By Nobu Kikui

Summary: Activated carbon fiber (ACF), made from coal pitch, is manufactured using a spinning process to create the fiber, which is activated and chopped. The end product can be molded into virtually any shape for use in POU water filter applications. ACF filter elements can offer longer life or smaller size and are suited for NSF Standard 42 compliant applications.

Activated carbon fiber (ACF) is relatively unknown in the U.S. market but is widely used in Japan in point-of-use (POU) filter devices because of superior chlorine adsorption compared to granular activated carbon (GAC), and its smaller size, i.e. less than a half size of a carbon block with the same performance. ACF offers a high chlorine chemisorption rate, longer life, a low-pressure drop, a smaller size compared to granular or powdered activated carbon, and unlimited molded shapes or composites with other materials. The faster adsorption rate and higher capacity combined with a smaller size is of particular interest in the Japanese market where taste and odor control are of primary interest.

ACF can be produced from several raw materials such as phenolic fiber, cellulose, acrylonitrile and petroleum or coal-pitch. Pitch-based ACF, the focus of this article, is said to be the most cost effective and has the highest carbon content (99 percent). GAC, in general, made of coconut or coal contains about 95-98 percent carbon, although it’s not unheard of for some to have ash content of 10 percent (90 percent carbon). The higher carbon content offers a better yield rate from raw material and a higher chlorine removal performance. Very fine and uniform micropores are dominant on the ACF surface and generate a higher van der Waals force and offer a higher adsorption rate for various contaminants, whereas macro and micropores co-exist on the surface of GAC, which adsorbs a wider range of contaminants than ACF.

Typical ACF properties include a specific surface area from 700 to 2,500 square meters per gram (m2/g), fiber diameter from 15-18 microns (µm), and iodine numbers from 950 to 2,500 square meters per gram (m2/g) with chopped or milled fiber form. The average pore size ranges from 17 to 21 angstroms while GAC offers more micropores in the range of 103 and 106 angstroms.1, 2 ACF can be vacuum molded to a variety of shapes (see photo), with molded elements commonly used for water filtration applications.

Chlorine removal
The chlorine removal mechanism is simple (see Figure 1). The two chemical reactions are believed to be dominant in water. First, ClO- reacts with carbon on the fiber surface where the active reaction sites are located, generating Cl- and CO2. The other reaction is between O (from the decomposition of ClO-) and carbon on the fiber surface. Oxygen is bonded with carbon and Cl- is generated. Also, the reaction rate increases at higher temperature, and the amount of chlorine removed also increases as temperatures rise. The amount of organic compounds, which may block the reaction site on the carbon, decreases in the higher temperature water, causing the higher adsorption volume.3 As specific surface area increases, active sites for those reactions increase and result in a higher removal performance. The fibrous form provides a larger physical contact area than granulated carbon. This is the fundamental chlorine removal mechanism for ACF.

Reasons for the tests
ACF was compared with GAC and copper-zinc powder (Redox) in a shower filter application. Short and long-term tests were conducted. The purpose of the short-term test was to evaluate the chlorine removal capability of the different media. The purpose of the long-term test was to determine the real life of the material.

Test conditions included: water temperature at 105°F (typical of shower water); water pressure at 60 pounds per square inch (psi); water flow rate at 2.0 gallons per minute (gpm); and the influent chlorine level at 2.0 parts per million (ppm). The short-term test was done at a local chemical lab, and Spectrum Labs handled the long-term test. Instead of using an on-off cycle flow used for faucet-mount water filtration devices (per NSF Standard 42), continuous flow was used to place a heavier burden on the filter material. This also is the standard procedure followed in Japan.

Costs in the end
From the test results, the chlorine removal cost based upon the chlorine amount removed with the cost of materials used was estimated. Roughly speaking, Redox material cost is $0.29/chlorine reduction per gram and the filter life reaching at 60 percent reduction level is 2,000 gallons, whereas the ACF cost for two different grades is $0.11-$0.18/chlorine reduction per gram and filter life using one type of processed fiber is 3,000 gallons and another is 5,000 gallons, respectively.

ACF—combined with organic materials or surface treated by chemical functional groups produced in sheet and other forms—is used in many applications including gas and air treatment. The activation process conditions can change the pore size and its distribution. For instance, a larger pore size ACF has already been developed and is being effectively used for trihalomethane (THM) reduction for faucet-mount water treatment devices. ACF, however, needs to be combined with other media such as carbon or ceramic blocks in order to satisfy the Cryptosporidium claim and some other NSF Standard 53 claims.

Conclusion
ACF has a tremendous capacity for removing chlorine and other contaminants. ACF is lightweight, offers smaller size elements than alternative media, has a low- pressure drop, and is competitively priced. Molding technology enables versatility of design. Present uses for ACF include shower filters and water filters that comply with NSF Standard 42. ACF molded elements, which typically offer 15-to-20-µm filtration, must be combined with carbon blocks to meet cyst and some other removal standards for compliance with NSF Standard 53. The next generation of POU element will be a combination ACF/carbon block. Some preliminary tests were already conducted and ACF combined or mixed with a carbon block showed good performance that will be available in the market shortly.

References

  1. Dubinin, M.M., Uspekhim, Khim, 24, P3, 1955.
  2. Kitagawa, S., et al., “Activated Carbon Industry” Jyukagaku-Kogyo-Tushinsya, 1975.
  3. Maeda, T., Japanese Water Filtration Association seminar proceedings, 1998.

About the author
Nobu Kikui is president of Osaka Gas America Inc., of Pasadena, Calif. Osaka offers pitch-based carbon fiber, activated carbon fiber and related products that are manufactured by their sister company, Osaka Gas Chemicals, a manufacturer of faucet mounted type filtration devices in Osaka, Japan. Kikui can be contacted at (626) 304-9081, (626) 304-9327(fax) or email: kikuiy@ogamerica.com.

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