Physical Adsorption

Common industrial adsorption systems often use activated carbon, zeolite and polymeric adsorbents. Activated carbon, which is neither fully hydrophobic nor hydrophilic, has an affinity for both polar and non-polar molecules. Because of this, humidity has a noticeable effect on activated carbon. Polymers and hydrophobic zeolite are generally much less sensitive to humidity, and much less susceptible to fire, crumbling or powdering. Therefore, they do not need to be replaced as often. However, activated carbon has a lower initial cost.

During adsorption, contaminant molecules from the air stream make contact with, and are retained on, the surface of a solid adsorbent where they are physically taken up at ‘active sites’. This attraction to the solid surface can lead to condensation of the contaminants in the micropores. As the active sites are occupied by adsorbed solvent molecules, the adsorbent becomes progressively exhausted. Contaminant breakthrough occurs once the effective working capacity of the adsorbent is reached. Desorption of the adsorbent to regenerate the active sites is then required.

Adsorption capacity generally increases with:

  • increasing molecular weight/boiling point of the adsorbed compound;
  • reducing polarity;
  • increasing cyclic (rather than straight chain) structure of organic solvent.

As the adsorption capacity increases, the size and cost of a unit needed to achieve a given level of adsorption decreases.

The most common adsorbent is activated carbon. Figure 1 below shows typical adsorption potential of activated carbon for various types of chemicals.

Figure 1. Adsorption Potentialk of Activated Carbon for Various Types of Chemicals.

The major disadvantages of activated carbon are:

  1. Labor cost for periodic carbon replacement;
  2. Cost of carbon replacement is significant;
  3. Inability of the carbon to simultaneously adsorb all the odorous contaminants, rendering it ineffective, in this application;
  4. Carbon is a fire hazard; and
  5. Significant process downtime during carbon replacement.