Study on the thermal performance of rhubarb and intermittent multi-frequency ultrasound-enhanced vacuum far-infrared energy saving drying process

Ultrasonic technology, as an emerging technique, holds broad application prospects in the field of drying. This study proposes an intermittent multi-frequency ultrasonic-enhanced vacuum far-infrared (UEV-FIR) drying method with the optimisation objectives of enhancing drying efficiency, improving product quality, and reducing energy consumption. Performed at 55 °C using 25 kHz + 28 kHz + 40 kHz multi-frequency ultrasonication (120 W) with WSH-60A equipment. It aims to evaluate the impact of multi-frequency ultrasound at varying intermittent ratios and pause durations on the energy efficiency and quality of vacuum far-infrared drying. Results indicate that the average drying rate achieved through tri-frequency UEV-FIR drying reaches 0.561 g/(g·min). The drying rate of intermittent multi-frequency UEV-FIR drying exhibits a positive correlation with intermittent duration and a negative correlation with the intermittent ratio. A combination scheme featuring a 20 min intermittent cycle paired with a 1:1 intermittent ratio can significantly enhance energy efficiency during the drying process, achieving an optimal energy efficiency equilibrium point. Moreover, compared with continuous multi-frequency processing, the retention rates of dianthracene and free anthraquinone components in samples under intermittent multi-frequency ultrasound increased by 35.98% and 25.65% respectively. Ultrasonic intermittent processing can reduce thermal damage caused by prolonged exposure to high temperatures, thereby preserving more functional components. The application of intermittent ultrasound technology, leveraging the thermal properties and phase transition behaviour of rhubarb, offers significant advantages in reducing energy consumption, enhancing drying efficiency, and improving product quality.