Study of Methane Storage and Adsorption Equilibria in Multi-Walled Carbon Nanotubes

Adsorbed natural gas has various advantages and is relatively more economical than liquefaction and compression. Carbon nanotubes can be introduced as a new candidate for natural gas storage. In this study, adsorption of methane was firstly studied on the as-prepared multi-walled carbon nanotubes, and then chemical and physical treatment of MWCNTs was performed to enhance the methane adsorption. Treatment by acid washing and annealing with air improved purity, surface area and methane adsorption. The adsorption and equilibrium isotherm data of T-MWCNTSs, were measured by a static volumetric technique at different temperatures of 298, 291, 273 and 263 K and pressures up to 7 MPa. The maximum value of methane storage at normal temperature of 298 K was achieved to 2.81 mmole/g in our experiments. This amount of methane adsorption is equal to 108 v/v, meanwhile the target value of the adsorbed natural gas is 120 v/v to become as the accepted material for ANG process. The isosteric heat of adsorption of T-MWCTs was determined in the studied range of pressures and temperatures. The results revealed an energetically heterogeneous surface in methane adsorption. Furthermore, different isotherm models were fitted on the experimental adsorption data and the model parameters were correlated. Within the different studied isotherms, Sips equation provided best fitting to the experimental data.