Lewis number in the context of air-drying of hygroscopic materials

Drying is a complex process to model. Certain assumptions, such as an isothermal process, a uniform temperature process and a heat transfer limiting process, where no mass transfer resistance is considered and so on. Lewis number (Le) is defined as the ratio of the thermal diffusivity to the mass diffusivity. It indicates if the heat and mass transfer processes are whether mass transfer limiting (Le ≫ 1), heat transfer limiting (Le ≪ 1), or are much coupled processes (Le ∼ 1). The Lewis number for drying is calculated using the properties that may not be realistic, e.g. that by neglecting the effect of the water sorption capability of the material being dried. Sometimes, quite different conclusions on whether or not drying is limited by heat transfer can result. Here, two new approaches to Lewis number estimation are presented for hygroscopic materials. Sample calculations are made for milk particle drying as an illustration of the principles outlined.