Drying process second falling rate period

As drying proceeds, the point is reached where the evaporating surface is unsaturated. The point of evaporation moves into the solid, and the dry process enters the second falling-rate period. The drying rate is now governed by the rate of internal moisture movement the influence of external variables diminishes. This period usually predominates in determining the overall drying time to lower moisture content. 

When drying continues the liquid film near the surface first breaks up into isolated pockets (pendular state) and finally this state spreads over the complete thickness of the gel. This situation is called the second falling rate period (FRP2) where evaporation takes place inside the gel body and the principal transport process is expected to be Knudsen diffusion of vapour. 

As shown in Fig. 5.17, the drying process can be divided into two major stages (1) a constant rate period (CRP) where the evaporation rate is nearly constant and (2) a falling rate period (FRP) where the evaporation rate decreases with time or the amount of liquid remaining in the body. In some materials it is possible to further separate the FRP into two parts. In the first falling rate period (FRPl), the evaporation rate decreases approximately linearly with time, while in the second falling rate period (FRP2), the rate decreases in a curvilinear manner. 

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