High temperature convective drying of wood chips with air and superheated steam

Abstract

High temperature convective drying of single wood chips with air and superheated steam respectively is studied theoretically. The two-dimensional model presented describes the coupled transport of water, vapour, air and heat. Transport mechanisms included are the convection of gas and liquid, intergas as well as bound water diffusion. In the initial part of the drying process, moisture is transported to the surface mainly due to capillary forces in the transversal direction where evaporation occurs. As the surface becomes dry, the drying front moves towards the centre of the particle and an overpressure is simultaneously built up which affects the drying process. The differences between drying in air and superheated steam, respectively, can be assigned to the physical properties of the drying medium. The period of constant drying rate which is comparatively short in air drying becomes more significant with increasing humidity of the drying medium and is clearly visible in pure superheated steam drying. The maximal drying rate is higher in air drying, and shorter drying times are obtained since the heat flux to the wood chip particle increases with increasing amounts of air in the drying medium. The period of falling drying rate can be divided into two parts: in the first, the drying rate is dependent upon the humidity of the drying medium whereas in the second, there is no such correlation. The influence of intergas diffusion in air drying was found to be of minor importance.