(22d) Validated DEM Simulations of Wet Granular Matter in a Bladed Mixer

Flow and aggregation of powders is of central interest for applications like powder mixing, granulation or drying in the pharmaceutical industry. Despite intensive research has been carried out in this field, the description of wet powder flow is still demanding. For example, the complex flow in equipment with rotating internals, e.g., bladed mixers, is poorly understood from a theoretical point of view.

In our work we use specialized simulation techniques and a sophisticated measurement system to obtain a deep insight into wet granular flow. We use the discrete element method (DEM) to simulate the flow and mixing in a bladed mixer. Sophisticated particle interaction models were used to study both dry and wet bulk solid flow behavior (Radl et al. 2009). Furthermore, we have significantly improved our experimental setup (Lekhal et al. 2006) and are the first groups that is able to perform particle image velocimetry (PIV) of wet granular matter under well defined process conditions (pressure, temperature, stirrer speed, etc.).

Our goals were the investigation of the effects stemming from (a) the relative particle size, (b) the effect of moisture content, and (c) the effect of impeller speed on the mean and fluctuating velocity fields in the mixer. Our ultimate goal was the monitoring of a drying process by means of a video imaging system to yield improved process control. Such a system does not exist yet and would significantly contribute to process analytical technology (PAT) for particle processes.

Both numerical and experimental results reveal that the particle size has a significant effect on the instantaneous flow pattern in the mixer. This is due to a ?roll-and-lock? effect that is caused by the gaps between big particles, i.e., the flow field has a higher fluctuation amplitude for large particles compared to small particles. Furthermore, we show that an identical granular Bond number is not sufficient to predict cluster formation in wet granular matter. Also, our experimental results show that there is a clear transition in the flow pattern when the moisture content is increased. This transition, however, also depends on the stirrer speed, i.e., the shear rate. This knowledge can be used to monitor, control and optimize drying processes in industrial applications.

References

Lekhal, A., Conway, S.L., Glasser, B.J., Khinast, J.G., 2006. Characterization of granular flow of wet solids in a bladed mixer. Aiche Journal 52, 2757-2766.

Radl, S., Kalvoda, E., Glasser, B.J., Khinast, J.G., 2009. Mixing Characteristics of Wet Granular Matter in a Bladed Mixer. Powder Technology (submitted).