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... 

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. 

Fig. 3.15, The CARS spectrum rotational width versus methane density for various values of parameter y (1) y = 0, (2) y = 0.3, (3) y = 0.5, (4) y = 0.7, (5) y = 0.75, (6) y = 0.9, (7) y = 0.95, (8) y = 1. Curves (4) and (6) are obtained by subtraction of the dephasing contribution from the line width calculated taking account of vibrational broadening. The other dependences are found assuming purely rotational broadening (vibrational relaxation neglected).

Buoyancy. Hydrogen would rise more rapidly than methane (density at standard condition is 1.32 kg/m3), propane (4.23 kg/m3), or gasoline vapor (5.82 kg/m3). 

TITc) 2.0, is very high compared to that of carbon dioxide, rR l.l. To overcome ideal gas behavior, it is necessary to compress methane to very high pressures to obtain reasonable methane densities and, hence, solvent power. 

Figure 1.17. Methane-density derivative of water-water PCF solid lines, water-water PCF dashed lines, the density derivative (or perturbation due to methane). Top panel, the 0-0 pair middle panel, the O-H pair bottom panel, the H-H pair.

Density profiles of the hydrocarbon mixture components are calculated by MD simulations. The examples of the profiles are presented in Fig. 1 for temperature 330 K at two pressures. The liquid phase is butane-rich the vapor phase is methane-rich. The density profiles turn out to be non-typical for a liquid film. Absolute value of the methane density does not change remarkably at the transition from vapor to liquid phase. Moreover, the absolute methane density is lower in liquid phase with respect to vapor at some conditions. It is interesting to note that there is a maximum of the methane density near the phase boundary at 40 atm. It points to the methane adsorption on the interface. Similar phenomena are observed at the modeling of the liquid in a contact with solid walls [15-17, 20], as well as in Coulomb clusters [45, 46]. 

LNGs are colourless, multi-component liquid mixtures of methane (density 422 kg/m ), ethane (density 544 kg/m ) and smaller proportions of propane (density 581 kg/m ), higher hydrocarbons, carbon dioxide and nitrogen (density 807 kg/m ), with normal boiling point at 1 bar in the range 112-120 K (or -161 to -153 °C). 

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