Approximate isothermal drying curves of hygroscopic porous materials with given desorption isotherm

The drying curves of hygroscopic porous materials can usually only be predicted by solving the mass transfer equation (isothermal case) numerically with the corresponding limit conditions because of the strong dependence of the apparent diffusion coefficient on the moisture content and the influence of the desorption isotherm on the drying rate. A simple approximate method which gives an analytical expression for the drying curve under isothermal conditions for both drying periods (penetration period and regular regime) for any given diffusion coefficient and desorption isotherm, both defined as a function of the moisture content, is presented here. In this method, the observation that the drying rate normalized by an appropriate steady-state flux hardly depends on the form of the diffusion coefficient is used.