Fume Hoods, Laminar Flow, PCR

How Do Activated Carbon Filters Capture Vapors?

The filtration of gas molecules is based on the concept of adsorption. This process involves the capture and retention of chemical vapors on the surface of the carbon filter media and primarily occurs through two complementary mechanisms: Physical Adsorption and Chemisorption.

Physical Adsorption
Physical adsorption is a non-specific process driven by physical forces. Adsorption of the gas molecule is by diffusion or adsorption/condensation using Van Der Waals’ forces.
- Gas molecules move into the empty pores of the activated carbon media by diffusion (Brownian Movement).
- The molecules are attracted and captured within the space by Van Der Waals’ forces.
- Because the number of pores present in the carbon is vast, the total surface area is extremely large. The aggregate surface area can be up to 2,000 m²/g, which is roughly equivalent to four football fields.
Chemisorption
Chemisorption follows the physical adsorption process.
- This is a chemical reaction in which the two substances react together and the resultant chemical is trapped on the filter material.
- The impregnation of filter media can greatly extend the range of gases that can be removed from the air stream.

Factors Affecting Adsorption Efficiency

The ability of a filter to function efficiently depends on a number of factors, including temperature, humidity, residence time, filter age, evaporation rate and chemical concentration.

Temperature
- The higher the temperature of the gas, the lower will be the adsorption capacity. This is especially true for gases with a low boiling point.
- A relatively high temperature can even generate desorption, with the filter releasing previously adsorbed gas molecules. Generally, the temperature must be kept below 40°C.
Humidity
- The relative humidity is an important factor that can affect the efficiency of the filter.
- Molecules of gases with low boiling points will be less adsorbed, because the molecules of the water vapor will be adsorbed in their place, leaving less free surface in the pores for the gas molecules to impact the carbon.
- Generally, relative humidity must always be kept below 60%.
Residence Time
- Residence time is the time needed by the air to cross the filter, during which it stays in contact with the carbon itself.
- This is the time during which gas molecules can be adsorbed by the pores of the carbon before leaving it. This time should be as long as possible.
- To maximize time, filter thickness must be sufficient and the airspeed crossing must be kept to a minimum.
- The airspeed across the front opening of the fume hood (face velocity) must be kept at a rate sufficient to ensure the containment of the fumes within the enclosure.
Filter Age
- Normally the filter, if not properly stored before use, becomes less efficient with time because of “poisoning” of the charcoal due to the presence of gasses in the atmosphere, as well as the effect of humidity in the environment.
Evaporation Rate and Concentration
- The rate at which the chemical is being evaporated and the concentration of chemical vapors within the enclosure will likewise impact the efficiency with the higher evaporation and concentration having a negative impact on efficiency.

Applications Not Recommended

Activated carbon filters are not recommended for use where:

Very large quantities of contaminants are produced, such as in acid digestions or evaporation of solvents to dryness.
Highly toxic substances are in use.
Unknown reactions are carried out.

WARNING

Failure to check chemical handling and failure to replace filters regularly can result in the recirculation of harmful vapors into the air.

CHEMICAL GUIDE

Use this guide to identify which ductless filtration solution is best suited for your chemicals and applications.

Enhanced Filtration Technology™ (EFT), first introduced in 2010, is an innovation in ductless fume hood applications associated with activated carbon filtration and combinations with HEPA/ULPA filters.

Multiplex^™ Filtration System consists of a pre-filter, main filter and optional safety filter to create a combination of chemical and physical architecture customized to each application.