Pressure swing adsorption
Pressure swing adsorption ( PSA - Pressure Swing Adsorption ) is a physical method of separation of gas mixtures under pressure by means of adsorption .
Special porous materials (e.g. zeolites , activated carbon ) are used as adsorbents . The separation effect can be based on two different principles: separation due to equilibrium adsorption or separation due to molecular sieve action . In the first case, one of the components to be separated is more strongly adsorbed than another, which means that the poorly adsorbed component is enriched in the gas phase. In the second case, certain molecules penetrate the porous structure of the adsorbent more quickly. If the gas mixture flows through the adsorbent in a reactor bed, the component that penetrates the pores more poorly needs less time to flow past, i.e. it is more likely to reach the reactor bed exit.
Litigation
The gas is introduced under increased pressure (usually 6 bar to 10 bar) into a fixed-bed reactor which is filled with the adsorbent, so that it is flowed through. One or more components of the mixture (these are called "heavy components") will now be adsorbed. At the exit of the bed, the so-called “light component” can be taken in concentrated form. After a while, the adsorber bed is largely saturated and part of the heavy component also escapes. At this moment, the process is switched over via valves so that the outlet for the light component is closed and an outlet for the heavy component is opened. This is accompanied by a drop in pressure. At the low pressure, the adsorbed gas is now desorbed again and can be recovered at the outlet. Two adsorbers that are alternately charged and discharged enable continuous operation. In order to drive the supernatant of the desorbed heavy component out of the adsorber bed, it is rinsed with a portion of the desired product in order to avoid contamination.
The exact adjustment of the switching times takes place according to the desired purity of the gases. In the case of one component, their increase is always at the expense of their recoverable amount and the purity of the other component.
If you work at pressures below atmospheric pressure, the method is also known as VSA (Vacuum Swing Adsorption). If the pressure is partly above atmospheric pressure and partly below it, this is called VPSA (Vacuum Pressure Swing Adsorption). Except for the pressure range used and the precautions required thereby, these procedures are all identical.
The main cost factor in pressure swing adsorption is the provision of mechanical energy . That is why it is preferred for material systems with low adsorption enthalpies. The cycle times are usually only a few minutes.
Applications
Pressure swing adsorption is used when the desorption energy is low and the concentration is high. It is used to break down air to extract N 2 (> 99.9%), O 2 (<97%) or argon as well as extract / purify hydrogen e.g. B. used for fuel cells . Another application is the removal of CO 2 from biogas or the removal of water from compressed air.
Individual evidence
- ↑ VDI 3674: 2013-04 Exhaust gas cleaning through adsorption; Process gas and waste gas cleaning (Waste gas cleaning by adsorption; Process gas and waste gas cleaning). Beuth Verlag, Berlin, p. 19.
- ↑ a b Jürgen Klein: Regeneration of adsorbents. In: Dust - cleanliness. Air . 36, No. 7, 1976, ISSN 0949-8036 , pp. 292-297.
- ↑ VDI 3896: 2015-10 Emission Reduction; Preparation of biogas to natural gas quality (Emission Control; Preparation of biogas to natural gas quality). Beuth Verlag, Berlin, pp. 15-17.