Activated Carbon Adsorption for Gas Storage

Natural gas is composed mostly of methane (85 to 90%) with only small amounts of ethane, propane, and butane. It is almost free of harmful contaminants and can bum efficiently, and may release only small amounts of harmful gases and vapors. However, the low energy density of natural gas retards its wide application as automobile fuel. One liter of petroleum on combustion produces about 3.5 x 10 KJ 

The natural gas consists mainly of methane, and the adsorption of methane on coals and activated carbons has been studied by a number of workers. The adsorption on coal has generally been found to be physical as indicated by small heats of adsorption and the reversibility of the adsorption isotherms. The adsorption at the ambient temperature takes place in the micropores. Ruppel et al. studied the adsorption of methane on coals at temperatures of 0 to 50°C and at pressures from 1 to 15 mPa, and applied Langmuir and Polanyi adsorption models. Ruppel found that the adsorption could be well explained by the Langmuir equation. 

Clarkson et al. studied the adsorption of methane on wet coals at elevated temperature and pressure above the critical point and applied the BET, DR (Dubinin-Radushkevich), and Dubinin-Astakhov (D.A.) equations to the adsorption data and compared the results with the Langmuir equation. The three-parameter DA equation gave the best curve fit to the high-pressure methane adsorption data. The two-parameter DR and BET equations were also found to be better than the Langmuir equation. The nonvalidity of the Langmuir equation to the adsorption data was attributed to the failure of the assumption of an energetically homogenous surface that was not strictly valid in these cases. 

FIGURE 5.15 Methane eifective adsorption capacity vs. storage pressure. (After Xing, W. and Yan, Z.F., Carbon 02, Intern Carbon Conf., Beijung 2002, Paper PI 94D 106. With permission.) 

Tijdscher, Algem. Tech. Vereen Bectwortelskikerfabr., Reff., 31, 91, 1936. 

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