Effect of Natural Gas Composition

Besides methane, which is the main component in a proportion usually above 90%, natural gas contains small amounts of nitrogen, carbon dioxide, and decreasing volumetric percentages of other hydrocarbons with an increasing number of carbon atoms: ethane, propane, butane, pentane, etc. (Table 4).

TABLE 4. Chemical composition (%-vol) of natural gas that Portugal imports from the Algerian Hassi R'Mel well. (Gas de Portugal.)

Methane CH4 83.7% Propane C3H8 1.9%

Ethane C2H6 7.6% Butane C4H10 0.7%

Nitrogen N2 5.4% Pentane C5H12 0.2%

Like methane, the other hydrocarbons are also adsorbed in the micro-porous volume of the carbon during refueling of the reservoir. However, the higher hydrocarbons, due to their higher adsorption potential, are easily desorbed during discharge. As the number of refueling and discharge cycles increases, the higher hydrocarbons progressively occupy the available micro-porous volume of the carbon which should be filled with methane [12]. This gives rise to a decrease of the net storage capacity of the reservoir (Figure 9).

In order to solve this problem, it is necessary an efficient and economic means of controlling the contaminants that are introduced into the reservoir during charge. The most economical solution is to install a purification system for natural gas at the refueling station [13]. The filter is quite simply a carbon bed because, as has been referred, it preferentially retains the higher hydrocarbons over methane.

' Amount of iN available in practice

The net capacity of the ith component (1 = CH4, 2 = C2H6, . . .) is defined as n

' amount of N pure CH4 available ^ isothermallt j where zt is the mole fraction of species i in the source gas. groups all hydrocarbons (HCs) and gives the total net capacity of the reservoir, because all HCs are combustibles. After many cycles the tank attains a cyclic steady state.

w

Pure methane, Isothermal conditions

Pure methane, real conditions

vixc,

C2

C3

C4

y y yCs

10 100 N.0 of cycles

0.2 2 Life span of the cylinder (years)

10 100 N.0 of cycles

0.2 2 Life span of the cylinder (years)

Figure 9. Decrease of storage capacity with the number of charge/discharge cycles, due to the effect of gas composition given in Table 4.

The carbon for this guard bed should, nevertheless, have slightly larger pores than those of the carbon employed in the storage tank. This way, kinetic effects that could degrade the selectivity of the guard bed are avoided. Regeneration is done by heating in an inert atmosphere.

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