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# Air - Diffusion Coefficients of Gases in Excess of Air

## Diffusion coefficients, D12, for gases in large excess of air at temperature varying from 0 - 400 °C

Fick's first law states that a substance will flow from a region with high concentration to a region of low concentration. 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 /(m2 s)]
D = diffusion coefficient [m2/s]
dφ = change in concentration of substance [mass or mol/m3]
dx = chenge in length [m]

For simple one-dimensional transport, the diffusion coefficient describes the time–rate of change of concentration. The diffusion coefficient varies from substance to substance and with temperature and pressure.

Figure and table below shows the diffusion coefficients, D12, for binary gas mixtures where air is in large excess.

 Gases in large excess of air Diffusion coefficient, D12, [cm2/s]at atmospheric pressure and given temperatures 0 °C 20 °C 100 °C 200 °C 300 °C 400 °C Name Formula 32 °F 68 °F 212 °F 392 °F 572 °F 752 °F Argon Ar 0.167 0.189 0.289 0.437 0.612 0.81 Methane CH4 0.21 0.321 0.485 0.678 0.899 Carbon monoxide CO 0.208 0.315 0.475 0.662 0.875 Carbon dioxide CO2 0.16 0.252 0.39 0.549 0.728 Hydrogen H2 0.668 0.756 1.153 1.747 2.444 3.238 Water vapour H2O 0.219 0.242 0.399 0.638 0.873 1.135 Helium He 0.617 0.697 1.057 1.594 2.221 2.933 Sulfur hexafluoride SF6 0.15 0.233 0.329 0.438

## Related Topics

• Air Psychrometrics - The study of moist and humid air - psychrometric charts, Mollier diagrams, air-condition temperatures and absolute and relative humidity and moisture content
• Fluid Mechanics - The study of fluids - liquids and gases. Involves velocity, pressure, density and temperature as functions of space and time
• Gases and Compressed Air - Air, LNG, LPG and other common gas properties, pipeline capacities, sizing of relief valves
• Material Properties - Material properties for gases, fluids and solids - densities, specific heats, viscosities and more

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## Citation

• Engineering ToolBox, (2018). Air - Diffusion Coefficients of Gases in Excess of Air. [online] Available at: https://www.engineeringtoolbox.com/air-diffusion-coefficient-gas-mixture-temperature-d_2010.html [Accessed Day Mo. Year].

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