Role of Air Pressure in Drying of Weakly Permeable Materials

Modeling of the drying of porous materials is often approached by assuming that the gas pressure of the vapor-air mixture remains constantly equal to the outer atmospheric pressure. Use of both experimental and theoretical results reveals that such an assumption is inadequate to account for the weight loss observed during the drying of weakly permeable materials. For such materials, the gas pressure cannot remain constant because no significant convective Darcean transport of the gas can actually occur. In contrast, the evaporation coupled with the diffusion of dry air generates a gas overpressure that propagates within the material and works actively toward a uniform vapor molar concentration. As a consequence, the diffusion of vapor becomes rapidly nonactive as a driving force of moisture transport. Paradoxically, the drying of weakly permeable materials is eventually achieved by the transport of moisture in its liquid form and its evaporation at the sample boundary in contact with the surrounding air. The analysis is carried out through a modeling of which the formulation is based upon macroscopic thermodynamic considerations. It involves a dry-air component and a water component, the liquid and the water-vapor phases being addressed separately.