An investigation and proposed integrated interaction law for moist agglomerate.

In process industries powders are frequently granulated in order to promote their handleability and flowability. During these operations, a liquid binder is usually mixed with the powders to increase their cohesiveness. For binder granulation processed, coalescence and collision of moist agglomerate during the mixing process is the main concern. This thesis considers the computer simulation of moist agglomerate collisions using the discrete element method (DEM). The study is confined to pendular state moist agglomerate, at which liquid bridge is presented as a pendular liquid bridge and interparticle force is modeled as the adhesive contact force, the static liquid bridge force and the dynamic liquid bridge force. This thesis briefly described various algorithms developed by other researchers for modelling the contact force due to surface adhesion and particle deformation and the pendular static liquid bridge force between mono-sized spherical particles due to hydrostatic pressure deficiency and surface tension. A theoretical study of the pendular dynamic liquid bridge force between mono-sized spherical particles has been made and it has been found that a numerical instability occures from a DEM simulation with a fine time step size. As a result, an interaction law is proposed and implemented into a DEM code, DEMC, by specifying a minimum separation distance, namely the 'breakdown distance' two particles. It essentially avoids an infinite value of dynamic liquid bridge force predicted by typical expressions derived from lubrication theory for the dynamic liquid bridge, when two particles just contact. Two dimensional computer simulations of a 'two particles head-on collision test' and 'agglomerate free falling tests' are reported. The results from the former simulation test demonstrate the effectiveness of adding viscous binder to prevent disintegration. Results of the agglomerate free falling test show that a repulsive liquid bridge force occurs between a pair of particles just about to contact. The magnitude of this repulsive force increases with a decreasing breakdown distance. It is also found that an excessive repulsive force occurs for agglomerate with too small value of breakdown distance. As a result, a range of appropriate value of the breakdown distance is recommended such that an appropriate amount of viscous effect is involved from the liquid bridge to the agglomerate system