A study of the energy efficiency of convective drying systems assisted by ultrasound in the production of clipfish

High intensity, airborne ultrasound is a promising hybrid technology for reducing the drying time for convective drying processes. The drying kinetics of the convective drying of clipfish with and without the assistance of ultrasound are compared in a laboratory scale pilot plant. Tests carried out showed that the drying time at a temperature of 20 °C can be reduced by 43% at an ultrasonic intensity of 25 W kg−1. The drying rate is especially increased during the initial period of the drying process, while the effect is minor toward the end of process. The data obtained was used for a dynamic simulation of a batch tunnel drier with a capacity of 40 tons, assisted by ultrasound. Process simulations for heat pump drying (HPD) and heated ambient air drying (HAAD) were performed and compared with ultrasonic assisted drying. Heat pump drying without ultrasound is the most energy efficient dehydration process for clipfish (206 kWh ton−1), followed by heat ambient air drying without ultrasound (915 kWh ton−1). The energy consumptions for ultrasonic drying increase multiple times despite its faster dehydration. In order to be energy efficient, ultrasonic intensities in the convective drying of clipfish should not exceed 2 W kg1, while resulting in a drying time reduction of at least 50%.