By Henry Nowicki and Bud Carr

The test method – Gravimetric Rapid Pore Size Distribution (GRPD) is the best available technology to characterize activated carbons and many other sorbents.

GRPD quickly provides clients with the broadest and deepest information about their materials physical and performance opportunities. GRPD provides an evaluation of cleanliness in the initial thermal cleaning step. GPRD provides the characteristic adsorption/desorption curve and its commercial comparison to the world of activated carbons, figure 1. GRPD provides adsorption as a function of temperature, figure 2. GRPD provides the number of adsorption sites per gram or volume at different adsorption energies, in differential (figure 1b) and cumulative (figure 1) presentations. GRPD provides a wide range of adsorbate(s) aqueous adsorbabilities in Freunlich isotherm presentation form. Methyl-tertiary-butyl ether (MTBE) is relatively difficult for activated carbon adsorption figure 3. Benzene is a better carbon adsorbate than MTBE, figure 4. Phenol at pH 7 or lower is the most strongly adsorbed of the three, figure 5. Clients can quickly place their specific application adsorbates into the range of the three compounds given in the standard GRPD test results. GRPD gives the materials pore size distributions, figure 6. The GRPD fits the sorbent into the six application types, figure 2b. The GRPD data can provide the BET surface area in squared meters per gram and the trace capacity number. No other single test method can deliver this much relevant sorbent information.

Recently we have reported a case study using GRPD to compare two activated carbon felt materials to help a client select the best material for their application1-2. If you would like an electronic copy of these two recent WQA presentations let us know. We regularly provide needed education by activated carbon users and suppliers3-4.

Our purpose here is to present GRPD data on the comparison of two samples; W-113 and W-114. No other present test method can get the information obtained from the Greenbank GRPD technology. GRPD is cost effective and superior compared to other activated carbon test methods. We have written this article using a format similar to a GRPD report of a sample run at our laboratory. Due to space considerations we have shortened our typical GRPD report by fifty percent because our reports run 16-25 pages. If you would like to get a full electronic copy of this abbreviated report contact Henry@pacslabs.com.

Executive Summary
Two activated carbon felt samples were characterized by gravimetric rapid pore size distribution (GRPD). The conditioned W-113 sample had 12 times more total adsorption pore volume than W-114 (Figure 1 as shown on the Cumulative Characteristic Curves). Sample W-113 was similar to coconut shell based activated carbon on the differential plot (Figure 1B) and W-114 performed more like wood based activated carbon in that it was without much small pore volume. W-113 has superior isotherm performance, see Figures 4, 5, and 6.

GRPD Results: Sample W-113, was a light felt and W-114 the more ridged and felt stronger. The W-113 and W-114 samples were characterized by measuring the entire characteristic curve using the GRPD as shown in figure 1.

The samples were only labeled W-113 and W-114. No other information was available.

These client sample carbons were then compared to three standard commercial activated carbon products made from a range of raw materials.

The samples were run as about 50 milligrams of cloth pieces. Standard testing conditions were used as previously reported. The W-113 sample lost 13.6 weight percent and the W-114 sample lost 22.56 weight percent on conditioning (heating to 240C in argon and holding for 25 minutes). Conditioning consists of heating the sample in a sweeping inert gas atmosphere. To compare samples it is necessary to clean them before GRPD analysis. Losses of less than 8 percent indicate a well stored activated carbon sample that has been protected from the small amount of moisture pick-up from ambient air during handling and storage and was also fresh and not oxidized. Old carbons can add oxygen functional groups. Sample W-114 had over three times the weight loss indicating either a spent carbon or one that was not protected (stored in a proper container) from oxidation or picking up humidity. All activities and adsorption capacities are calculated on a clean carbon weight basis. To observe these capacities in the field may require additional processing of the carbon in the field.

The GRPD runs were typical. The difference between the adsorption and desorption curves was minor through out the experiment, therefore there was no hysteresis present, as is normal for commercial activated carbons. This report extends the comparison of the five carbons beyond the presentation of the characteristic curves. The plots of the differential and cumulative characteristic curve data are presented in Figures 1 and 1b are on a weight-basis volume-based comparisons. The specific GRPD run data and results are below.

GRPD Raw Data
The GRPD (gravimetric rapid pore size distribution method) measures over 700 adsorption and desorption data points, covering seven orders of magnitude in relative pressure (isothermal basis) and three orders of magnitude in carbon loading. The mass adsorbed was also divided by the mass carbon to generate a weight percent loading for easier comparisons. After cleaning to 240C the adsorbent gas, C134a or 1,1,1,2-tetrafluoroethane (TFE), is introduced and the loading increases as the temperature is decreased. The activated carbon mass loading was plotted against temperature but the relative pressure was also changing. There are three variables affecting loading performance that change from point to point: vapor pressure, partial pressure, and temperature.

To make comparisons easier, the large data file of adsorption/desorption points at different temperatures and relative pressures was simplified. First the data was interpolated to get 30 evenly spaced points covering the entire 700 points in data range. Next the adsorption and desorption results were averaged to get the equilibrium values (the difference between adsorption and desorption was minimal for both samples – no hysteresis). The y-axis is converted to pore volume measures, in cc liquid adsorbed or cc pores filled/100grams carbon, instead of weight percent, by dividing TFE adsorbed weight by its density. The average/interpolated data for these characteristics curves is presented Figures 1 and 1b.

Performance Prediction Models
These characteristic curves are the only carbon related information required to predict physical adsorption performance using Polanyi Manes Adsorption Potential theory. These single and multicomponent, gas and liquid phase, computer models are used to predict carbon performance and are available from PACS. To do performance predictions the following polynomial equations describes these carbon samples:

In the equations, y is common logarithm of pore volume in cc/100g carbon and x is the e/4.6V adsorption potential in cal/cc. Characteristic curve polynomials are also available for some 5,000 samples run on the GRPD technology.

Performance in the Six Types of Applications
The simplest comparison of carbon for a specific application is to run the performance prediction calculations for a specific set of conditions: concentrations, and components present in the application. However, our experience with years of carbon optimization and performance comparisons has found that all physical adsorption applications can be placed into six application types. The proof is part of a 16 hour/800 slide training course on carbon fundamentals given by PACS at least once a year in conjunction with the annual International Activated Carbon Conference every October near the Pittsburgh, PA airport.

The comparative results demonstrate the value of the different carbons for use in the different types of applications on a weight basis. For a given application type, the results are related to the amount of carbon required to get a certain level of performance. Therefore a carbon with twice the cc/100g adsorption performance in an application type requires half the pounds of carbon to achieve a level of performance in that application type.

Since this client application was not space limited, performance on a weight basis is appropriate. These results can be multiplied by the samples apparent densities (AD) for the volume analysis. Volume based analysis is needed for applications where space for the activated carbon media is critical.

A series of two slides are attached as Appendix B which describe the 6 application types and the classification process to determine what the application type. Wastewater applications tend to be Type II or Type III, municipal water purification varies from Type III, Type IV or Type V applications. Removal limits are not low enough and analytical testing is not sensitive enough at this date for Type VI (purifying hydrogen of CO and N2 at room temperature is one of the few current Type VI applications). Municipal plants with surface water sources tend to be Type III or Type IV plants with ground water sources tend to be Type IV or V.

Figure 1 gives the values of the comparative results for the sample carbons versus the performance for the standard commercial carbons for the six application types.

Adsorption Isotherms
The characteristic curves are also translated into adsorption isotherms using the PACS software programs: Figure 3 for MTBE (weakly adsorbed material), Figure 4 for benzene (more strongly adsorbed species) and Figure 5 for phenol at pH=7 (quite strongly adsorbed material).

Pore Size Distributions
The Kelvin equation, modified by Halsey, can be used to convert the characteristic curve data to bet surface areas or pore size distributions. This is not useful in terms of performance evaluations, but some audiences are more comfortable with the concepts of pore radius and a series of capillary sizes when thinking about activated carbon. Figure 6 shows the cumulative pore size distributions which we include but find of little use (the BET surface area and trace capacity numbers (TCN) can be calculated from the GRPD data file. Calculated BET and TCN are provided to clients upon request, at no extra cost for our clients.

Interpretation of the GRPD results

  1. Both samples were activated carbon felts with no other information. The W-114 sample lost 22.56% and W-113 lost 13.62% by weight on conditioning indicating contaminated samples or exposure to humidity or an oxidizing atmosphere.
  2. The conditioned W-113 sample had 12 times more total adsorption pore volume than W-114 on a weight basis.
  3. The W-113 was similar to coconut shell carbon on the differential plot and the W-114 looks more like the wood carbon without much pore volume.
  4. The comparison of W-114 to W-113 showed 12 to 20 times worse performance in the six application types. W-113 was superior to all three commercial carbons in the six application types.
  5. The Polanyi-Manes isotherms for sample W-113 was significantly better than W-114.
  6. Material with PACS sample identification W-113 was the best material for the client application. No other existing test method could reveal the GRPD results reported here.
  7. The bottom line is: W-113 has 12 times more total adsorption space, 24 times more high energy adsorption spaces, and list 13.6% of its weight on conditioning compared to 22.y% lost by W-114. W-113 performs like a coconut shell based carbon and W-114 performs like a wood carbon.

References

  1. Henry Nowicki and Wayne Schuliger. “Modern Activated Carbon Characterization Method” Water Quality Association Conference. Orlando, FL, March 28, 2007.
  2. Henry Nowicki and Wayne Schuliger. “Selecting the Best Activated Carbon Adsorption Systems” Water Quality Association Conference. Orlando, FL, March 30, 2007.
  3. Wayne Schuliger, P.E. PACS Short Course: Design, Operation, Troubleshooting of Liquid and Vapor Phase Activated Carbon Adsorbers. See www.pacslabs.com for a full description and schedule.
  4. Henry Nowicki, Ph.D. PACS Short Course: Activated Carbon Adsorption: Principles, Practices, and Opportunities. See www.pacslabs.com for more information and a full list of some 57 courses, eleven courses for the activated carbon industries.

About the authors
Henry Nowicki, Ph.D./M.B.A. directs the day-to-day Testing R&D, and Consulting Services at PACS: Testing, Consulting, and Training. You can reach Nowicki by e-mail: Henry@pacslabs.com, phone: 724. 457. 6576 or web site www.pacslabs.com.

Bud Carr is a GRPD instrument specialist for PACS laboratory services. He has run over 5,000 samples on the GRPD instrument, mostly activated carbon materials, but has run a wide variety of other commercial materials. Carr can be reached at Budpacs@aol.com.

About the Company
PACS is in its third decade of providing services for the activated carbon and related industries. Dr. Henry Nowicki is the chairperson for the 20th International Activated Carbon Conference

(IACC-20) near Pittsburgh PA and invites presenters and vendors to attend and/or make presentations October 18-19, 2007 at IACC-20.

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