Falling-rate period of drying

In the falling-rate period of drying, the area for mass transfer is smaller than the area for heat transfer since solid material will account for some of the area. The three basic relations are... 

Figure 12-13 shows the transition between constant- and falling-rate periods of drying occurring at the critical point. The critical point refers to the average moisture content of a material at this transition. 

Figure 4.3 Phenomena of transport involved in (a) the constant-rate period of drying and (b) the falling-rate period of drying. The solvent migrates from the left to the right of the figures. Microconvection is not mentioned in (b) for the sake of clarity.

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