The drying rate –dY/dt depends primarily on the moisture content Y of the product. Figure 1 shows a typical drying rate function –(M_s/A)(dY/dt = Ṁ_v = f(Y) for constant drying conditions such as temperature T_∞, humidity X_∞, pressure P, and mass transfer coefficient β_g between the surface of the product and the drying agent (air). M_s is the amount of dry product, and A is the interfacial area. Starting the drying process from the initial moisture content Y₀, the drying rate remains constant until the so-called critical moisture content Y_cc is reached. Within this “constant rate period”-regime I-the liquid evaporates directly at the external surface area A of the product. The capillary suction of the liquid-filled pores is strong enough to replace the liquid being evaporated at the surface. The temperature of the product in the steady state is identical with the wet-bulb temperature T_WB as derived in the aforegoing section.

Figure 1 Drying rate function of a porous slab

At the critical moisture content Y_cc, the moisture level starts to retreat. The capillary forces are no longer sufficient to transport the liquid to the surface. Thus a layer of dry material forms with additional heat and mass transfer resistances, which lowers the drying rate in regime II.

At low moisture contents a second critical moisture content Y_ch emerges due to sorptive bonds of the product, which lower the vapor pressure of the liquid. This additional effect brings the drying rate down to zero at the hygroscopic equilibrium moisture content Y_h∞[T_∞, 𝜑_∞(X_∞), P ], which depends on the temperature T_∞, the humidity 𝜑_∞(X_∞), and the pressure P of the drying agent according to the so-called sorption isotherms as shown in Figure 2 [𝜑_∞ = p_v∞/p^*_v∞ (T_∞) = x̃_∞/x̃_∞^* (T_∞) = relative humidity at T_∞]. Regime III is called the hygroscopic one. As the hygroscopic bonds vanish, one observes an apparent terminal drying rate, provided that the sample is a slab (e.g., not a sphere).

Figure 2 Sorption isotherms for various materials at 20 °C