Methane isotherms

The issue of the theoretical maximum storage capacity has been the subject of much debate. Parkyns and Quinn [20] concluded that for active carbons the maximum uptake at 3.5 MPa and 298 K would be 237 V/V. This was estimated from a large number of experimental methane isotherms measured on different carbons, and the relationship of these isotherms to the micropore volume of the corresponding adsorbent. Based on Lennard-Jones parameters [21], Dignum [5] calculated the maximum methane density in a pore at 298 K to be 270 mg/ml. Thus an adsorbent with 0.50 ml of micropore per ml could potentially adsorb 135 mg methane per ml, equivalent to about 205 V/ V, while a microporc volume of 0.60 mEml might store 243 V/V. Using sophisticated parallel slit... 

Fig. 5. Methane isotherms at 298 K on two potassium hydroxide activated carbons.

Thus, while models may suggest optimal pore spuctures to maximize methane storage, they give no indication or suggestion as to how such a material might be produced. On the other hand, simple measurement of methane uptake from variously prepared adsorbents is not sufficient to elucidate the difference in the pore structure of adsorbents. Sosin and Quinn s method of determining a PSD directly from the supercritical methane isotherm provides an important and valuable link between theoretical models and the practical production of carbon adsorbents... 

The storage capacity of an ANG storage system is always greater than its delivered capacity, usually by about fifteen percent. For some carbons, however, it can be as high as thirty percent because of the large amount of methane which is held by the adsorbent at less than one bar, (0.1 MPa). Carbons which are very microporous, such as polyvinylidene chloride (PVDC) carbons, tend to have very steep initial slopes to their methane isotherm, and as much as thirty percent of their overall uptake occurs at less than 0.1 MPa. These carbons have a high storage capacity but a much lower deliverable VfV. 

Figure 4. Methane isotherms on Pocahontas coal solid symbols represent desorption solid curves indicate isotherms of ascending temperature sequence and broken curves, descending temperature sequence...

Figure 5. Methane isotherms on Pittsburgh coal determined in increasing temperature sequence desorption points are solid...

In Figure 4 for Pocahontas coal the methane isotherms at —195°, —78°, 0°, 30°, and 50°C., determined in the sequence indicated, are shown as solid curves, and the isotherms at 0°, —78° and —195°C. after the initial sequence are shown as dashed curves. For the Pittsburgh coal, only the isotherms in a rising series of temperatures were determined (Figure 5). Figures 4 and 5 show a plot of methane isotherms at —195°C. on a relative pressure basis (pressure of methane/vapor pressure) because the vapor pressure is only about 10 torr. Isotherms determined at —195°C. represent metastable equilibrium and those at 30°, 50°, and possibly 0°C. equilibrium. Adsorption was... 

Surface areas reported in Table II show some of the difficulties in estimating a significant surface area for coal. Areas from the initial nitrogen and methane isotherms at —195°C. probably represent only the external area of... 

Equation 4.29 corresponds to pore sizes between 0.35 and 1.3 nm, and Equation 4.30 to higher pore sizes. PSDs were determined in the form of histograms (following the approach developed by Sosin and Quinn for high-pressure methane isotherms [59]). In each pore size range, the amount of gas adsorbed per total MPV was calculated as... 

FIGURE 4.7 PSDs obtained by different methods on the CMS1 sample. DFT and C02 are the PSDs obtained by applying the DFT method and the Dubinin-based method proposed by Cazorla-Amords et al. [10] to the C02 adsorption isotherm at 273 K (Figure 4.5), respectively. CH4 is the PSD obtained from the high-pressure methane isotherm at 298 K by applying the method developed by Sosin and Quinn [59]. 

Sosin KA and Quinn DF. Using the high pressure methane isotherm for determination of pore size distribution of carbon adsorbents. J. Porous Mat., 1995 1(1) 111-119. 

Sosin, K.A., Quinn, D.F., and MacDonald J.A.F. (1996). Changes in PSD of progressively activated carbons obtained from their supercritical methane isotherms. Carbon, 34, 1335—41. 

The consistency test of DFT was carried out by Jing et al. [1161 and more recently by Quirke et al. [117], Quirke et al. used the PSD obtained from nitrogen adsorption data to predict the adsorption isotherm of methane at normal temperature. As a result, the methane isotherm is considerably overpredicted. This is attributed to several reasons, two of which are that (1) at higher temperatures other nondispersive molecular interactions are weakened and (2) the methane has no dipole or quadruple, which may enhance the adsorbate-adsorbent interaction for N2 adsorption. 

Answer by author Initially in the determination of the 76°K pure methane isotherm, reactivation conditions were varied from room temperature and no evacuation to 250°F with evacuation for 24 hours with no noticeable effect on the reproducibility of data points. The authors feel that this maximum reactivation temperature is not high enough to affect a significant change in the surface of the adsorbent. Furthermore, the same effect of temperature on the adsorption of methane was observed in two separate sets of experiments. 

Figure 10.43. High-pressure nitrogen and methane isotherms on purified clinoptilolite at 22 °C (Jayaraman et al., 2002).

Figure 10.49. Nitrogen and methane isotherms on Sr-ETS-4 (heat-treated at 315°C) at 22°C (Jayaraman et ai., 2002).

We have mentioned in the previous part of this chapter that it is very important to determine the gas (especially the methane) isotherms at very wide temperature range including the critical temperatures as well. Such type of isotherm measurements have never been published in the literature, and we were convinced that these isotherms would give more informations on the energy structure of the sorption and desorption processes. We were not disappointed in our expectations. 

Methane isotherms measured at 77 K on TS and CFC samples are displayed in Fig. 3. CFC isotherm is a type II isotherm according to the lUPAC classification [10], while TS samples isotherms are type I (as evidenced by the logarithm plot of Fig. 5). A type IV isotherm should be measured for mesoporous adsorbents the NTR isotherm is flierefore not consistent with the previous TEM results, indicating that the mesopore network is not fully open. This result is consistent with the NTR low density (p = 0.8 g cm ) we have measured by helium picnometry. 

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