(267g) DEM Study of a Vibrational Powder Transport System

Vibrational transport systems are used in the pharmaceutical industry to transport the powder from one unit operation to the next downstream one. The vibration source can be attached to a transport tube or some other powder containing vessel. The important parameters used to control the vibrational powder transport are the amplitude and the frequency, whose range is dictated by the controlling unit. Challenges may arise when transporting powders that have poor flowability, high cohesivity and adhesion levels.

The vibration system used in this work is shown in the Fig. 1. The powder is introduced from the top side in the filling hopper. The 90° elbow and the transport tube, which has a diameter reduction in one place, fill the downstream unit with the powder. The vibration source is located below the diameter reduction on the transport tube, and the whole system oscillates horizontally. The difficulties in this system include particle agglomeration, system clogging and flow blockage in the hopper and transport tube, especially because the transported material has poor flowability and is highly cohesive.

During experimental treats, it was determined that the operating frequency affects the flowability of the powder. However, to resolve the clogging and flow blocking problems using experiments is challenging.

Therefore, the Discrete Element Method (DEM) was used to model the process, accounting for millions of particles. With this approach it is possible to investigate various process conditions and equipment designs in short period of time [1], [2]. The process was closely replicated in the DEM model, with the same operating conditions and equipment design. The particles were calibrated prior to the investigation (DEM reproduction of the FT4 and compression tests), in order to closely represent the behavior of powder used in the experiments. The particle size was scaled in comparison to the original ones, keeping the distribution width as close to reality as possible.

The DEM results identified an interesting mechanism, i.e., the segregation of the smallest particles in the fill hopper, along the side walls of the system and just before the tube reduction. The investigation of the particle stress distribution in the system showed a peak of the normal and shear stress at the position of the segregated particles. The fact that these positions overlap led to the conclusion that the system clogging is caused by the small particles agglomerating when exposed to high stress. The investigation of the operating frequency showed that reducing the oscillation frequency disrupts the small particle ensembles, and works in favor of the powder flowability. This was also confirmed by experiments.