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Activated Carbon Filters

Use of Activated Carbon Filters

Air filters incorporating trays of activated carbon for odor removal are being specified more and more for commercial, institutional and industrial air-handling systems. These filters can be used wherever smoke, fumes, gases and "people odors" may affect personal comfort and health as well as product quality and preservation. Such filters are often installed in convention homes, museums, libraries, entertainment centers, sports arenas, gymnasiums, locker rooms and also in certain areas of hospitals, commercial establishments and light industry.

What Is Activated Carbon?

The odor-removing ingredient in filters is activated carbon. This is prepared from naturally occurring carbons, coconut husks and organic materials from which foreign substances such as hydrocarbons have been removed by distillation and steam treatment. It is used in gas masks, in removing fusel oil from alcohol, and in adsorption of vapors, smoke, fumes and odors.

Carbon filters are usually specified as containing a certain number of pounds of activated carbon for a given CFM. However, it is possible for two similar cells to hold the same volume of carbon and have different adsorptive capacities. Therefore, a specification by weight is not truly meaningful, since it ignores both the density and the adsorption capacity of the carbon.

Adsorption capacity is defmed as the amount of carbon tetrachloride which a given weight of carbon will adsorb.

The procedure for determining the adsorption capacity as a percent of carbon weight is covered by U.S. Government Specification MIL-C-17605B. A "Carbon Tet Number" can be calculated from the carbon tetrachloride adsorption capacity and carbon density - yielding a more accurate measurement of the life expectancy of a carbon cell than a mere specification of carbon weight.

For example, consider a tray containing carbon whose inside dimensions are 12" x 12" x 1". The volume contained in the tray is 1/12 (0.0833) cubic foot. The "Carbon Tet Number" for two different carbons would be calculated as follows:

One pound of extremely porous carbon contains more than 5,000,000 square feet of surface, which will adsorb up to a half-pound of odor. When the carbon in a filter tray approaches saturation, it can ususally be regenerated and used again. (See "Reactivation of Carbon" below.) 

Type of Carbon*	 Density   Wt.in 	Activity%**    Carbon
			 (Lbs./Ft3)	12x12x1 Tray   Tet No.
				
Some Pelletized   24.5	   2.04		    83		1.69
  Granular*** 	  31.5	   2.62		    60		1.57

* Typical specifications for illustrative purposes only

** Per MILC-17605B.

*** Coconut Shell Base

ADsorption

Not absorption, but ... adsorption is the adhesion in extremely thin layers of molecules of gases (odors) to the surfaces of solid bodies (carbon) with which they are in contact (in passing through filters). By this process, activated carbon will remove most odors from the air.

One pound of extremely porous carbon contains more than 5,000,000 square feet of surface, which will adsorb up to a half-pound of odor. When the carbon in a filter tray approaches saturation, it can ususally be regenerated and used again. (See "Reactivation of Carbon" below.)

ABsorption

As mentioned previously, the principal mechanism by which gases (especially those associated with occupancy odors) are removed is adsorption. There are certain gases for which the adsorptive capacity is low. Typical of these gases is hydrogen chloride. In order to improve the ability of carbon to capture these gases, the carbon is treated by impregnating it with other chemicals. These chemicals react with the gases to prevent their desorption. This reaction is called absorption. Eventually all the impregnating chemicals are used up, and new chemical compounds are formed by the reaction of the gas and the chemical. Carbons which have been impregnated cannot be reactivated because the new chemicals which are formed cannot be decomposed back to the original materials. When impregnated carbon is used up, it must be discarded and replaced by new material.

Carbon's Effectiveness on Various Odors

Activated carbon adsorbs virtually all organic vapors and many inorganic compounds. Its capacity to remove odors varies with gas concentration in the air, with humidity and temperature, and with the velocity at which the odorous gas passes through the carbon.

The following table gives the relative capacity of activated carbon for removing most of the common air contaminants and many others found in specific applications. The figures (4 through 1) are based on removal of the substance from dilute concentrations encountered in air recovery and purification applications, assuming air temperature of 100 degrees F or less. The numbers given represent typical or average conditions and have the following meaning:

4. Activated carbon has high capacity for all materials in this category, which includes most odor-causing substances. One pound takes up about 20% to 50% of its own weight (average about 33 1/3%).

3. Activated carbon has satisfactory capacity for items in this category, taking up about 10% to 25% of its weight (average 16.7%).

2. Includes substances not highly adsorbed by activated carbon but which might be taken up sufficiently for good service under particular conditions. In this in-between class, activated carbon can remove some materials but not others (which includes materials like ammonia, amines and sulfur dioxide) depending on circumstances. Check with your contractor in cases involving these materials.

1. Adsorption capacity is low for these materials (like carbon dioxide and carbon monoxide). Activated carbon is unsuitable for removing these except under most unusual circumstances.

In general, a substance which has a boiling point above 0 degress F is adsorbed and the higher the boiling point the more readily it is adsorbed. Conversely, if the boiling point is below 0 degress F, the substance is not readily adsorbed and the lower its boiling point, the less it is adsorbed. Gases in this category usually require a special treatment (depending on the gas) of the carbon for removal.

High humidities usually decrease the carbon's capacity for gas. Exceptions to this are where the gas is soluble in water, or where the gas reacts with water. Carbon readily takes on water vapor, but also readily gives it up when dry air is passed through it.

The higher the operating temperature, the less the efficiency and capacity of carbon for a given substance. Again, this depends upon the boiling point of the substance. In the adsorption process, heat is liberated. In low concentrations this cannot normally be detected, but may become a factor where extremely high concentrations or high temperature operations are contemplated. The efficiency of a carbon bed is dependent upon the amount of time that the gas remains in contact with the carbon on its passage through the bed. With a given carbon granule size and depth of bed, the higher the air velocity, the lower the efficiency. It should be borne in mind, however, that the adsorption process is very rapid and that dwell times or contact times of a ftaction of a second are sufficient for a very high degree of removal. The adsorption sites on carbon are not identical. Just how they vary is not known. They are identified, however, by their selectivity in adsorbing various substances. For instance, an early researcher found that adsorbed carbon dioxide gas could be partly displaced from a typical carbon by hydrogen cyanide, but complete displacement could not be accomplished. Similarly, a partial displacement of the carbon dioxide could be accomplished using carbon tetrachloride. When both carbon tetrachloride and hydrogen cyanide were added, the carbon dioxide was completely displaced. The deduction is that carbon dioxide can be adsorbed by two different types of activation centers, one of which is also able to adsorb carbon tetrachloride, and the other hydrogen cyanide. This example, which indicates different types of active centers, also shows the displacement effect, in which a gas which is preferentially adsorbed can displace another gas which has previously been adsorbed. This is often the case when the carbon is nearing the end of its useful life and the least adsorbed gas or odor is displaced by the more easily adsorbed gases, and the light gas or odor is detected downstream.

The efficiency of the carbon bed remains relatively constant throughout its life; efficiency drops off rather slowly, until near capacity. Then it drops off quickly. When reaching capacity, fluctuations in temperature and humidity can have rather drastic effects upon carbon efficiency.

The values in this table have been assembled from many sources: lab tests, field work and, in some cases, opinion based on experience. There are countless chemical compounds that are objectionable in vapor form, but only a few can be included in any practical list. A compound not listed here is probabiy adsorbed in about the same quantity as a similar or chemically related material that is included below. This table should be used as a general guide only.