Advanced Activated Carbon Test Method: New GRPD Versus Classical Iodine Number and BET Surface Area
By Henry Nowicki and Barbara Sherman
We previously provided information about an exciting advanced test method for the activated carbon industry.1 This method is now in the literature with two names: Gravimetric Rapid Pore Size Distribution (GRPD) and determination of Adsorption Energy Distributions (AED). We plan to help make the sorbent industry aware of this powerful new test method. Perhaps the GRPD is the best available test method to provide sorbent characterizations.1
We are providing here a case study from 2003 work at our laboratory where we used both classical test methods, long used by the activated carbon industry and the relatively unknown (to you) GRPD technology.
GRPD versus iodine number and BET surface area
Nine samples of powdered activated carbon were received for American Society for Testing and Materials (ASTM) iodine number determinations, Brunauer, Emmett and Teller (BET) surface areas, and Gravimetric Rapid Pore Size Distribution (GRPD) determinations. We also provided other ASTM test methods: apparent densities, total ash, water soluble ash, hardness and moisture on these client samples. The client numbered samples 1. through 9. were used for their sample identifications (ID) and we added our laboratory accession numbers to each client sample; i.e., S -180 through S -188.
The test results (clients sample identification and PACS sample identification) for the iodine numbers, surface areas and GRPD ultimate capacities, total (ultimate) pore volume and high energy pore volume determinations are tabulated in Table 1.
We have not added any background information or method description material for iodine number and BET surface area determinations because of their long-term acceptance and use by the activated carbon industry. We want to make you aware of the Gravimetric RPD because the activated carbon industry, as a whole, is less familiar with this more powerful activated carbon selection method. A data interpretation aid is provided in the executive summary paragraph, followed by more detailed information.
When interpreting data from the adsorption potential distributions on Tables 1 and 2, the adsorption energy in the first column relates to the pore binding strength. The higher the number (29 is the highest) the stronger the adsorption. The pore volumes (cubic centimeters per 100 grams of powdered activated carbon) for each of the nine samples is accumulative. In the instrument measurement process the sample starts out hot. At this temperature, the sample has the least amount of gas phase adsorbate pick-up—only the high-energy sites can capture and hold the adsorbate. The sample is temperature programmed and cooled to allow the weaker adsorption potential sites in the PAC to pick up the gas phase adsorbate. Our coolest sample temperature inside the instrument gives the highest amount of adsorbate pick-up and highest total pore volume. The analysis cycle is fully pre-programmable and automated for linear ramp rate, temperature range and temperature dwell periods. This ultimate total pore volume is read across from the zero adsorption potential in column one into the row for each of the nine samples. The volume for high energy binding sites in the powdered carbons between 25-29 calories per cubic centimeter is listed in Table 1 from raw data in Table 2. Data in Table 1 shows the correlation of GRPD total pore volumes (ultimate volumes) with iodine numbers. More importantly, the GRPD method reveals the volume of high energy binding sites but the Iodine Number does not. These high-energy sites clearly differentiate the sorbents.
Differentiating between activated carbons
The structure of activated carbon is composed of interconnected domains of fused benzene rings, called graphite plates, organized with some localized order on a molecular level. The adsorbing pores are defined by the gap, angle and spacing between the graphite plates. The adsorption forces in activated carbon are due to intermolecular interactions between carbon atoms in the structure and the adsorbing molecule. These intermolecular attractions (called London forces) are analogous to gravitational forces, but are significant only on a molecular scale (they are additive, temperature independent and nonspecific but very short ranged). The individual interactions between the adsorbate with nearby carbon atoms, propagate into a force field within the voids of the structure. The adsorption sites are simply a region within a void, the size of the molecule of interest, within the structure within this force field. The observed adsorption force for a specific adsorption site within an activated carbon structure is the sum of all the interactions with all the nearby carbon atoms. The adsorption properties of activated carbon can be fully characterized by measuring the statistical distribution of numbers of sites present with different adsorption forces.
The physical chemistry and fundamental knowledge to appreciate and apply this advanced test method is provided each October in Pittsburgh, Pa. with Dr. M. Manes, Dr. H. Nowicki, Dr. M. Greenbank and Wayne Schuliger, P.E. in a practical series of educational courses. The authors recommend that you acquire and apply this knowledge from these instructors while you can.
Full adsorption pore characterization
The goal of a full characterization of activated carbon is the direct measure of this distribution of adsorption sites. The adsorption potential, or adsorption energy, of the site is actually what is measured but is closely related to the adsorption force. Also the number of sites is multiplied by the volume of the adsorbate molecules and the distribution is reported as pore volume versus adsorption potential, called a characteristic curve. The characteristic curve can be fit to a low order polynomial (5th degree or less) if a logarithmic y-axis is used.
This characteristic curve data can then be translated into single and multicomponent adsorption isotherms using computer prediction programs based on Polanyi adsorption potential theory.2 This is also the only carbon adsorption data required to be input into these performance prediction programs that can model most any gas or liquid phase application. A computer data library of these activated carbon characteristic curves can then be used to optimize the carbon selection for a given application.
Our library has over 500 activated carbons, including commercial carbons, plus a variety of exotic carbons from unique raw materials. A given activated carbon product performance can be compared to standard commercial carbons for any application area selected. This quickly allows the determination of true differentiation in product properties or in adsorptive performance before spending time, effort and money on activation equipment or process development.
Experimentally the characteristic curve can be defined by measuring adsorption over the range in pressure, or concentration, that utilizes all of the different adsorption sites. Many different experimental methods and any gas or liquid-phase adsorption system can be used. For best results, the following criteria should be used for selecting a characterization method:
- Measure the full distribution: for a typical activated carbon this means about seven orders of magnitude range in relative pressure or concentration, which can be an analytical challenge.
- Define the curve: generate enough data points to fully define the curve shape in the distribution (a minimum of 10).
- Assure equilibrium is attained: the foolproof method is to approach equilibrium from different paths by measuring both the adsorption and desorption curves. If they coincide equilibrium is attained.
- Condition the sample to a clean surface: a clean surface is the only reproducible state, even though it may not represent the carbon sample as received or as used.
For a full characterization of activated carbon pore structure, a fully automated, modified thermogravimetric analyzer generates over 200 points over seven orders of magnitude adsorption isobars. This instrument accomplishes the above goals by directly measuring the amount adsorbed gravimetrically, using a flowing stream of pure gas adsorbate to minimize concentration gradient complications and changing the relative pressure by changing temperature.2 The carbon sample is conditioned to 250°C in helium. As a standard adsorbate 1,1,1,2 Tetrafluoroethane (H-134a) is used because it is safe (nonflammable and low toxicity), inexpensive and readily available. By cooling, then heating the sample, both adsorption and desorption isobars are measured and if the results coincide, equilibrium is demonstrated (in practice, small differences in the curves are accounted for by using an average of the two curves). The resulting characteristic curves are fit to log/linear polynomials for use in the performance prediction programs.
Activated carbon specifications
The problem with full characterization of carbons is that few people have the means or equipment to satisfy the above criteria, yet. A full characterization is not presently a viable means of specifying activated carbons. However, one of the key characteristics defined in the full distribution is total adsorbing pore volume. Therefore, simple tests that measure these three characteristics directly can be used as specifications. The following are test methods that directly measure the three parameters of the chi square distribution.
1. Total adsorption pore volume approximated using standard ASTM test methods
Unfortunately, existing standard test methods for activity numbers and the specification tests for commercial activated carbons tend to measure just total pore volume and not the pore size and volume distribution or the full adsorption isotherm. The existing specification tests are only about one third of what is required to truly define an activated carbon.
The activity level for activated carbon is measured by adsorbing a gas or solute (iodine, butane, carbon tetrachloride and nitrogen are popular) at near saturation conditions, thus filling all the pores with any significant adsorption force present. The measured amount adsorbed on the carbon is roughly proportional to the total adsorbing pore volume converted and is reported as an activity number, or index. For relative comparison, or a specification test, the units are not important. For comparison to actual total pore volume, the values are converted to units of cubic centimeters pore volume occupied per gram carbon (the observed density for iodine in the carbon pores is only about 40 percent of the handbook value). This density discrepancy is due to restrictions of iodine crystal packing in nano-spaces.
These existing test methods do define the carbons in some instances. For example, when you have only a single family of activated carbon products (with apparent densities ranging from 0.2 grams per milliliter to 0.8 grams per milliliter) that can be made from a given starting material. As a result, there are distinct relationships between activity numbers and pore size distributions for a given starting material and activation process. Thus, if the activation process and starting material are defined, then the activity number can indirectly define pore size distribution—and thus performance—and activity numbers work as specifications.
2. Average adsorption force using a heat of adsorption test
The heat of adsorption, in the presence of an adsorbate under saturated conditions, is a function of the average adsorption force times the total pore volume. Measuring the heat of adsorption and correcting for the effect of the total pore volume (measured using standard methods) results in an indirect measure of average adsorption force without determining the entire distribution.
A simple, fast and portable test is heat of immersion. A small portable heat of adsorption measurement equipment is commercially available. This equipment will allow a direct comparison of the adsorption characteristics of two carbons in a test that takes only a couple of minutes. The adsorbate is typically mineral oil, because it is inexpensive, safe and readily available; but can be glycerin, glycol or corn syrup to make the test more relevant to the specific customer application. The results translate to relative comparisons between carbons for average adsorption force (adjusted for total pore volume).
3. Maximum adsorption force using a trace adsorption test
A reproducible equilibrium adsorption test in the trace region will be heavily influenced by the maximum adsorption force parameter. Correcting the results for the total adsorption pore volume and the average adsorption force, it is possible to extract the contribution from the maximum adsorption force.
There are a variety of trace adsorption test methods that can be used. A gas phase adsorption measure is faster, simpler and less expensive that most liquid phase approaches. For an accurate measure of a single isotherm point, activated carbon loading curves versus time are generated, at pressures from 100 microns to 6 atmospheres, using an automated molecular probe device. For estimating the maximum adsorption force, typically the adsorbing gas is pure methane at atmospheric pressure onto a vacuumed activated carbon. The carbon is conditioned by vacuuming to 10 microns and equilibrium is determined when the loading curve no longer changes with time. The results can also be correlated directly to carbon performance in many trace adsorption applications.
Heterogenity testing of activated carbon
A problem arises when the carbon product is not homogeneous in its activity number, either between granules (interparticle), or within a granule (intraparticle). In the case of a heterogeneous sample, the activity number averages to the measured value for the overall sample, but the pore size distribution is distinctly different for different granules (or parts of a heterogeneous granule). For the pore size distributions, a different shape is observed when combined, generating multiple modes (or humps) rather than simple numerical averaging. As a result, characteristic activity numbers or specifications relate to performance only when the sample is homogeneous, both particle to particle and within each particle. Heterogenity can cause as much as a tenfold decrease in performance for municipal drinking water carbons in removal of trace contaminants such as MTBE.
A heterogeneity test measures the difference in adsorption characteristics (heat of adsorption is standard) between the inside and outside of the particle, large and small particles and high and low density particles (mixed products). A differential heat of adsorption instrument is used for making relative comparisons of adsorbent samples by directly measuring the difference between the adsorption characteristics of different samples or portions of a sample (or particle).
Acknowledgment for outstanding achievements
During Dr. Mick Greenbank’s three years with PACS Pittsburgh/Orlando based science firm, he lowered the cost to manufacture the testing instrument and most importantly found the ideal challenge gas 1,1,1,2-Tetraflourethane (TFE) for his methodology. Previously his test method was performed with methane, ethane and propane as a suite of challenge gases.3 This tri-fold suite covered six orders of relative gas concentration, whereas TFE covers eight orders of concentration. To detect and quantify the number of the highest energy binding sites in a sorbent you need the lowest concentration of challenge gas and a difficult molecule to adsorb. TFE is the ideal challenge gas. We take our hats off to Mick.
- Henry Nowicki and Barbara Sherman, WC&P March 2006.
- Dr. Milton Manes (www.pacslabs.com) provides a two-day course each October in Pittsburgh, Pa., which covers the Polanyi-Manes model in great detail. Manes next class is October 14-15, 2006 before the 18th International Activated Carbon Conference and Courses program in Pittsburgh, Pa.
- M. Greenbank, T.M. Matviya, and W.G. Tramposch, “Rapid Carbon Adsorption Characterization Using Temperature Programmed Adsorption and Desorption,” 1991 Annual American Institute of Chemical Engineers (AICHE) Meeting, Los Angeles, CA, Nov.17-22, 1991.
- Homer Yute and Henry Nowicki, “Software Programs for the Activated Carbon Industry”, 2nd biannual International Activated Carbon Conference and Courses program Pittsburgh, Pa., 1993.
About the authors
Henry Nowicki, Ph.D./MBA directs and provides testing and consulting for PACS: Testing, Consulting and Training. Dr. Nowicki is in his second decade of providing products and services for the activated carbon industry: routine and non-routine stationary and mobile laboratory testing, R&D for new sorbents and technologies, consulting, expert witness services and an introductory activated carbon two-day short course titled Activated Carbon Adsorption Principles, Practices and Opportunities.
Barbara Sherman, MS directs over 50 short courses, conferences and expositions for PACS, which has an extensive offering of activated carbon continuing education short courses. For more information, call (724) 457-6576, e-mail: HNpacs@aol.com or visit www.pacslabs.com.