What is the diffusion mechanism

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Diffusion processes in the pores

Various diffusion mechanisms can take place in the pores:

  • Free molecular diffusion / normal diffusion
  • Knudsen diffusion
  • Configurational diffusion

Which of the three diffusion mechanisms dominates depends on the pore diameter and the gas pressure - i.e. on the free path of the individual molecules.

Normal diffusion

In large pores, the molecules will seldom hit the wall. There is free molecular gas diffusion, which can be described with the binary gas diffusion coefficient.

Since the structure of the porous catalyst is not taken into account in this diffusion coefficient, the effective diffusion coefficient became

introduced. The porosity factor reflects the area proportion of the pore openings on the outer surface, the labyrinth factor the internal cross-linking.

Knudsen diffusion

If the pore size is smaller than the mean free path of the molecules, these will pass their momentum on to the pore wall more often than to other molecules in the pore structure. This area is called the Knudsen diffusion. The associated effective diffusion coefficient is calculated as follows:

Configurational diffusion

If the pore diameter assumes the order of magnitude of a molecule, the mass transport is described by the configurational diffusion. A typical example of this is the transport of substances within the pore structure of a zeolite.

Small changes to the structure of the solid can shift the value of the diffusion coefficient by several powers of ten. Slight structural changes in the diffusing species can cause the same effect (cf. diffusion ofiso- andn-Alkanes in zeolite A).

It is not yet possible to estimate the speed of the configurable diffusion in advance.