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Kirsch R.M.,University of Leeds | Kirsch R.M.,Institute for Micro Process Engineering and Particle Technology | Williams R.A.,University of Leeds | Brockel U.,Institute for Micro Process Engineering and Particle Technology | And 2 more authors.
Industrial and Engineering Chemistry Research | Year: 2011

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. © 2011 American Chemical Society.

Kozhar S.,TU Hamburg - Harburg | Dosta M.,TU Hamburg - Harburg | Antonyuk S.,University of Kaiserslautern | Heinrich S.,TU Hamburg - Harburg | Brockel U.,Institute for Micro Process Engineering and Particle Technology
Advanced Powder Technology | Year: 2015

Bulk solids are exposed to time-dependent mechanical stressing due to particle-particle and particle-apparatus contact interactions during various manufacturing processes and transportation steps. These interactions can be described by discrete element method (DEM) based on the contact models of particles. Usually in DEM simulations the particles are assumed to be spheres and as a consequence shape effects are neglected. However, most bulk solids processed in industry consist of irregular shaped particles. Therefore, in order to improve accuracy of numerical simulation the real shapes of particles must be considered in DEM. In this work, amorphous irregular shaped micrometer-sized titanium dioxide agglomerates were investigated. The force-displacement curves at compression were obtained with the help of a self-designed experimental setup. Based on the experimental data, several material parameters were determined and implemented in viscoelastic and elastic-plastic contact models. To consider the shape effect in the estimation of contact parameters the DEM simulation of studied agglomerates was performed by multi-sphere approach and bonded-particle model. The shape and position of the agglomerates on the loading pin were obtained by X-ray computer tomography and used in DEM simulations. From the obtained results it was pointed out that the bonded-particle model based on Maxwell viscoelastic model gives the best agreement with experimental data from compression tests with titania agglomerates. © 2015 The Society of Powder Technology Japan.

Kirsch R.,Institute for Micro Process Engineering and Particle Technology | Brockel U.,Institute for Micro Process Engineering and Particle Technology | Brendel L.,University of Duisburg - Essen | Torok J.,University of Duisburg - Essen
Granular Matter | Year: 2011

In this paper we present a novel experimental setup created to measure the strength of solid bridges between double-particle systems in the millimeter range. Beside the tensile strength, shear and torsional strength can be measured as well. Urea particles are used which are known for creating solid bridges under well defined climatic conditions. The measurements indicate that unlike to isotropic materials the bridge between two particles has higher shear than tensile strength. Moreover, the strengthening of the bridges with storage time is very inhomogeneous, suggesting the use of wide distribution for bridge strength instead of mean values in modelling. © 2011 Springer-Verlag.

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