Abstract
Typical flow problems, like arching or bridging, of ultrafine cohesive powders are caused by undesired particle adhesion, poor flowability and large compressibility and intensified by poor permeability. Thus, the physical understanding of micromechanics of ultrafine particle flow and permeation is very essential to design properly the product quality and to improve the process performance in particle technology.A force balance at a homogenously assumed, dynamic bridge is formulated at the hopper outlet that includes the dead weight, inertia, wall and drag forces. The permeation resistance is calculated as sum of a microscopic flow-around resistance of single particles plus the macroscopic bed resistance of the moving cohesive powder bridge. The resulting differential equation is shown for turbulent flow-through conditions of air. The first integration gives analytical discharge velocity-time function, steady-state discharge velocity and the characteristic discharge time of incipient (accelerated) flow. The second integration results in analytical height-time and velocity-height functions, discharge and residence times. This method results in physically consistent, analytical models that are comfortable to handle and easy to prove. The steady-state discharge velocity is compared with measurements of full-scale silos at Coperion and Zeppelin companies.
This publication describes the results of a closed collaboration between university and industry. The practical objective of the project was to solve the serious discharge problems of ultrafine, compressible, hardly permeable, cohesive powders by applying mechanical vibrations during their gravitational flow.
©2012 Walter de Gruyter GmbH & Co. KG, Berlin/Boston