Functional principles of adsorption

The word “adsorption” comes from the Latin and means: to gather (a gas, liquid, or dissolved substance) on the surface of solid bodies in a condensed layer.

The optimal selection of the adsorbent (the adsorbing agent) will depend on the physical and the chemical characteristics of the substance to be adsorbed. It is necessary to consider the following factors for assurance of obtaining the optimal adsorbent (all factors pertain to the substance to be adsorbed):Application of substances and techniques such as the following have proved highly effective in reduction of gaseous pollutant concentrations:

  • Boiling point and/or the boiling range
  • Molecular weight
  • Molecular structure (i.e., straight-chain or branched-chain)
  • Chemical reactivity
  • Chemical stability
  • Wetting ability
  • Explosion limits
  • Solubility in water.

Application of substances and techniques such as the following have proved highly effective in reduction of gaseous pollutant concentrations:

  • Reduction of concentration by the admixture of air
  • Activated carbon
  • Granules with catalytic converters
  • Gas scrubbers
  • Biofilters
  • Catalytic combustion.

Activated carbon:
In this context we shall restrict our description to work with activated carbon as adsorbent. The characteristic of activated carbon that is of primarily significance here is its large surface area. One gram of activated carbon has a surface area of 800 … 2000 m². In comparison, a fibre with a mass of one gram, a diameter of 1 µm, and a length of 750,000 m has a surface area of only 2.3 m². The effective gathering of the adsorbed agent onto this large surface will depend on several processes, such as the following:

  • Diffusion
  • Adsorption
  • Capillary condensation
  • Chemical sorption
  • Ion exchange
  • Hydrolytic molecular cleavage
  • Isotope exchange.

Which of the processes of separation will play the predominant role in separation will depend on the gases to be filtered, their concentration, the pressure prevailing, and the temperature. The following basic rule applies: the smaller the molecular size and the smaller the gas concentration, the smaller the pores should be. Large, long-chained, and perhaps branched molecules require a greater share of transport pores, as well as larger micropores.

The most important separation processes are:

If the pollutant gas flows into the activated-carbon layer, the molecules of this gas can adsorb onto the inner surfaces of the activated carbon, as a result of the effects of van der Waal's forces.

Schematic representation of an adsorbent grain

Capillary condensation:
If the temperature falls below the critical temperature of the adsorbate (the pollutant gas that is adsorbed), then the gas will condense. The adsorbate, now in liquid state, will fill the pores of the activated carbon. This procedure quantitatively enhances the adsorption capacity of the activated carbon.

In the case of odours to be filtered, the design of the activated-carbon filter will generally depend on the so-called primary gases. This means that – of all the many gases found in the mixture – design efforts are concentrated on those substances that are the most odour-intensive, that are characteristic for the gas mixture, that ideally make up the main components of the gas mixture, and that can be detected on a specifically analytical basis and to a sufficiently exact extent.

In the case of toxic gases, on the other hand, an exact investigation of the entire spectrum of the existing gases must initially take place: because all components of the mixture must by all means be separated. Once the various substances have been defined, design will be based on that substance that requires the longest contact period (also see Filter media / Adsorbent filters / Line of activated-carbon products).

Definition of terms:

Adsorbent (n.)
The adsorbent is the medium by which the pollutant gases are adsorbed (e.g., activated carbon, silica gel, etc.)

Adsorptive (n.)
The adsorptive is the pollutant gas still found in the air, which must yet be separated.

Adsorbate (n.)
The adsorbate is the gas that has been adsorbed on the adsorbent.

Desorbate (n.)
The desorbate is the gas that is released from the activate carbon by the process of desorption (e.g., by application of steam).