The standard for filtering VOC’s and odors —that is, molecular filtration—is a packed bed of activated, impregnated carbon granules. A carbon filter employs two different processes to remove molecules from an air stream—physical adsorption and chemical reaction.
Physical adsorption, the trapping of molecules in the micropores of the carbon, is effective against chemicals of low vapor pressure. Activated carbon is a very effective sorbent because its pore sizes vary and have a large surface area. Typically the pores in activated carbon have effective surface areas of about 1,200 square meters per gram. Each piece of carbon is designed to provide a large section of surface area, in order to allow contaminants the most possible exposure to the filter media. One pound (454g) of activated carbon contains a surface area of approximately 100 acres. The carbon in a single 2000 cubic-ft-per-minute military filter provides more than 5 square miles of active surface area.
Physical adsorption works well only against large molecules. As a rule of thumb, compounds of vapor pressure less than 10 mm Hg at the temperature of the filter bed are strongly adsorbed and retained in the pores of the carbon. To filter chemicals of higher vapor pressure, carbon is impregnated with salts of copper, zinc, silver, molybdenum, and triethylene diamine (TEDA), which react to form products that are innocuous or that can be physically adsorbed.
Carbon filters used for collective protection are designed for high efficiency and capacity. Efficiency is the percentage of agent removed in a single pass, and capacity is the quantity of agent a filter can remove before it ceases to filter at the specified efficiency of 99.999%. A standard carbon filter can be expected to physically adsorb about 20% of its weight in agent or to remove about 5 to 10% of its weight in reactive gas.
The service life of an impregnated carbon filter is defined by both its reactive gas life and physical adsorption life. Filter life is site-specific, as both capacities are affected by the environmental air quality. Physical adsorption life is reduced by adsorption of air pollutants. Capacity for reactive gases diminishes gradually over time and is typically lost within about three to four years of exposure to humidity. The rate at which the reactive life degrades varies with the temperature and amount of water adsorbed by the carbon bed, and degradation begins once a filter is opened to the atmosphere.