Movement of Water during Drying

It is important to understand how the moisture moves to the drying surface during the falling-rate period, and two models have been used to describe the physical nature of this process, the diffusion theory and the capillary theory. In the diffusion theory, the rate of movement of water to the air interface is governed by rate equations similar to those for heat transfer, whilst in the capillary theory the forces controlling the movement of water are capillary in origin, arising from the minute pore spaces between the individual particles. 

Since diffusion theory is applicable to non-porous solids, the following assumptions are made to solve the equation. 

For the falling-rate period the initial moisture content is equal to the critical moisture content and at the surface the moisture content is equal to equilibrium moisture content. The following boundary conditions result for a solid slab with a thickness equal to 21 

Using these boundary conditions, and solving Equation 11.16 results in  

For long drying times, when Dt/l exceeds approximately 0.1, only the first term can be considered in the above equation. Differentiating this term results in the following drying rate. 

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