Simulation of Mixing in Hot Melt Extruders Using Smoothed Particle Hydrodynamics

Introduction

Hot melt extrusion (HME) attracted increasing attention in pharmaceutical manufacturing during recent years due to its mixing capabilities for high-viscous materials and its continuous processing characteristics. The potential of HME to achieve solid solutions is a possible way to overcome the challenge of bioavailability of poorly soluble drug molecules, where the majority of newly developed drug molecules belongs to.

The most common devices used for pharmaceutical HME are co-rotating twin screw machines. Compared to single screws and counter-rotating twin screws the co-rotating twin screws are frequently preferred due to their mixing performance and self cleaning screw profiles. The typical modular screw design of co-rotating twin-screw extruders allows flexible use of a single device.

The HME process is used in the polymer industry for many years. Nevertheless, it is still a challenge to predict non-Newtonian, non-isothermal flow and mixing in the complex, rotating geometry of twin-screw extruders. This is especially the case for partially filled screw sections, for which conventional (mesh based) CFD simulations are currently not suitable¹. To overcome these challenges, we follow a different strategy and investigate the applicability of the Smoothed Particle Hydrodynamics (SPH) method to the simulation of mixing in HME processes.

Method

SPH is a mesh-less Lagrangian particle method, which approximates the governing equations for fluid flow and can inherently handle free surface flows². We use the open-source particle simulator LIGGGHTS (www.liggghts.com), which was originally developed for the simulation of granular flow via the Discrete Element Method (DEM), and provides an SPH module.

Different approaches to model wall boundaries in SPH exist, mostly based on particles. However, it is not obvious in SPH how to handle complex shaped geometries (as extruder screws), typically represented by a set of wall triangles. We developed a new approach to achieve an appropriate interaction between wall triangles and SPH particles.

Results

Figure 1 shows snapshots of the distribution of two different particle types (yellow and red) in a cross section of a co-rotating twin-screw extruder, calculated with our method. Figure 2 shows the top view of a similar extruder, and illustrates how a slice of tracer (shown in red) moves and disperses in axial direction due to the action of the screw. This allows a detailed analysis and comparison of the mixing performance of different screw geometries, e.g., conveying elements or kneading elements.

Figure 1: Snapshots of mixing in a co-rotating twin-screw extruder (cross section; top: initial state; center after 0.05 revolutions; bottom after 1.05 revolutions).

 

Figure 2: Axial mixing of a tracer in an extruder (top view; left: initial state; right: after 0.2 revolutions).

References

1. Sarhangi Fard A, Anderson PD. Simulation of distributive mixing inside mixing elements of co-rotating twin-screw extruders. Computers & Fluids. 2013. (in press)

2. Monaghan JJ. Smoothed particle hydrodynamics. Reports on Progress in Physics. 2005;68(8):1703-1759.