Rational Scale-Up of Hot Melt Extruders: 1D Predictive Modeling based on 3D Smoothed Particle Hydrodynamics
Josip Matić*,**, Andreas Eitzlmayr*, Johannes Khinast*,**
*Institute for Process and Particle Engineering, Graz University of Technology, Inffeldgasse 13/III, 8010 Graz, Austria
**Research Center Pharmaceutical Engineering, Inffeldgasse 13/III, 8010 Graz, Austria
Email for correspondence: firstname.lastname@example.org
Hot melt extrusion (HME) is a continuous manufacturing process primarily using co-rotating twin-screw extruders (TSE). The potential to increase the bioavailability of poorly soluble drugs, the steady product quality of a continuous process, the increased efficiency as well as a reduction of operational costs led to the specific interest of pharmaceutical industries in HME. In order to effectively predict the impact of screw configuration and process parameters on the HME process, an improved understanding of flow and mixing in individual screw elements as well as in the entire extrusion process is of high interest.
In our previous work, we showed that the smoothed particle hydrodynamics  (SPH) simulation method provides significant benefits for extruder screw simulations. In contrast to computational fluid dynamics (CFD), SPH inherently accounts for convective mixing and free-surface flows, allowing a detailed investigation of flow and mixing in fully filled and partially filled extruder screw elements. In order to provide correct boundary conditions for the complex screw geometries, a novel wall interaction method was developed .
Due to the high computational expense of spatially resolved flow modeling, simplified modeling approaches are still required for the modeling of entire extrusion processes. One-dimensional (1D) models consider the spatial dependencies only along the screw axis and thus, provide the benefit of comparably low computational costs, suitable for the industrial use in design, optimization and scale-up of extruders. Following the idea of a continuous stirred tank reactor cascade with back-mixing, as shown by Choulak et al. , a 1D extruder model was developed ,  in order to predict profiles of filling ratio, pressure, specific energy input, material temperature and mixedness along the screw axis. Due to its strong simplifications, the 1D approach requires pre-computed data about flow and mixing in individual screw elements, which were obtained from the spatially resolved SPH model. This combination of the three-dimensional (3D) SPH approach for the detailed investigation of individual extruder screw elements and the 1D approach for the description of process variables along entire screw configurations results in a comprehensive tool, which increases the process understanding, supports design, optimization and scale-up, and reduces the need for experiments.
An extruder scale-up scenario was analyzed based on proposed scaling laws for process parameters as throughput and screw speed which result in similar conditions at different length scales. Our results show the impact of the extruder dimensions (scale) on filling ratio, pressure, specific energy input, temperature and mixedness.
 A. Eitzlmayr, G. Koscher, and J. Khinast, “Co-Rotating Twin-Screw Extruders: Detailed Analysis of Conveying Elements Based on Smoothed Particle Hydrodynamics. Part 1: Hydrodynamics.” Chemical engineering science, Oxford, p. In Press, 2015.
 J. J. Monaghan, “Smoothed particle hydrodynamics,” Reports Prog. Phys., vol. 68, no. 8, pp. 1703–1759, Aug. 2005.
 A. Eitzlmayr, G. Koscher, and J. Khinast, “A novel method for modeling of complex wall geometries in smoothed particle hydrodynamics,” Comput. Phys. Commun., vol. 185, no. 10, pp. 2436–2448, May 2014.
 S. Choulak, F. Couenne, Y. Le Gorrec, C. Jallut, P. Cassagnau, and a. Michel, “Generic Dynamic Model for Simulation and Control of Reactive Extrusion,” Ind. Eng. Chem. Res., vol. 43, no. 23, pp. 7373–7382, Nov. 2004.
 A. Eitzlmayr, J. Khinast, G. Hörl, G. Koscher, G. Reynolds, Z. Huang, J. Booth, and P. Shering, “Experimental Characterization and Modeling of Twin-Screw Extruder Elements for Pharmaceutical Hot Melt Extrusion,” AIChE J., vol. 59, no. 11, pp. 4405–4450, 2013.
 A. Eitzlmayr, G. Koscher, G. Reynolds, Z. Huang, J. Booth, P. Shering, and J. Khinast, “Mechanistic Modeling of Modular Co-Rotating Twin-Screw Extruders,” Int. J. Pharm., vol. 474, no. 1–2, pp. 157–176, 2014.
See more of this Group/Topical: Pharmaceutical Discovery, Development and Manufacturing Forum