(335c) DEM-Based Refill Strategy Optimization of a Twin-Screw Feeder

Residence time distribution (RTD) modeling has proven to be a valuable tool for material tracking in continuous pharmaceutical processes. Refilling is thoroughly studied in the literature, but the main focus lies on the feed rate disturbances. The impact of the feeder themselves on material tracking is often overlooked. Since the experimental methods to measure the RTD feeder discharging processes feeder are complex and material intensive, there is are only limited experimental RTD data available in the literature.

A DEM (discrete element method) simulation of a discharge of a Coperion KT-20 feeder in volumetric mode is available in an open access repository [1]. The results show that the simplest of material tracking assumptions – first in, first out (FIFO) – does not hold in this feeder because a large fraction of material that is moved and intermixed by the agitator. The DEM simulation shows that it is a much better approximation to assume discharge of a perfectly mixed material. In addition, the simulations show that there is a tendency to discharge material located above the agitator early. In order to predict the behavior during multiple refill events, three models in order of increasing complexity are presented:

(1) A simple reduced order model that assumes perfect intermixing of old and new material, leading to an exponential decay of the concentration of the old material in the hopper. This is a good approximation when discharging down to 30% before refilling (around the agitator axis)

(2) A RTD model based on the discharge of material layers. This model captures the preferred discharge of the top material layers, but fails at very early refills above 70% fill level.

(3) A DEM extrapolation method colloquially called "Relay Race" [2]. This model accurately describes the trajectories of individual particles over multiple refills.

Refilling at fill levels below 30%, which corresponds to the agitator axis, can be accurately described by any of these models. However, cohesive powders often require refilling at higher fill levels in order to ensure a stable mass flow rate. In this case, discharge of material above the agitator axis plays an important role and one of the more complex refill models is then required.

References

[1] Toson and Khinast, 2019. Particle-level residence time data in a twin-screw feeder. Data in Brief, 27: 104672. doi:10.1016/j.dib.2019.104672. dataset doi: 10.17632/D76RZZD8R7.2

[2] Siegmann et al., 2021. Massively speeding up DEM simulations of continuous processes using a DEM extrapolation. Pow Tech, 390: 442–455. doi:10.1016/j.powtec.2021.05.067.