By Henry Nowicki, Ph.D. and M.B.A.
Activated carbons (AC) are used in many process applications. Your drinking water and indoor air probably have been purified with AC, and the white granulated sugar used in your food and coffee was decolorized with activated carbon. Some AC industrial applications: hydrogen sulfide and odor control, military respirators, solvent recovery, recovery of gold from mine tailings, taste and odor in beverages (like beer, wine and soft drinks), biotechnology and support for catalysts and chemicals and many others. There are many more opportunities and applications for low-cost/high- performance activated carbon materials.
Pollution control is the main market for AC. Commercial activated carbons are manufactured from bituminous coals and coconut shells. About four pounds of coal yields one pound of activated carbon. Any material with high carbon content is a candidate raw material for activated carbon manufacture. Activated carbons are available in many forms: powder, granular, pellet, carbon blocks, cloth and felts. Chemical impregnation of the above forms to produce specialty AC with additional functionality is also available.
Activated carbon has long been classified as a specialty chemical, but in the last few years has become a commodity, with price decreases of two to three times, largely due to global competition and manufacturing productivity. These competitive price decreases have been good and bad for activated carbon users and manufacturers. Unfortunately price may not directly correlate to the user’s needed quality. A lower cost product may be better than higher priced AC. Testing is needed to make this decision.
Laboratory testing is needed to determine quality. There are independent commercial laboratories that provide testing services in addition to captive manufacturer’s and reactivator’s laboratories. Activated carbon users should use laboratory testing in their activated carbon applications. AC does not last forever and all AC is not the same.
The American Society for Testing Materials (ASTM) provides approved test methods with the voluntary cooperation of the activated carbon industry, made up mostly of manufacturers. These ASTM testing methods provide technical communication between buyers and sellers, and they are often the main part of the purchasing specifications. Vapor phase AC test methods are received and dry apparent density, water percentage, butane activity and butane working capacity, particle sizing and hardness or abrasion determinations, total ash content, water soluble ash, dusting attrition, thermogravimetric analysis, gravimetric rapid pore distribution (GRPD) and estimated remaining service life determinations. There are many other ASTM methods available for users and manufacturers to apply to their AC applications.
Many vapor phase applications need to provide periodic testing to assure that the activated carbon adsorption media is still working. All activated carbons are not the same and need to be monitored to document continued performance. This sorbent does not last forever and it must be replaced with new a sorbent or reactivated when it fails to perform. Users should send laboratory samples from the top, middle and bottom of the carbon beds to determine the location of the mass transfer zone (MTZ) and the remaining adsorption capacity. The MTZ location lets the user know how much of the bed or column is still providing adsorption. It also lets the user know the contaminants breakthrough order and concentration as a function of volume or time. Staying within regulatory effluent specifications at the lowest cost is the user’s responsibility. Laboratory tests allow the user to have confidence and documented proof in the control performance and to plan for media replacements. Periodic testing also helps avoid non-compliance with environmental laws and possible fines. Independent laboratories provide a check on the vendor’s testing results, and often receive six to 10 AC samples associated with a bid package for analysis.
Municipalities often request multiple activated carbon suppliers to submit AC samples, which are representative of the material they will supply if awarded the contract. The municipality then contracts an independent laboratory to run ASTM tests on the vendor samples and compare their results with each AC supplier. The purchasing decision depends on the physical and chemical properties of the activated carbons supplied by the vendors and the economics, as well as possible post-purchase services.
Advanced test methods and software
New instruments and testing methods are available to estimate the remaining service life of an activated carbon adsorber. When AC adsorbs organics, heat is given off. The amount of heat is directly related to the available adsorption space in that carbon. As activated carbon is used the adsorption space fills with adsorbates. By running the unused and used activated carbon in an AC TESTER (which measures the heat-of-adsorption in a five gram sample) it is possible to estimate the remaining service time. The AC TESTER is a simple, low cost screening device to measure the heat-of-adsorption in activated carbon samples, and even non-technical staff can use it on-site as a screening tool.
Labs specializing in activated carbon evaluations can do additional approved ASTM confirmation testing. Users of AC should retain samples of newly installed carbon, unfortunately most users do not keep retained samples. These retained samples are useful to monitor their adsorbers with the AC TESTER and ASTM test methods. Comparing lab results from used and unused activated carbon is useful when processing problems arise or purchasing new AC.
The Gravimetric Rapid Pore Distribution (GRPD) method solves challenging carbon adsorption applications and many other materials analysis problems. The GRPD method was invented by Dr. Mick Greenbank. It has long been observed that several (10-15) manufactured activated carbons can have similar ASTM tests results (iodine or butane adsorption activity), but one of the carbons proves superior during field performance, such as removing vinyl chloride or toxic gases from air or MTBE or THMs from water. The GRPD instrumental methodology offers carbon users a way to quickly and at low cost identify and select the best activated carbon for their applications. The GRPD method reveals the heterogeneous adsorption energy distribution of the binding sites and their relative binding energies to other commercially available activated carbons and other new sorbents. Water purification professionals have adsorbates, which need specific binding sites with threshold binding energies. Different carbons have characteristic adsorption energy distributions. The GRPD capability differentiation of activated carbons based on binding site information is a very important development in the activated carbon industry.
Software programs are useful to quickly determine a possible go or no-go application for activated carbon. If the software reveals a loading capacity of only 0.1 grams of adsorbate per 100 grams of carbon, this will probably not be a good carbon application. However, if the software calculates a capacity of 50 grams of your contaminant per 100 grams of carbon, you probably have an excellent opportunity for carbon to solve your environmental or process problem. Most vendors, consultants, labs and engineering firms have software that help potential carbon users determine in a few hours if their application is a go or no-go for applying AC as a solution to the problem. Typically isotherms and accelerated milligram quantity activated carbon mini-column tests are provided to help determine the best amount and usage rate of carbon, i.e. 5,000 pounds in an adsorber and 950 gallons of process water treated per pound of activated carbon to obtain process effluent specifications. This information will help plant operators determine whether or not activated carbon treatment is suitable for their facilities problems and help engineers design appropriate carbon treatment units.
Knowledge is power. Historically there were no regularly scheduled independent, non-commercial training courses covering the activated carbon and other sorbents subjects. Vendors sent out sales people to good prospects to purchase their sorbent materials and provided them only selected knowledge. Today there are noncommercial sources for continuing education on activated carbon adsorption. These provide a broad base of knowledge for carbon users and manufacturers, and reinforce the need for an internal expert knowledgeable about carbon. These internal (or part-time consultants) experts can facilitate the selection and proper use of the best activated carbon for their applications. This knowledge results in lower costs, decreased liabilities and better vendor performance for the activated carbon users. In the present competitive AC marketplace, vendors are most helpful to knowledgeable users. Knowledgeable users place purchasing orders and send staff to training courses.
The bottom line is there have been many changes in the activated carbon industry the last few years. Users now have the largest selection and best process media at the lowest cost, but users need to play an active role in educating themselves to take full advantage of the opportunities. Decreased vendor AC prices also means decreased technical services, compared to the days when prices were 2-3 times higher.
About the Author
Dr. Henry Nowicki is director of testing and consulting services at PACS Testing, Consulting, and Training Inc. PACS activated carbon ‘application and user groups’ are open to anyone and the next are Pittsburgh, Penn., October 6-7, 2005 and March 6-7, 2006 in Orlando, FL. and Meetings include professional interactions with individuals who have some of the same problems as a way to help solve processing applications with activated carbon and other technologies. Present working groups are: taste and odor in drinking water, arsenic removal from drinking water, hydrogen sulfide and odor removal from air, solvent recovery, design of carbon adsorbers and laboratory analytical methods for the activated carbon industry. For additional information, email Nowicki at: firstname.lastname@example.org, call (724) 457-6576 or visit http://www.pacslabs.com.