Within that context it becomes possible for proponents of ductless portable fume hoods to demonstrate a positive environmental impact as compared to the alternative use of conventional vented fume hoods. Ductless filtration systems are designed to cleanse and re-circulate the indoor air. Thus eliminating the need for dumping toxic air into the atmosphere along with the associated heated or conditioned air. The benefits are twofold. No pollution is introduced into the atmosphere and energy savings sufficient to generate typical payback periods of less than two years are claimed.

• Chemical Activation
  • This technique is generally used for the activation of peat and wood based raw materials.
  • The raw material is impregnated with a strong dehydrating agent; typically phosphoric acid or zinc chloride mixed into a paste and then heated to temperatures of 500 - 800°C to activate the carbon.
  • The resultant activated carbon is washed, dried and ground to powder.
• Steam Activation
  • This technique s generally used for the activation of coal and coconut shell raw material which is usually processed in a carbonised form.
  • Activation is carried out at temperatures of 800 - 1100°C in the presence of steam.

• Physical Adsorption
  • Physical adsorption is non-specific and adsorption of the gas molecule is by diffusion (Brownian Movement) or adsorption/condensation using Van Der Waals’ forces. 
  • The gas molecules move into an empty area and diffuse into the pore. 
  • Attracted and captured in the space by the Van Der Waals’ force, the molecules penetrate into the pores, impact the walls and are trapped.
  • The number of pores present in the carbon is vast and therefore the total surface area is extremely large. 
  • The specific surface is a measure of the surface area per unit of weight, that is m²/g. 
  • Depending on the carbon is use and the type of filter, aggregate surface area is in the range if 2000m²/g.  This represents a surface area roughly equivalent to about 4 football playing fields.
• Chemisorption
  • The physical process of adsorption is followed by chemical adsorption (chemisorption). 
  • 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.

• Temperature
  • The temperature of the gas is very important because the higher the temperature, the lower will be the adsorption capacity, especially for a gas with a low boiling point. 
  • A relatively high temperature can even generate desorption, with the filter releasing previously adsorbed gas molecules. 
  • Generally temperature must be kept below 40°C
• Humidity
  • Relative humidity is an important factor that can affect the efficiency of the filter. 
  • The molecules of gases with low boiling points will be less adsorbed, because the molecules of the water vapor will be adsorped 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 air speed crossing it must be kept to a minimum. 
  • This minimum is influenced by the fact that the air speed across the front opening of the fume hood (face velocity) must be kept at a rate sufficient to insure 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 efficiency with the higher evaporation and concentration having a negative impact on efficiency.

Warning. Activated carbon filters are not recommended for use

• where very large quantities of contaminates are produced, such as in acid digestions or evaporation of solvents to dryness,
• where very highly toxic substances are in use,
• or where unknown reactions are carried out.

• Checking the airflow to ensure the pre-filter is not clogged with dust
• and checking the exhaust air for chemical contaminants to ensure the main filter has not reached the breakthrough point.

Airflow can be measured with an anemometer. There are a number of anamometer designs available including propeller, hot wire and vane anemometer. In addition, most ductless fume hoods are fitted with a low airflow alarm which indicates a low airflow situation as well as fan failure.

Adsorption takes place in a filter bed in what is known as the active filter zone (represented above as dark saturated area). As the filter is used this active zone progressively moves up the filter bed until it approaches the top surface of the filter. At this point there is an initial breakthrough by the contaminant vapor(s), and thereafter the percentage of contaminant gas that escapes filtration increases.