Direct observation of the dynamics of bridge formation between urea prills

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.