By Kenneth E. Schaeffer
Activated carbon adsorbent media is easy to use and easy to handle. If stored properly, it has an indefinite shelf life. It is nontoxic and relatively nonflammable. If it has become wet or damaged through improper storage, it usually can be dried out or restored easily. After it has become saturated with contaminants, it often can be reactivated to virtually its original adsorptive capacity with only a slight volume loss during reprocessing.
Activated carbon is a form of commercially pure carbon typically manufactured from coal, wood, peat or coconut shell having a microscopic physical structure. Each granule contains millions of tiny passageways or pores that provide a tremendous internal surface area, which account for its adsorptive power. The surface area of activated carbon is typically 600 to 1,200 m2/g depending upon the type and grade of the product. It is used to purify air and water, recover valuable vapors and liquids, catalyze chemical reactions and aid in the manufacture of highly purified products. Activated carbon is available in a wide variety of granular sizes, powders, beads, shaped pellets, cloth, pads and fibers. The physical form used is dictated by the service application and in some cases, pressure drop considerations.
Handling and storage
Activated carbon is stable and can be stored for many years in its shipping container or package if protected from contaminating vapors and liquids. Typical packaging options include large supersacks (1,000-2,000 pounds), mini-supersacks (200-500 pounds), small bags (20-55 pounds), Gaylord-type boxes (200-1,000 pounds) and steel or fiber drums (50-200 pounds). Activated carbon deteriorates only by doing work; that is, by acting as a catalyst or adsorbing vapors, gases and dissolved substances. Because it is relatively pure carbon, it is resistant to most acids, alkalis, chemicals and corrosive substances. Repeated heating and cooling has little effect on its physical structure. In solvent recovery plants, the adsorbent has been subject to as many as 10,000 heating and cooling cycles with no appreciable breakdown of the granules or pellets.
The best grades of granular activated carbon are hard and abrasion resistant, but methods of handling that tend to grind the product should be avoided. In vapor treatment systems, a high-velocity or non-uniform airflow may cause movement of the particles and rapid wear or breakage of the granules, particularly if the top of the carbon bed is not held in place by a screen. Upward flow of fluid at high velocities during adsorption should be avoided, because it tends to lift the granules and causes channeling. The method of handling activated carbon must be suited to the exact character of the product being used. For example, a powder has different considerations in connection with its handling than does a granular material. Impregnated carbons generally have somewhat lower ignition points than non-impregnated carbons. There are exceptions to this, but one must give special attention to ignition points in connection with carbons, which have been impregnated or given other special treatments. Manufacturer product specification sheets and material safety data sheets (MSDS) information provide important details such as carbon ignition temperature.
For the purification of air in living and working spaces, the adsorbent should be reasonably dry. It is not sensitive to moisture, but if it is dripping wet, its capacity will be reduced. If it has become saturated, it can be dried in an oven at 200°-250° F. Wet activated carbon tends to corrode mild steel, aluminum and some other metals. For wet applications, stainless steel, monel nickel, plastic, glass or ceramics can be satisfactorily employed as container vessels. For most dry applications, ordinary steel, aluminum, plastic or paper are suitable container materials.
If a particular grade of granular carbon contains too much dust or fine granules for a given application, they may be removed by screening, air-hosing or water-washing. Adding 10 to 30 percent moisture can reduce the dustiness of most powdered carbons.
Health and safety
Activated carbon is not toxic. Many types meet U.S. FDA Codex requirements for food grade applications and many are also approved by the American Water Works Association and NSF for drinking water treatment. In addition, some grades of activated carbon are U.S. Pharmacopeia (USP) certified and are taken internally for various medicinal treatments. Dust exposure is a concern with some activated carbons and normal dust handling procedures should be used such as eye protection and dust masks for personnel handling the carbon, as well as room dust filters to keep dust level under control.
Activated carbon is combustible, but not easily ignited. In the absence of a forced draft, even hot activated carbon supports combustion with difficulty and, under some conditions, will extinguish on its own accord. However, it should not be subjected to a blast of air at temperatures above approximately 400°F. Where high concentrations of vapors are adsorbed, the heat of adsorption (adsorption is an exothermic reaction) may be sufficient to raise the temperature of the carbon appreciably. In such cases, it may be best to operate with the carbon partially wet with water, as is typically done in solvent recovery plants, or to reduce the vapor concentration by adding additional air.
Vessels containing activated carbon
Care must be exercised in entering tanks or other enclosed spaces which contain large quantities of activated carbon. It is recommended that before entering such spaces, the atmosphere be tested for oxygen content, carbon dioxide or monoxide content, or other contaminants which might conceivably be present in the space. As an example of the precautions stated herein; if a vessel which contains activated carbon and which is large enough for the entry of a man needs to be repaired, inspected or have the carbon replaced, blowing in air should ventilate the air space. This precaution should be observed even though the carbon may not have been used in service.
Activated carbon can react slowly with oxygen under certain conditions even at ambient temperatures and, although this rate is sufficiently slow and no important amount of carbon is consumed nor any great weight of oxygen used, it may be an important factor in the oxygen content in a space, which has been tightly closed for a long time.
Activated carbon has a definite life, or capacity, in performing its function. When the limit has been reached, the adsorbent should be replaced or reactivated. It is good practice to have at least one spare charge on hand so that the original one can be replaced whenever necessary.
Special service conditions
Activated carbon is a powerful adsorbent and catalyst. Although most of its reactions and properties are reasonably well known, it must be regarded with care in unusual circumstances. Activated carbon can adsorb most types of molecules (there is a great difference in degree, depending on the type of molecule adsorbed and the ambient conditions) and if the conditions change, the carbon can give up these molecules into the surrounding space.
Restoration and reactivation
If contaminated with low boiling impurities, activated carbon may be either partially or completely reactivated in an oven or furnace at 200°-400° F or treated with low-pressure steam and then dried. Granular activated carbon usually can be factory-reprocessed (reactivated) to make it almost equal in adsorptive capacity to the original product.
Carbons impregnated with metallic salts may require special handling because of irritation due to dust particles. Carbons having especially high purity or low moisture content must be handled carefully to avoid contamination or moisture pick-up. For special carbons, be guided by instructions furnished for that type or class of carbons.
Shipping activated carbon
There are some transport regulations and restrictions with regard to the shipping of activated carbon by land, air and ocean and these should be carefully observed. There are no special hazards with most types of activated carbon, but apparently, there have been a few instances where carbon was the suspected source of ignition, which is the reason for the regulations.
Special impregnated activated carbons (used in respirators or for specific applications such as ammonia, formaldehyde or radioactive iodine treatment) have chemicals added and require special handling and need to be kept away from oxidizing agents and MSDS warnings should be heeded. In general, chemically activated carbon (such as phosphoric acid) needs a higher level of attention and should also be stored separately from oxidizing agents. There is no history of well-prepared steam activated carbon causing problems in shipping or storage. Activated carbon should not be compared with crude charcoal or barbecue charcoal, which has entirely different properties and which will from time to time spontaneously combust.
New, pure activated carbon will not support the growth of bacteria. However, carbon can adsorb nutrients required by bacteria and, after such adsorption has taken place, it is possible for bacteria to grow on the outside of the carbon granule and, to some degree, within the large pore entrances. Bacteria can be eliminated by heat sterilization, high pH soak, low pH acid soak or through the use of some other chemicals. There are types of carbon available that contain biocides, which do not add any poisonous substances to the water, but will control bacteria growth. Bacteria that are free-floating in the water can be removed by activated carbon because it can act as a filtering agent as well.
Activated carbon should be properly mixed with the fluid to be treated (powder form) or suitably supported in the flow stream (granular form). Where it is used for the purification of liquids in a filter tank, the flow system should be arranged so that pockets of air are eliminated, permitting the liquid to be in contact with the full charge of carbon. Air or water that is to be purified by activated carbon should be free from dust or suspended matter that might tend to plug the carbon bed. A particulate filter ahead of the carbon will eliminate this potential problem.
In addition to its use as an adsorptive agent, activated carbon can be used as an electrical conductor, an insulator, a sound-deadening agent or for the adsorption of light.
Most spent activated carbon can be properly disposed of at a reactivation facility, landfill or incinerator. Hazardous spent activated carbon must be handled accordingly and sent to a RCRA approved facility for disposal.
About the author
Kenneth E. Schaeffer has a master’s degree in environmental science from the University of Texas at Dallas and has been in the activated carbon industry for over 25 years. He’s President of Carbon Resources, LLC, a supplier of activated carbon products, equipment and services in Oceanside, Calif. and Vice Chairman of the ASTM Committee on Activated Carbon, D-28, as well as current Chairman of the Information Exchange Committee of IACMA (International Activated Carbon Manufacturers Association). Schaeffer can be reached at (760) 630-5724 or email: [email protected]