Contact forces of particles and their influence on processes in simulation and experiment
The processing of particles is an important step in diverse pharmaceutical and food processes. At the chair for systems process engineering, the complete processing chain from conveying, milling, mixing, granulation and tableting can be performed. The complete range can as well be investigated by simulation by the Discrete Elemente Method, coupled with computational fluid dynamics. Contact forces, specifically adhesive forces, have an important impact on all process steps.
Capillary forces are important in the presence of fluids, the most important adhesion forces are hence van-der-Waals, electrostatic and forces created by elastic deformation. Adhesion forces are used for agglomeration, an example is tableting or spray-granulation. A mathematical description of contact forces is possible using the Lennard-Jones-Potential. An established model for adhesion was published by Johnson, Kendall and Robers (JKR), which takes the adhesion as balance between elastic energy and loss of surface energy. An alternative is the Derjaguin-Muller-Toporov (DMT) model, which works with Hertz contact forces, but models additionally attractive forces outside of the contact surface.
An example is the tableting in the pharmaceutical industry. Tablets are pressed, their stability depends from interparticular contact forces. Blistering or braking in consequence of insufficient adhesive forces may already happen during ejecting of the tablet for wall contact forces. Additives may reduce braking or sticking of the tablet to the matrix. The comprehension and prediction of necessary additives based on the knowledge of the medium is one part in the current research.
Tablets which break during the experiment
Pneumatic conveying depends strongly on interparticular contact forces. Particles are transported by a gas stream. Product which tends to cake is difficult or impossible to fluidize and transport. The influence of the contact forces can be simulated based on a contact model. Experimentally the transport is investigated by computer tomography and contact force measurement at the wall.
Measuring part of the pneumatic conveying test rig
During granulation, e.g. in fluidization, the medium granulation depends on the contact forces of the particle, and depends strongly on the properties of the medium itself. The agglomeration of particles needs to be avoided, while at the same time the increase of the particle diameter by spraying on it is realized. Usually this is possible by quick hardening at high temperatures. The contact forces again need to be in a specific range to allow granulation, and to allow a hardness sufficient to resist the process.
For further information please contact:
Dr.-Ing. Johannes Lindner