As you may recall, parameters such as diffusivity are typically found from exper

Question

As you may recall, parameters such as diffusivity are typically found from experiments Consider a simple experiment where a slab 2 cm thick was initially weighed at 250 g It is dried for 1 hour and weighed again to be 200 g. The weight of dry solids was know n to be 70 g The slab is thin enough in one direction compared to the other two directions so that the moisture transfer is along the thin direction only. Also, it is being dried from faces equally, i e, drying is symmetric. The dry air maintains a moisture content of zero at both faces of the slab. Using the weight loss data, calculate the diffusivity of moisture in the slab.

Explanation / Answer

1) (Cav - Cs) / (Ci - Cs)   = 8 * exp( - D * (pi / 2*L)^2 * t ) / pi^2

     L = 0.02 m t = 3600 sec   Cav = 200 g   Cs = 70 g Ci = 250 g

     (200 - 70) / (250 - 70)    = 8 * exp( - D * (pi / 2*0.02)^2 * 3600 ) / pi^2

     D * (pi / 2*0.02)^2 * 3600   = 0.1154

     D = 5.197 * 10^-5 cm^2 / s

2) The diffusivity will reduce as the material dries because diffusivity is proportional to the amoun of moisture in the system.

     D   =

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