There is a clay plate with a uniform distribution of water of 14% mass. The plate is 5.6 cm thick and the sides of the plate are insulated from mass transfer. A current of air with low moisture content passes over the exposed faces of the plate and the water begins to desorb. The process it is controlled by the diffusion of moisture within the plate. The average moisture content within the plate after 360 min is 9% mass. The moisture content at equilibrium is 1.9% mass. (a) Derive the differential equation for the diffusion process. Explain the boundary conditions. (b) Find the effective diffusivity of water in the porous solid.