We present a visco-elastic coupling model between caked spheres, suitable for DEM simulations, which incorporates the different loading mechanisms (tension, shear, bending, torsion) in a combined manner and allows for a derivation of elastic and failure properties on a common basis. In pull, shear, and torsion failure tests with agglomerates of up to 10000 particles, we compare the failure criterion to different approximative variants of it, with respect to accuracy and computational cost. The failure of the agglomerates, which behave according to elastic parameters derived from the contact elasticity, gives also insight into the relative relevance of the different load modes.
Solidification and caking of bulk solids often occurs during storage or while being transported to the customer. To investigate the formation and growth of solid bridges between two discrete particles, the modification of the contact region between these two particles stored in a climatic chamber is examined. The effect of load, temperature, relative humidity, and storage time on the formation of a bridge is analyzed. The objective is to describe the behavior of crystalline or salt-like granules under real storage conditions.
To investigate the formation and growth of solid bridges between urea particles, the changes in the contact region between two discrete particles are measured. The work is carried out using a Double-Particle-System (DPS). The DPS was stored in a climatic chamber. The effect of load, temperature, relative humidity, and storage time on the formation of a bridge between these particles was analyzed. The objective was to describe the geometrical changes in the contact region and to measure the strength of the resulting interparticle bridge.
A novel experimental method has been developed that enables the direct observation of bridge formation between urea particles under very carefully controlled climatic conditions (±<0.2 °C, ±1% humidity) providing new insight into the fundamental system behavior. Results demonstrate that liquid bridges are formed rapidly between urea prills, often within 30 min, when exposed to conditions close to the critical relative humidity and following adsorption of water at the surface of the prills. Observations over a longer time period show that a time-dependent mass transfer into the liquid bridge takes place, transforming it into a solid bridge. This complies with earlier experimental results reported elsewhere [Wahl, M.; Kirsch, R.; Bröckel, U.; Trapp, S.; Bottlinger, M. Chem. Eng. Technol.
2006, 29, 674]. Elucidation of the mechanism shows that the phenomenon cannot be considered a simple crystallization of dissolved urea by evaporation of water in a distinct drying step, as widely assumed in the past. We report here evidence of the time dependence of solid-bridge formation on mass transfer. These observations present a new challenge for future studies seeking to develop appropriate models to describe the bridge formation in urea.
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