Synthetic polymers-based controlled release urea (CRU) leaves non-biodegradable coating shells when applied in soil. Several alternative green materials are used to produce CRU, but most of these studies have issues pertaining to nitrogen release longevity, process viability, and the ease of application of the finished product. In this study, we utilized tapioca starch, modified by polyvinyl alcohol and citric acid, as coating material to produce controlled release coated urea granules in a rotary fluidized bed equipment. Response surface methodology is employed for studying the interactive effect of process parameters on urea release characteristics. Statistical analysis indicates that the fluidizing air temperature and spray rate are the most influential among all five process parameters studied. The optimum values of fluidizing air temperature (80 °C), spray rate (0.13 mL/s), atomizing pressure (3.98 bar), process time (110 min), and spray temperature (70 °C) were evaluated by multi-objective optimization while using genetic algorithms in MATLAB^®. Urea coated by modified-starch was double coated by a geopolymer to enhance the controlled release characteristics that produced promising results with respect to the longevity of nitrogen release from the final product. This study provides leads for the design of a fluidized bed for the scaled-up production of CRU.

To impede nitrogen loss due to leaching and NH₃ volatilization, pristine urea particles are coated with synthetic polymers such as poly(acrylic acid) and polyethylene for the controlled release of nitrogen. However, due to nonbiodegradability and environmental and economic issues, these synthetic polymers are replaced with cheap, biodegradable, and green coating materials. In this study, borax-modified starch biopolymer is used to produce controlled release urea (CRU) in a tumbling fluidized bed. Central composite rotatable design is used to study the interactive effect of process parameters on time and kinetics of nitrogen release. Nitrogen release in distilled water is monitored by the UV–Vis spectrophotometer and soil burial test is used to observe release in soil. The optimum values of process parameters and response objectives generated by the analysis of variance are validated by triplicates of confirmation runs and the % error is reported as a precision indicator. A mathematical model is used to determine the diffusion coefficient of nitrogen release. Biopolymer-coated urea results in much better release performance when compared with starch-based CRU reported in the literature. The most influential process parameter both for time and kinetics of nitrogen release appears to be the coating time. Release time increases linearly with coating time and the diffusion coefficient decreases with increase in coating time. The kinetic study reveals that some of the samples followed non-Fickian diffusion and others followed Case-II transport during the release of nitrogen.

Pristine urea is lost due to ammonia volatilization and leaching causing toxic emissions and eutrophication. Controlled-release urea is employed as an abatement strategy. Most of the synthetic polymers used to produce controlled-release urea are non-biodegradable and expensive. To offset this problem, modified-starch biopolymer is used as coating material to produce controlled-release urea in fluidized bed. The effect of different process variables is studied on release characteristics and coating uniformity of coated urea. The product has better release characteristics. The statistical analysis reveals that fluidizing gas temperature and coating time are the most influential variables. The nutrient release time increases with increase in coating time and decreases with increase in fluidizing gas temperature beyond a certain limit. Coating uniformity, significant thickness and film integrity are required for promising release characteristics. Urea release followed non-Fickian diffusion and Case-II transport. This study can help produce green fertilizer at bigger scale.

Controlled-release fertilizers are employed in precision agriculture to optimize technology-enabled farming without environmental deterioration. In this study, almond-shell lignocellulosic waste particles are chemically processed to synthesize a coating suspension for the production of controlled-release urea (CRU) in a Wurster fluidized-bed reactor. The interactive effect of process parameters such as atomizing air pressure (P_air), fluidized-bed temperature (T_fb), spray rate (R_spray), and fluidizing-air flow rate (Q_air) on the (i) coating-film uniformity of CRU particles and (ii) longevity of nutrient-release from CRU is reported. CCRD is used with RSM to design the experiments for the optimization of nutrient-release performance and coating-film uniformity in terms of a coefficient-of-variance (CoV) of film thickness. The regression models indicate a good prediction of coating-film uniformity and nutrient-release time, with R² = 0.971 and R² = 0.98, respectively. The optimum conditions for coating-film uniformity are determined to be P_air = 3.5 bar, T_fb = 80 °C, R_spray = 0.15 mL/s, and Q_air = 72 m³/h, with a predicted CoV film thickness of 11.5%. Similarly, P_air = 3.2 bar, T_fb = 78 °C, R_spray = 0.125 mL/s, and Q_air = 75 m³/h are the optimum conditions for nutrient-release performance, with a prediction nutrient-release time = 56 h. The experimental validation yields a CoV of film thickness = 12.6% and a nutrient-release time = 49.5 h, indicating good agreement between predicted and experimental values. In addition, T_fb appears to be the most significant parameter.