Methane, adsorption/storage

Mota, J.P.B. Rodrigues, A.E., and Tondeur, D., Charge dynamics of a methane adsorption storage system Intraparticle diffusional effects. Adsorption, 3(2), 117-126 (1997). 

Adsorption of supercritical gases takes place predominantly in pores which are less than four or five molecular diameters in width. As the pore width increases, the forces responsible for the adsorption process decrease rapidly such that the equilibrium adsorption diminishes to that of a plane surface. Thus, any pores with widths greater than 2 nm (meso- and macropores) are not useful for enhancement of methane storage, but may be necessary for transport into and out of the adsorbent micropores. To maximize adsorption storage of methane, it is necessary to maximize the fractional volume of the micropores (<2 nm pore wall separation) per unit volume of adsorbent. Macropore volume and void volume in a storage system (adsorbent packed storage vessel) should be minimized [18, 19]. 

Of these four regions, methane adsorption only occurs in the micropores of an activated carbon. At ambient temperatures, there is negligible methane adsorption in the macropores, and the macropore volume can be considered as void space methane is present in these voids only at the density of the compressed gas. Hence, the volume occupied by macropores and interstitial voids must be minimized for optimum storage capacity, consistent with the need to allow sufficient... 

From a practical standpoint, to increase the methane adsorption capacity we need to develop not only micropore volume but also control carefully the micropore size distribution. Thus, samples with high surface areas and very narrow micropore size distributions are required for this application. For this reason, in our studies of methane storage KOH instead of NaOH was used as the activating agent for the preparation of ACs because of the narrower MPSDs that can be obtained by KOH, as shown in Sections II and III. 

Effects of chemical modification of carbons on their methane storage capacities have been reported. Kaneko and Murata (1997) modified AX-21 by MgO impregnation and showed a substantial increase in methane adsorption at 10 MPa and 303 K. However, as pointed out by Cook et al. (1999), on a V/V basis there would be no advantage as the micropore volume is significantly reduced. Surface treatment by oxidation and reduction of activated carbon (BPL) showed essential no effects on methane adsorption (Barton et al., 1991). 

Grand canonical Monte Carlo simulations performed for natural gas adsorbed on carbon have demonstrated that the optimum pore size is 0.76 nm, as mentioned earlier (methane adsorption (capillary condensation) does not takes place at room temperature) thus, a further increase of the slit width would lower the particle density without a significant increase in amounts adsorbed. These calculations predict that the theoretical maximum methane storage capacity of carbon at 3.5 MPa is 209 VA for a monolithic carbon that fills the vessel and contains a minimum amount of macropore volume and no external... 

The enhanced concentration at the surface accounts, in part, for the catalytic activity shown by many solid surfaces, and it is also the basis of the application of adsorbents for low pressure storage of permanent gases such as methane. However, most of the important applications of adsorption depend on the selectivity, ie, the difference in the affinity of the surface for different components. As a result of this selectivity, adsorption offers, at least in principle, a relatively straightforward means of purification (removal of an undesirable trace component from a fluid mixture) and a potentially useflil means of bulk separation. 

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