EXAMPLE 14.3

 

The efficacy of pharmaceutical products is reduced by prolonged exposure to high tempera- ture, light, and humidity. For water vapor–sensitive consumer products that are in tablet or capsule form, and might be stored in humid environments such as bathroom medicine cabi- nets, blister packaging is used to limit the direct exposure of the medicine to humid condi- tions until immediately before its ingestion.

Consider tablets that are contained in a blister package composed of a flat lidding sheet

and a second, formed sheet that includes troughs to hold each tablet. The formed sheet is

 

L = 50 µm  thick  and  is  fabricated  of  a  polymer  material.  Each  trough  is  of  diameter D = 5 mm  and  depth  h = 3 mm.  The  lidding  sheet  is  fabricated  of  aluminum  foil.  The binary diffusion coefficient for water vapor in the polymer is DAB  = 6 X 10-14  m2/s while the aluminum may be assumed to be impermeable to water vapor. For molar concentrations of  water  vapor  in  the  polymer  at  the  outer  and  inner  surfaces  of  CA,s1  = 4.5 X 10-3 kmol/m3  and CA,s2  = 0.5 X 10-3  kmol/m3, respectively, determine the rate at which water vapor is transferred through the trough wall to the tablet.

Formed sheet

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