Diffusion of Gases through Porous Solids

The permeation process considered in the previous section was somewhat unusual. It required the prior dissolution of the gas in the solid matrix before it could make its way through the medium by a process of solid phase Fickian diffusion. A more conventional permeation process occurs when the solid involved is porous or is composed of loosely packed particles. Here the permeating gas enters the solid through the pore openings and continues its way through the porous passages, all the while remaining in the gas phase. Diffusion is strictly Fickian and no penetration of the solid matrix per se takes place. 

Diffusion in a porous solid (a) straight-through pores (b) tortuous pores with branching (c) molecular diffusion (d) Knudsen diffusion. 

Mass Transfer and Separation Processes Principles and Applications 

Illustration 3.6 Transpiration of Water from Leaves Photosynthesis and Its Implications for Global Warming 

The principal transport processes in a leaf, apart from the conveyance of nutrients, are the uptake of carbon dioxide from the air and the release of oxygen and water vapor to the atmosphere. The latter process is referred to as transpiration. 

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