Adsorbents

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Two understand the role that the adsorbent beds play in the Pressure Swing Adsorption process first we need to know what is Adsorption. Adsorption is the process that happens when a gas or liquid solute gets accumulated on the surface of a solid or, sometimes, a liquid (adsorbent), forming in the process a molecular or atomic film (known as adsorbate). The term adsorption refers and encompasses both processes, while the reverse process is known as desorption.

To understand how the molecular sieves of the adsorbent beds works by taking the example of oxygen concentration when air is passed through a PSA process. The efficiency of the whole process depends on conditions of high pressure, low temperature, and higher concentrations of the gases that is to be adsorbed. As can be seen from the figure as air is allowed to flow through a column (or bed) of molecular sieve, the component gases it contains gets adsorbed and stratified in the degree of their relative affinity to the material used in the molecular sieve.

The process continues till next to the last gas component stratifies at the end of the bed. After using the full bed length, the bed must be again regenerated by desorbing (or purging) the adsorbed gases. Purging is done by a method of reducing the pressure in the bed and back-flushing with some quantity of the concentrated gas product. Adsorption and desorption processes are completely reversible and can be carried out without end. To ensure that molecular sieve beds never "wear out" or for that matter become "clogged,'' a proper cycle of adsorption and desorption must be followed through.

Features of Adsorbent beds:
It has been found that for two adsorbents that have similar nitrogen-to-oxygen, selectivity but different Nitrogen and Oxygen capacities, placing the low-capacity adsorbent at the feed end and the high-capacity adsorbent at the product end of the adsorption bed gives certainly a better performance.

• The adsorbents has a very strong discriminating power for different gases.
• Typically the adsorbent material used as a bed for PSA systems are very porous.

PSA Technology – Adsorbent Beds:
Some of the typical material generally used in the adsorbent bed are the following:

• Activated Carbon
• Carbon Molecular Sieve
• Zeolitic Molecular Sieve
• Silica Gel
• Alumina