abstract

Controlled-release coated urea (CRCU) is an important agrochemical in precision farming, with its effectiveness reliant on the quality of the coating film and the nutrient-release kinetics. This study explores the use of a chemically modified lignocellulosic biopolymer derived from almond shells as a coating material for producing CRCU using Wurster fluidized-bed equipment. The study examines how process parameters—namely fluidized-bed temperature (T_fb), spray rate (R_spray), fluidizing-air flow rate (Q_air), and atomizing-air pressure (P_air)—influence coating quality and nutrient-release kinetics. These are assessed through the inter-particle coefficient of coating mass variance (CMV) and the diffusion coefficient, respectively. The mechanism of nutrient release was studied using the Ritger and Peppas empirical model, specifically by calculating the diffusional exponent, n, for all samples. A Response Surface Methodology (RSM) approach coupled with a CCRD was applied to plan the experiments, perform statistical analysis, predict outcomes, and optimize the process conditions. The Analysis of Variance indicated that T_fb significantly impacts the studied parameters. Optimal coating quality (CMV = 6.7%) was achieved under conditions of T_fb = 75 °C, Q_air = 80 m³/h, R_spray = 0.17 mL/s, and P_air = 3.1 bar. The optimum diffusion coefficient (2.2 × 10⁻⁷ cm²/s) was obtained at T_fb = 78 °C, Q_air = 75 m³/h, R_spray = 0.125 mL/s, and P_air = 3.2 bar. The experimental and predicted responses showed close agreement that validates the regression models for predicting quality of coating films and kinetics of nutrient-release. Based on the n values, some samples exhibited Fickian diffusion, while others followed a non-Fickian nutrient-release mechanism.