Analysis of powder mixing and axial liquid distribution within a twin screw granulator
Creators: name: Karkala, Subhodh Kumar
Publisher: name: No Publisher Supplied
Year: 2021

description: A variety of processes in industries involve the handling of liquid-solid particulate systems. The amount of liquid and its distribution can affect the material's cohesivity, flow behavior, and strength. These properties can then affect the operational efficiency of a process handling wet granular systems. The wet granulation process combines dry powder and liquid into larger agglomerates. It is a vital operation in the pharmaceutical industry as it improves flowability and prevents powder segregation. The twin-screw granulator (TSG) is often used for wet granulation when a continuous operation is required due to its flexible screw design. This work examines the factors that influence powder mixing and liquid distribution along the axial length of the TSG using a combination of mathematical models and experimental studies. The powder mixing was studied using the discrete element method (DEM) model. As the first aim of this work, a calibration method was developed to simulate cohesive particles for the powder mixing study. Dynamic yield strength (DYS) and shear cell experiments were used to calibrate the parameters of the JKR cohesion contact model. DEM models of these experimental setups were developed. Parameter sensitivity analysis on the DYS model showed that the DYS results in the simulations were highly sensitive to surface energy. Hence, the surface energy parameter was calibrated using the DYS. A combination of DYS and shear cell simulations was used to identify the set of frictional parameters that represented the flow behavior shown by the experimental system. The second aim of this work was to develop a DEM model of a co-rotating TSG and study the mixing of a cohesive binary system of particles (P1 and P2). This model was used to study the influence of particle size, density, and flowability of P2 on powder holdup, mean residence time (MRT), and degree of mixing along the axial length of the TSM. The simulated particles were calibrated using the angle of repose and dynamic yield strength simulations. The model provides fundamental insights into the differences in mixing in the conveying sections of the mixer for materials with different flowability. The total steady-state holdup and the mean residence time (MRT) of particles inside the TSM were heavily influenced by particle flowability compared to the other two tested particle parameters. The model was also used to suggest and test different screw design configurations to improve the mixing of powders with different flowabilities. The final aim of this work was to improve the understanding of the axial variation of liquid distribution in a TSG. First, a method was developed to quantify liquid concentration using colorimetric image analysis and validated against experimental results. Experiments were conducted to investigate the effects of screw design and granulation process parameters on the liquid distribution inside a TSG using a dyed liquid. A photo of the TSG screws was taken at the end of each experiment and analyzed using image analysis. The results showed that mixing in process settings of low screw speed and low liquid to solid ratio resulted in poor mixing. Additionally, the type of screw element used in the experiment significantly influenced the liquid distribution and the final liquid concentrations in the wetted powder.