MATHEMATICAL-MODEL OF STEAM DRYING OF WOOD CHIPS AND OTHER HYGROSCOPIC POROUS-MEDIA

A model is presented that is focused on the drying kinetics of single wood chips as a function of time and external conditions such as tempe rature, pressure and velocity of the superheated steam. A multiphase a nd 2-D approach was used to model the coupled transport of water, vapo r, air and heat in anisotropic hygroscopic porous media. The model was verified by drying experiments where measurements of the average mois ture content, center temperature and pressure in a single wood chip co uld be performed simultaneously. A comparison between the calculations and the measurements showed that the drying behavior was well predict ed. The drying can be divided into three stages: a heat-up period when condensation on the surface initially increases the moisture content; a period of constant drying rate when the external heat transfer cont rols the drying rate; and a period of decreasing drying rate when the drying is controlled by internal mass transfer Many interesting featur es of the drying could be assigned to the strong anisotropicity of woo d, which makes a 2-D model necessary.