Comminution of brittle solid materials in fluidized bed processes

The objective of this work is the description of the particle size distributions of brittle solids materials in the different parts of complex fluidized bed processes taking the effects of comminution into account. Three main sources of attrition in fluidized bed systems were identified in the literature: the bubbles and jets in the fluidized bed, and the gas cyclones. Two brittle materials were selected to study comminution in these sources. They represent two particle structures that are characteristic of minerals that are treated in complex industrial fluidized bed processes. These particle structures are associated with different production processes of the minerals. Minerals with a rock-like structure originate from a crushing process of the raw material. Iron ore was chosen as test material as it is a characteristic example of this group of minerals. Industrial precipitation or spray drying, on the other hand, produces agglomerated minerals that are composed of primary particles. The second test material was a catalyst carrier on the basis of aluminum oxide that represents this latter group of minerals. The test materials were mechanically stressed in a laboratory-scale gas cyclone and a laboratory-scale fluidized bed. The bubble motion in fluidized beds did not lead to particle breakage. Under technically relevant operating conditions the gas jets caused abrasion only. For mechanical stress in the gas cyclone, however, particle breakage was observed for both test materials. Models were available from literature for the description of abrasion in the fluidized bed. To calculate particle breakage in gas cyclones no models existed in literature. Within the scope of this work population balance models were proposed that describe the change in the particle size distribution of solids through mechanical stress in gas cyclones. For the breakage probability three empirical and one semi-empirical approach were developed based on physical relations. The breakage probability was modelled as a function of a material constant, particle size, stress history, as well as gas velocity and solids load in the cyclone inlet. For the breakage function a theoretical approach with one material-dependen model parameter was chosen. Comminution models and a fluidized reactor model were integrated into a flowsheet simulation tool. The simulation software SolidSim provides a stream structure that is suitable for the handling of disperse properties. The extended simulation tool was succesfully applied to calculate two technical processes: The synthesis of maleic anhydride from n-butane (Du Pont process) and the chemical looping combustion process. By accounting for comminution effects the proposed models allow for a more realistic description of fluidized bed processes.