Influence of high-intensity ultrasound on drying kinetics in fixed beds of high porosity

• The application of high-intensity ultrasound could improve traditional convective drying systems.
• High-intensity ultrasound application requires analyze its influence during the convective drying of non-porous materials.
• The development of an mathematical model is essential to analyze the effect of the operating conditions on drying kinetic.

Hot air drying is an energy intensive process and can affect bioactive components. A common method of food drying is in a fixed bed. The application of high-intensity ultrasound could constitute a way of improving traditional convective drying systems. Therefore, the main aim of this work was to assess the influence of high-intensity ultrasound on transfer phenomena during the convective drying in a high-void bed of non-porous materials, like thyme leaves. For this purpose, drying kinetics of thyme leaves were carried out at 1, 2 and 3 m s−1 air velocity at different air temperatures (40, 50, 60, 70, and 80 ± 1.2 °C), and different levels of acoustic power density (0, 6.2, 12.3, 18.5 kW m−3). To address the effect of US on the drying kinetics, a mathematical model was developed considering time varying boundary conditions for heat and mass transfer between the air and the product. Due to the physical characteristic of the material, the influence of the ultrasound power density applied on the internal resistance to the mass transfer was significantly lower than its influence of the external resistance; therefore process intensification is mainly linked to external resistance. Nevertheless, the influence of the application of ultrasound on transport phenomena was only observed at air temperatures and air velocities below 70 °C and 3 m s−1 respectively.