Ultrasonic Intensification of Grape Stalk Convective Drying: Kinetic and Energy Efficiency

Abstract

Grape stalk is a by-product of the winemaking process with a high antioxidant content. Drying is a necessary stage before antioxidant extraction, which may affect not only kinetic and energy efficiency but also product quality. Coupling non-thermal technologies, such as power ultrasound, to convective drying is considered a strategy for process intensification in order to prevent certain drawbacks of conventional technologies. In this work, the use of power ultrasound in the convective drying of grape stalk was tested in order to estimate its influence on kinetic and energy efficiency. For this purpose, convective drying kinetics of grape stalk were carried out with and without power ultrasound application (21.8 kHz, at two ultrasonic power levels: 45 and 90 W). In addition, the inlet and outlet air temperatures of the drying chamber were monitored. The drying process was modeled considering heat and mass transfer phenomena jointly with the total energy consumption and the energy efficiency of the system. Power ultrasound application involved a shortening of grape stalk drying time, which was dependent on the drying air temperature (40 and 60°C) and the ultrasonic power applied (45 and 90 W). The modeling showed the increase in diffusion and convective heat transport phenomena produced by ultrasound application, despite grape stalk being a low-porosity product and, therefore, only slightly sensitive to ultrasonic effects. In addition, it was also highlighted that ultrasound application increased the energy efficiency during the drying of grape stalk.

Keywords:

• Biomass
• Simulation
• Ultrasound-assisted drying

ACKNOWLEDGMENTS

The authors acknowledge the financial support from the Ministry of Economy and Finance of Spain through Project DPI2009-14549-C04-04 and from the Generalitat Valenciana through project PROMETEO/2010/062.