Adsorption Isotherm

Despite the presence of the adsorbent inside the reservoir, its operation is identical to that of a regular compression tank. The pressure inside the reservoir decreases progressively with fuel consumption.

The operation of the reservoir under vacuum conditions is not easily and economically achievable onboard a vehicle. The impediment for this is that the gas can only flow to the combustion chamber if the tank pressure is higher than that of the engine. For this reason, the reservoir is considered depleted when its pressure attains a value which is slightly above atmospheric (1.2-1.4 atm).

Due to this operational constraint, adsorptive storage poses a challenge which is directly related to the shape of the adsorption curve. As opposed to compression storage, the amount of stored gas by adsorption does not vary linearly with pressure: the adsorption curve is concave, showing a steeper slope in the region of lower pressures (Figure 4). As a consequence, a significant amount of gas remains in storage when the pressure attains its minimum operating value [7]. Depending on the adsorbent, the amount of undelivered gas can total up to 30% of the amount stored at charge pressure (35 atm).

For this reason, a carbon which adsorbs a large amount of NG at charge pressure (35 atm), is not necessarily a good candidate for a storage reservoir. In fact, what defines the available (net) capacity of the reservoir is the difference between the amount of gas stored at charge pressure and that stored at depletion pressure (Figure 4).

Adsorption Pressure Curve

Figure 4. Adsorption storage curves for three different adsorbents. The reservoir represented by curve (a) has a total capacity which is not enough; the system characterized by curve (b) has an insufficient net deliverable capacity although its total capacity is large; the reservoir represented by curve (c) has more desirable characteristics.

Figure 4. Adsorption storage curves for three different adsorbents. The reservoir represented by curve (a) has a total capacity which is not enough; the system characterized by curve (b) has an insufficient net deliverable capacity although its total capacity is large; the reservoir represented by curve (c) has more desirable characteristics.

A promising solution to reduce the effect of depletion pressure is to introduce a gaseous additive in the NG that can selectively compete with it for the available space in the micropores [8]. If the affinity of the carbon towards the additive over natural gas decreases with increasing pressure, then the net result is an increase of the deliverable capacity of the reservoir.

In practice, this method corresponds to artificially changing the shape of the adsorption curve by introducing a competitive species. The optimum operating conditions depend on the nature and concentration of the additive. An inherent difficulty associated to this method is that the additive is dragged by the discharge stream and has to be reintroduced into the reservoir during the subsequent charge.

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