Gas permeation

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The gas permeation (Latin permeare - to penetrate) is a process for the separation of gas mixtures . In the case of permeation , a substance ( permeate ) penetrates a solid.

functionality

The gas to be separated is fed into a gas separation module with excess pressure . In this, the substances to be separated migrate through a selective membrane due to the pressure difference . This separates the raw gas flow into a purified permeate (the clean gas) and the concentrating retentate (a rich gas).

The transport processes taking place during gas permeation are a result of the structure of the separating membrane and the pressure and concentration conditions that occur.

The membrane has a multi-layer structure:

  • Solvent-selective , pore-free polymer film (thickness <0.5 pm)
  • microporous support membrane: high porosity , no separation effect for gases
  • permeable fleece

A gas molecule migrates through the membrane by dissolving in the polymer film and then diffusing to the low-pressure side due to the driving force of the concentration gradient. The process takes place in three steps:

  1. Absorption on the pressure side ( Henry's law )
  2. Diffusion through the membrane ( Fick's law )
  3. Desorption on the vacuum side (Henry's law)

application

Gas permeation is mainly used to purify technical gases , fuel , synthesis and exhaust gases .

Exhaust gas cleaning

The emission values prescribed by law can usually not be achieved with gas permeation alone . It is therefore used in combination with other methods. The combination with pressure swing adsorption is particularly important, since the pressure generated during gas permeation or the vacuum is used for adsorption and desorption. Adsorption and desorption take place alternately. Adsorption under the overpressure conditions on the pressure side, desorption under the underpressure conditions on the vacuum side.

Biogas processing

Gas permeation is also increasingly being used for processing biogas . Biogas is raised to natural gas quality by processing with gas permeation and can then be fed into the natural gas network. A corresponding system has been in place in Bruck an der Leitha since early 2007.