Activated Carbon Particle Size: Balance Between Kinetics and Capacity
By Robert J. Potwora
Activated carbon (AC) is a universal adsorbent. It is used to purify the water we drink, the air we breathe, many of the foods we eat and numerous industrial chemicals that we use every day. In drinking water treatment, AC is used to remove taste and odor, VOCs and a myriad of unwanted organic materials. US EPA lists granular activated carbon (GAC) as a best available technology (BAT) to remove specific organics from drinking water. In home water filtration devices, it is also used to remove disinfectants such as chlorine and chloramines, as well as DBPs.
Activated carbon types
The three most common types of raw materials used to make AC are coal, coconut shell and wood. Other types of raw materials are used to a much lesser extent, including fruit and olive pits, nut shells, bamboo and rice hulls. The most common types of AC used for water treatment are coal-, coconut- and wood-based. Activated carbon can be used in powdered form (PAC), in granular form (GAC) or in pellet form, known as extruded activated carbon (EAC). Powdered activated carbon is dosed into the water and then filtered out. Granular activated carbon and, to a lesser extent, EAC, is used in fixed-bed-adsorber columns.
For small POU AC systems, such as faucet mounted and pitcher devices, 20 x 50 mesh size GAC is preferred. This small size provides fast kinetics for adsorption of impurities and chlorine removal, while maintaining an acceptable pressure drop. Larger POE systems use 12 x 30 or 12 x 40 mesh size. This larger particle size still allows for fast kinetics and an acceptable pressure drop, while more easily covering a large area. Both forms of GAC and EAC can be thermally reactivated after they are spent, whether for cost savings or for ecological benefits and waste minimization.
Drinking water treatment
The drinking water segment is clearly one of the largest uses of AC using PAC and GAC. Several hundred-million pounds of AC are used annually in the US. Besides the ever-increasing need to clean drinking water, a newer market driver is US EPA’s final implementation of the DBP rules. These regulations are designed to help prevent the formation of hazardous compounds, such as trihalomethane (THM) and haloacetic acids (HAAs), and by removing organics from municipal drinking water supplies before chlorine or chloramines are added as disinfectants. An estimated 50 to 70 million pounds/year of GAC are currently needed for this market segment and these regulatory drivers are expected to provide tremendous growth for the AC market in the future.
Another growing area of concern is in the use of AC to remove trace amounts of pharmaceuticals and personal care products (PPCPs), as well as endocrine disrupting compounds (EDCs) from drinking water. Many adverse ecological effects have been attributed to these chemicals, but it is not yet clear what risks they pose to human health. Previously, the presence of these chemicals in water was known but was difficult to quantify. Today, new analytical methods can quantify the presence of these chemicals down to the number of parts per trillion (ppt). One part per trillion is equal to one drop in 26 Olympic-sized swimming pools.
The overwhelming majority of EDCs and PPCPs are not currently regulated and the levels found in drinking water are usually very low. Human health risks associated with these low levels, however, are not yet fully understood. Since conventional municipal water treatment does not remove ppt levels of these chemicals very well, POE and POU water treatment devices with AC can help fill this void. A new drinking water standard, NSF/ANSI 401, was recently developed to evaluate the performance of POE/POU devices on these potential contaminants.
Fine-mesh activated carbons and carbon blocks
Kinetically, the adsorption rate of organic impurities and the dechlorination rate of ACs goes up rapidly as the particle size goes down. This kinetic effect is utilized in making carbon blocks for POU and POE markets. Carbon blocks are manufactured from so-called fine-mesh AC that are typically 10 to 15 times smaller in particle size than the GAC used in municipal water filters.
Typical fine-mesh sizes include 30 x 200, 50 x 200 and 80 x 325. The fine-mesh AC is molded or extruded with binders to form a solid block. Additives may also be incorporated into the carbon block to target certain contaminants such as lead and to enhance performance. The carbon block filters typically have a 0.25- to 1-inch (6.35- to 25.4-mm) block thickness and have an empty bed contact time of just seconds, as compared to the minutes of contact time required for effective contaminant removal with GAC.
One of the more common uses of AC is for the removal of chlorine to improve the taste and odor of chlorinated drinking water. As the particle size of the AC is reduced, chlorine-removal life of the AC filter is dramatically increased. For example, a filter containing 150-cubic centimeters of a 12 x 40 mesh size AC has a capacity of 80 gallons (302 liters). For the same volume of AC, reducing the carbon particle size to 100 x 200 mesh increases the bed life to 10,000 gallons (37,854 liters).
Precise particle size control
As smaller and smaller AC particles are used to make carbon blocks, the need for precise particle size control becomes more important. Small variations in carbon particle size can have a significant impact on water flowrate and contaminant removal performance. To illustrate, three examples of particle size distribution (Types A, B and C) are shown in Table 1. Type A shows an example of coarser particle size and narrower distribution. This leads to carbon blocks that produce a higher flowrate of > 4.0 liter/min, but a relatively low efficiency of contaminant removal. At the other end, Type C shows an example of finer particle size and wider distribution. This leads to carbon blocks that produce a lower flowrate of < 2.5 liter/min, but a relatively high efficiency of contaminant removal. In between, Type B shows an example of optimal particle size distribution with precise control. This leads to carbon blocks that produce the desired flowrate of 3.5 to 4.0 liter/min, while maintaining a high efficiency of contaminant removal.
To achieve the optimal particle size distribution, precise particle size measurements are needed. (Traditional sieving methods originally developed for GAC were found unsatisfactory for fine-mesh carbon.) Modern instruments are based on laser diffraction. Such instruments can deliver rapid, accurate particle size distributions for both wet and dry dispersions with a minimum of effort over the nanometer to millimeter particle size ranges.
There are many different types of AC available. Selecting the right type and the right particle size distribution is critical to ensure proper performance. Work closely with your supplier to ensure that you select the most economical AC to meet your performance objectives.
About the author
Robert J. Potwora, of Oxbow Carbon LLC, has over 30 years experience in the activated carbon industry. He is also Chairman of ASTM’s Committee D28 on Activated Carbon. Potwora may be reached by phone (760) 630-5724 or email firstname.lastname@example.org.
About the company
West Palm Beach, FL-based Oxbow Activated Carbon LLC, one of the largest private US companies, provides one of the most diverse lines of AC solutions backed by expert technical support and knowledgeable customer service. The company has advanced processing capabilities in North America and Europe with direct sales and distribution operations around the world. Oxbow is the world’s leading marketer, upgrader and distributor of petroleum coke, calcined petcoke and sulphur products. The company’s scale, expertise and leading market positions have allowed it to create a powerful global sourcing platform with superior market intelligence and longstanding relationships with its key suppliers and customers. Oxbow activated carbon has greatly benefited from its ability to leverage Oxbow’s global operations and extensive global distribution network. The company’s website is www.oxbowactivatedcarbon.com.