(578b) 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.