How Molecular Filtration Works

 How Molecular Filtration Works

 

What is it?


Molecular filtration is a term used to describe filtration of chemical contaminants having sizes at the molecular scale. While particulate filters, such as HEPA filters, are effective in filtering particles down to the 0.3 micron size range (1 micron is 1/1,000,000 of a meter), they are ineffective against chemical contaminants such as gases and volatile organic compounds (VOCs) present in the vapor phase or as dissolved organics in the liquid phase. Molecular size is measured in Angstroms (1 angstrom is 1/10,000,000,000 of a meter). As an example, the size of a benzene molecule is approximately 6 Angstroms or 0.0006 microns.

Sources of Molecular Contamination


Sources of molecular contamination can exist both inside and outside building environments. Outdoor pollution sources include vehicle exhaust, combustion byproducts from manufacturing processes, emissions from process equipment and chemical supply lines, cross-contamination between manufacturing areas, chemical storage areas, waste management facilities, agricultural and animal farming.
Indoor sources of pollution include human metabolic byproducts, off-gassing from building and construction materials, ozone and fugitive chemical emissions from electronic equipment and housecleaning solvents.

Types of Molecular Contamination

 

Molecular contamination may be classified in four categories each requiring different control strategies. These categories are as follows:

Toxic: A substance is said to be toxic if it exhibits the ability to cause damage to living tissue, impairment of the central nervous system, or in extreme cases, death when ingested, inhaled, or absorbed through the skin.
Corrosive: Those compounds which are likely to cause deterioration or damage to the interior of a building or its contents are considered corrosive. They may also have a detrimental effect on human occupants as well.
Irritant: Chemicals that can be said to cause discomfort, and potentially permanent damage, to an exposed person may be considered irritating. Many of the gases considered to be irritants produce symptoms of pain or discomfort to the eyes, skin, mucous membranes, or respiratory system.
Odorous: Materials that primarily affect the olfactory senses are considered odorous and usually carry negative connotations.

There are essentially three strategies used to control molecular contamination:

Source control: Removal of the contaminant source or control of its emissions
Dilution: Ventilation with clean dilution air
Air cleaning—for either particles or gases, or both

 

Source control for outdoor air contaminants is often neither feasible nor practical; therefore, ventilation control should be the next option. However, this may not prove viable in all cases either as the use of large amounts of outdoor dilution air is neither cost-effective nor energy-efficient. Further, bringing in additional quantities of outside air could result in substituting one group of contaminants for another— those with sources outside the building for those internally generated. In areas with poor outdoor air quality, neither source or ventilation control can prevent the introduction of contaminants into a facility; therefore, air cleaning must be employed. Air cleaning is often used as an alternative to source control and ventilation. A gas-phase air filtration system can effectively reduce the concentration of the contaminant to levels that are at or below the level of detection for the monitoring techniques employed.

 

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.

 

Applications


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.