Calculation of the drying process of hygroscopic polymer pellets in desiccant dryers and with additional microwave application

Hygroscopic polymers absorb water and bind it within; therefore an effective drying down to an acceptable moisture level is essential for a successful production. If not dried properly, the residual moisture can cause major problems for converting and also significantly affect the product quality. Usually, data sheets give standard recommendations for drying temperatures and times; however, these do neither take into account the actual current moisture up-take of the plastic pellets, nor the current conditions. In this work, a simulation model is presented, calculating the heat and mass transfer within the polymer pellet in order to predict the drying progression. In addition to approaches from the literature describing the diffusion coefficient of water in plastic with a complex function and a high number of material parameters, a linear function was used to model the dependence of diffusion from moisture and temperature. Moreover, the effect of microwave application for an accelerated drying was included by calculating the resulting temperature rise from dielectric heating. The simulation of the drying process shows a very good conformity with experimental results. The linear approach used in this work leads to good results and allows to reduce the complexity of the whole model compared to approaches from the literature. The calculation of a temperature rise through dielectric heating also shows satisfying agreement with that measured in the real process from a prototype for microwave drying on an industrial scale. Therefore, the model is a reliable tool to predict required drying times for different polymers and under variation of drying conditions.