Comparative analysis of physical and thermal properties of prilled and granulated urea

Solid urea is the largest nitrogen fertilizer product, produced in two forms: granules and prills. Although the chemical properties of both prills and granules remain similar, their differing physical and mechanical properties distinguish them, making them suitable for different applications, either as fertilizers or raw materials for the chemical industry. This research aims to understand the differences between prilled and granulated urea by comparing their physical and thermal properties. The characterizations were conducted using thermal gravimetric analysis, differential scanning calorimety, Karl-Fischer titration, Scanning Electron Microscopy, High-Performance Liquid Chromatography, sieve analysis, strength measurements, and pycnometer. Karl Fischer titration revealed that granules contain more moisture (0.48%) compared to prills (0.34%). The moisture content of a fertilizer during storage and handling affects its overall quality. HPLC analysis on biuret content showed that the prilled samples contained higher biuret content. The results of the sieve analysis technique used to determine the size distribution indicated that granules had a larger mean diameter of 2.81 mm compared to prills, which had a mean diameter of 1.64 mm. SEM images revealed clear differences in terms of shape and structure between the prills and granules. The outer surface of the prills was smoother and glass-like, while the granules appeared rougher. Interestingly, on the inside, the granules had a denser and more solid middle layer. In contrast, the prills exhibited cracks throughout their inner structure.

To understand how the urea samples break under pressure, all the samples were subjected to a compression test. This test was carried out using an Instron (i.e. Bio-momentum Mach-1). This machine can apply a maximum force of 100 N and steadily squeeze the samples at a rate of 0.05 mm min−1. For reliability purposes, approximately 50 granules were examined for every sample. The selection process involved randomly picking the granules from their containers, resulting in variations in size among the granules of the same type. The granules were placed randomly on the anvil of the equipment. The Bluehill software of the Instron instrument was utilized to monitor the failure load of the granules. The size of each sample was determined by subtracting the original extension (penetration) from the machine extension. To calculate the apparent strength, the applied failure load (force) was divided by the cross-sectional area perpendicular to the force. The full procedure for strength measurement and analysis has also been reported elsewhere by Rahmanian et al. in (2013b) and (2009) for different types of granules, with the same methodologies applied in this work.

Keywords

Urea fertilizer, Granulation, Prilling process, Agricultural application, Thermal properties