Simulation of moisture transfer during wood vacuum drying

Abstract

Vacuum techniques are commonly used in material drying applications as they provide good quality at fast rates. In this study, wood samples were dried under absolute pressure of 0.02 MPa at 80 °C to assess the drying characteristics. A heat and mass coupling transfer model was established and verified to determine the wood temperature and interior moisture content at various time points during the drying process. Results indicated that the average drying rates for wood with moisture content above fiber saturation point (FSP) is about 2.91 times than that below the FSP. Wood temperature increases over time and nears the ambient temperature at about 60 min. The diffusion coefficient decreases as wood moisture content decreases; it varies from 1.32 × 10−7 to 2.65 × 10−7 m2/s when wood moisture content is above FSP and from 0.46 × 10−7 to 1.32 × 10−7 when wood moisture content is below FSP. The pressure gradient and water volume fraction gradient can be considered wood vacuum drying forces, and the moisture content from the heat and mass coupling transfer model is similar to the actual measured value. The proposed model effectively predicts wood moisture content during wood vacuum drying. Keywords: Wood, Vacuum drying, Heat and mass transfer, Diffusion coefficient, Drying rate