Burn-off degree

FIGURE 4.2 CXs-plot obtained from the high-resolution N2 isotherm at 77 K of an ACF prepared from Nomex (E.I. DuPont de Nemours Company, Inc., Wilmington, DE) by physical activation with C02 (72% burn-off degree). Spheron 6 carbon black is used as reference. 

Thus, the objective of this section is to make a comparison between the results obtained by SAXS technique and by gas adsorption (nitrogen at 77 K and C02 at 273 K). These results include pore size and porosity development. For this study, ACFs, which have been prepared using C02 and steam as activating agents up to different burn-off degrees, have been used. 

ACFs were prepared from commercial carbon fibers according to the procedure described in the literature [85], Two series of ACFs obtained from C02 (series CFC) and steam (series CFS) activation have been used in this study. The burn off of the fibers is between 11% and 54%. The nomenclature of each sample includes the burn-off degree. 

Figure 4.14 shows the SAXS plots for the ACFs prepared by C02 and steam activation. All the scattering curves have the same trend, and the main difference between them is that the intensity increases with the burn-off degree due to the development of porosity. In order to estimate the mean pore size from the SAXS data, the general approach based on Guinier equation has been used (see Equation 4.50). Table 4.3 presents the Guinier radii for the ACF and the original fiber. These values are quite similar to those obtained in a previous study done with ACF [87], Table 4.3 also contains the pore width calculated for spherical particles (see Equation 4.51). 

It has to be mentioned that the micropore surface area values, calculated by Stoeckli-Ballerini equation [9], present a maximum value with the burn-off degree of the samples, either from the N2 or CO2 adsorption isotherms data. This is due to the increase in the pore size (L values), and the assumptions made in the equations. Thus, as the pore size -L- increases, the micropore surface area for a determined micropore volume -Wo- is smaller. 

The effects of the adsorption mechanism, the pore geometry and the energetic heterogeneity of the pore walls on the determination of the micropore size distribution of activated carbons from adsorption isotherms are evaluated by means of Monte Carlo simulation. Results are applied to the characterization of two series of activated carbons with different burn-off degrees. 

Our characterization method, based on Monte Carlo simulation in the continuum, was applied to predict the MSD of two series of activated carbons, obtained by carbonization of olive stone For series D, the activation step took place in a flow of carbon dioxide at 1098 K, while for series H a flow of water vapor at 1023 K was used Activated carbon samples D8, D19, D52, D70 and H8, H22, H52 and H74 were obtained, where the number represents the burn-off degree Details concerning the preparation of the samples and the measurement of N2 adsorption isotherms at 77 K are given in [19-22]... 

Both the behavior of MSD predicted by the slit geometry and the one predicted by the triangular geometry are generally consistent with the increase in burn-off degree for all carbons of the series, so that this analysis is not sufficient to decide which geometry is more appropriate to describe the structure of activated carbons. However, our simulation and characterization method allows us also to obtain the behavior of the isosteric heat of... 

Understanding the Activation-Pore Structure Relationship of ACFs Effect of Activating Agent and Burn-Off Degree... 

FIGURE 4.38 USXE spectra CK of AC (40% bum-off) samples with different burn-off degree (30%-47%). (Adapted from Appl. Surf. Sci., 258, Gun ko, V.M., Zaulyehnyy, Ya.V., Ilkiv, B.I. et al.. Textural and electronic characteristics of mechanochemically activated composites with nanosilica and activated carbon, 1115-1125, 2011f, Copyright 2011, with permission from Elsevier.)... 

FIGURE 1.21 Pore size distribution by DPT method of ACF prepared from carbon fibers (Donac) by (a) KOH activation, (b) NaOH activation using different hydroxide/car-bon fiber ratio, and (c) by physical activation with CO2 at 890°C to different burn-off degrees (redrawn from Macia-Agull6, J.A., Moore, B.C., Cazorla-Amords, D., and Lin-ares-Solano, A. Carbon 42(7) 1367-1370, 2004. With permission). 

Measured radial carbon profiles in the decoked particles are depicted in Figure 6.9.13. The experimental times to reach 50% burn-off are compared wiffi the numerically calculated data. [Coked catalyst particles were regenerated at different temperatures up to a burn-off degree of 50% (Kern, 2003).] The calculated and... 

According to this model, the influence of pore diffusion is restricted to a coke-free shell (r > rc) whereas in reality (at least for medium temperatures and not too high burn-off degrees) coke is still present in this outer zone. This leads to an underestimation of the carbon conversion by the model. Conversely, the assumption that the carbon load in the core region (0 < r < rc) is still equivalent to the initial value overestimates the burn-off rate compared to reality, where both Ic and C02 decrease in the core region of the particle. As shown below, these two effects compensate each other quite well. 

Figure 6.9.14 shows the influence of regeneration time on the burn-off degree at different temperatures. The agreement between the exact numerical solution and the approximation by the closed solution of the combined model - Eqs. (6.9.19)-(6.9.21) - is very satisfying. 

Figure 4 represents, in histogram form, the evolution of the different pore volumes for activated carbons of series A and B as a function of burn-off degree obtained in CO,. Uncatalyzed and catalyzed process have been included for comparative purposes. 

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