APPROXIMATE ISOTHERMAL DRYING CURVES OF HYGROSCOPIC POROUS MATERIALS WITH VARIABLE SURFACE CONCENTRATION

The governing equations for isothermal drying of slabs of hygroscopic porous material are analyzed to develop a method for the approximate c alculation of drying rate curves. Gradual decrease in surface moisture content is incorporated in the model by considering the effect of the desorption isotherm. The proposed method enables one to estimate dryi ng rate curves for any conditions without solving drying equations whe n the dependence of moisture diffusivity and the desorption isotherm a re given as functions of moisture content. Accuracy of the drying rate s calculated by the method is examined by comparing with numerical sol utions of the drying equations. When one of the internal and superfici al mass transfer resistances is dominant, approximate drying rate curv es show very good agreement with numerical solutions. When both resist ances are comparable, the method underestimates drying rates with erro rs of less than 6%.