Enhancement of heat and mass transfer during drying: A process intensification approach - integrated radio frequency (RF) and ultrasonic (US) assisted drying of paper and board

Drying of paper and board conventionally relies on alternating conduction—via steam-heated cylinders—and convection, where heated air flows over the paper web surface. These surface-dominated methods predominantly utilize steam from fossil fuels, resulting in high energy consumption and thermal inefficiencies. To address these challenges, this study investigates an experimental drying system that sequentially applies two volumetric energy techniques: Radio Frequencies (RF), and Ultrasonics (US), integrated with conventional drying. In situ measurements of drying characteristics assess process intensification. Intermittent RF and US applications combined with convection-only and alternating conduction and convection drying environments produced substantial improvements to overall drying rate, drying time, and input energy intensity, with some localized behaviors showing up to 230 % improvements. Radio frequency focuses on volumetrically induced dipole rotation, whereas ultrasonics using pressure-driven bulk flow, removes moisture through rapid atomization. The synergistic RF-US approach intensifies the rate of dehydration and extends the constant-rate drying period by enhancing capillary flow at higher moisture contents and improving vapor diffusion and boundary layer disruption during the falling-rate phase. When powered by renewable energy sources, these volumetric methods can potentially offer significant carbon intensity reductions compared to steam-based systems. These insights contribute to future modeling, simulation, and optimization of sustainable drying technologies using electromagnetic and acoustic energy.