Comparison on Modelling of Drying Kinetics of Granular Polymers PA6 by Diffusion Models and Normalization Method

Abstract

In this work, the drying behaviour of single granular PA6, hanging in the flow direction of hot air in a drying chamber, was investigated. In the experiments, the air was passed through the chamber at contant very low velocity in the drying chamber of about 0.01 m/s but in a variety of temperatures (60, 70 and 80 °C). In order to obtain drying data, the changes in the mass of the polymer were measured by magnetic suspension balance. The variations of moisture content and drying rates with time were used to test two diffusion models (i.e. Arrhenius and Vrentas Duda) and normalization model (developed by van Meel) given in the literature. A one-dimensional model for diffusion inside isothermal and spherical single particle was used. The simulation was performed by solving the diffusion equation by pdepe command editor in Matlab and fitting the value of D simultaneously. The fitting procedure was conducted by minimizing the sum square of error (SSE) between mean particle moisture obtained from measurement and simulation. The measured drying rates do not indicate the first constant drying rate period and then the critical moisture content for any of the investigated temperatures of drying. By Arrhenius model, it founded the diffusion coefficient of moisture migration is 6.15x10-12 m2/s for 60°C, and by linear regression from three different temperatures drying data result the activation energy 54.3 kJ/mole. However, the Arrhenius model given not a good agreement. Furthermore, the normalization of drying curves does not work well due to each temperature has a different normalized drying curve. Finally, the Vrentas-Duda model was found the best for explaining the drying behaviour of PA6. By using this model, the phenomenon of drying tail or plateau diffusion can be properly described.