(641e) The Simulation of Mixing and Free Surface Flows in Co-Rotating Twin-Screw Extruders

Hot melt extrusion provides high potential to overcome the bioavailability challenge of poorly soluble drug molecules via solid dispersions. The continuous process characteristics and the combination of different functions in a single device provide high potential for increased efficiency and reduced operational costs.

Extrusion processes are well established in the polymer and food industry. However, the simulation is still highly challenging due to the complex geometry of extruder screws and the variety of the involved phenomena. For example, for conventional, mesh-based CFD (Computational Fluid Dynamics) methods the rotation of the screws requires sophisticated remeshing techniques [1]. Similarly, partially filled sections are extremely challenging for mesh based simulation methods due to the free surface flows. The rheological properties of polymer melts are usually complex (non-Newtonian, viscoelastic, strongly temperature dependent). The melting zone is even more complicated, since the material there exhibits the entire spectrum from granular to molten state on a scale that requires extremely high resolution.

In order to develop a comprehensive simulation approach with sufficient potential for the above challenges, we investigated the applicability of the Smoothed Particle Hydrodynamics (SPH) method. SPH is a Lagrangian method for the simulation of fluids, i.e., it represents the flow by the movement of small fluid elements [2]. SPH is mesh-free, which is beneficial compared to conventional CFD methods in this application. Furthermore, SPH inherently accounts for free surface flows and convective mixing. Since the typical methods to model wall boundaries in SPH (e.g., boundary particles, ghost particles) were not appropriate for the complex geometry of extruder screws, we developed a novel wall interaction method which allows the direct use of flat wall surfaces without additional particles (e.g., generated by CAD software in the *.STL format). For the implementation we used the open-source particle simulator LIGGGHTS (www.liggghts.com) and the RCPE in-house code XPS (“eXtended Particle System”), which is a GPU-based high-performance particle simulator. The method was validated with analytical solutions for simple flow scenarios and data for a twin-screw conveying element from the literature, based on conventional CFD. Based on that, we investigated flow and mixing properties of different types of screw elements, as usually used for co-rotating twin-screw extruders with modular screw design.

For the future, we want to extend our developments based on Newtonian fluids to more realistic rheologies and implement the thermal energy equation to account for the temperature distribution.

References:

1. A. Sarhangi Fard et al., Int. J. Numer. Meth. Fl. 68 (2012), 1031-1052.
2. J.J. Monaghan. Rep. Prog. Phys. 68 (2005) 1703-1759.