Modelling of hot-melt extrusion using the discrete element method (DEM)
Loading...
Date
2023
Authors
Βασιλάκη, Μιχαέλα
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Hot melt extrusion (HME) involves the melting of materials and/or mixing and pushing of various solids under elevated temperature and pressure. It is a mature technology that has been extensively and successfully used for a broad spectrum of applications in the polymers, food and pharmaceutical industry [Crowley et al., 2007]. HME is a promising downstream processing method due to its high efficiency and economic value, however mathematical modelling of HME processes has always been challenging due to the limited understanding of the material behavior and the demanding experimental techniques. The process usually consists of two co-rotating screws enclosed in a barrel. The powder is fed through a hopper and the product is formed by melting and forcing the raw powder through a die. Two zones are observed: the solids conveying and the melt zone. The first transports the material within the barrel and the latter is fully filled creating pressure built-up.
There has been extensive research in developing efficient mathematical models which can be used for process synthesis. The objective of this study is to evaluate such classical mathematical models which have been extensively used to model extrusion processes and to use DEM simulations to extract the flow characteristics of the extruder that can be used in low order models to perform steady and dynamic state simulations. In Chapter 2 a review of mathematical models for modelling extrusion processes is presented. The first model proposed by (Kulshreshtha and Zaror, 1992) and (Fernando and Garnica, 2017) describes the dynamics of a twin-screw extruder within each zone separately, while deriving the equation which best describes the movement of their interface boundary. The second one is based on the work of (Kulshreshtha et al., 1991) and it describes the extrusion process in steady sate flow for non-Newtonian fluids which requires an additional analysis on the local shear rate. The third model discussed is the residence time distribution model where extruder regions and complex geometrical configurations can be modelled as ideal reactors, such as CSTR, PFR and LFR. A combination of reactor types is also examined to determine the optimum assumption. The fourth model is an efficient approach where the extrusion process is analyzed in both dynamic and steady state as a linear complementarity problem (LCP). Even though classical models have been employed for years, there has been a recent turn for the use of discrete element method (DEM) which is a numerical technique appropriate for designing manufacturing processes through particle-scale simulations resulting in in-depth analyses of a system’s flow dynamics. In chapter 3 the DEM theoretical background is addressed, while an analysis on base contact and rolling friction models is presented. Key definitions for particle-scale simulations are explained.
In chapter 4 Altair’s EDEM software is employed to monitor the dynamic response of a system containing a hopper and a conveying zone and simulations are performed to establish the key parameters which affect the system’s dynamic state. Contrary to their promising applications, DEM simulations require significant computational cost and considerable time [Morrissey, Hanley and Ooi, 2021]. Hence, an essential prerequisite of this study is to produce particle-scale analyses while minimizing such limitations. At last, this research aims in developing models that can be efficiently applied for designing full-scale industrial processes producing high-quality pharmaceutical products.
Description
Keywords
Hot-melt extrusion, Twin-screw extruder, Discrete element method, Modelling