Modelling the Process Behavior of Dry Granulation
Christian Eichler, M.Sc.
Roll compaction is a process for the dry granulation of powdery bulk material. It finds application in pharmaceutical, chemical and food industry. In the first stage the powder is compressed between two counter-rotating rolls. Therefore the material is compacted due to the effective pressure the rolls exert (1). The resulting intermediate product is a high-density consolidated ribbon. Subsequently, the ribbon is comminuted into coarse pieces (2) and drops into the sieving area of the machine (3). Within the latter the ribbon is granulated to a specific size.
The granulation process improves flow characteristics of the raw material and prevents dust formation by particle agglomeration. In contrast to wet granulation techniques, dry granulation can be used for processing moisture-sensitive and thermo-labile substances, as neither wetting nor high temperature drying is involved. Further advantages lie in a reduced production time, as no drying step is required. The main drawback of rolling compaction is the lack of quality assurance during processing. The state of the art is taking samples and analyzing them offline.
The goal of this project is the development of an inline measurement system in real-time to enable control and documentation of relevant quantities during processing. Based on these, a fully automated process control system will be installed to identify and adjust deviations during the continuous granulation process. This control system requires a computational material model to describe the influence of the recorded quantities on the product properties (e.g. strength, compaction, tendency of agglomeration). Concerning this matter, creating a comprehensive model that is suitable for real-time predictions will be a substantial challenge.
In the course of the project, the dry granulation process is simulated using a multiscale model. It consists of the microscopic discrete element method (DEM) and the macroscopic population balance model (PBM). In this regard the DEM is used to describe particle behavior in each of the three sub-processes (Fig. 1) during processing. Additionally, an advanced bonded-particle model (BPM) characterizes particle-particle interactions as bond formation or agglomerate destruction. This enables the heterogeneous description of both ribbon formation and comminution. The DEM results are used to implement a macroscopic PBM to reduce computational time. The PBM of all sub-processes are integrated to yield a model of the whole process to consider material properties as well as process parameters. The goal output value should be the particle size distribution of the granulate material. Additionally, characterization experiments are used to calibrate contact model parameters for the different sub-products (Fig. 2). Furthermore, experiments of the sub-processes are needed to validate the simulations.
The German Federation of Industrial Research Associations (AiF)