Prediction of the residence time required for drying a product from an initial moisture content Y₀ to some final moisture content Y_e is based on the normalized drying rate function

\[\label{eq1} \dot{M}_{v}=\dot{M}_{v , I}(T_{\infty}, X_{\infty}, \beta_{g})\dot{\nu}(\eta) \tag{1}\]

where Ṁ_v,I is the drying rate in the constant rate period (regime I in Figure 333.1) and depends on the external mass transfer coefficient β_g and is to be predicted by Equation 333.1 or (Equation 333.1.a, Equation 333.1.b), respectively, whereas \(\dot{\nu}\)(η) usually follows from laboratory-scale experiments.

The basic rate Equation 1 is combined with the respective conservation law for the evaporating liquid (e.g., water),

\[\label{eq2} \dot{M}_{v}A=-M_{s}\frac{\partial Y}{\partial t} \tag{2}\]