Drying kinetics of granular Nylon-6
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media (T2-7P)
Keywords: drying, diffusion, Arrhenius, free volume, nylon.
In the production of polymers, drying is one of the major recovery operations employed to obtain a final saleable product from a reaction process. Since it consumes large amounts of energy, the drying process deserves attention towards both energy savings and improvement of the quality of dried polymer. Furthermore, a study of drying kinetics could be helpful in the selection of adequate drying systems.
This study presents a detailed investigation of the drying kinetics of granular nylon-6 (PA6). PA6 is a very hygroscopic polymer, for example in 50% RH and 23°C (= normal condition) can absorb moisture about 3%. Therefore, drying is needed to decrease the moisture content below 0.1% for product quality.
In the first step, drying experiments with single particle were carried out in a magnetic suspension balance (MSB) using dry air. Hereby the temporal change of mean particle moisture was determined at different process temperatures. The dry mass is defined by constant mass in the end of drying by nitrogen gas at 80°C, which the time series analysis is applied to determine time and the value of the constant mass.
In the next step, the prediction of residual moisture is based on the assumption that the drying process is mainly controlled by diffusion of moisture within the particle and mass transfer kinetics at the phase boundary. The equilibrium moisture content at the surface of particle is corresponding to their sorption isotherm curve. The diffusion equation has been solved numerically to choose an adequate diffusion model capable to predict experimental results by minimized the sum square of error (SSE) between mean particle moisture obtained from measurement and simulation.
It has been found out, that simple diffusion models such as Arrhenius approach gives activation energy 54 kJ/mole will not produce satisfactory results since such models fail to predict the end of drying. To obtain better prediction, the more complex Vrentas-Duda diffusion model has been applied. This model bases on free volume concept in polymer where the free volume is the major factor controlling the diffusion rate. All free volume parameters within the Vrentas-Duda diffusion model can be determined from thermodynamics, except constant pre exponential factor and ratio of solvent and polymer jumping unit. These two parameters need to be estimated from experimental data by fitting. Furthermore, the comparison of measured data and simulated using Vrentas-Duda diffusion model shows a satisfactory prediction of mean particle moisture content. The value of diffusion coefficient is range 1E-15 to 1E-11 m2/s depend on temperature and moisture content.
In addition the sorption isotherm curve of has been determined and shows good agreement with Flory-Huggins model.
See the full pdf manuscript of the abstract.
Presented Monday 17, 13:30 to 15:00, in session Transport Phenomena in Porous/Granular Media (T2-7P).
