Diffusion Coefficients
A substance will flow from a region of high concentration to a region of low concentration
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Fick's first law states that substances flows from regions with higher concentrations to regions of lower concentrations. Fick's law can be expressed as
J = -D dφ/dx (1)
where
J = diffusion flux - the amount of substance that flows through an unit area per unit time (mass or mol /m2s)
D = diffusion coefficient (m2/s)
dφ = concentration of substance (mass or mol / m3)
dx = length (m)
Diffusion Coefficients for some common Gases
| Substance | Diffusion Coefficient - D - (m2/s) |
| Water vapor, 0oC | 22 |
| Methane, 1 bar | 20 |
| Ethane | 11 |
| Air | 19 |
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