Adsorbed methane density

Several alternative methods have been considered in order to increase the energy density of natural gas and facilitate its use as a road vehicle fuel. It can be dissolved in organic solvents, contained in a molecular cage (clathrate), and it may be adsorbed in a porous medium. The use of solvents has been tested experimentally but there has been little improvement so far over the methane density obtained by simple compression. Clathrates of methane and water, (methane hydrates) have been widely investigated but seem to offer little advantage over ANG [4]. Theoretical comparison of these storage techniques has been made by Dignam [5]. In practical terms, ANG has shown the most promise so far of these three alternatives to CNG and LNG. 

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... 

From isotherm measurements, usually earried out on small quantities of adsorbent, the methane uptake per unit mass of adsorbent is obtained. Sinee storage in a fixed volnme is dependent on the uptake per unit volume of adsorbent and not on the uptake per unit mass of adsorbent, it is neeessary to eonvert the mass uptake to a volume uptake. In this way an estimate of the possible storage capacity of an adsorbent can be made. To do this, the mass uptake has to be multiplied by the density of the adsorbent. Ihis density, for a powdered or granular material, should be the packing (bulk) density of the adsorbent, or the piece density if the adsorbent is in the form of a monolith. Thus a carbon adsorbent which adsorbs 150 mg methane per gram at 3.5 MPa and has a packed density of 0.50 g/ml, would store 75 g methane per liter plus any methane which is in the gas phase in the void or macropore volume. This can be multiplied by 1.5 to convert to the more popular unit, V/V. 

Mcntasty el al. [35] and others [13, 36] have measured methane uptakes on zeolites. These materials, such as the 4A, 5A and 13X zeolites, have methane uptakes which are lower than would be predicted using the above relationship. This suggests that either the zeolite cavity is more attractive to 77 K nitrogen than a carbon pore, or methane at 298 K, 3.4 MPa, is attracted more to a carbon pore than a zeolite. The latter proposition is supported by the modeling of Cracknel et al. [37, 38], who show that methane densities in silica cavities will be lower than for the equivalent size parallel slit shaped pore of their model carbon. Results reported by Ventura [39] for silica xerogels lead to a similar conclusion. Thus, porous silica adsorbents with equivalent nitrogen derived micropore volumes to carbons adsorb and deliver less methane. For delivery of 150 V./V a silica based adsorbent would requne a micropore volume in excess of 0.70 ml per ml of packed vessel volume. 

From the above data, it would appear that methane densities in pores with carbon surfaces are higher than those of other materials. In the previous section it was pointed out that to maximize natural gas or methane storage, it is necessary to maximize micropore volume, not per unit mass of adsorbent, but per unit volume of storage vessel. Moreover, a porous carbon filled vessel will store and deliver more methane than a vessel filled wnth a siliea based or polymer adsorbent which has an equivalent micropore volume fraction of the storage vessel. 

Because of the observed increase in apparent density of the particles of active carbons AG and RN (see Figure 3a and 3b) and variations of the specific surface area Sbet, especially for the samples of the carbon RN (Table 2), from which the most external layers were removed, it was decided to carry out investigations of their storage capacity. Total amount of gas contained in the adsorption system is used as the measure of storage capacity, and in the case of methane it includes adsorbed and non-adsorbed methane present hoth in adsorbent particles and in interparticle spaces. 

Should the activated carbon be used for methane storage in a vessel at a pressure of 3.4 MPa, it is necessary to distinguish between (i) the volume of the carbon skeleton, which reduces the effective volume of that vessel (ii) the non-microporous volume, which is the summation of the volumes of mesopores, macropores and inter-particle voids (in this non-microporous volume the density of adsorbed methane is low, similar to that of compressed methane at 3.4 MPa, i.e. 0.023 g cm"- ), and not very useful for storage and (iii) the volume of micropores where the density of the adsorbed methane will be high due to the high adsorption potential within the micropores. The later is the important volume for gas storage when in competition with compressed natural gas (21.0 MPa). Consequently, the carbon for this application must have the maximum volume of micropores and a minimum volume for other pores and voids. 

The density of the adsorbed methane has been calculated for the disks activated with ZnCl2 and H3PO4 and for carbons obtained by gasification with carbon dioxide (800 °C) of some of the disks. In this way, a relatively large variety of carbon disks covers a wide range of micropore size distributions, wider than when attempting the preparation by only one activation method. 

As the adsorption of CO2 provides the volume of narrow micropores (up to say 0.7 ran) and the adsorption of N2 the volume of total microporosity (up to 2.0 ran), it is possible to estimate the density of adsorbed methane in each pore size range. The mass of methane adsorbed at a given pressure is the summation of that in narrow micropores (V iC02) X density in narrow micropores) and that in wide micropores [(V iN2) - (V iC02)] X density in wide micropores) ... 

The density data discussed above correspond to methane adsorbed at 3.4 MPa. The method has been applied to the adsorption isotherms for methane at different pressures, when plots similar of that of Figure 6.12 are obtained, one for each selected pressure. The values of d Equation (6.1) show that the density of adsorbed methane in narrow micro-pores increases rapidly with pressure up to about 0.25 gcm" at 2.5 MPa. This means that the maximum capacity for an activated carbon with only narrow micropores is reached at 2.5 MPa. The presence of wide micropores increases the adsorption capacity, but the density in these pores is much lower, and is significantly influenced by pressure. The density of methane adsorbed in these wide micropores at 0.5 MPa is negligible and it increases linearly with pressure up to the value of 0.9 gcm at 3.4 MPa, a value much larger than the density for compressed methane at that pressure (around 0.023 gcm" ). 

The models of Matranga, Myers and Glandt [22] and Tan and Gubbins [23] for supercritical methane adsorption on carbon using a slit shaped pore have shown the importance of pore width on adsorbate density. An estimate of the pore width distribution has been recognized as a valuable tool in evaluating adsorbents. Several methods have been reported for obtaining pore size distributions, (PSDs), some of which are discussed below. 

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