How Molecular Filtration Works

Activated Carbon Adsorption

One of the most effective means to eliminate organic compounds and gases is by the process of adsorption. Gas phase filters capture contaminants by physical adsorption or chemisorption. Physical adsorption or physio-sorption results from the intermolecular attraction (VanderWaals forces) of gas or vapor molecules to a surface. Due to the relatively weak forces involved, physical adsorption is (essentially) totally reversible. Adsorption is regarded as a surface phenomenon, whereby the removal capacity of a specific adsorbent is directly related to its total surface area.

Activated carbon is considered as the universal adsorbent to remove gaseous organic contaminants. The main attributes of activated carbon are an extremely high surface area (upto 1400 square meters per gram), a highly irregular pores structure with pore sizes ranging from 5 to 500 Angstroms and a non-polar chemistry which enables adsorption of a wide range of chemicals preferentially to moisture. Activated carbon is derived from a variety of raw materials such as coal, coke, wood and coconut shells, each imparting its own unique characteristic making them suitable for specific applications. Amongst these, coconut shell based carbon is considered to be most suitable for gas phase applications due to high surface area, highest content of micropores and low ash content resulting in high removal activity levels. Activated carbon works well under a wide range of temperature and humidity conditions, is inert, safe to handle and easily available in various particle (mesh) sizes.



Chemisorption occurs when gas or vapor molecules chemically react with adsorbent material or with reactive agents impregnated into the adsorbent. These impregnates react irreversibly with gases and form stable chemical compounds that are bound to the media as organic or inorganic salts, or are broken down and released into the air as carbon dioxide, water vapor, or some material more readily adsorbed by other adsorbents. Many different chemicals may be impregnated on activated carbon; potassium permanganate is a common chemisorbent, as it reacts with many common air pollutants, including formaldehyde and sulfur and nitrogen oxides.

There are many factors affecting the removal of gaseous contaminants. The rate of adsorption depends on the rate at which adsorbate molecules reach the surface of the adsorbent, the percent of those making contact that are adsorbed, and the rate of desorption. Other significant factors include the type of adsorbents, adsorbent particle (mesh) size and bed depth, resistance to airflow, air velocity, concentration and characteristics of the contaminant(s) in the airstream, and the temperature and relative humidity of the air-stream.


Gas phase filtration utilizing activated carbon adsorption is considered one of the most effective and economical for HVAC applications, where the concentrations are very low and the contaminant loading varies constantly. A wide range of filter formats and sizes are used ranging from refillable loose fill deep-bed systems to disposable bonded filter designs.