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GAED Comparison of Two Activated Carbon Cloth Materials
Gravimetric Adsorption Energy Distribution
Full Characterization of W-113 and W-114 using the GAED
PACS
Executive Summary
Two felt samples were characterized by gravimetric adsorption energy distribution (GAED). The conditioned W113 sample had 12 times more total adsorption pore volume than W114 (Figure 7a). The W113 was similar to coconut shell based activated carbon on the differential plot and the W114 performed more like wood based activated carbon in that it was without much small pore volume.
GAED Results:
Sample W113, was a light felt and W114 the more rigid and felt stronger.
The W113 and W114 samples were characterized by measuring the entire characteristic curve using the GAED.
| Sample ID | Sample Description |
| Style 'E' | W113 Carbon Felt |
| Style 'G' | W-114 Carbon Felt |
| BG-HHM - wood base | BG-HHM Wood-base |
| CAL Coal-base Liquid phase | CAL Coal-base Liquid phase |
| PCB coconut-base | Pcbstd |
These carbons were then compared to three standard commercial activated carbon products made from a range of raw materials.
The sample was run cloth pieces. A summary of the actual test data and conditions used is listed in the data summary table at the end of the report in Appendix A. The W113 sample lost 13.6 weight percent and the W114 sample lost 22.56 weight percent on conditioning (heating to 240C in argon and holding for 25 minutes). Losses of less than 8 percent indicate a well stored 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. The W114 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 basis. To observe these capacities in the field may require additional processing of the carbon in the field.
The GAED 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 four carbons beyond the just the presentation of the characteristic curves. The plots of the differential and cumulative characteristic curve data are presented in Figures 1a and 1b in both weight-based and volume-based comparisons. The specific run data and results are attached as Appendix A.
GAED Raw Data
The GAED (gravimetric adsorption energy distribution) 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. The raw data is plotted in Figure 2. At 240C the adsorbent gas, C134a or 1,1,1,2-tetrafluoroethane, is introduced and the loading increases. Note in Figure 2 the mass loading was plotted against temperature but the relative pressure was also changing. There are three variables affecting 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 data range. Next the adsorption and desorption results were averaged to get the equilibrium values (the difference between adsorption and desorption was minimal for this sample - 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. The average/interpolated data for these characteristics curves is presented in Table 1, and Figure 1a and 1b.
In the equation, 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 listed in Appendix A.
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 conditions, concentrations, and components present in the application. However our experience with years of carbon optimization and performance comparison have 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.
The comparative results in Table 2 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.
Unfortunately the AD's were not known so we are forced to use Table 2b, which compares performance on a weight basis. These results can be multiplied by the AD when available for the volume analysis.
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.
Table 2 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 programs mentioned above; Figure 3 for MTBE (weakly adsorbed material), Figure 4 for benzene (more bly adsorbed species) and Figure 5 for phenol at pH=7 (quite bly 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 was not calculated but could have been from these curves).
Interpretation of the GAED results:
- Both samples were felts with no other information. The W114 sample lost 22.56% by weight on conditioning indicating a contaminated sample or exposure to humidity or an oxidizing atmosphere.
- The conditioned W113 sample had 12 times more total adsorption pore volume than W114 (Figure 7a).
- The W113 was similar to coconut on the differential plot and the W114 look more like the wood without much pore volume.
- The comparison of W114 to W113 showed 12 to 20 times worse performance in the six application types. W113 was superior to all three commercial carbons in the six application types.
Appendix B: The Six Application Types and Classifying an Application
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