What is the Difference Between Knudsen and Molecular Diffusion?

The key difference between Knudsen and molecular diffusion lies in the processes involved in the transport of gas molecules through a system.

Knudsen Diffusion:

• Occurs when the scale length of a system is comparable to or smaller than the mean free path of the diffusing gas molecules.
• Named after scientist Martin Knudsen.
• Involves the collision of gas molecules with pore walls more frequently than with each other.
• The molar flux of Knudsen diffusion is given by: $$J{K} = \nabla nD{KA}$$.
• Knudsen diffusivity, $$D_{KA}$$, depends on the pore diameter, species molar mass, and temperature.
• A Knudsen number much greater than one indicates that Knudsen diffusion is important.

Molecular Diffusion:

• Describes the net flux of molecules from a region of high concentration to a low concentration.
• Involves the movement of molecules from one system to another according to their concentration gradient.
• Molecular diffusion is driven by a concentration gradient, which in most cases is embodied as a pressure gradient.

In summary, Knudsen diffusion is a process that occurs when gas molecules collide more frequently with pore walls than with each other, typically in systems with small scale lengths. On the other hand, molecular diffusion involves the movement of molecules from regions of high concentration to low concentration, driven by a concentration gradient.

Comparative Table: Knudsen vs Molecular Diffusion

The main difference between Knudsen and molecular diffusion lies in the mechanism of diffusion. Here is a comparison table highlighting the differences between the two types of diffusion:

In summary, Knudsen diffusion involves the collision of gas molecules with pore walls and is more prevalent in systems where the scale length is smaller than the mean free path of the particles. On the other hand, molecular diffusion describes the net flux of molecules from a region of high concentration to a low concentration, and it is more common in systems with larger scale lengths and less direct dependence on pressure.