(T1330-08-52) Development of a Discrete Element Method (DEM) Model to Study the Impact of Process Parameters and Material Attributes on Continuous Twin Screw Wet Granulation Process

Purpose: Granulation, a widely used process in the pharmaceutical, agriculture, food, and paper industry, is the technique of agglomerating particles together into semi-permanent granules (aggregates) made up of easily distinguishable original particles. In wet granulation, the binder liquid is sprayed onto the surface of a mixed granular bed, resulting in the formation of aggregates, in order to improve the powder properties such as flowability and compactibility. Despite its importance and widespread use, the granulation process is poorly understood. In this study, a Discrete Element Method (DEM) model has been developed to evaluate the impact of different process parameters (screw speed and powder feed rate) and material attributes (powder cohesion and particle size distribution) on a continuous twin screw wet granulation process. The developed model was later validated using experimental particle residence time and torque data. The designed model will be useful in optimizing the overall process and also decrease the number of experimental trials.

Methods: In this study, we have developed a DEM model for a commercial ConsiGma 25 Twin Screw Granulator. The geometry was designed using SolidWorks with a screw length of 100mm (as shown in figure 1) and the simulations were carried out using commercial software Altair EDEM. Johnson-Kendall-Roberts (JKR) and Edinburgh-Elasto-Plastic-Adhesive (EEPA) were evaluated as the contact models for particle-particle interactions and Hertz-Mindlin model was used for particle-equipment contacts. For the process parameters, different screw speeds (100 rpm, 300 rpm, and 500 rpm) and powder feed rates (5 kg/h, 10 kg/h, and 15 kg/h) were evaluated. The level of cohesion representing different L/S ratios was incorporated into the model using varying surface energy values. The simulations were performed for 5 seconds. After completion of the simulation, the average particle coordination number, average particle residence time, and torque were quantified. The developed model was validated using the experimental data.

Results: It was observed that the mean residence time increases with a decrease in the screw speed and an increase in the cohesion level. Moreover, the average coordination number was calculated under different conditions, and it was observed that an increase in cohesion increases the average coordination number indicating the formation of particle agglomerates (as shown in figure 2). An increase in mean residence time was observed at lower screw speeds and higher levels of cohesion with a decrease in the particle velocity (as shown in figure 3). Using the mean residence time distribution data for the twin screw wet granulation process, the mean residence time was observed to be approximately 2 seconds which correlated well with the experimental data.

Conclusion: DEM simulations were performed to study the twin screw wet granulation process to evaluate the impact of process parameters and material attributes. According to the simulation data, screw speed, material cohesion, and feed rate significantly influence the twin screw wet granulation process. In conclusion, the developed DEM model could be a really useful tool to improve the mechanistic understanding of the overall process in addition to decreasing the number of experimental runs.

Figure 1: Geometry of Twin Screw Wet Granulator

Figure 2: Twin Screw Granulator showing loose agglomerates

Figure 3: Twin Screw Granulation at 300 pm with 15kg/h feed rate at high cohesion level