Adsorbents

Features

Material Used

Uses of it

Selection of it

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

The following table highlights the use of different adsorbents in gas separation processes:

Process	Gas Mixture	Adsorbent
Gas Bulk Separation	Acetone/Vent streams Acethylene/Vent streams Normal Paraffins/Iso-paraffins, Aromatics N2/O2 CO, CH4, CO2, N2, Ar, NH3, H2	Activated Carbon Activated Carbon Zeolite Zeolite, CMS * Activated Carbon
Gas Purification	Organics/Vent streams Odors/Air H2O/Olefin-containing cracked glass Natural gas, Air, Synthesis Gas, etc. CO2/C2H4, Natural Gas, Hydrogen, LPG NOx/N2 SO2/Vent streams Hg/Chlor-alkali cell gas effluent	Activated Carbon Activated Carbon Silica, Alumina Zeolite Zeolite Zeolite Zeolite Zeolite
* CMS: carbon molecular sieve

Selection of proper Adsorbents in a PSA process:

• Mass Transfer Rates: Preferred adsorbents that is used in PSA separation processes should be able to exhibit quick mass transfer rates. This is specially important when cycle times are shorter and gas flow rates are relatively higher.
• Equilbrium: The adsorbent beds must have high equilibrium selectivity.
• Capacity: In selecting the appropriate adsorbent for a PSA process high working adsorption capacity is a must.
• Purge Gas requirement: Requirement of purge gas must be very low.
• Shows mechanical properties such as:
  • Higher crush strength
  • Lower attrition
  • Lower dust
  • Higher stability against aging