By Dr. Mick Greenbank and Jim Knepper

Summary: Powdered activated carbon (PAC) is occasionally confused with granular activated carbon (GAC) as having similar physical and performance attributes. This article addresses some of those misconceptions and expands on the practical use of PAC in small system applications.


The basic difference between granular activated carbon and powdered activated carbon is in the cost of manufacture. For a given starting material and activation process, the only difference between PAC and GAC is particle size. The inherent adsorption and transport pore structures (smaller than the particle dimensions) are equivalent. In fact, most PACs are activated as GACs between 4 and 40-mesh size, and are physically crushed or pulverized to reduce them to powders that typically pass through a 325-mesh screen.

The processing advantage of making a PAC product is that particle size and strength aren’t a concern to the end-user since the material is already a fine dust. Density is also less of a concern to end-users of PAC because it’s usually used in a water suspension and not packed into fixed spaces, such as a large carbon adsorption column. The only concern with density is in storage and handling of PAC bags. Consequently, lower-cost starting materials with lower densities and weaker structures can be used as raw materials for PAC.1

The overall yield in the activation process may be as low as 10-weight percent (or percent by weight, i.e., 10 pounds of raw material yields one pound of activated carbon); therefore, the cost of the raw material can multiply as much as tenfold when factoring in the final cost of the PAC product (on a weight basis). This is why so many PAC products are based on low-cost wood sources or low rank lignite coal, whereas GAC products are usually based on denser and harder materials such as coconut shells or high rank bituminous coals. In addition, fines and dust generated during GAC manufacture can be fed to a pulverizer and generate a low-cost PAC by-product. This is the case for many bituminous coal and coconut shell-based PACs. The result, as shown in Table 1, is that the cost of PAC can be as little as half of an equivalent GAC structure, either due to low-cost raw material or the use of a GAC by-product in the PAC manufacturing process.

Advantages of powdered carbon
PAC has two advantages over GAC—it’s less expensive and has a finer particle size. The low cost means that it’s generally practical to use on a throwaway basis without requiring reactivation or recycling to make it more economical. This is important since it’s difficult to design activation or reactivation equipment that can handle 325-mesh powders.

The small particle size means faster adsorption kinetics. The PAC particle diameter is 10 to 100 times smaller than GAC. The diffusion path to access the interior of the particle is 10 to 100 times smaller (see Figure 1). Therefore, if the particle size is 100 times smaller, the rate of adsorption in water solution will be more than 100 times greater for PAC than for GAC. This occurs when PAC is packed into an HPLC (liquid chromatograph) column. Unfortunately, the pressure drop across a bed of carbon also increases with decreasing particle size. The pressure drop across a bed of PAC can be several thousand pounds per square inch (psi) and too high to be practical for potable water applications.2

The other advantage of reducing the particle size is increasing the number of particles. When the GAC diameter is reduced, the number of particles generated increases to the third power. So pulverizing one 4-mesh particle will generate about a million 400-mesh particles (this represents roughly a 100-fold reduction in particle diameter).

Water treatment applications
The tremendous increase in the number of particles improves the ability of the PAC to contact all the water when applied as a dilute slurry. In addition, the decreased particle size also makes it easier to disperse the PAC and keep it suspended with agitation. PAC is usually removed from the treated water using basic tried and true filtration techniques.3 Filtration processes that reduce the water turbidity are also generally adequate to remove PAC. If problems persist using existing water filtration for PAC removal, a slightly larger mesh size and/or higher density PAC should be applied. This process is referred to as batch application of PAC.

The main PAC disadvantage is in how it’s usually applied in a batch fashion. Even though the PAC is well dispersed in the water with good agitation, the time the contaminant takes to diffuse and mix to contact the particle exterior is much longer than it takes to diffuse through the pores and adsorb.4,5 Therefore, it can take 16 hours for 10 parts per million (ppm) of chloroform in distilled water to reach 95 percent of its thermodynamic equilibrium capacity on 325-mesh PAC in a batch application5, yet the equivalent 12×40-mesh GAC product only requires less than three minutes actual contact between the water solution and carbon in a dense-packed bed in an adsorption column.4 The same PAC in a dense packed bed in an adsorption column would require less than 10 seconds contact but the pressure drop would be very high.6 At present, batch processing is the only practical way to apply PAC for large-scale water treatment. The required 16 hours contact is also reduced to less than one hour to be practical and, as a result, only 30 to 60 percent of equilibrium capacity is achieved in the batch process. We’ve found that most of this adsorption in batch processing occurs during the filtering step, when the PAC forms  crude adsorption columns and the water filters through the cake.

When to use PAC
Batch processing with PAC is best used for applications like taste and odor removal, where the objective is removal of 50-75 percent of the contaminants. For toxics and regulated materials where greater than 95 percent removal is required, dense-packed beds GAC should be applied in adsorption columns. Batch application of PAC for water treatment requires almost no capital equipment such as adsorption columns or their ancillary equipment. In addition, only a minimal amount of PAC must be purchased and stored on site. For GAC treatment, over a year’s supply of carbon must be purchased and “stored” in the adsorption columns to provide enough time to contact the water for efficient operation. Plus, GAC columns treat all water to a high removal limit, whether or not such treatment is required.

Another advantage of batch processing is that the PAC is used for a short period of time, and then discarded. As a result, there’s little chance for bacteria multiplying within the carbon bed before it’s removed from the system. Also, the dose or mass of PAC applied per volume of water can be varied continuously by the end-user. If the contamination problem is severe enough, extra bags of PAC can be added and filtered to get a higher percentage removal. The percent of equilibrium capacity achieved might still be low due to the batch treatment scheme, but additional equilibrium capacity is achieved with the extra PAC applied.

Conclusion
PAC is often used in this fashion to solve water problems involving taste and odor. The operator continues to add bags of PAC until the taste and odor of the effluent water is below the detection limit. Therefore, the PAC dose is close to the bare minimum of PAC required to solve the problem. If the problem disappears, the carbon dosing can be reduced. We recommend a continuous small dose of PAC to eliminate problems near the detection limit before they are identified.

References

  1. Jacobi Carbons AB, Kalmar, Sweden, website: www.jacobi.net
  2. Rosene, M. R., et al., “High Pressure Technique for Rapid Screening of Activated Carbons for use in Potable Water,” Activated Carbon Adsorption of Organics from the Aqueous Phase, Vol. 1, I. H. Suffet and M. J. McGuire, editors, Ann Arbor Science, Ann Arbor, Mich., Chapter 15, 1980.
  3. MacDowall, J.D., and E. Polman, “Filtration of Powdered Activated Carbon,” Filtration & Separation, August 1977.
  4. Ebach, E.A., “The Mixing of Liquids Flowing through Beds of Packed Solids,” Ph.D. Dissertation, University of Michigan, Ann Arbor Microfilms, Ann Arbor, 1957.
  5. Liu, Kuangtsan, “Determination of Mass Transport Parameters with the Micro-column Technique,” Ph.D. Dissertation, University of Michigan, Ann Arbor, 1980.
  6. Greenbank, M., Ph.D. Dissertation, Kent State University, 1981.

About the authors
Dr. Mick Greenbank is the chief technical consultant at Jacobi Carbons, a Swedish company with subsidiaries in England and Germany. He runs a laboratory in Ohioville, Pa., which is dedicated to activated carbon characterization and optimizing performance. He teaches a course twice a year on “Selecting the Best Activated Carbon for the Application.” Greenbank can be reached by email: mick.greenbank@jacobi.net or (707) 222-2296 (fax).

Jim Knepper is the U.S. national sales manager for Jacobi Carbons. He holds a bachelor’s degree in chemical engineering from Virginia Tech University and has over seven years experience in the activated carbon industry. He can be reached at (215) 546-3900 or email: jim.knepper@jacobi.net

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