Water distribution during ultrasound-assisted vacuum drying of wood
Wood ultrasound drying is an innovation method, it can reduce overall drying time, increase the mass transfer rate, and increase the effective water diffusivity. In this paper, poplar was taken as experimental material. The drying process was carried out under the conditions that the drying medium temperature is 60°C, the absolute pressure is 0.02 MPa, the ultrasound power is 100 W and the ultrasound frequency is 20 kHz. The moisture content distribution and water diffusion coefficient were studied, and the model among wood moisture content variation, drying time and water diffusion coefficient was established. Results indicated that the moisture gradient increases along with the increase of drying time during the drying process, free water and bound water are dried simultaneously when wood moisture content is above the fiber saturation point; Wood moisture decreased linearly when moisture is above the fiber saturation point, while the descending rate decreases when the moisture content is below the fiber saturation point. The water diffusion coefficient decreases along with the increase of drying time and increases exponentially along with the increase of moisture content. The moisture diffusion coefficient is 2.89×10-4 at the beginning stage, it is 3.02×10-6 when the moisture content is at the fiber saturation point, and it is 2.27×10-7 cm2.s-1 when the moisture content is 10 %; The equation between the water diffusion coefficient and the moisture content was established and it could be used to predict the water diffusion coefficient during ultrasound-assisted vacuum drying.