Heat and mass transfer during low intensity convection drying

A diffusion model of heat and mass transfer in moist porous media is obtained for low intensity convection drying found typically in food products and vegetable dehydration. The effective diffusivity was analyzed based on the coupled effects of moisture content and temperature on mass transfer in capillary flow, evaporation-condensation, and transition regions. The temperature effect was found to be significant in the transition and evaporation-condensation regions. The model was solved numerically using the finite difference method. Physical properties of banana were determined. The results show that the most intensive heat and mass transfer occur in the transition region where both capillary flow and vapor diffusion play a significant role. The length of the transition region is determined clearly by analyzing the liquid flux distribution in the material. Its value depends very much on the permeability of the material. The model proves that the assumption of evaporation front is valid for drying of very permeable materials. For the case of food and vegetable dehydration with the permeability below 10-19m2, more comprehensive analysis is necessary.