Study on the Cooling Chracteristics of Urea Fertilizer Particle by Blower Reforming in Urea Prilling Tower

Abstract

The purpose of the urea granulation tower is to establish the cooling process of fertilizer granules by scientifically and theoretically exploring the granulation tower tuyere structure, air temperature and the characteristics of temperature change of fertilizer granules with change of exhaust air volume, moisture change characteristics and residence time of granules.

Introduction

Urea is produced either as a solution or in solid form, with solidification occurring in a prilling (granulation) tower where molten urea droplets solidify into prills as they fall through a cooling air stream. Earlier studies on prilling modeling began with Mehrez, followed by Rahmanian et al., who proposed a simplified shrinking-core model assuming uniform particle temperature, and Alamdari et al., who developed a more advanced model incorporating continuity, momentum, and energy equations for both gas and particles.

The current study focuses on the mathematical modeling and numerical simulation of heat and mass transfer during urea solidification inside a granulation tower. The tower consists of air inlets, fans, and outlets that allow ambient air to cool liquid urea droplets into solid particles. The process involves phase change from liquid to solid urea at around 132.6°C, with minor evaporation of water and decomposition of carbamate (less than 1% of total gas). The goal is to determine the phase change characteristics, outlet temperature of fertilizer particles, and the air tuyere structure that maintains product temperatures below 60°C.

The mathematical model includes equations for particle motion (using the Stokes–Cunningham damping law), heat transfer during phase change (accounting for latent heat of evaporation and solidification), and mass transfer governed by diffusion and Raoult’s law. For the continuous air phase, continuity, momentum, energy, and species equations are solved.

The numerical simulation was performed using ANSYS Fluent 2023, employing about 454,726 hexahedral elements. Air with a mass flow of 440,000 m³/h at 30°C cools urea droplets injected at 141°C with various diameters (1.25–2.5 mm). Thermophysical properties and tower wall boundary conditions were defined, including concrete and stainless-steel walls with specified heat transfer coefficients. The Species Transport Model and Discrete Phase Model were applied, along with the energy and turbulence equations, to predict the cooling and solidification behavior of urea prills inside the tower.

Conclusion

We have considered the temperature distribution of urea solution particles along the tuyere structure in the urea tower and scientifically confirmed that the tuyere structure with a 45° angle flow guide across one tuyere with a half reduction in the tuyere area is a reasonable tuyere structure to achieve the cooling characteristics of the fertilizer. This structure revealed that the outlet fertilizer particle temperature, which was 75°C on average in the conventional structure, could be reduced to 55°C when the draught rate was 550,000m3/h in the tower in summer.

References

[1] Ali Mehrez, Ahmed Hamza H. Ali, W. K.Zahra, S. Ookawara, and M. Suzuki. Study on Heat and Mass Transfer During Urea Prilling Process, International Journal of Chemical Engineering and Applications, 2012. Vol. 3. No. 5. P.347-353. [2] N. RAHMANIAN, M. HOMAYOONFARD, Simulation of Urea Prilling Process: An Industrial Case Study, Chemical Engineering Communications. 2013.V. 200. P.764–782. [3] A. ALAMDARI, A. JAHANMIRI, N. RAHMANlYAN, MATHEMATICAL MODELLING OF UREA PRILLING PROCESS, Chemical Engineering Communications. Comm. 2000. V.178. P.185-198.

Copyright

Copyright © 2025 Chol-Ryong Kim, Chol Jin Jon. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.