Mechanisms of heat and mass transfer during drying of mate (Ilex paraguariensis) twigs

Abstract

ABSTRACTExperimental results of surface temperature and moisture content of twigs of mate were obtained in a conveyor-belt dryer operated batchwise. The first response was determined with an infrared sensor, while the second was by conventional gravimetry. A set of 0.04-m-long cylindrical twigs classified manually into three different subgroups on the basis of their diameters (3.5 × 10−3, 6.5 × 10−3, and 10 × 10−3 m) were used in the experiments. Drying always took place in a chamber fed with a thin single layer of material 0.5 m in length and 0.05 m wide. The fresh twigs without leaves at ambient temperature (≈27.2 ± 2.6°C) and with an initial moisture content close to 0.8 ± 0.1 were dried at three different average air temperatures (65.5, 80.2, and 83.8°C) for 7200 s. A full set of nine (31 × 31) drying experiments were performed by varying the examined factors (particle diameter and drying temperature) at three levels. The low estimated Biot numbers (<0.55) indicate that convection plays a much more important role than conduction in heat transfer. Because of this and since heating was much faster than drying, the Newton’s law of cooling alone was successfully applied to describe the increase of particle temperature with time. From a similar analysis involving a convective mass transfer coefficient calculated with the Chilton-Colburn analogy emerged high-mass-transfer Biot numbers (≈5.37 × 103 − 3.65 × 105) that reveal drying of twigs is governed by diffusion. In fact, the equation that represents the Fick’s second law of diffusion in a long cylinder (one-dimensional transfer), solved analytically and coupled to the model of heat transfer, was able to describe the kinetics of drying of mate twigs.