Sherwood's models for the falling-rate period: A missing link at moderate drying intensity

An exact model is presented for the falling-rate period of drying of solids based on Sherwood's diffusion theory, where an external evaporation-controlled constant-rate period is followed by an internal diffusion-controlled falling-rate period. Both the constant-rate period and the falling-rate period are characterized by a constant diffusion coefficient. The exact model is compared with two existing models by Sherwood for the falling-rate period, and with experimental data on water-based alumina suspension for tape casting and on wood. Good agreement is obtained between predictions of the exact model and the experimental data in both cases. It is shown that Sherwood's two models for the falling-rate period are adequate at low drying rates (μ<1.5)(μ<1.5), and high drying rates (μ>9)(μ>9), where μμ is the drying intensity. The exact model is valid for all values of μμ and hence provides a missing link between Sherwood's models at moderate drying intensity.