Knudsen flow describes the movement of fluids with a high Knudsen number

Knudsen number

The Knudsen number is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length scale could be, for example, the radius of a body in a fluid...

, that is, where the characteristic dimension of the flow space is of the same or smaller order of magnitude as the mean free path

Mean free path

In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...

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Separation processes using Knudsen Flow

For a gas passing through small holes in a thin wall in the Knudsen Flow regime, the number of molecules that pass through a hole is proportional to the pressure of the gas and inversely proportional to its molecular weight. It is therefore possible to effect a partial separation of a mixture of gases if the components have different molecular weights. The technique is used to separate isotopic

Isotope

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 mixtures such as uranium

Uranium

Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...

 using porous membranes. It has also been successfully demonstrated for use in hydrogen production

Hydrogen production

Hydrogen production is the family of industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Many other methods are known including electrolysis and thermolysis...

, as a technique for separating hydrogen from the gaseous product mixture created when water is heated at high temperatures using solar or other energy sources.