THE INFLUENCE OF THE DRYING MEDIUM ON HIGH-TEMPERATURE CONVECTIVE DRYING OF SINGLE WOOD CHIPS

High temperature convective drying of single wood chips with air and s uperheated steam respectively is studied theoretically. The two-dimens ional model presented describes the coupled transport of water, vapour , air and heat. Transport mechanisms included are the convection of ga s and liquid, intergas as well as bound water diffusion. In the initia l part of the drying process, moisture is transported to the surface m ainly due to capillary forces in the transversal direction where evapo ration occurs. As the surface becomes dry, the drying front moves towa rds the centre of the particle and an overpressure is simultaneously b uilt up which affects the drying process. The differences between dryi ng in air and steam respectively can be assigned to the physical prope rties of the drying medium. The period of constant drying rate which d oes not exist (or is very short) in air drying becomes more significan t with decreasing amounts of air in the drying medium and is clearly v isible in pure superheated steam drying. The maximal drying rate is la rger in air drying, and shorter drying times are obtained since the he at nux to the wood chip particle increases with increasing amounts of air in the drying medium. The period of falling drying rate can be div ided into two parts: in the first, the drying rate is dependent upon t he humidity of the drying medium whereas in the second, there is no su ch correlation.