By: Luz Nadiezda Naranjo Gómez, Kensaku Matsunami, Paul Van Liedekerke, Thomas De Beer & Ashish Kumar
In the continuous manufacturing of pharmaceutical solid dosage forms (i.e., tablets), precise and consistent feeding is crucial to ensure the final formulation content and, therefore, the quality of the final product. Powders that flow well and tend to maintain a constant density typically pose few handling challenges. On the other hand, when handling difficult materials that tend to stick to the hopper walls and bridge, density conditioning, flow facilitation, and consistent screw filling are required. To address these issues, an agitator is installed in the feeder hopper, thus promoting a stable and accurate mass flow rate.
An important but often disregarded factor in this feeding process is the intermixing effect induced by the agitator, which modifies the powder's bulk density, flow rate, and flow patterns. This contribution proposes a framework to assess feeding performance by combining Discrete Element Method (DEM) modeling and experiments on a commercial-scale twin-screw feeder. The study focuses on understanding the complex flow and mixing behavior caused by the agitator when changing the screw and agitator rotational speed ratio for pharmaceutical powders with different flowability levels.
The experimental results and model predictions allowed the visualization of three main flow features induced by the equipment design, feeding operation, and material flow characteristics: non-uniform flow with a bypass trajectory, stagnant zone formation in the feeder corners, and preferential back drawdown powder extraction. Additionally, zones prone to material accumulation were identified near the corners of the hopper bottom. On the other hand, through scenario analysis, the impact of the screw-agitator speed ratio and screw speed was analyzed, taking into account the cohesive properties of the powder. The findings suggest that powders with poor flow characteristics require restrictive operational constraints, as the screw-agitator ratio is susceptible to variations in mass feed rate.
Figure 1. Graphical abstract. Left: system of interest with front view of LIW. Right: main model predictions including main flow features (top) and scenario analysis (bottom). Main flow features: bypass path, powder from the top layer (pink powder and blue particle A) to the bottom following the agitator trajectory, stagnant regions (white powder, bottom corner). Scenario analysis: mass flow rate over time for a poorly flowable powder with agitator blade positions (red) and most favorable operating range (i.e., pink area) based on screw-agitator ratios and screw speed.
This work demonstrated DEM's capabilities as a decision-support tool in determining suitable operating conditions for powders with different flowabilities, considering the minimum agitation required to induce unimpeded flow and reduce variability in mass flow rate. This framework could, therefore, be used as a de-risking tool in feeder design when handling challenging materials (e.g., cohesive powders), helping to select a feeder with the required capability.
The most impactful strategy was the use of GPU computing; therefore, the GPU nodes available at the Flemish Supercomputing Centre (VSC) played a crucial role in accelerating the simulations and allowing the research to be completed in a time-efficient manner.
Due to this study's heavy reliance on DEM model predictions, the computational demands were high. Several strategies were employed to reduce the computational time, including domain simplification, particle upscaling, and time-step increment analysis. The most impactful strategy was the use of GPU computing; therefore, the GPU nodes available at the Flemish Supercomputing Centre (VSC) played a crucial role in accelerating the simulations and allowing the research to be completed in a time-efficient manner.
Read the full publication in Nature's Scientific Reports here