One of the oldest membrane materials used for dialysis is porous cellophane, a thin, transparent sheet made of regenerated cellulose. Typical values of parameters for commercial cellophane membranes are as follows: thickness = 80 pm, porosity = 0.45, tortuosity = 5.0, and pore diameter = 40 8, (Seader and Henley, 2006).
(a) Estimate the effective diffusivity of urea through this membrane at 298 K. The diffusivity of urea in dilute aqueous solution at this temperature is 1.38 x 10-5 x cm2 . The molecular diameter of urea is 5.28 A.
(b) The artificial kidney of uses a cellophane membrane. If the urea concentration difference across the membrane is 25 mg/dL, estimate the membrane area required.
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