A question that is often asked – “What is the difference between ABSORPTION and ADSORPTION?” ABSORPTION can be understood if we think of sugar being dissolved in water and mixing evenly throughout, or cream being mixed into coffee. Industrial absorption would be a gas being ABSORBED (taken into and mixing evenly) by a “scrubbing” liquid. In contrast, ADSORPTION is the physical attraction and adherence of gas or liquid molecules to the surface of a solid. The attractive force is very small, van der Waal’s forces, and exists between any two bodies, such as between the earth and the moon. Gas molecules are ADSORBED by activated carbon.

Activated carbon is truly a unique material. There are no other materials, natural or man-made, that will do all that it will do. Activated carbon:

1. Has a capacity for virtually any vapor contaminant; it will adsorb “some of almost any vapor”.
2. Has a large capacity for organic molecules, especially solvents.
3. Will adsorb and retain a wide variety of chemicals at the same time.
4. Has an extremely large capacity to catalytically destroy ozone, a major component of smog.
5. Works well under a wide range of temperature and humidity conditions.
6. Adsorbs odors and chemicals preferentially to moisture. It is not a desiccant and will release moisture to adsorb chemicals.
7. Can be used as a carrier of one material to attract and hold or react with another material.
8. Is inert and safe to handle and use.
9. Is available and affordable.

It is a material that has been treated (activated) to increase the internal surface area of the structure to the range of 950 to 1150 square meters per gram for gas phase applications. The internal area is the area that holds the adsorbed chemicals, in effect, this is where the “work” is done.

Using the number 1000 square meters per gram, which equals 1197 square yards, and multiplying 1197 by 454 (454 grams/lb.), results in 543,438 square yards of available surface area per pound of carbon. Comparing this to a football field, which is 50 by 100 yards, or 5000 square yards, this would be the surface area of more than 100 football fields. When the 543,438 sq. yards is multiplied by 9 (9 sq. ft/sq. yd.), it equals almost 5 million square feet of available surface area per pound of carbon. Utilizing a maximum working capacity of 33%, up to 1,650,000 square feet would be available for adsorption.

Consider the gas molecule benzene, which has a molecular weight of 78 and is approximately 6 x 10 –4 microns in diameter. Utilizing one pound (454 grams) of carbon and a maximum capacity of 33%, then 150 grams of a gas, (in this case, benzene) can be adsorbed. Dividing 150 grams by 78 gram moles of benzene yields 1.9 gram moles of benzene that can be adsorbed by one pound of carbon. Multiplying the Avogadro number*, 6.023 x 1023, by 1.9 yields 11.44 x 1023 molecules. Therefore, 1 pound of carbon would have enough internal surface area to adsorb (hold) 11.44 x 1023 molecules the size of benzene.

The national debt is over seven trillion dollars, ($7,000,000,000,000). Two trillion times the national debt would be an approximation of 11.44 x 1023. Simply put, whether the working capacity for a specific application is 33%, 5%, or 2% – this is a huge number of gas molecules! *The Avogadro number states that the number of molecules in a gram mole of a gas is 6.023 x 1023.

There are many types and grades of carbon that can be used for adsorption. There are carbons used for liquid and gas phase applications; they are not interchangeable. For the purpose of this presentation, only gas phase carbons that are used for general HVAC applications will be addressed. The commercial quality HVAC carbons that are being manufactured today are produced from either coal or coconut shells. These materials are interchangeable, as long as the activity level and the average particle size are the same.

The term CHARCOAL is used widely, and the question often asked “What is the difference between activated carbon and activated charcoal?” There is only one material, and the terms are used interchangeably. Normally, those who are in the carbon business will use the term carbon. Those outside the industry, or just learning, will use the term charcoal. ASTM – The American Society of Testing Materials – utilizes the term carbon.

Activated carbon ADSORBS a wide variety of gases and vapors – chemical pollutants. Whether there is one gas molecule and one carbon particle, or many of each, the adsorption process is the same. The physical process begins with a gas molecule coming into contact with the surface of an activated carbon particle and coming to rest in a large surface pore. Then, due to unbalanced forces on and within the carbon particle, the gas molecule will begin to move “down” into the carbon particle – into the smaller pores, where it will finally stop and be held in place. At some point between the surface and the “stopping point”, this gas molecule will condense and become a liquid particle. For those who would like a more technical version: The adsorbate diffuses thru the surface film to the macropore structure. Then, due to van der Walls’ forces, the gas molecule migrates into the micropore structure, condensing during this movement, and finally stopping when either the forces become balanced or it becomes physically blocked. This molecule, which was an objectionable gas, will remain a liquid inside the carbon until it receives enough energy, in the form of heat, to excite it. If this condition arises, the molecule will begin moving toward the surface. If enough energy (heat) is absorbed, it will be vaporized, returned to a gas and be released back into the air stream, i.e. the process will be reversed. In HVAC applications, there is normally not enough heat to excite, or re-energize, adsorbed molecules. With regular carbon filter or bulk carbon “changeouts”, sufficient capacity will always be available. It is interesting to note, the adsorbed gases that condense and become liquid molecules will “line” the internal surface area – and this “lining” will be one (and only one) molecule thick.

As a reminder, this presentation deals only with the removal of gaseous and vapor contaminants. The application of carbon to HVAC installations is considerably different when compared to how carbon is applied to solvent recovery systems and other industrial removal applications. For the latter, air streams with hundred to thousands of parts per million (ppm) of gas molecules are recovered, or removed, with beds that can be several feet thick and contain thousands of pounds of carbon. For HVAC applications, thin bed filters with comparatively small amounts of carbon are utilized. Therefore, it is not only important that these differences be recognized, but that the parameters be defined. First, air in a structure can be treated as it:

1. Enters.
2. Leaves.
3. Is re-circulated.
4. Or a combination of the above.

For the majority of HVAC applications, the air to be treated is a combination of air entering (make-up) and air that is re-circulating. The gaseous contaminant loading that should be removed:

1. Is always a combination of many odors and pollutants – chemicals.
2. Is normally unknown either as to the chemical make-up, the amount of each pollutant, or the total loading.
3. Varies constantly with changes in occupancy levels, activity within or without the structure, and shifts in the wind.
4. Is in the 1 ppm range.

In four above, the “1 ppm range” has been used. When the loading drops below 1 ppm, down to the ppb [parts per billion] or to the .1 ppb range and below, there is no concern. When the loading increases, which it can do for short periods, there are concerns and questions. Knowing that the industry has utilized, and continues to utilize, thin bed carbon filters, the following situations must be considered: How high are these higher loadings (peaks), and how long do they last? Answer:Unless the structure has been studied over a period of time, there is no definite answer. Ten to 20 ppm would be a heavy overload and may be within reason for this application. This condition may last for minutes to hours as opposed to days. During the peaks, how effective is the filtration system? Answer:The percentage of these peaks, which exceeds the design capability of the filter or the capacity of the carbon at that time, will pass thru and into the inside air. Since the air is being re-circulated, these peaks will be adsorbed on future passes. As stated earlier, carbon must be changed on a regular basis. This regularity, which should be established during the first year of operation by utilizing test samples, will insure that these peaks, typical to that installation, will be quickly adsorbed.

The old ASHRAE specification of 45 pounds per 1000 cfm per year has been used for many years as the design guideline. In all probability, the vast majority, if not all, of the older carbon V-banks that are being utilized in airports, sports stadiums, and commercial buildings were designed with these numbers. When this amount of carbon is changed regularly, these filters perform effectively. Is this the optimum amount of carbon? From experience with many installations, the answer would certainly be “This has proven to be enough carbon.” A remaining question is “Would somewhat less carbon be as effective; and if so, how much less?” The older V-banks were “designed” to be changed out in approximately one [1] year. This time would be adjusted, based on the specifics of the application. Obviously, utilizing less carbon per filter, but changing more frequently, would produce similar results. For the newer “clean room” type applications, which may call for the removal contaminants in the ppb or less range, filters with significantly less carbon are being used successfully.

The carbon adsorption products that have been designed and utilized for HVAC applications are very effective. When they are misapplied in industrial applications, problems can and do arise. Normally, it is not that the carbon does not adsorb, but rather the concentration exceeds the capability of thin bed products, and neither the pass efficiency nor the life (capacity) are satisfactory. Industrial applications, although they will contain many of the same chemicals found in HVAC applications, can be defined as having:

1. Primarily one contaminant (chemical).
2. A chemical that is known, both as to it’s formulation and quantity (ppm).
3. Either a constant quantity or a constant range.
4. A loading well above the 1 ppm range.

Very simply, this will be, by comparison, a heavy-duty application, and the information concerning the contamination will be known. One example that will crystallize the differences between these applications: A printing company with a large printing press may have a solvent recovery system to recover the solvents from the inks. This system will remove the high concentrations of solvent with large amounts of carbon. However, the solvent “leakage” that escapes into the general air and into the air handling system will be heavily diluted and be just one of the many other chemicals in the HVAC system. This air, that re-circulates thru the offices, lunchrooms, laboratories, etc., would now be treated as an HVAC application, with thin bed products. Adsorption may not be the most effective process for industrial applications. Companies skilled in these fields should be consulted to make the determination regarding the most effective process and system.

Activated carbon will adsorb without the air being drawn thru it, i.e. without a fan or blower. This is of importance when there are rooms or closed areas that do not have ventilation ducts, electricity and/or where the use of a portable air cleaner would not be practical. Carbon pads can be placed in these areas and be a very effective solution. Gas molecules will disperse (mix) themselves evenly throughout a given area, in accord with the “Perfect Gas Laws”. Activated carbon will attract and adsorb the odor or pollutant molecules to which it is closest, causing an unequal mixture to result. Whereupon the gases will redistribute themselves, the carbon will again adsorb the closest, and the process will continue until the air is “cleaned” or the carbon reaches its capacity. A relatively small amount of carbon can, and will, keep these types of areas odor and pollutant free. An example of utilizing a small amount of carbon is to maintain a refrigerator odor free. By adsorbing gases, it thereby eliminates the transfer of odors and tastes from one food to another, and to ice cubes.

For HVAC applications, adsorption products can be divided into several categories: 1. Panel Filters: Panels filters are available in a variety of designs and configurations. They are available in a variety of thickness’, 1″, 2″ and 4″, with 2″ being the predominant commercial product. Panel filters can in turn be divided into two groups: A. The newer products: Carbon filter products that are designed to have a low pressure drop and are normally interchangeable with particulate filters. Filters using either pads or pleats, contain either granular or powdered carbon. Carbon is either integrated with fibers during manufacturing or impregnated via a slurry process into an existing non-woven product. For 2″ filters, in either pad or pleat form, carbon loadings will vary from approx. 300 grams per square foot of filter face area, down to about 35 grams per square foot. Products include, three stage filters, pads, pads in die cuts, pleats and ring panels. B. The older products: Filled or honeycomb filters have relatively high pressure drops and therefore can not be interchanged with particulate filters. They are utilized in V-banks. Filled filters have perforated metal faces, held in place with welded metal spacers, surrounded with a metal frame and filled with carbon. Honeycomb filters utilize a honeycomb shaped material as the internal structure, which is either partially or completely filled with carbon. The carbon is contained within the honeycomb structure by a thin netting and a metal frame which surrounds the structure. As filters, they are available in ¾”, 1″ and 2″ thickness’. When used in V- banks, the ¾” and 1″ “trays” account for almost all HVAC applications. The 2″ thickness is used only in special applications. Both filters contain packed granular carbon. Loadings vary from approx. 2.1 lbs/sq. foot of filter face area for a 1″ filled filter, to slightly under 1 lb/sq. foot for a ¾” honeycomb filter. 2. Extended surface filters [Rigid Cells]: Deep Pleat, rigid cell extended surface filters, are the newer products. They are applied in many normal HVAC applications, previously reserved for the older V-banks. They are also used in clean room and microelectronic applications where contaminants are in the ppb or ppt [parts per trillion] range. These “Rigid Cells” utilize several different types of “thin bed” activated carbon media, that are formed into pleats that are 4″, 6″, or 12″ deep, the later being by far the most popular. The “deep” pleats are then sealed into a metal frame [rigid cell]. They can be used individually, but are normally used in a filter bank. Activated carbon loadings for a 24″ x 24″ x 12″ cell, vary from as high as approx. 7 lbs, to as low as slightly over 1 lb, depending upon the carbon filter media that is utilized. 3. V-Banks, the older products, are still in use. They are used in airports, stadiums and commercial buildings and in many clean room applications. These products will offer from approx. 30 to 45 pounds of carbon per 1000 cfm of air, and are available for flow rates of 500, 1000 or 2000 cfm. Virtually all units will have a face area of either 24″ x 24″ or 12″ x 24″; utilize removable trays, V sections, or have a serpentine bed configuration; can be applied individually, or be built into filter banks.

The application of these products can be divided into two areas – either new construction or retrofitting existing installations. When a new structure is being designed, the engineer has the option of utilizing any of the above products. When adsorption applications arise in existing installations, panel filters, although they will need to be changed more frequently to offer similar capacities, offer the advantage of interchangeability, thereby eliminating the need to alter the existing air handling system.