A thin slab of solid material dries first by evaporation from the top surface and then by diffusion from the interior of the solid. The water movement through the solid is approximated by the Fick's diffusion equation.
The Fick's diffusion equation involves the two independent variable, time and length. The distance variable can be eliminated by finite differencing the soild into N equal sized layers (N=10 in this model) of length X such that N*X = L (total thickness of the solid).
The boundary condition used for the solid drying are
At X = 0, C0 (surface equilibrium concentration) = 0
At X = L, dC/dX = 0 corresponding to zero flux
Conditional control using the control variables Z and K determines the end of the free surface drying and the start of diffusional drying.
Assumption
The solid is broken into 10 layers here. Thus if the thickness of solid is quite large , the user might consider breaking it into more no. of layers to get a better result and that would imply building in more blocks for drying of the additional layers in Solid Drying compound block.
Reference: J. Ingham, I.J. Dunn, E. Heinzle & J.E. Prenosil, Chemical Engineering Dynamics : Modelling with PC Simulation, VCH Publishers Inc., New York (USA).
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